An experience from the Mediterranean islands in Europe (2003)

Following the emergence of bluetongue (BTV-2) in the island of Corsica (France) in 2000, annual vaccination campaigns were conducted between 2001 and 2003. Only the ovine population was vaccinated with the South African BTV-2 live attenuated vaccine. Despite the vaccination campaign, outbreaks were reported from several areas in 2001. No more outbreaks occurred in 2002, but viral circulation was revealed in different areas of the island through serological conversion in sentinel cattle. This appeared to be associated with an insufficient level of vaccination. A cross-sectional survey was conducted in 2001: around 60°h of the vaccinated sheep had antibodies. In 2002, the estimation of the level of vaccination was close to 90%. The prevalence in the other ruminant populations (mainly bovine) was estimated at 50% for the whole Island. Finally, the overall coverage - taking into account the census data - was estimated at 70% (+/- 5%). The immune coverage was probably not regularly distributed across the Island and could explain the occurrence of viral circulation in various areas. A key element of the success of vaccination campaigns is the cooperation of farmers. Efficacy of the vaccination and the absence, or a low level of side effects, are then necessary. The efficacy of the vaccination is described based on experimental animals and field studies. In the literature, some attenuated bluetongue vaccine strains were shown to cause abortion or to be teratogenic in sheep when administered during the first half of pregnancy. Corsican farmers (mainly sheep breeders) reported a decrease in ram fertility. However, no quantitative studies have been done to assess these negative side-effects. Using published and available data, the strategies of vaccination in Mediterranean islands are examined and compared with the Corsican one. Four main strategies for vaccination have been identified: the prevention of the establishment of BT in an area, the reduction of the number of clinical cases, the eradication of the infection and the immunisation of animal that will be introduced in an infected area. Given that the large-scale herd immunity threshold (HIT) is not known for bluetongue, the level of vaccination to achieve the eradication of the disease is unknown. The absence of clinical cases in Corsica since November 2001 demonstrates that vaccination is efficient to avoid clinical cases. Nevertheless it is unclear if eradication has been achieved and if vaccination against BTV2 should be stopped. Two elements have to be taken into account: firstly, the strong connexion of the Corsican epidemiological situation with the one in Sardinia (where BTV4 appeared in August 2003) and secondly, the possibility that BTV2 could persist in some places in Corsica still threatening livestock if the level of protection should decrease. Studies in small islands are attractive because a restricted and limited area allows investigating epidemiological patterns in a laboratory-like environment. Nevertheless, conclusions drawn in this environment are specific as the susceptible population in some of the Mediterranean islands is limited so the disease can die out more easily. Comparison between Balearic islands on one side and Corsica, Sardinia and Sicily on the other side should be pursued to understand the contrasts between the 2 situations. Mathematical models for determining the basic reproductive number (R0) of BT appear essential in order to help decision makers. Precise field appraisals of prevalence (natural infection) in cattle and goats, and vaccination coverage in sheep have to be carried out through cross-sectional studies to determine the efficacy of control measures. (Texte intégral)