Marek's Disease: What Backyard Keepers Should Know

Marek's disease is one of the more commonly discussed diseases in small chicken flocks, especially when a young bird develops weakness, paralysis, or another neurologic sign. Penn State Extension describes it as one of the most common illnesses seen in small flocks. [1]

The most important point is that a neurologic sign is not a diagnosis. Marek's can cause characteristic nerve and tumor changes, but diagnosis relies on the bird's history, clinical signs, necropsy findings, histology, and, when needed, additional laboratory testing. [2]

What Is Marek's Disease?

Marek's disease is caused by an alphaherpesvirus that primarily affects chickens. Infectious virus is produced in feather-follicle epithelium and is released into the environment in feather dander and dust; susceptible birds can become infected by inhaling contaminated material. [1,2]

Not every infected chicken develops obvious disease. Clinical disease is most often associated with younger birds, but older chickens can also be affected. [1,2]

What Signs Can Marek's Cause?

The signs depend on which tissues are affected. Nerve involvement can cause weakness, ataxia, leg or wing paresis or paralysis, difficulty standing, and abnormal posture. A classic presentation is one leg extended forward and the other backward, although that posture is not present in every case. [2]

Marek's can also produce lymphoid tumors in internal organs, skin, muscles, and other tissues, and ocular disease can change the iris or pupil. Some affected birds show weight loss or poor condition rather than obvious paralysis. [2,5]

These findings are not unique to Marek's disease. Other infectious, neurologic, traumatic, toxic, nutritional, and neoplastic conditions can overlap with parts of the same presentation, which is why guessing from one symptom is unreliable. [2]

How Does It Spread?

Infected chickens shed virus from feather follicles into dander and dust. That contaminated material can remain infectious in the poultry environment for months or longer. [1,2]

Penn State Extension specifically warns that virus-laden dander can be moved between groups of birds on hands, clothing, shoes, hair, skin, and equipment. [1]

Marek's disease virus is not vertically transmitted through the hatching egg. Chicks can hatch uninfected and then become exposed after hatch. [1,2]

Can Marek's Spread to People or Other Animals?

People, Cats, Dogs, and Other Mammals

Marek's disease is not considered zoonotic. Penn State Extension states that it will not make people sick, and Merck Veterinary Manual notes that mammals appear refractory to infection. On that basis, household dogs and cats are not considered susceptible hosts for Marek's disease virus and are not expected to develop Marek's disease. [1,2]

Turkeys, Quail, and Other Galliform Birds

Chickens are the most important natural host, but turkeys and quail can also be infected, particularly when they are raised near chicken farms. Marek's disease has also been reported in pheasants and peafowl. Turkey herpesvirus (HVT) is a related avirulent virus commonly used as a Marek's vaccine in chickens; HVT should not be confused with the virulent chicken MDV strains that cause Marek's disease. [2,6]

Geese and Ducks

Research has detected MDV-1 genetic material in wild geese and ducks, and a rare clinical Marek's disease case has been reported in a white-fronted goose. Surveillance work has suggested that some wild waterfowl may act as carriers or reservoirs, although clinical Marek's disease appears to be uncommon in geese. [6,7]

Parrots and Other Pet Birds

Current authoritative host summaries do not identify parrots or other psittacines as established natural hosts of Marek's disease virus. In an older broad zoo-bird survey of 111 birds representing 49 species across 14 orders, researchers isolated MDV only from birds in the genus Gallus. That does not prove infection is impossible in every parrot species, but the available evidence does not support treating parrots as typical susceptible Marek's hosts. [2,8]

Mechanical Transport Is Different From Infection

A species can be resistant to infection and still potentially move contaminated material. Penn State specifically notes that virus-laden dander can be moved on hands, clothing, shoes, hair, skin, and equipment. By inference, a dog, cat, or other animal that physically enters a dusty chicken area could potentially pick up contaminated dander on fur or feet and move it elsewhere even though the animal itself is not infected. Direct evidence quantifying household pets as Marek's fomites is limited, so this should be treated as a sensible biosecurity precaution rather than a proven pet-to-chicken transmission route. [1,2]

What Does Vaccination Do?

Vaccination is the central prevention tool for Marek's disease. For backyard keepers, vaccination at hatch—often performed by the hatchery—is the preferred approach because protection needs to be established before exposure. [1,2,3]

Vaccination greatly reduces the risk of clinical disease, but it does not prevent a vaccinated bird from becoming infected with field virus or from shedding virus. [2,3]

There is no treatment that eliminates clinical Marek's disease once it develops, so prevention and exposure reduction matter more than trying to treat the virus after signs appear. [1,2,5]

Penn State Extension advises that chicks need time after vaccination to develop protection and recommends protecting newly vaccinated chicks from older birds and contaminated dander during that early period. Its current guidance describes roughly 4–7 days for vaccine protection to develop. [1]

What Should You Do With a Suspect Bird?

