Marek’s disease is an infectious viral condition affecting chickens globally, caused by a highly contagious herpesvirus. This virus induces cancerous tumors and can lead to nerve damage and paralysis. The disease primarily impacts young chickens, typically between 6 and 20 weeks of age, and can result in significant health problems and mortality. Once infected, a chicken remains a carrier for life.
Clinical Signs and Forms of the Disease
Marek’s disease manifests in several distinct forms. The classical, or neurological, form impacts the bird’s nervous system, resulting in progressive paralysis. This often affects the legs, leading to a characteristic stance where one leg is stretched forward and the other back. Paralysis can also impact the wings, causing them to droop, or the neck, resulting in a twisted appearance.
Another common manifestation is the visceral form, characterized by the growth of tumors on internal organs like the liver, heart, spleen, and kidneys. Birds with visceral tumors may not show external signs until the disease is advanced. At that point, they might exhibit weight loss, lethargy, diarrhea, and pale combs and wattles.
The disease can also present in cutaneous and ocular forms. In the cutaneous form, tumors develop in the feather follicles, appearing as raised, rough bumps on the skin that can form crusty scabs. The ocular form involves lymphoid cells infiltrating the eye, causing the iris to turn a gray color and the pupil to become irregular or misshapen, which can lead to partial or total blindness. A temporary form of paralysis, causing incoordination for a day or two, can also occur.
Transmission and Contagion
Marek’s disease spreads primarily via contaminated dust and dander. An infected chicken sheds the virus through its feather follicles, which are specialized structures in the skin from which feathers grow. This virus-laden material becomes airborne and can be inhaled by other chickens.
This contaminated dust is resilient and can persist in a chicken coop, on equipment, and in the soil for many months. The virus is easily carried on the clothes and shoes of people or on shared equipment. The virus is not transmitted from a hen to her chick through the egg; the disease’s high level of contagion is due to this environmental shedding.
Vaccination as Primary Prevention
Vaccination is the most effective method for protecting chickens from developing the severe symptoms of Marek’s disease. The vaccine works by preventing the formation of tumors and the onset of paralysis. However, it does not stop a chicken from becoming infected with the wild virus or from shedding it. Vaccinated birds can still carry and spread the disease to unvaccinated birds.
For vaccination to be effective, it must be administered before a chick is exposed to the virus. This is why chicks are typically vaccinated at the hatchery, either while still in the egg (in-ovo) or on their first day of life via subcutaneous injection. After administration, it takes approximately four to fourteen days for the chicken to develop an effective immune response. There are different types of vaccines, including some that combine strains like HVT and MDV2 for broader protection.
When acquiring new chicks, it is a sound practice to source them from hatcheries that provide documentation of day-old vaccination. This early intervention is the most reliable way to ensure a flock is shielded from the debilitating effects of the disease.
Biosecurity and Flock Management
While vaccination is a powerful tool, it should be supported by strong biosecurity and management practices, especially since there is no cure for Marek’s disease. Maintaining a “closed flock,” which means not introducing new, unvaccinated, or adult birds, can limit the potential for bringing the virus onto your property. If new birds must be introduced, a quarantine period of at least four weeks is recommended.
Controlling dust is a direct way to manage the virus. Regular and thorough cleaning of the coop, followed by disinfection with products effective against herpesviruses, is a necessary step, particularly between flocks. All litter should be removed before washing and disinfecting surfaces. Proper ventilation can also help reduce the concentration of airborne dander.
Separating young, susceptible chicks from older birds is another management technique. Since vaccinated chicks need time to build immunity, keeping them isolated from the general flock prevents early exposure to the virus they may be shedding. Simple measures like using dedicated boots and tools for each coop can also prevent the transfer of the virus.