Mareks Virus- The Big Informational

  1. seminole wind
    This was written by a friend of mine Jennifer Miller, who gathered the most correct information I know. This FAQ has been checked over for accuracy by a group of people who are known to be very very knowledgeable about Marek's Virus and have spent hours/years researching information from the best resources in the world. Still, there are many unknowns.

    The Great Big Giant Marek's Disease FAQ
    BY Jennifer Miller

    By: Nambroth (user name)

    Posted 10/25/13 Last updated 9/18/14 87,115 views 42 comments

    The Great Big Giant Marek's Disease FAQ

    Note from the author: As this article deals with a viral disease, new information is always forthcoming. As such, this is a work in progress and I will come back in and edit to add (or remove) information as I learn more, in order to keep this information as up to date and factual as possible.

    This article is long! If you are just looking for specific information, I have broken it up into groups. Look for the chapter headers in ALL CAPS to help you find what you are looking for, faster!

    This article is written for the average home chicken keeper. It is not aimed at larger production style chicken keeping, nor is it intended to be a scientific paper. It is not intended to be a compilation of every bit of Marek's information out there, but enough to get you started in case you need to research deeper. When I started researching this disease, I found there was a lot of information out there, but it was scattered to the seven winds and a lot of it was hard for the layman to understand. It is my hope to keep this article up-to-date as possible with the limited resources that the public has, in an effort to help average backyard chicken keepers understand, prevent, and manage this disease in their flocks. When in doubt, seek the professional advice of a licensed veterinarian, poultry examiner, or other qualified person in regards to this disease.

    If you have concerns about Marek's in your bird or flock, or need help, see if any of the following FAQs can help you. Please be sure to check out the resources list at the bottom.

    Disclaimer: I am not a veterinarian or an expert in any of the following. This information is a constantly evolving work of art that has been gleaned from months of research. Above all, you must never take this as medical or veterinary advice and when in doubt please contact an actual expert (see resources). My sources include: Avian veterinarians, published veterinary papers, diagnostics labs, personal research and the anecdotal experiences of others with this disease. While this article is written in my own words, my knowledge has amassed from all of these sources.

    It is important to be open to new breaking research and findings about this disease. Like many human retroviruses, things can change, and what was once a rule is now the exception. Use common sense, don't be afraid to research, and never be afraid to ask questions. This disease is estimated to cost the commercial poultry industry in excess of $1 billion dollars annually, so new research is constantly being funded, and new vaccines are under trial. I will do my best to keep this up to date with any useful new information.

    What is Marek's disease?

    Very simply, Marek's disease is a viral disease of poultry, primarily chickens. It can come in several "forms" and symptoms can include, but are not limited to: Partial or total paralysis of the legs, sometimes wings, and even neck; visceral lymphoma (cancerous tumors in the body), blindness, tumors or growths on the skin, general listlessness, wasting away, or poor health, to name a few. Some chickens get only one symptom, and some get more than one. Some chickens get symptoms, while some don't show any signs. It is a very common disease, as it is spread via chicken dander (dust) and is easily spread on new chickens, the wind, your clothes and shoes, and even wild birds can transport it. It does not always show up in quarantine. It can be fatal, and there is no cure. Some chickens do survive it, and many build resistance to it. If you suspect you have Marek's disease in your flock, do not panic, but please do educate yourself. This is a somewhat
    complicated disease and the very best thing you can do for yourself and your chickens is to learn about it and take an educated approach to managing Marek's. I will discuss this disease in great detail below, so please continue reading.




    Scroll down to see the different chapters! They are in a large red font.

    To Start:
    A few Facts about Marek's Disease. Most of these are re-hashed in the various questions below, but it will get you started on this complicated journey:

    Marek's disease is a virus. Specifically, it is a type of alpha herpesvirus, and more specifically it is a DNA virus.

    Marek's cannot be transmitted vertically. This means that a mother hen cannot pass Marek's on through the egg to her chick. The chick would have to be exposed to the virus after hatching to catch Marek's.

    There is no cure for Marek's.

    Chickens exposed to Marek's disease must be considered carriers for life, even if they were previously vaccinated and/or never develop symptoms.

    Current research indicates that Marek's is dominantly spread via chicken dander (chicken dust) and is contracted by inhalation.

    Marek's is highly contagious among chickens.

    Marek's can be contracted in a chicken by:

    1) Exposure to other chicken(s) that have Marek's disease, even if they show no symptoms;

    2) Exposure to an environment that has had Marek's disease shed upon it;

    3) Exposure to dander transported unwittingly from infected chickens by wild birds, the wind, rodents, and human traffic (shoes/clothes). The chance of transport of dander by wind, wild birds, and rodents increases if there is an infected flock nearby.

    Marek's disease virus is known to survive in an exposed environment (such as the soil where chickens are kept) for at least 5 months, and possibly for several years.

    Marek's disease is not zoonotic (contagious) to humans.

    The meat and eggs from infected birds are perfectly safe to eat.

    To date, Chicken Marek's disease virus (MDV) is not known to be zoonotic (contagious) to other commonly kept fowl, except rarely in quail, and in some cases inclosed commercial breeding of turkeys in Europe (said turkeys had been housed in closed quarters close to infected broiler chickens).

    Chickens will become infected with Marek's virus if exposed, regardless of vaccination. Vaccination only gives chickens a chance to built resistance and reduces the instances of symptoms developing.

    A vaccine derived from Turkey Herpesvirus (MDV-3) is the most common vaccine administered to non-commercial chickens. It alone can not cause the chicken to become infected with or shed Chicken Marek's Disease Virus (MDV-1).


    What are the symptoms of Marek's?

    Symptoms in animals are more accurately called 'signs', as an animal cannot literally describe symptoms to us. However, to keep things simple and easier to understand, I shall use the word "symptoms" in this article.

    There are several forms or ways that Marek's can present itself in chickens. These symptoms may overlap, or not. They may also present in ways not listed below if the chicken is experiencing more than one illness at once. Marek's can be frustrating to diagnose.

    Symptoms can include but are not limited to the following.

    Some chickens will have multiple symptoms. Some chickens will have only ONE symptom, and some will have none (frustratingly enough):

    Classical Marek's (nerolymphatosis), also known separately as neural and visceral forms:

    Paralysis of one or both legs, and sometimes wings. This can include staggering, increasing loss of motor control in one or both legs, inability to stand or balance. This may present sometimes as one leg forward and one leg back (the splits) or simple paralysis of the legs. This is caused by lesions to the sciatic nerve, that controls the legs movement.

