Serological evidence for a non-protective RHDV-like virus

The data were recorded during a Rabbit haemorrhagic disease outbreak that occurred in France in 2001 in a wild population of rabbits that we have been monitoring since 2000. These data suggested the existence of non-protective antibodies due to a putative RHDV-like virus. Twenty-one blood and 22 liver samples were taken from the 26 corpses of recently dead rabbits that were found. RHDV was found in all liver samples. A first screening for RHD antibodies, carried out using an ELISA based on the detection of VP60-RHDV antigen, showed that 20 of the rabbits were seropositive. Moreover, we determined antibody titres for 13 of these 20 seropositive samples. All were greater than or equal to 1/400. Such titres normally indicate antibody levels sufficient to confer protection to all known RHDV or RHDV-like strains. For 16 samples, we determined whether these rabbits had died of a chronic or an acute form of the disease, by employing monoclonal antibody (Mabs)-based differential ELISA. All had died of an acute form of RHD. Because the antibodies detected by this VP60-ELISA test are known to appear 5-6 days after infection and since acute RHD generally kills the rabbits 2-3 days after infection, we assumed that the detected antibodies must have been present before the exposure to the virus that killed these rabbits. A second detection of antibodies was made with Mabs that are specific for RHDV. The results were negative, showing that the antibodies detected with the VP60 ELISA test were not specific for RHDV. We sequenced a portion of the VP60 gene of viruses isolated in 17 rabbits. All RHDV isolates were very similar to the RHDV strains commonly isolated in France during this period, suggesting that this viral strain was not a putative variant that is not neutralised by antibodies. Therefore we conclude that the detected antibodies were probably due to a RHDV-like virus that induces the production of detectable but non-protective antibodies.