Use of Marek's disease vaccines: could they be driving the virus to increasing virulence?

Marek's disease (MD) is an economically important neoplastic disease of poultry. MD almost devastated the poultry industry in the 1960s but the disease was brought under control after Marek's disease herpesvirus (MDV) was identified and vaccines were developed. This is the first effective use of an antiviral vaccination to prevent a naturally occurring cancer in any species. MDV infection has many effects. Initially causing a cytolytic infection in B-lymphocytes, MDV infects activated T-lymphocytes where it becomes latent. In susceptible chicken genotypes MDV transforms CD4+ lymphocytes, causing visceral lymphomas and/or neural lesions and paralysis. Fully productive infection and shedding of infectious virus only occurs in the feather-follicle epithelium. Vaccination of newly-hatched chicks with live vaccines has been widely used to successfully control MD since the early 1970s. However, vaccinated chickens still become infected and shed MDV. Vaccine breaks have occurred with regularity and there is evidence that the use of MD vaccines could be driving MDV to greater virulence. MD continues to be a threat and a number of strategies have been adopted such as the use of more potent vaccines and vaccination of the embryonic stage to provide earlier protection. Recombinant MD vaccines are useful vectors and are being exploited to carry both viral and host genes to enhance protective immune responses. The future aim must be to develop a sustainable vaccine strategy that does not drive MDV to increased virulence.