    Going off of food or inability to connect with food when trying to eat. Sometimes mistaken for a sour crop or other crop problems, lesions on the vagus nerve can cause dilation of the crop and/or proventriculus. The inner lining of the gizzard may also be effected.

    Difficulty breathing, darkening comb. Caused by several factors; brachial nerve lesions can restrict respiration. Additionally, lymphomas are known to grow into the heart muscle, reducing the chicken's ability for proper respiration. The comb may become very dark red or purple in appearance and gasping or trouble breathing may occur. Marek's does not cause discharge, watering eyes or nose, or gurgling-- however these symptoms may be the result of a secondary viral infection by a different disease.

    Lymphomas / Neoplasms (cancerous tumors) throughout the chicken. Though these symptoms can usually only be observed after death and a necropsy is conducted, lymphomas typically grow on the thymus (located in the neck, near the crop). The tumors then move on to grow on any of the following: gonads, spleen, liver, kidneys, lungs, heart, proventriculus, adrenals, muscles, and sometimes skin (enlarged feather follicles). These lymphomas are often aggressive, fast-growing, and are usually fatal as they cause organ failure.

    Weight loss, wasting, depression. Inability to eat, connect with food, or digest food because internal tumors can cause rapid weight loss in birds. They may also waste away more slowly with no other obvious symptoms.

    Loose, watery, and/or bright green stool. As the digestive system shuts down, or because the bird is not eating enough, the stools become increasingly loose and consisting of very little to no solids. This may be accompanied by green, bright green, or yellowish coloration.

    Ocular Marek's (ocular lymphatosis):

    Discoloration of the iris. The iris may turn grey or a pale blue color.

    Deformity of the pupil. The pupil may change shape. A typical form is a keyhole shape but the pupil may also look as if it is melting or might become amoeba-like in shape.

    Pupil with no reaction to light changes. The pupil in one or both eyes may become constricted in appearance and might not react to changes in light. This can create a 'tiny pupil' appearance, called Miosis.


    Cutaneous Marek's:

    Lesions or deformities at the feather follicles. This may be minor to severe and can range from large bumpy nodules to crusty looking lesions. They may be rounded or hard.

    General symptoms:

    Immunosuppression. Birds infected with Marek's may have a periodic or lifelong supression of the immune system. Marek's virus is known to cause impairment of T-lymphocytes which weakens the immune system. Secondary diseases or illnesses may then present as the chicken's ability to fight them off is reduced considerably. Marek's infected birds with immunosupression are known to be much more susceptible to coccidiosis and viral respiratory diseases. Immunosuppression may be transient (does not last) or it may be permanent. Some birds experience full immunosuppression (they do not have any immune system left) in which case a secondary disease is usually the ultimate cause of death.

    Come and go symptoms with no obvious resolution. Some birds may present some symptoms, appear to get well, and then become symptomatic again at a later point. Problems might seem to come and go without much reason (this can be very frustrating to diagnose).

    My chicken has one or more of these symptoms! Is it Marek's?

    Maybe. There is no way to 100% diagnose Marek's disease (see Diagnosis below). However, there is a lot of good guesswork you can do. It is important to look at the big picture and try to make a flock history. Sometimes it is useful to write everything down as you discover "clues". Some questions to ask yourself: Was this bird properly vaccinated? Has the bird been exposed to any possible Marek's disease sources (see Control below) in the last several months? What symptoms do I observe? Do the symptoms fit any other disease? How old is the bird? Have I lost any other birds with these symptoms or due to a "mystery illness"? Is Marek's common in my area (you can call your county or state poultry extension to find out)? Has this bird or my flock been stressed or sick in the last several months? Does anyone near me have chickens that might have had Marek's?

    Are there any other diseases that look like Marek's?


    It is very important to note that many of the symptoms of Marek's are also symptoms of other problems in chickens. In terms of biosecurity, it is very wise to proceed cautiously, as if you have Marek's, but as there is no cure for Marek's it would also be wise to explore treatments for disease that look like Marek's.

    Vitamin Deficiencies can cause weakness or paralysis, for example, and those can be reversed with careful supplementation. If you are experiencing paralysis or weakened muscle control in your chicken(s), be sure to explore this possibility: Note that B vitamin deficiencies can especially look like Marek's disease, due to the interactions of B vitamins with a healthy nervous system. If you experience leg/foot problems, lameness, paralysis, etc... with toe-curing especially, be sure to investigate B2 deficiency: The body's need and ability to use vitamins to maintain general and immune health is complex, and can stem from simple problems such as old feed that has lost nutritional value over time (several vitamin complexes break down quickly with age!), to
    incomplete feed, to a deficiency in the bird's mother (which is passed onto the chick through the egg), to complex genetics that interfere with the uptake of vitamins. The good news is that vitamin deficiencies are sometimes correctable, and may save a bird's life.

    Avian Lymphoid Leukosis is a disease that can cause Marek's like symptoms, though signs are usually only visible upon necropsy. This virus causes lymphomas, much like Marek's disease, throughout organ tissue. Most chickens with this virus will experience weakness, and will "waste away" over time, becoming more and more emaciated as the tumors spread. This viral disease is often thought to be the 'sister disease' to Marek's, as it is very similar in many ways. Unlike Marek's disease, it can be transmitted through the egg (vertically) from parent to chick. Of important note, there have been some signs that in individual chickens with a genetic predisposition, that Serotype-2 Marek's vaccine (only hatcheries have this vaccine) may cause this disease to more rapidly harm the infected chicken.

