Marek’s disease is a serious and highly contagious viral illness in poultry, particularly chickens. This alphaherpesvirus causes T-cell lymphomas and nerve enlargement, leading to tumors in nerves and internal organs, paralysis, and death. Vaccination is the primary method for preventing and controlling Marek’s disease.
Vaccine Administration and Timing
Vaccination for Marek’s disease is most effective when administered to chicks at one day old, ideally before any potential exposure to the ubiquitous virus. Some commercial operations also utilize in-ovo vaccination, where embryos receive the vaccine at approximately 18 days of incubation. For day-old chicks, the vaccine is typically given as a subcutaneous injection, usually beneath the skin at the back of the neck. A standard dose for this method is 0.2 milliliters per chick.
The vaccine is cell-associated, meaning the live virus is suspended in cells and kept frozen in liquid nitrogen at about -196°C (-320°F). Proper handling involves carefully thawing ampoules in a water bath at 25-30°C (77-86°F) for 70-90 seconds, avoiding vigorous shaking. After thawing, the vaccine is mixed with a diluent and must be used promptly, within 1-2 hours, to ensure effectiveness. Many hatcheries offer vaccination services, which is the most common way for small flock owners to acquire vaccinated chicks.
Common Types of Marek’s Vaccines
Several types of live virus vaccines are available for Marek’s disease, each offering protection against different serotypes. One widely used type is Herpesvirus of Turkeys (HVT), a naturally avirulent virus (Serotype 3) that does not cause disease in chickens but stimulates an immune response. Another common vaccine is CVI988/Rispens (Serotype 1), an attenuated strain of chicken herpesvirus. SB-1 (Serotype 2), a naturally avirulent Gallid alphaherpesvirus, is also used in vaccination programs.
To provide broader protection, particularly against more virulent field strains of Marek’s virus, bivalent vaccines are often utilized. These vaccines combine different serotypes, such as HVT and CVI988/Rispens, or HVT and SB-1. CVI988/Rispens, sometimes in combination with HVT, is frequently chosen for commercial operations or in regions where highly virulent Marek’s virus strains are prevalent. For smaller flocks, freeze-dried HVT vaccines are available and can be stored at 2-7°C (35-45°F) until they are reconstituted, providing a basic level of protection.
How the Vaccine Protects Chickens
The Marek’s vaccine is highly effective at preventing the clinical manifestations of the disease, such as the development of tumors in nerves and internal organs, paralysis, and associated mortality. The vaccine functions by introducing a weakened or non-pathogenic form of the virus to the chicken’s immune system. This exposure allows the bird to build an immune response, equipping it to combat the virulent Marek’s disease virus if encountered in the field, thereby preventing the onset of severe symptoms.
It is important to understand that the Marek’s vaccine does not prevent a chicken from becoming infected with the wild Marek’s virus. Vaccinated birds can still harbor the virus and shed it into their environment, a characteristic often described as a “leaky” vaccine. The virus replicates within the feather follicles of infected chickens and is then released through feather dander, which can persist in poultry house litter or dust for several months. Despite this shedding, studies indicate that vaccinated birds shed significantly lower amounts of the virus, potentially reducing the viral load transmitted to unvaccinated flock mates. This reduction in viral transmission can lead to less severe disease and lower mortality rates even in unvaccinated chickens exposed to the virus from vaccinated birds, highlighting a population-level benefit.
The Role of Biosecurity with Vaccination
Since vaccinated chickens can still shed the Marek’s virus into the environment, vaccination alone does not provide a complete solution for disease control. Implementing robust biosecurity practices is therefore necessary to reduce the overall viral presence in the environment and minimize exposure, particularly for young, susceptible chicks. Maintaining a clean brooder and coop involves regular cleaning and disinfection of poultry houses and equipment, reducing the habitat for the virus.
Minimizing direct contact between newly vaccinated chicks and older or unvaccinated birds is also recommended to delay potential exposure until the chicks’ immunity has fully developed, which typically takes about 10 days post-vaccination. Managing dust is a biosecurity measure, as Marek’s virus spreads effectively through infected feather dander, which can remain infectious in litter and dust for months. Limiting foot traffic, vehicles, and equipment from outside sources into poultry areas helps prevent the introduction and spread of the virus into the flock. These biosecurity measures, including the use of footbaths and protective clothing, work in conjunction with vaccination to create a safer environment and reduce the risk of Marek’s disease outbreaks.