Bluetongue vaccination in Europe: the Italian experience.

The incursion of bluetongue (BT) into Italy in August 2000 caused heavy economic losses, partly due to the disease itself, but mostly because of disruption caused to the national animal trade structure. To limit direct losses and the circulation of BT virus (BTV), the Italian Ministry of Health ordered, on 11 May 2001, the vaccination of all susceptible domestic ruminant species (i.e. sheep, goats, cattle and water buffalo) in both infected and surrounding areas. The vaccination strategy was based on a risk assessment that suggested it would prevent direct economic losses and significantly reduce virus circulation. Vaccination of the target animal populations commenced in January 2002, prior to the epidemic peak of BT that began in July 2002. The proportion of vaccinated animals differed between the various regions and the varying levels of vaccination of these populations had clear consequences on the occurrence of clinical disease and the spread of BTV infection. In those regions where more than 80% of the target population were properly vaccinated, the disease disappeared almost completely and virus circulation was reduced significantly. The importance of this reduced circulation of BTV (i.e. infection did not spread from affected areas) was immediately obvious in areas affected by the less virulent BTV serotype 9 where, despite the virtual absence of clinical disease, trade of animals to other areas was prohibited. The areas affected by the highly virulent BTV-2 also benefited from vaccination because it eliminated clinical disease while animal movements were prohibited. The main consequence of the reduction of virus circulation after vaccination, as documented by serological surveillance, was a significantly reduced expansion of the areas that were subjected to animal movement restrictions. Subsequently, analysis of surveillance data, coupled with specific risk assessments, led to a progressive relaxation of movement restrictions even in areas where the infection was still present but where most of the population had been adequately vaccinated. The effectiveness of the strategy used in Italy (i.e. vaccination of all domestic ruminants) was reinforced by extensive experimental and field studies. The aim of these studies was to: a) evaluate levels of individual and herd immunity and resistance to challenge conferred by vaccination, and b) quantify the frequency and severity of the adverse effects of vaccination on domestic ruminants. Ongoing research has focused on the ability of vaccination to suppress or reduce viraemia in ruminants following natural challenge by a virulent BTV strain. These studies address the issue of safety of the trade and movement of vaccinated animals that originate from areas in which BTV continues to circulate and could justify the reversal in current policy that restricts the international trade of animals vaccinated against BT.