    Heavy Metal Toxicity in chickens (and other fowl, such as ducks) can look very similar to Marek's paralysis. Lead toxicity seems to be the most common for birds like chickens, that unlike other pet birds such as parrots, generally do not chew on metal objects but may swallow small metallic objects whole. Lead shot, BBs, pellets, (etc) are often mistaken for stones and ingested to aid the bird in digestion. A single BB or piece of lead shot is enough to cause serious illness in a large fowl chicken, or even eating old lead paint flakes, or finding them in the soil is enough to harm a chicken-sized bird. Heavy metal toxicity is one of the leading medical problems that vets and wildlife rehabilitators see in ground dwelling birds such as chickens, ducks, and geese, so it can be somewhat common. Symptoms include neurological issues such as partial or total paralysis of one or both legs and sometimes the wings. With lead toxicity, lesions of the nervous
    system and elevated white blood counts can also mimic Marek's infection. X-rays and/or blood tests might be necessary to diagnose this problem. Treatment generally involves injections of a chelating agent such as Calsenate. Large metal objects may need to be surgically removed. This problem is very hard to diagnose without veterinary help. For more reading:

    Botulism in fowl can also mimic the symptoms of Marek's, in that it often causes neurological distress and paralysis. Often birds with botulism will present leg weakness, and neck weakness or paralysis. This can come on quite suddenly or gradually, depending on how much of the Botulism toxin has been consumed by the bird. Botulism is caused by the consumption of the toxin, either from decaying material (usually decaying carcasses) or eating an abundance of invertebrates that have been infected with the Botulsim toxin (such as maggots that have been feeding on decaying material). Generally, if a bird survives more than 48 hours, it will recover, so if Botulism is suspected in birds with sudden paralysis, immediate treatment is necessary. For more reading on Botulism consult the following: and
    Egg Binding is obviously only a concern in hens, but an egg bound hen will often squat, lay down, act lethargic, and seem as if she can not use her legs properly. She may waddle, or fall over easily, or use her wings for balance. Severe egg binding or internal laying and other reproductive problems can also cause a hen to "walk like a penguin". If you have a hen displaying ANY of these signs, it is important to check her for egg binding right away before trying any other treatments. Egg bound hens must be assisted quickly as they can die rapidly depending on where the egg is. A cloacal exam may be necessary. Though this seems rude, it may save her life. Using a glove and a lubricated finger, feel inside and up her vent to see if you can feel an egg. More information and treatment:

    Hypocalcemia / Layer Fatigue is another problem that is only a concern in hens. A pullet or hen that becomes very suddenly paralyzed may be experiencing this condition. It arises from hypocalcemia (extreme lack of calcium) while her body is putting the shell on her egg during her egg laying cycle. Even hens that are being supplemented with calcium in their diets can experience this either due to lack of uptake in calcium (osteoporosis), simply not eating the supplied calcium, or from a lack of phosphorus or vitamin D3 which are both essential in the proper processing of calcium in her body. Death can occur rapidly. If this condition is suspect, offering a large and immediate dose of calcium can save the hen's life. Crushed calcium tablets (such as Tums), crushed oystershell or even limestone can be fed directly. This condition is more often seen in production breeds such as red sex-links, ISA browns, etc. More reading on this and long term care:

    Parasites can cause otherwise healthy chickens to exhibit a variety of sickly symptoms that might be mistaken for Marek's. An abundance of external (mites, lice, ticks) and/or internal (various worms and protozoa) can cause lethargic behaviors, poor balance, anemia, off-colored combs and wattles, poor quality droppings, decreased appetite, and various other "generic" poorly symptoms. Internal parasites are NOT always visible in droppings and so a visual diagnosis is not an accurate way to determine if your bird has internal parasites. External parasites such as mites can be very tiny and may require a lot of examination to see. If you see these symptoms, consider using an appropriate treatment to take care of worms and/or external parasites in your flock. Birds can "get" worms, mites, and lice in all weather, and some individuals might have it worse than others. It is very important to treat properly if you suspect an active infestation; preventative
    measures such as (Diatomaceous Earth, Garlic, Pumpkin Seeds, Wood Ash, Apple Cider Vinegar, etc) may help prevent parasites but can not hope to cure a bird that is acting ill from a large parasite load. Valbazen is generally regarded by many chicken keepers to be the best broad-spectrum wormer: For external parasites, attempt to find out what sort of "bug" you are dealing with, and administer treatment based on your findings and unique coop situation. There are many products that work well, but many insecticides can have negative health impacts (both on chickens and their keepers) if used improperly, so it is best to research and use the one you are most comfortable with. Always use such products as directed.

    Inner Ear problems or Infection can cause very Marek's-like symptoms in afflicted birds. Much like in humans, inner ear function is important to the balance in chickens, and if the bird is experiencing an infection or growth in the ear, the bird may demonstrate any of the following: dizziness/vertigo, nausea, weakness, loss of appetite, balance issues (etc). It is easy to see why this might look like Marek's. A few keepers with this problem have reported the chicken doing flips, and even tumor-like swelling near the ear. It is important to do a careful exam of both ears. Look for swelling, discharge, clogging, or discoloration. The small, stiff bristle-like feathers covering the ear opening may need to be gently spread to look into the ear canal (a small flashlight can be of value here!). Deep inner ear infections may have very few outward signs. Chickens with ear problems sometimes shake their head repeatedly, or scratch often at the head/ear
    region. Treatment includes careful and gentle cleansing of the ear/removal of physical blockages, and a round (minimum 7-10 days) of appropriate antibiotics. It is best to consult with a veterinarian if possible regarding dosage and proper antibiotics.

    Avian Encephalomyelitis is a disease that can cause paralysis in chickens. It can also affect turkeys, quail, pheasants and pigeons. Like Marek's, this disease can show up as birds losing coordination, leg paralysis and/or weakness, including sitting on the hocks, neck spasms, and tremors. This disease is mostly observed in chicks, under the age of three weeks. As chicks less than three weeks can not develop Marek's paralysis, be sure to investigate the possibility of Avian Encephalomyelitis if you experience these symptoms in young chicks. Read more:

    Viral tenosynovitis is a viral form of arthritis that is transmitted in chickens and turkeys. Transmission is generally via fecal material of infected birds. Infected birds experience lameness and hock inflammation, swelling of the tendon sheaths (the 'tubes' that the leg tendons are encased in), and general lack of mobility of the legs. It is most commonly seen in commercial meat birds and has been reported less commonly in commercial leghorns in the past. Photos with more info (warning, necropsy images):

    Mycoplasma synoviae (MS) is another disease of poultry that might look like Marek's. This bacterial disease affects chickens and turkeys, but may also infect other commonly kept fowl. It is a relatively common disease, and easily transmitted. Like Marek's, symptoms include lameness/paralysis of the legs and reluctance to stand and walk, as well as blue/purple comb and or wattles due to respiratory distress. The hock (ankle) and wing joints may become swollen. Birds may also experience rales (roughness of breath, rattling, wheezing) and may have some respiratory discharge.

