Marek’s Disease (MD) is a highly contagious illness primarily affecting domestic chickens worldwide, posing a serious threat to poultry health and production. It is one of the most common diseases found in chicken flocks, caused by a persistent viral agent. Marek’s Disease is definitively not contagious to humans; it is not considered a zoonotic disease.
Defining Marek’s Disease
The causative agent of Marek’s Disease is an alphaherpesvirus, specifically Gallid alphaherpesvirus 2 (GaHV-2), also known as Marek’s Disease Virus (MDV). This virus causes a lymphoproliferative disease, characterized by the uncontrolled growth of lymphocytes, a type of white blood cell. This results in a virally induced cancer that primarily targets young birds.
The symptoms of MD vary depending on which tissues the virus infiltrates. The classic nervous form causes progressive, asymmetric paralysis of the legs, wings, or neck. For example, a bird with leg paralysis may sit with one leg extended forward and the other tucked under its body. The virus also causes a visceral form, leading to tumors in major internal organs such as the liver, spleen, kidney, and heart. In some cases, the skin develops firm lesions around feather follicles, or the eye is affected, resulting in blindness or a graying of the iris.
Risk to Human Health
The concern about human transmission is understandable, but MDV does not pose a health risk to people. The virus is highly species-specific, having evolved only to infect chickens and a few closely related avian species, such as turkeys. Experimental data indicates the virus cannot infect mammalian cells or animals.
This species barrier exists because the virus cannot recognize or bind to the specific receptor molecules found on the surface of human cells. The necessary biological mechanisms for the virus to enter, replicate, and establish an infection in the human body are absent. Therefore, an infected bird cannot pass the virus to a person through physical contact or environmental exposure.
Concerns regarding food safety are also unfounded. Consumption of meat or eggs from infected chickens does not transmit the disease to humans, as the virus is not a foodborne pathogen. It is readily inactivated by standard cooking temperatures. Although tumors may be present in the internal organs of an infected bird, they do not transfer the disease to the consumer.
How the Disease Spreads Among Poultry
Marek’s Disease is highly contagious and spreads readily among chickens in a flock. Transmission occurs horizontally, meaning from bird to bird, rather than vertically through the hatching egg. The virus matures into a fully infective form within the epithelium of the feather follicles of an infected bird.
The virus is continuously shed into the environment within poultry dander, which consists of skin flakes and feather material. This virus-laden dander becomes airborne and is easily inhaled by susceptible birds, which is the primary route of infection. The virus is resilient and can survive for months in poultry house dust and litter, making the environment a long-term source of infection.
Infected birds become carriers for life and continue to shed the virus, even without developing clinical signs. Controlling the disease relies heavily on vaccination, typically administered to chicks shortly after hatching. The vaccine prevents the development of severe symptoms and tumors, but it does not prevent the bird from becoming infected or shedding the virus into the environment.