All Cooped Up: Marek’s Disease

All Cooped Up: Marek’s Disease

Reading Time: 5 minutes

All Cooped Up is a new feature, profiling poultry diseases and how to prevent/treat them, written as a collaboration between medical professional Lacey Hughett and University of Pennsylvania poultry specialist Dr. Sherrill Davison.

The facts:

What it is: One of the most common viral neoplastic diseases seen in poultry.  

Causative Agent: Three species within the genus Mardivirus, although only one, Gallid alphaherpesvirus, is virulent. 

Incubation period: About two weeks, but it can be three to six weeks before clinical signs are evident. The incubation period for this disease is highly variable.

Disease duration: Chronic.  

Morbidity: Incredibly high. 

Mortality: Once a bird begins showing symptoms, 100%.

Signs: Paralysis, neurological disease, and severe weight loss. Postmortem examination will show tumors and enlarged nerves.

Diagnosis: Diagnosis can be done with flock history, clinical signs, postmortem lesions of tumors and enlarged nerves, and cell histopathology.

Treatment: No treatment exists, but severe infection can be prevented with good sanitation and vaccination.

Chicken with leg paralysis from Marek’s disease. by Lucyin CC BY-SA 4.0,

The scoop:

Marek’s Disease Virus (MDV) is one of the best-known poultry diseases. It causes tumors and immunosuppression in mainly chickens, but is occasionally seen turkeys and quail. Once infected, a flock generally shows clinical signs of the disease between six and 30 weeks of age; however, the disease can affect older birds as well. Not all infected birds will show signs of being sick, but they will be a carrier for life and will continue to shed the virus.

Marek’s Disease Virus (MDV) is one of the best-known poultry diseases.

MDV replicates in the feather follicles of infected birds, where it is shed via dandruff and easily spread from bird to bird. An uninfected bird will inhale the virus, where immune cells become infected in the lungs. B and T lymphocytes are the first cells to become infected, and both are responsible for different types of immune responses. The bird then becomes immunocompromised, opening it up to opportunistic pathogens.

As the disease progresses, tumor cells will begin to appear in the bird’s nerves, spinal cord, and brain. Tumors infiltrating these vital areas are responsible for the classic signs of Marek’s, which are paralysis in the legs and/or wings and head tremors. The paralysis alone can be enough to kill a bird, as it struggles to get to food and water and is at risk for being trampled by its flock mates. Birds may recover from this paralysis, but it is extremely rare.

Postmortem examination will show enlarged nerves and diffuse tumor growth, including many internal organs such as the liver, gonads, spleen, heart, kidneys, lungs, and muscle tissue. Externally, birds may have tumor cells infiltrate the iris of the eye making it appear grey in color. In addition, the birds may exhibit enlarged feather follicles due to tumor cell infiltration of the skin. These eye and skin lesions are rare.

Egg type breeds appear to be more vulnerable to falling ill than meat type breeds.

Interestingly, different breeds of chickens show different levels of susceptibility to MDV. Egg type breeds appear to be more vulnerable to falling ill than meat-type breeds. Silkies are reported to be highly susceptible to MDV.

Although MDV is common in flocks, diagnosis is important to rule out other similar diseases such as lymphoid leukosis or Reticuloendotheliosis. Lymphoid leukosis and Reticuloendotheliosis are rare. Diagnosis is based on the enlarged peripheral nerves and the presence of tumors, along with microscopic examination of the lesions. Immunohistochemistry and PCR testing can be done to look for MDV antigens. Tested birds will exhibit high quantities of the virus and the viral DNA, and tests should show there is no other tumor viruses present. Unfortunately, birds can be concurrently infected with MDV and other tumor-related diseases.

Since MDV is released from the feather follicles of infected birds, the environment where the bird is living is considered contaminated. The virus can live years without a host in the dust and litter, so even if all infected birds are gone from an area, the area is still considered contaminated.  

Preventing birds from getting sick from MDV is possible. Raising birds in an “All-in, all-out” way can help prevent the infection from spreading to new flocks. Between batches of birds, thoroughly disinfect the living area or move the new flock to a new area if possible. Most backyard owners have multiple generations of birds, so this isn’t possible. This is where excellent biosecurity comes in.

New chicks should ideally have a separate caretaker from the established flock and should be housed in a sanitized area away from any other birds. If having separate caretakers is not possible, begin feeding, watering, and cleaning the chicks, and finish with the older birds. Going from youngest birds to oldest birds is going from “clean” to “dirty.”

Broiler with cutaneous lesions from Marek’s disease. ROMAN HALOUZKA / CC BY-SA

MDV can be carried back to the younger birds on the owner’s clothes, feed, equipment, hands, and anything else that can get dusty. If it is necessary to return to the younger chicks for any reason, change clothes and shoes and wash your hands before handling or caring for the youngest birds. It may seem tedious but it keeps the new generation of birds safe. Additionally, keeping the chick equipment and feed separate from the regular flock’s supplies is good practice.

When bringing new chicks home, have the hatchery vaccinate them. Home vaccination is possible, but not ideal. The MDV vaccine must be refrigerated and reconstituted, then used in exact amounts no later than two hours post reconstitution. If a suboptimal dose is administered, the bird will not be effectively vaccinated. The vaccine takes up to a week to circulate and begin working, so wait at least that long before introducing chicks to an area that previously held infected birds.

Vaccination prevents the development of tumors in healthy birds and reduces the spread of MDV, but it doesn’t fully eradicate the disease. Even vaccinated birds can be carriers of the disease and can be a source of infection to younger birds. Sanitation to reduce the amount of virus in the environment is a key preventive measure. An excess amount of viruses in the environment can overcome vaccination and the birds can come down with clinical disease. Since clinical disease is not always evident, it is presumed that subclinical infection is present and the environment is contaminated with the virus. This is one of the reasons that it is essential that the birds be vaccinated in the hatchery for Marek’s Disease.


Originally published in the October/November 2020 issue of Backyard Poultry and regularly vetted for accuracy.

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