    Mycoplasma gallisepticum (MG) is similar to MS, in that it is a mycoplasma infection, is very common in chickens, and is easily spread. It can cause paralysis and lameness in birds, similar to Marek's, but is generally accompanied by respiratory distress, sometimes severe in nature. Respiratory distress can include but is not limited to: rales (rasping, wheezing), coughing, sneezing, nasal discharge, bubbling or discharge around the eyes, expelling mucus, and overall difficulty breathing.

    Mold, Moldy Feed, Stale/Old Feed (Aspergillosis) is a serious problem, as mold in grain, especially corn, is known to grow aflatoxins. Most of the aflatoxin problems on corn are caused by Aspergillus flavus, and the most potent toxin produced by this mold is called aflatoxin B1. These toxins may have degenerative effect on the nervous system of birds, causing Aspergillosis, which appears similar to Marek's such as weakness, paralysis, or spasms, along with pale combs, weight loss, and lethargic behavior. Aspergillosis is often reported alongside with respiratory issues, but it is very important to note that it often presents without them as well! It is a common ailment that can be attributed to overall poor health in a bird. It is very important to investigate feed, both bagged and in the bird's environment, to make sure it is fresh and free of mold. Check manufacturing dates when possible. Stale and old feed may not be moldy in appearance, but due to
    age might have a greater chance to harbor aflatoxins. Molds may not be visible to the naked eye, so when in doubt, discard old feed and supply birds with fresh feed. If you find moldy feed or suspect moldy feed and see any changes in health in your flock, immediately discontinue access to affected feed! A single source of aflatoxins in grain can affect many birds depending on the source of the feed. Read more: More information, and treatment:

    Injury to the body, and especially to the head, can cause paralysis-like symptoms that look similar to Marek's. It is important to carefully check for hidden injuries, gently palpate bones for breakage, and observe your bird carefully to determine if injury is a factor. Injuries may be internal and not visible via simple physical exam. Head injuries can cause paralysis or weakness, and loss of motor control. Breeds with vaulted skulls (such as Silkies) are especially prone to brain damage, and can sustain brain injury and swelling that can create physical disability.

    Bumblefoot and other leg and foot issues can cause chickens to limp and favor their feet and legs. Carefully investigate for bumblefoot, and consider leg sprains and other injury when assessing your bird.

    If you suspect ocular Marek's, be aware that there is a condition that may look like ocular Marek's, called Coloboma. Colomba is a physical irregularity in the iris, which usually looks like a black 'notch' or sometimes a 'drip'. The notch is caused by genetics or trauma to the eye. It is possibly hereditary and may show up in chicks too young to have Marek's symptoms. Not much is known about Colomba in chickens; in humans, Colomba of the iris does not cause blindness, and so if your chicken seems blind in the irregular eye, it might not be Colomba.

    Erysipelas is a bacterial disease, not common but it can affect all commonly kept fowl. Turkeys seem especially susceptible. Generally few signs appear, though birds may become weakened and may have seeming leg paralysis very shortly before death. Mortality is generally less than 15% in infected birds. Birds that succumb will generally die within 24 hours. It is very sudden. This disease is serious, and while uncommon, if you suspect it, you may wish to test it because it can infect other animals and humans (through open cuts).

    Pasteurellosis is also a bacterial disease. It is also not overly common, and most often seen in turkeys, but a few of the symptoms can look like Marek's disease. Lameness in the legs, difficulty breathing, and twisted neck (wry neck) are often observed, as well as loose stools and swollen wattles.

    Equine Encephalitis, or EE (WEE, EEE), can infect chickens as well as other commonly kept fowl, such as turkeys, ducks, pheasants, and other game birds. This disease causes paralysis, staggering, going off of feed, and spasms. Birds that survive this disease may become blind and have lasting paralysis and similar neurological signs. It does not cause the lymphomic tumors of Marek's disease. If you experience "Marek's Like" symptoms in non-chicken poultry, be sure to read about EE. Be aware that, as the name implies, this disease is contagious to horses and other mammals (including humans). It is spread mostly by mosquitoes. For this reason, it is closely monitored. You can check these maps to see if it has been reported near you, as this will help you potentially diagnose this problem. If you live in an area that is reporting active EE and you have poultry with these symptoms, it may be worth taking seriously. If you suspect EE in your poultry,
    consider contacting your county for testing as it may be free. "Sentinel" poultry is an important tool for charting the yearly spread of this virus.
    Eastern Equine Encephalitis Map (Sentinel):
    Western Equine Encephalitis Map (Sentenel):
    Please note that "Sentinel" maps means that sentinel species, such as chickens, have tested positive in the indicated areas. Use the tabs at the top navigation bar to select other options. More reading on EE in poultry: and

    Because of these many similar diseases, it is important to look at all the clues before self-diagnosing Marek's disease.

    How long after being infected will a chicken show symptoms?

    A bird may never show symptoms. Generally, Classic Marek's (with paralysis and/or lymphomas) has an incubation period of 3-25 weeks. Meaning, the 'soonest' that a chicken might show visceral symptoms after being infected is about three weeks... but on the other hand it may not show symptoms for up to 25 weeks.

    After initial infection, (when the chicken breathes the virus in), the general timeline is as follows for classic Marek's disease only:
    Approximately 7 days: Virus latency (meaning the virus has now stored a 'blueprint' of itself in the chicken's cells).
    Approximately 10 days - death: Full replication of the virus is carried out and the chicken begins to "shed" the virus.
    Approximately 7 days -3 weeks: Lymphocytes carrying latent Marek's virus travel through the body, to visceral organs and nerves.
    Approximately 3-4 weeks: In chickens that do not develop resistance, the lymphocites in the organs and nerves undergo a transformation and become gross lymphomas. It is only at this point that symptoms appear and, sadly death often follows shortly after.


    Infected chickens generally do not present with paralysis and tumor growth after one year of age (though some individuals still can, it is more uncommon). Infected birds may present any of the other symptoms at any age after 4 weeks, however. There are no hard and fast rules as to when a bird may become symptomatic.

    As it can be very hard to pinpoint the time of exposure/infection, this can be very frustrating.

    Are older chickens less likely to get Marek's?

