SERUM NEUTRALIZATION TEST FOR EPIDEMIOLOGIC STUDIES OF SALMONID RHABDOVIROSES IN FRANCE

Serological examination is not yet accepted as being a suitable diagno stic method for fish that are asymptomatic virus carriers. Nevertheles s, encouraging preliminary results using an endpoint serum neutralizat ion test (SNT) in several French trout farm populations have demonstra ted an excellent correlation between the SNT and the previously establ ished virus histories of the tested populations. Following the isolati on of infectious haematopoietic necrosis virus (IHNV) in France, serol ogical screening of fish for a neutralizing antibody (NAb) to IHN was conducted on a national scale. This survey confirmed the relationship between the serum-neutralizing immune response of the fish and the pre sence of IHNV in a given trout farm population. Insofar as many trout populations underwent dual rhabdovirus infections with both IHNV and v iral haemorrhagic septicemia virus (VHSV), NAbs to both viruses were a lso detected in the fish from such populations, often in distinct indi viduals. NAb-responding fish became detectable 2-3 months post-infecti on (pi). The number of responding fish reached a mean prevalence of 20 % between 3 and 6 months pi and disappeared after 8 months. The neutra lizing serum titres (NST) were considered positive at greater than or equal to 32 and 64 for VHSV and IHNV, respectively. Both the NST resul ts and the prevalence varied greatly according to individuals, populat ions and the number of repeated stimuli involved in a given serum samp ling series. Conversely, the thousands of sera collected from trout in virus-free farms did not display any neutralizing activity against ei ther VHSV or IHNV. The SNT thus seems to constitute a reliable tool fo r the assessment of the health status of trout farm populations for rh abdoviruses, and would be useful in the implementation of fish health surveillance programmes.