Molecular and serological studies of Egyptian strains of rabbit haemorrhagic disease virus and their comparison with vaccine strains.

Abstract

Vaccination is the major control measure for rabbit haemorrhagic disease virus (RHDV). The co-circulation of different RHDV genotypes in Egypt has led to the need to determine the most effective vaccine strain and the cross-protection between these genotypes. Rabbits seronegative for RHDV were vaccinated with the commercial GI.1a (RHDVa) vaccine strain Giza2006 and the GI.1d (G5) vaccine strain Giza97. The rabbits were challenged three weeks post vaccination with GI.1a (RHDVa) strains Giza2010 and Kal2012 and GI.1d (G5) RHDV Giza97 and RHDV2014 to determine the degree of cross-protection and evaluate immunity and cross-reactivity by haemagglutination inhibition (HI) and indirect enzyme-linked immunosorbent assay (iELISA). Both vaccines were fully protective three weeks post vaccination, with 95% protection for the GI.1a vaccine and 94.7% for the GI.1d vaccine, with no direct relationship between mortality rates and the genotype of the challenge strain. The antibody titres obtained using the HI test were one log higher for the GI.1a compared with the GI.1d vaccine, but post-challenge titres showed increased responses, expressed as 1?3 log2 higher titres, for the GI.1d vaccine. Sequence and phylogenetic analysis of the Egyptian strain RHDV2014 revealed its relatedness to the GI.1d genotype and showed no evidence of the presence of GI.2 in Egypt until 2014. In conclusion, both GI.1d (G5) and GI.1a (RHDVa)-based vaccines are protective against both RHDV genotypes present in Egypt but continuous monitoring of circulating strains is essential because the arrival of GI.2 in Egypt will require new vaccination strategies.