    There is a common misconception that once a chicken makes it to a certain age, it is safe from Marek's. This is not so. It is less likely to be fatal to older chickens (less likely to cause cancerous lymphomas), but birds can be infected at any age and can develop symptoms at any age after 4 weeks.

    How does the virus work?

    Marek's is a type of DNA alpha herpesvirus. Without getting too technical, it is inhaled by the chicken on carrier dander/dust. At this point the chicken is infected. What happens next matters on a number of factors, such as the overall health of the bird, immune system response, genetics, and stress level.

    In a chicken that has built resistance (either via vaccination or natural resistance) and has an overall strong immune system, the virus will be overcome by the immune system and the chicken may not develop any symptoms. In this case, the virus will sneakily make copies of its gene and inserts it into an RNA strand in the cells of the host. This is called latency and in this way a chicken will carry the virus for life, and may shed the virus. If, later in life, the chicken becomes immunosuppressed, the Marek's virus can wake back up and become active in the system again, thus showing symptoms months or even years after initial infection.

    In a chicken whose immune system cannot fight the virus off (or into dormancy), the chicken will display symptoms. When there is an active infection taking place in a chicken, the virus inserts itself into the bird's cells, and tricks the cells into manufacturing many, many copies of the virus. In this way the virus can overwhelm the host chicken, in addition to 'shedding' large amounts of virus into the chicken's environment.

    What are strains? What are serotypes?

    There are (currently) three described serotypes of marek's. Serotype refers to the species of virus, and as such there are: MDV-1, which is chicken Marek's herpesvirus, MDV-2 which is attenuated Marek's and does not cause disease, and MDV-3 which is the turkey herpesvirus/Marek's. All three serotypes are closely related, but can have much different effect in a chicken's body. Only MDV-1 causes the symptoms that we see, above. The other two are considered 'benign' when a chicken becomes infected with them.

    When testing blood or tissue samples for Marek's, testing is able to determine which serotype your chicken has. This can be important to know, so that a true Marek's diagnosis can be confirmed... vs. a "false positive" from having one of the other serotypes from vaccination.

    A "strain" refers to a mutated or changed virus within the same species. As such there are MDV-1 strains, which represents all virulent Marek's disease virus strains and is further divided into pathotypes, designated as mild (m), virulent (v), very virulent (vv), and very virulent plus (vv+).

    This information may be helpful when trying to understand the results from diagnostic testing, or when discussing with your vet or poultry expert. Be aware that if having PCR (blood) testing done for Marek's diagnosis, that the Rispen's vaccination may give false positives:


    How does the vaccine work?

    The most important thing to know about chicken Marek's vaccines is that they are not a cure, they are not an immunity, and they are non-sterilizing. In short, this means that giving a chicken a vaccination gives the chicken's immune system exposure to a related virus (usually HVT MDV-3, HVT stands for HerpesVirus of Turkeys). This related virus cannot cause symptoms in a chicken, but gives the immune system something to target and build a resistance. The hope here is that if the chicken is ever exposed to Marek's virus later in life, that the immune system will react appropriately to fight off progression of the virus into lymphomas, and the chicken will not develop the symptoms.

    Effective vaccination in chickens will not ever prevent the chicken from becoming infected with Marek's virus. In fact, it can be assumed that any chicken exposed to the virus will become infected. Instead of preventing infection, the vaccine builds resistance within the immune system which in turn will prevent lymphoid tumors from forming. As these tumors (neoplasms) are the most fatal part of the disease, effective vaccination can save the chicken's life.

    Should I vaccinate for Marek's?

    This is a personal choice. The best way to make this choice is to determine the risks for exposure to Marek's, evaluate your goals with your poultry, and understand the disease. If you are not sure if you want to vaccinate at all, you are in the right place. Keep reading and make the best informed decision that you can!

    How effective is the Vaccine?

    This is a very complicated question with a complicated answer. There are many variables at play, including: Which vaccine(s) were administered, if they were administered correctly, the breed of the chicken, the genetics of the chicken, the natural resistance vs. vaccinated resistance in the chicken, how the chickens are kept, overall health, what "strain" of Marek's the chickens are exposed to, etc.

    All things being equal, you will see many hatcheries and vaccination manufacturers claim 90% or better effectiveness. This has been true in years past. Today we are seeing mutated strains of Marek's that are challenging the older vaccines. The most commonly used vaccine (derived from MDV-3, which is Turkey Herpesvirus/ HTV) is also unfortunately the 'oldest' and has been used so widely on so many chickens (mostly commercial chicken keeping) that mutated strains can now challenge it and overcome. Some research suggests that the virus is mutating faster than previously thought, to remain virulent in spite of vaccinations. It is also remember that the 90% figure comes from proper vaccinations of chickens kept in commercial farms, most of which in the USA employ long indoor housing, with chickens in great densities, that often practice "all in - all out" keeping, and can properly deep clean all indoor surfaces. Our backyard chicken keeping methods are quite
    a bit different. We have much less control over environmental factors, and often (and to the woe of some) we swap chickens at meets, sales, state fairs, retired battery hens, craigslist, etc etc.

    These conditions and the existence of new strains (for more details, read Disease Details below) are causing the common vaccine to be 'challenged'. So, in any given flock, more than 10% of vaccinated birds that are exposed to a more virulent strain might succumb to Marek's. It's not necessarily common that this happens, but it has to be mentioned because it's a lot more than the 10% of yesteryear. Some flock keepers have experienced more than 50% loss to vvMDV-1 (very virulent Marek's).

    When should I vaccinate?

    Marek's vaccinations should be administered to chicks that are less than 36 hours out of the egg. Within 24 hours is recommended. This is to give the undeveloped immune system the proper amount of time to build resistance to the harmless MDV-3.

    Most hatcheries will vaccinate day-old chicks for you. The cost of vaccination is typically less than $0.30 at the time of writing. It is even less at some hatcheries.

    You can vaccinate your hatched chicks at home within this 36-hour period, but all proper vaccination procedures must be followed in order to guarantee an effective vaccination. Always follow all pharmaceutical guidelines offered by the manufacturer of the vaccine.

    Chickens that were not vaccinated by 36-hours-old should never be considered properly vaccinated and should not be sold or traded under the pretenses that they were vaccinated for Marek's.

    There is also some research that suggests that a follow-up vaccination at the age of two-three weeks old may increase the chances of immunity. Find out more:

    How long does it take for the vaccine to work?

