Long-Term Persistence of Vaccine-Elicited Bluetongue Serotype 8 Neutralizing Antibodies in Dairy Cattle

Introduction - Bluetongue virus serotype 8 (BTV-8) caused a massive epidemic in Europe in 2006-2007. In response to this epidemic, large-scale vaccination campaigns of susceptible species were implemented in most European countries starting from the spring of 2008, when inactivated BTV-8 vaccines became available on the market. In Italy, the first evidence of BTV-8 circulation was reported in March 2008 from five dairy cattle herds in the province of Verona. Compulsory vaccination was then implemented in the two adjoining provinces of Verona and Mantua.Aim - This study aims at determining the prevalence of vaccine-elicited BTV-8 neutralizing antibodies in dairy cattle 29-47 months after vaccination. As infected and vaccinated animals cannot be differentiated serologically, information on the post-vaccination antibody dynamic under field conditions is crucial to implement surveillance activities.Materials and methods - A cross-sectional serosurvey of vaccine-elicited BTV-8 neutralizing antibodies in dairy cattle was performed in the province of Verona; 110 cows housed in 12 farms vaccinated during the 2008 campaign, and 270 cows housed in 28 farms vaccinated during both 2008 and 2009 campaigns, were sampled in 2012. Eighty-four non-vaccinated heifers born from 2010 onwards were also sampled to confirm absence of virus circulation in the sampled herds after vaccination. All samples were tested for BTV-8 neutralizing antibodies using virus neutralization assay. Differences in seroprevalence between cows receiving one or two vaccinations (2008 vs. 2008 and 2009) were tested with chi(2) test. The analysis considered stratification of samples according to herd size and vaccination schedule, clustering of animals at the herd level, and sampling weights as to weight the sample back to the population from which it was drawn.Results and discussion - Overall seroprevalence was 43.1% (95% confidence interval: 41.8-44.5%), relatively low compared to that of similar surveys performed in other countries. Seroprevalence in cows vaccinated once was 30.4% (29.3-31.5%), significantly lower than that in cows vaccinated twice (48.3%, 46.5-50.2%). Antibody titres ranged from log(10) 1.00 to 2.51 (mean 1.36). None of the 84 non-vaccinated heifers had detectable BTV-8 neutralizing antibodies, confirming absence of virus circulation in the sampled herds after vaccination.Conclusions - A seroprevalence of 43.1% was evidenced in Italian dairy cattle 29-47 months post-vaccination. Although neutralizing antibodies are being waned sooner in formerly, non-repeatedly vaccinated animals, for at least 47 months vaccinated animals cannot be included in (sero) surveillance programmes. The fact that many vaccinated animals are likely to still be protected by vaccination during that period provides an explanation of why the devastating 2006-2007 European BTV-8 epidemic was so efficiently controlled despite most vaccination campaigns were stopped after two years.