Description of the first recorded major occurrence of equine viral arteritis in France.

REASONS FOR PERFORMING STUDY The vast majority of equine arteritis virus (EAV) infections are inapparent or relatively mild, but may occasionally cause outbreaks of equine viral arteritis. The event observed in France during the summer of 2007 was the most important seen in the country, with mortality and disruption of economic activity. OBJECTIVES To describe the different stages seen during the outbreak and to show how molecular tools were used for both the detection and management of the crisis. METHODS EAV detection was performed by real-time reverse transcription-polymerase chain reaction (RT-PCR) in blood, nasal swabs, semen or organ samples. Characterisation of EAV strains was performed by sequencing the ORF5 fragment. RESULTS The outbreak affected 18 premises in 5 counties in western France, which represented the index, 8 primary and 9 secondary premises. Artificial insemination in draught horses was responsible for the virus spread. Eight mortality cases were observed, including one fetus, 5 young foals and 2 mature horses. Forty-three individuals had positive results by real-time RT-PCR. The range of measured cycle threshold (Ct) values varied from 19.8 to 40.4 depending on the biological samples. Phylogenetic analysis revealed that the 33 isolated strains all clustered within the EU-2 subgroup. CONCLUSIONS The mortality rate attests to the virulence of the strain involved in this outbreak. Real-time RT-PCR was used for the first time in order to follow-up an epidemic disease in horses. POTENTIAL RELEVANCE The early detection of 3 signals with high Ct values attest the importance of taking low signals into account in field conditions.