    After vaccination at hatch (before 36 hours of age!), it is important to isolate (quarantine) chicks from exposure to Marek's disease for a minimum of 3 weeks, for the maximum benefit and best chance for the vaccine to develop resistance within the immune system. Some chicken keepers prefer to go longer periods of time, while others do not. Exposure to other chickens (or their dander, which is easily carried on your clothes and hair) before the 3 weeks is over is not recommended. Exposure before 3 weeks should be avoided at all costs if you suspect or know your flock already has Marek's disease and are carriers. Exposing vaccinated chicks before three weeks may compromise the effectiveness of the vaccine.

    My chicks were not vaccinated. Can I vaccinate after the 36 hour period?

    Here is where we enter into some theory, and you must not take any of this is rock solid fact. It's hardly close, and based only on the speculations of chicken owners and their real life experiences with this disease.

    Vaccinating chickens that have not been previously (properly) vaccinated cannot hurt healthy birds.

    It may or may not increase their chances of building resistance if they are later exposed to MDV.

    If you vaccinate chickens older than 36 hours, you should not consider them properly vaccinated, but at the same time it may give them a better chance than no vaccine at all. The younger the better, of course.

    If you have chickens that have already been exposed to Marek's virus, there is little chance and no findings to suggest that vaccination will help them at this point. It will not cause a healthy bird any harm but it is probably a waste of time. If it makes you feel better, you can try, but do not expect much from it.

    My chicken has Marek's symptoms right now! Should I vaccinate?

    I can find no sound reason to vaccinate an actively symptomatic bird. In fact, many manufacturers of the vaccine warn against doing so. Marek's vaccine is not a cure, it is not a medicine, and it is not even an immunity. It makes little sense to introduce another virus into a bird's system when the immune system is already fighting against MDV-1 (chicken Marek's virus). Read about the 'timeline' of infection below to understand why vaccination after symptoms show is probably useless and possibly even dangerous. Do so at your own risk.

    Aren't vaccinated chickens carriers for life?

    This is a misconception. The MDV-3 HTV (turkey herpesvirus) vaccine itself can not cause the chicken to shed live chicken Marek's disease (MDV-1). It simply does not contain any chicken Marek's disease virus and thus a chicken can not spread or shed Marek's simply by virtue of being vaccinated. What happens, and what has caused this myth, is that a vaccinated chicken that is later exposed to an outside source of Marek's disease can then become infected, not develop symptoms (because of proper and effective vaccination) and yet will shed the virus, thus becoming a silent carrier. Studies suggest that birds that are exposed to Marek's disease become carriers regardless of vaccination, and that there is no real difference in the amount of virus shed in vaccinated vs. unvaccinated individuals. Meaning, vaccinated birds will still shed the same amount of virus once exposed to it. Reference:

    The other two types of vaccines (Rispens and SB1, aka MDV-2) are not known to cause symptoms in chickens as they are a different serotype. These two vaccines are only administered by a few hatcheries and cannot be obtained for home vaccination (at this time).

    Can the vaccine cause the chicken to get Marek's?

    No, it cannot cause a chicken to get the Marek's disease that makes them sick. The commonly administered (MDV-3 HTV) vaccine alone can not possibly cause chickens to become infected with MDV-1, which is the type of Maerk's disease chickens get that makes them symptomatic. See above answer for more detail.

    If I do not vaccinate and my flock is exposed to Marek's virus, will they all die?

    Probably not. There are many, may variables at play that make an individual bird susceptible to the virus or not. Except in vvMDV+ infections (mutated forms, very virulent) approximately 60% of any unvaccinated flock might succumb. This number might be far less if the flock members have been bred for immunity, and have natural resistance the the virus... or might be far more in the flock members are particularly weak against it.

    Why does the vaccine fail?

    There are many reasons, but some of the most common are:

    1. 1.
    1.The chicken's immune system is challenged by (encounters) a very virulent strain of Marek's virus (vvMDV+). These are mutated forms of Marek's that have become especially nasty, and beyond the scope of the vaccinated chicken's immune system's ability to handle.

    2.Improper vaccination. Many things can go wrong with vaccination, such as not administering the vaccine within the 36 hour period of life, not administering enough vaccine or in the wrong location, or administering damaged vaccine (old, or torn vaccine).

    3.The chicken was immunosuppressed in other ways-- this is a fancy way of saying that the chicken was already sick or had been sick in the past, and the immune system was busy fighting another disease. Marek's is very opportunistic.

    4.Exposure to Marek's disease too soon after vaccination. Ideally, chicks should be isolated entirely from the disease for a minimum of three weeks after hatching and vaccination, to give their immune systems a chance to build resistance.

    5.Due to genetics and many other variables, some breeds seem much more susceptible to Marek's than others. Particularly vulnerable breeds include: Silkies, Polish, (others?).

    What vaccinations are available?

    There are currently three commonly used vaccinations for use in chickens against Marek's disease. They are derived from the different related viruses that will not cause a chicken to become infected but instead develop resistance. The first is HTV (MDV-3) which is a type of turkey herpesvirus. This is the vaccine discussed in this article, because at this time this is the only Marek's vaccine available to the consumer. The other two types are known as Rispens and SB1 (MDV-2), which are attenuated chicken Marek's virus (this is a fancy way of saying that they are benign).

    Some hatcheries use a combination of all three vaccines (HTV + Rispens + SB1) to give the chickens the best possible resistance to very virulent strains of Marek's. Most hatcheries and all home vaccinations use HTV (MDV-3).

    Be aware that if you are having PCR/Blood testing done for Marek's disease, and the Rispen's vaccination was used, that it may give a false positive:

    Should I only consider hatcheries that use all three vaccines?

    If you currently have a flock of chickens that are carrying a very virulent strain of Marek's and normal vaccination with HTV /MDV-3 dos not seem effective, then it might very well be worth getting chicks that have had the three-part vaccination. If you do not have Marek's on your property and are simply trying to prevent the disease, it may be less important to you. This is a personal decision based on your wishes, your flock's health, and experiences.

    Which hatcheries use which vaccines?
    If it is important to you as to which vaccines are administered, it is important to contact your hatchery of choice and ask them directly. This may change from year to year and season to season depending on availability and administrative decisions. Any hatchery worth your time should be willing to tell you which vaccine(s) they administer.

