Marek’s Disease (MD) is a highly contagious illness caused by a herpesvirus that poses a significant threat to chicken flocks worldwide. It is a common source of concern for both commercial producers and backyard poultry owners. This article addresses the question of human safety regarding MD transmission from chickens and provides essential details about managing the disease within a flock.
Human Transmission Risk
Marek’s Disease is not transmissible to humans and poses no risk to human health, even with close contact with infected birds or consumption of their products. The virus responsible, scientifically known as Gallid alphaherpesvirus 2, is highly species-specific. It is an avian herpesvirus that has evolved to infect chickens and a few closely related bird species, such as turkeys, but not mammals.
The cellular machinery required for the virus to successfully replicate is unique to the avian host, making cross-species infection extremely unlikely. Therefore, the widespread concern about Marek’s Disease being a zoonotic threat is unfounded.
What Marek’s Disease Looks Like in Poultry
Marek’s Disease is a highly contagious viral condition that can manifest in several ways. It triggers the proliferation of lymphocytes, a type of white blood cell, leading to the formation of tumors and nerve damage. Once a bird is infected, it remains a carrier for life, continuously shedding the virus into the environment.
Transmission occurs primarily through the inhalation of infected dust and feather dander shed by carrier birds. The virus is highly resistant in the environment and can remain infectious in dust and litter for months. The disease most commonly affects unvaccinated young chickens, typically between 6 and 30 weeks of age.
The disease manifests in several forms:
- The neural form, which presents as progressive, one-sided paralysis of a leg or wing, sometimes causing the bird to sit in a characteristic splay-legged position.
- The visceral form, characterized by the development of tumors in major internal organs (liver, spleen, kidney, and heart), which can cause weight loss and depression.
- The ocular type, which causes a graying or irregularly shaped pupil.
- The cutaneous form, which results in enlarged feather follicles and scabs on the skin.
Management and Prevention Strategies for Poultry
Since there is no treatment for Marek’s Disease once clinical signs appear, the control strategy is centered on rigorous prevention. Vaccination is the most dependable method for managing the disease and is typically administered to chicks when they are one day old or in ovo (in the egg) before hatching. The vaccine works by preventing the severe symptoms of the disease, but it does not prevent infection or the shedding of the virus.
Commonly used vaccines include the Turkey Herpesvirus (HVT) or a combination vaccine such as the bivalent HVT and Gallid alphaherpesvirus 3 strains. For optimal effectiveness, chicks must receive the vaccine before they are exposed to the field virus. It takes approximately one to two weeks for the bird’s immune system to build protection after vaccination.
Strict biosecurity measures are necessary to prevent early exposure and ensure the vaccine has time to work. New chicks, especially if vaccinated at home, should be kept in a clean, disinfected brooder completely isolated from older birds for at least the first week. Owners must also minimize the risk of carrying the virus to the chicks on their clothing or shoes.
Maintaining excellent sanitation, reducing dust, and using designated clothing and footwear when handling different age groups of birds helps decrease the viral load in the environment. Purchasing chicks that have been correctly vaccinated at the hatchery is the most practical preventative step for small flock owners.