Marek's disease in commercial turkey flocks (letter)

Marek's disease (MD) is a disease of herpesviral aetiology that causes immunosuppression and formation of tumours in many organ systems, primarily in chickens. The causative agent, Marek's disease virus (MDV), has been shown to regularly infect commercial chicken operations and cause production losses. Often these are seen as poor performance rather than mortality although, in an overtly infected flock, this can reach 100 per cent. In comparison, herpesvirus of turkeys (HVT) is not thought to cause clinical disease or any production loss in turkeys. HVT is used as a whole or part of vaccination strategies against MD in chickens. MD has been reported before in turkeys in the UK, although these have been small outbreaks compared with those seen in chickens. Tests on four sites suggest that HVT is likely to have infected these birds at an earlier age than MDV-1, but that HVT had given little or no immunity when the birds had faced challenge by MDV-1. It is of note that most of the grow-out sites tested have raised chickens at some point and these chickens may have been the source of infection for the turkey flocks. We are now conducting a trial to examine the practicality of vaccinating turkeys at the hatchery with a Rispens type vaccine and further testing is planned to assess the impact of this vaccination.