Prevention
Vaccination is the only known method to prevent the development of tumors when chickens are infected with the virus. However, administration of the vaccine does not prevent an infected bird from shedding the virus, though it does reduce the amount of virus shed in the dander, hence reducing horizontal spread of the disease. Mareks disease does not spread vertically. Before the development of the vaccine for Mareks disease, Mareks disease caused substantial revenue loss in the poultry industries of the United States and the United Kingdom. The vaccine can be administered to one-day-old chicks through subcutaneous inoculation or by in ovo vaccination when the eggs are transferred from the incubator to the hatcher. In ovo vaccination is the preferred method, as it does not require handling of the chicks and can be done rapidly by automated methods. Immunity develops within two weeks.Because vaccination does not prevent infection with the virus, Mareks is still transmissible from vaccinated flocks to other birds, including the wild bird population. The first Mareks disease vaccine was introduced in 1970. The disease would cause mild paralysis, with the only identifiable lesions being in neural tissue. Mortality of chickens infected with Mareks disease was quite low. Current strains of Marek virus, decades after the first vaccine was introduced, cause lymphoma formation throughout the chickens body and mortality rates have reached 100% in unvaccinated chickens. The Mareks disease vaccine is a "leaky vaccine", which means that only the symptoms of the disease are prevented.
Infection of the host and the transmission of the virus are not inhibited by the vaccine. This contrasts with most other vaccines, where infection of the host is prevented. Under normal conditions, highly virulent strains of the virus are not selected for by evolution. This is because such a severe strain would kill the host before the virus would have an opportunity to transmit to other potential hosts and replicate. Thus, less virulent strains are selected. These strains are virulent enough to induce symptoms but not enough to kill the host, allowing further transmission. However, the leaky vaccine changes this evolutionary pressure and permits the evolution of highly virulent strains. The vaccines inability to prevent infection and transmission allows the spread of highly virulent strains among vaccinated chickens. The fitness of the more virulent strains is increased by the vaccine. The evolution of Mareks disease due to vaccination has had a profound effect on the poultry industry. All chickens across the globe are now vaccinated against Mareks disease (birds hatched in private flocks for laying or exhibition are rarely vaccinated). Highly virulent strains have been selected to the point that any chicken that is unvaccinated will die if infected. Other leaky vaccines are commonly used in agriculture. One vaccine in particular is the vaccine for avian influenza. Leaky vaccine use for avian influenza can select for virulent strains. References
External links

Mareks Disease Information at poultrykeeper.com
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