Isolation of very virulent pathotypes of Marek's disease virus from vaccinated chickens in Europe.

Two very virulent strains of Marek's disease virus were isolated from two separate farms in northern Italy that had experienced outbreaks of Marek's disease in vaccinated flocks. The isolates were similar to very virulent strains reported in the USA in terms of their enhanced pathogenicity, both for chickens vaccinated with the herpesvirus of turkeys, and for genetically resistant chickens. This first description of very virulent strains of Marek's disease virus from outside the USA suggests that at least some of the increasingly frequent disease outbreaks reported in Europe may be associated with such strains and that the adoption of bivalent or polyvalent vaccines containing, for example, attenuated Marek's disease virus plus herpesvirus of turkeys may be beneficial in the field.