Move a bird with severe weakness or neurologic signs to a quiet, secure area where it cannot be trampled or pecked, and make food and water easy to reach. Severe weakness and neurologic signs warrant prompt attention because an impaired bird can rapidly become unable to eat, drink, or protect itself. [2,4]

Do not assume paralysis automatically means Marek's. A poultry or avian veterinarian can help assess the bird, and necropsy, histology, PCR, or other diagnostic testing may be needed to distinguish Marek's from look-alike conditions. [2]

If a bird dies or must be euthanized, ask a veterinarian or poultry diagnostic laboratory whether necropsy and testing would be useful before disposing of the body. Diagnostic laboratories can help distinguish Marek's from other tumor diseases when lesions are present. [2,5]

Protect the Rest of the Flock

Reduce movement of dust and dander between bird areas; clean or dedicate footwear and equipment when practical; care for young healthy chicks before older or suspect birds; and keep newly vaccinated chicks separated from older birds during the early period when vaccine protection is still developing. [1,2]

When buying chicks, ask the hatchery whether Marek's vaccination is available and whether it is administered at hatch. [1,3]

When to Get Help Quickly

Merck's backyard-poultry examination guidance identifies seizures or other neurologic signs, extreme respiratory difficulty, and severe weakness as signs needing immediate emergency care. [4] A chicken that cannot stand or reach food and water, has progressive paralysis, repeated seizures or tremors, or is rapidly declining should be evaluated promptly. [2,4]

If several birds become ill close together, especially if there are sudden deaths or many birds with respiratory signs, contact a veterinarian or the appropriate animal-health authority rather than assuming Marek's disease. Merck's backyard-poultry infectious-disease guidance specifically recommends veterinary or state animal-health contact when a flock has sudden high mortality or many birds with respiratory signs because diseases such as avian influenza require different testing and response. [5]

C&K Takeaway

Marek's disease is serious, but weakness or paralysis alone does not prove Marek's. The most useful response is to protect the affected bird, limit cross-contamination, involve veterinary or diagnostic help when signs are severe or unclear, and use vaccination before exposure as the main prevention strategy. [1,2,3,4,5]

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Sources & References

[1] Eva Wallner-Pendleton, DVM, MS, ACPV. “Marek's Disease in Chickens.” Penn State Extension. Updated November 17, 2025.

[2] Venugopal Nair, PhD. “Marek's Disease in Poultry.” Merck Veterinary Manual. Full review April 2024; last updated April 2025. Peer reviewed by David E. Swayne, DVM, PhD, DACVP, DACPV.

[3] Yuko Sato, DVM, DACPV. “Vaccination of Backyard Poultry.” Merck Veterinary Manual. Full review and last updated November 2024. Peer reviewed by David E. Swayne, DVM, PhD, DACVP, DACPV.

[4] Yuko Sato, DVM, DACPV. “Physical Examination of Backyard Poultry.” Merck Veterinary Manual. Full review and last updated November 2024. Peer reviewed by David E. Swayne, DVM, PhD, DACVP, DACPV.

[5] Yuko Sato, DVM, DACPV. “Common Infectious Diseases in Backyard Poultry.” Merck Veterinary Manual. Full review November 2024; last updated April 2025. Peer reviewed by David E. Swayne, DVM, PhD, DACVP, DACPV.

[6] Ruy D. Chacón, Christian J. Sánchez-Llatas, Claudete S. Astolfi-Ferreira, Tânia Freitas Raso, and Antonio J. Piantino Ferreira. “Diversity of Marek's Disease Virus Strains in Infections in Backyard and Ornamental Birds.” Animals. 2024;14(19):2867. doi:10.3390/ani14192867.

[7] Shiro Murata, Yuko Hayashi, Ayumi Kato, Masayoshi Isezaki, Sarah Takasaki, Misao Onuma, Yuichi Osa, Mitsuhiko Asakawa, Satoru Konnai, and Kazuhiko Ohashi. “Surveillance of Marek's Disease Virus in Migratory and Sedentary Birds in Hokkaido, Japan.” The Veterinary Journal. 2012;192(3):538–540. doi:10.1016/j.tvjl.2011.07.006.

[8] B. R. Cho and S. G. Kenzy. “Virologic and Serologic Studies of Zoo Birds for Marek's Disease Virus Infection.” Infection and Immunity. 1975;11(4):809–814. doi:10.1128/IAI.11.4.809-814.1975.