    I got my chicks/chickens at a Feed Store such as Tractor Supply, or private breeder. Were they vaccinated?

    You will need to contact the individual responsible for ordering the chicks, or the breeder, to find out. It is important to note that sometimes feed store employees are not aware if the chicks have been vaccinated or not and may have misinformation. While it is true that some feed stores will special order vaccinated chicks for you if you request them, in general, it is the practice of most chain feed stores to NOT vaccinate against Marek's. You may need to do some detective work to find out. Unfortunately, it has come to light that some customers are mislead (usually out of ignorance, not malice) on this issue, and may be told that chicks were vaccinated when in truth they were not. When in doubt, you can usually assume that they were not.

    Very few private breeders vaccinate, but it is worth asking. Some do, and some may vaccinate for you upon request and for an additional fee.

    Where can I get Marek's vaccine for my chickens?

    Different retailers come and go, but for now you can try the following (I am not affiliated with them in any way, shop wisely):

    Twin Cities Poultry Supply:

    Jeffer's Pet:

    First State Veterinary Supply: (you may need to call)


    I'm worried my chicken might have Marek's disease. How can I tell for sure?

    There are a few ways to figure out if you have Marek's in your bird(s). There are more ways that listed here, but some of the most common ones are:
    1. 1.
    1.Self diagnosis based on symptoms. Least accurate method, but is better than nothing. You can make some well educated guesses (see the question above, under the Symptoms header). These are still guesses though, as there are many diseases that can 'look' like Marek's.

    2.Necropsy (home). This can only be done on a dead bird. If the chicken showed signs of classical Marek's, sometimes tumors can be found on the internal organs as described above. Be sure to check out the resources links below to find photos of what to look for. While tumors are a good indicator of probably Marek's, be advised that Avian Lymphoid Leukosis Disease can also cause tumors that are visually identical.

    3.Necropsy (as preformed by an experienced avian examiner). This can only be done on a dead bird. This may be done at a state poultry lab or at an experienced avian veterinarian's office. Some states offer free or reduced cost necropsy of poultry. Contact your local extension office or state lab to find out about these services and any fees associated. Birds to be checked must not be frozen, but stored under refrigeration, and sent for diagnosis as soon as possible (within two days if possible). These necropsy reports are generally a good indicator of the presence of Classical Marek's but it is very important to note that even under skilled medical examination, not all Marek's infected birds will have symptoms. A necropsy is not a 100% for sure diagnosis. It is an educated guess.

    4.DNA blood test/ PCR testing. This is done via blood draw on a live bird or samples taken from a deceased bird. The blood or samples can then be shipped (as a medical sample) to a diagnostics lab for full testing for DNA markers of Marek's virus. This is especially useful in cases where necropsy is inconclusive or tumors are similar to Avian Lymphoid Leukosis Disease. See references for links to labs that preform this service. There is often an extra fee. See previous chapter for an explanation of the difference between serotypes and strains. PCR testing is a bit complex. Please continue reading on to learn more about PCR.

    Here is a scan of the results of one such DNA blood test I had run.

    If my chicken's gross necropsy came back as probable Marek's, what does this mean?
    A gross necropsy usually involves examining the bird inside and out for visual clues as to the cause of death. Common signs of Marek's disease include tumor growth in key organs, as well as possible neural lesions (sometimes visible under microscope), as well as swelling or enlargement of the sciatic or vagus nerves, and irregularities of the digestive system. Unfortunately, there are other diseases that are visually similar to Marek's, even upon examination. Avian Lymphoid Leukosis looks very similar to Marek's when lymphoid tumors occur. Other diseases may also appear similar, especially if they cause neoplasms (tumors). That said, Marek's is a very common disease, and often it genuinely is Marek's.

    My chicken had a PCR test run, how accurate is it?

    Some labs will do PCR testing during a necropsy if the bird in question was a Marek's suspect, and some will not unless it is requested. PCR testing can also be run on blood samples from live birds. You may need to ask ahead of time to determine what testing will be done. PCR stands for Polymerase Chain Reaction, and in short is a way of taking a sample and amplifying copies of select DNA in that sample. In this way, scientists "look" for copies of the DNA belonging to the Marek's virus. There are two main types of PCR testing, and this is important to know- I will explain why shortly. There is conventional PCR testing, and "Real time" PCR testing (this is referred to as qPCR). A conventional PCR test reads the DNA data at the end of the sequencing, whereas a Real-time qPCR test reads the sequencing data in real time using a laser. It is currently thought that qPCR (real time) is much more selective and sensitive to finding specific DNA in a sample.

    Whew. Are you still with me? This is complex.

    PCR testing in general is quite accurate, but only within a sample. Meaning, if the sample tested contains enough Marek's virus DNA, the PCR test is very likely to isolate and amplify it, therefore giving a "positive test" for Marek's. But, if a sample does not contain Marek's virus or does not contain very much of it, the PCR test may fail to amplify the DNA and therefore read as "Negative".

    The problem then arises... Marek's virus is wonderful at becoming latent in an otherwise healthy chicken. Put very simply, think of this as the virus being in remission. It is hiding out in some of the bird's cells, but not causing an active infection. It can cause an active infection later on (especially if the chicken experiences a weakened immune system) or not at all. But the chicken can still "shed" and spread the virus. So it is important to know if a bird currently has Marek's (in live birds) or if a bird had the disease (in a deceased bird being considered for testing).

    In birds experiencing latency (no active symptoms), PCR testing may not be sensitive enough to detect the virus, and may give a "negative" result, especially if the only sample submitted is a blood sample. When testing for Marek's in a bird that has been exposed but is not showing symptoms, qPCR is recommended because it is more sensitive, but it is not a guarantee.

    Even in birds experiencing active symptoms, PCR run on blood samples alone may not find enough of the virus DNA to read as a positive result.

    PCR is most reliable when it is a real-time test (qPCR) and it is run on sampled tissues where neoplasms are either found or are known to grow, such as: gonads, spleen, liver, kidneys, lungs, heart, proventriculus, adrenals, thymus. Unfortunately, such samples need to be taken from a bird that has already expired.

    In conclusion, PCR testing is a very useful and important diagnostic tool, but must be considered only one part of the puzzle piece and should be considered in combination with other findings.

