Variation of rabbit serum IgA in relation to RHDV vaccination and infection: implications for serological diagnosis and epidemiological investigations

Rabbit haemorrhagic disease (RHD) is a highly pathogenic and contagious disease caused by RHD virus (RHDV) and RHDV type 2 (RHDV2), two distinct Lagovirus europaeus belonging to the Caliciviridae family. Serological investigations have been widely and successfully used on both wild and farmed rabbit populations. However, interpreting serological data remains uncertain, especially for IgA results in previously infected or vaccinated rabbits. The first part of this study reports the results of two independent vaccination experiments that demonstrated (a) serum IgA was present only in trace amounts and for a few weeks in about half of the vaccinated rabbits and (b) the serological profile of vaccinated rabbits changed significantly, with a marked and rapid increase in IgA, only when they were challenged with live RHDV administered orally. The second part of the study was carried out on a farm over 3 months, during which an RHDV outbreak unexpectedly occurred. We repeatedly sampled nine multivaccinated does, finding constant and medium-to-high levels of IgA in only three of them, namely the oldest ones, which had already been present a year and a half earlier during a devastating RHDV2 epidemic. At least one of the does became IgA positive, coinciding with the RHDV outbreak. A parallel survey on 61 young rabbits living in separate sheds on the farm showed a high percentage of IgA-positive rabbits in just one shed. Overall, our findings definitively demonstrate that vaccinations do not induce serum IgA, except in previously infected rabbits, and that IgA can be used as a marker of RHDV reinfection.