    More reading: and

    Where can I get a PCR test run? Who will do the "real time" qPCR testing?
    Please see the reference links below. UC Davis is known to use only real-time qPCR testing. Read more:

    So, if my chicken's necropsy came back with no signs of Marek's, is my flock clean?
    Maybe. It is important to note that each bird can present symptoms differently, and not all of the symptoms can be seen during a necropsy. This is frustrating for many owners. You are not alone! This is a complex guessing game. Use every clue that your bird gave you to put the puzzle together as best you can.

    I'm confused. So can I find out 100% for sure if my flock has Marek's or not?

    You are not the only one! I am currently researching this very thing and hope to bring the best answers that I can to this FAQ. This is a very complex science and I am just a simple enthusiast. As best I understand, no single test can 100% confirm or deny Marek's disease in an individual chicken. It is when viewed as a whole, that tests, symptoms, flock history, and other clues can give us the best answers.


    I have Marek's in my flock! Do I cull them?

    This is a bit controversial, and ultimately it is a personal decision. Many backyard chicken keepers choose not to cull infected birds or flocks.

    Simply, if you confirm that one or more of your chickens has been infected with Marek's disease, you must assume that the virus has been shed by that bird (or birds). Any place that bird has been, any part of the yard, coop, house, etc... probably has some of the virus. The virus can live for months or years outside of the chicken's body, and has the chance to infect any new chicken that comes in contact with it.

    For most chicken keepers, this means that by the time you realize you have Marek's, it is already everywhere that all of your chickens have been. It is easily carried on the wind, by rodents, on your shoes, clothes, and hair. Even if you have separated coops and flocks, if you walk back and forth between them you have spread the virus. Essentially, what I am saying is, unless you take drastic daily biosecurity measures, you probably have Marek's disease everywhere in the places you keep chickens.

    So, culling would really not provide any benefit. All of the exposed chickens are probably infected. It is possible for infected birds to survive the disease.

    Only one of my chickens has symptoms of Marek's! Should I cull it?

    See the above answer. If that chicken does indeed have Marek's, and has been around your other birds or even been to the places that they go, it is too late. It has already spread. Culling the bird will not prevent the spread of the virus unless it has been in isolation for months.

    Should I isolate any chickens that are showing symptoms of Marek's disease?

    Yes, problably-- this is a good practice no matter what the disease. It's a good idea because even if it is too late to prevent the spread of the virus to the rest of your flock, it is documented that chickens that are presenting active symptoms also shed a lot more of the virus than those that do not show symptoms.

    It will also help to monitor their health, weight, food and water intake, and administer any care you need to give to the sick birds.

    In addition, if it is not actually Marek's but another disease, isolation might help prevent its spread!

    My chicken(s) are showing symptoms. What should I do now?

    First, isolation of the sick bird(s) would be a good idea.

    Give them an area that is a comfortable temperature, free of stress, and away from dangers or disturbances. If the bird is presenting with paralysis, extra support might be necessary. A laundry basket, small cage, or even rolled up towels/rags on either side of the bird can help prevent it from falling over due to paralysis.

    Make sure the bird has ready access to clean water and nutritious food.

    If you have a scale, it is extremely beneficial to monitor the bird's weight twice a day (once a day minimum). This can be used to determine if there is weight loss, gain, or stability.

    Observe the bird for behaviors and symptoms. Observe if the bird is eating, drinking, or defecating.

    Check the bird's crop. The crop should pass food over time.

    If the bird has symptoms matching another disease besides Marek's, consider trying to treat the alternative disease.

    Seek medical advice from your local avian veterinarian or state poultry department. If this is unavailable, it may be worthwhile to describe the situation in as much detail as possible to chicken-keeping peers, such as the Backyard Chicken Forum to seek aid.

    There is no cure for Marek's. You can provide supportive care and sometimes chickens will recover (but will never be cured), and sometimes they do not. Continue reading below for holistic therapy ideas.

    Are there any medications I can give specifically for Marek's?

    There are no cures for Marek's disease. Any medications administered in relation to Marek's are generally given to help with secondary problems, or are holistic/home remedy in nature. Some holistic medicines have claims to help ease symptoms, though it is important to understand the difference between the medicines treating actual Marek's symptoms and treating what appears to be Marek's but is possibly a different disease or problem. Even a bird that recovers from symptoms (with or without the aid of holistic remedy) is never truly cured, but is in remission, and the virus can still be spread and the chicken may still have a relapse, depending on the form that Marek's presents with.

    In an effort to try to save chronic birds, many chicken keepers have tried holistic medicines. Most of these are untested by science (as is a lot of medicine in poultry-keeping, sadly), but if you wish to explore them please do so at your own risk. It is important to carefully research holistic remedies because if used improperly, some of them can cause more harm than good. Some holistic remedies that others have tried include:
    St. Johns Wort, specifically Hypericum,
    Turmeric + Black Pepper, specifically Curcumin with Bioperene (thought to be a blood cleanser),
    Vitamin B ("Super B") Complex, Nutritional Yeast (check label to be sure of high vitamin B-12/Thiamin percentages!),
    Cold-pressed Coconut oil (medium-chain fats help vitamin absorption in general),
    Cranberry, specifically high quality whole berry (can be chopped, mashed, or cooked into a sauce) or no-sugar sauces/juices in a pinch,
    (others? if you know of one that has helped, please let me know).

    Hypericum has many claims in regards to Marek's disease: I do not claim to be an expert in its use, but please keep a cool head when reading about it. It may or may not help, but is not guaranteed and is not a true cure. Many of us with confirmed Marek's in our flocks have very mixed results. Study this carefully as it can cause harm if used improperly. A study was conducted on chickens (published in 2012) using Hypericum. The chickens were intentionally infected with a Bursal disease virus (NOT Marek's) known as IBDV BC-6/85, and were given Hypericum therapy. The study found that a middle dosage of Hypericum of 667.9 mg/kg of Hypericum per body weight (BW) per day created the greatest change in the reduction of damage to the organs studied in the body. It is important to note that the findings were for a different disease than Marek's, and the results were not terribly different than the positive control group, but it did help. Note that none of the
    chickens in the study were ever 'cured' but simply suffered less damage. Read the study here:

    It is currently unknown how cranberry works, however some claim success with its use, and fortunately feeding cranberry in moderate amounts has no known negative side effects. I have read that it is simila

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