Rabbit haemorrhagic disease: Cross-protection and comparative pathogenicity of GI.2/RHDV2/b and GI.1b/RHDV lagoviruses in a challenge trial

Abstract

European rabbits (Oryctolagus cuniculus) are severely affected by rabbit haemorrhagic disease (RHD). Caused by a lagovirus, the disease leads to losses in the rabbit industry and has implications for wildlife conservation. Past RHD outbreaks have been caused by GI.1/RHDV genotype viruses. A new virus belonging to the GI.2/RHDV2/b genotype emerged in 2010, quickly spreading and replacing the former in several countries; however, limited data are available on its pathogenicity and epidemiological factors. The present work extends these issues and evaluates cross-protection between both genotypes. Ninety-four and 88 domestic rabbits were challenged with GI.2/RHDV2/b and GI.1b/RHDV variant isolates, respectively. Cross-protection was determined by a second challenge on survivors with the corresponding strain. Mortality by GI.2/RHDV2/b was highly variable due to unknown individual factors, whereas mortality by GI.1b/RHDV was associated with age. Mortality in rabbits < 4 weeks old was 84%, higher than previously reported. Cross-protection was not identical between the two viruses because the ratio of mortality rate ratios for the first and second challenges was 3.80 ± 2.68 times higher for GI.2/RHDV2/b than it was for GI.1b/RHDV. Rabbit susceptibility to GI.2/RHDV2/b varied greatly and appeared to be modulated by the innate functionality of the immune response and/or its prompt activation by other pathogens. GI.1b/RHDV pathogenicity appeared to be associated with undetermined age-related factors. These results suggest that GI.2/RHDV2/b may interact with other pathogens at the population level but does not satisfactorily explain the GI.1b/RHDV virus's quick replacement.

Detection of Rabbit Hemorrhagic Disease Virus GI.2/RHDV2/b in the Mediterranean Pine Vole (Microtus duodecimcostatus) and White-Toothed Shrew (Crocidura russula)

“…All of the 37 wild rabbits were also qPCR-positive for the same virus at mean (SD) Cq values of 18.7 (2.04). This value is in agreement with Calvete et al (2018), who reported Cq values ranging from 18.03 to 18.30 using the same qPCR procedures in rabbits that had died from acute RHD caused by the same virus. Although qPCR was not developed to quantify viral RNA copy numbers (Calvete et al 2018), the high Cq values found in the small mammals suggested a lower quantity of viral particles than in rabbits killed by acute disease.…”

“…The results of the qPCR analysis were negative for all tissue samples except for the liver and the spleen of the surviving rabbit challenged with the Cr-Jun15-P4 inoculum. In this instance, GI.2/RHDV2/b RNA was found with high Cq values in the spleen and liver (40.5 and 42, respectively), which were, nevertheless, out of the lowest limit of detection (Cq¼38.7) established for qPCR (Calvete et al 2018). Seroconversion was detected in two surviving rabbits: one challenged with Cr-Feb15-P1 inoculum, and the survivor from the Cr-Jun15-P4 inoculum challenge.…”

“…Liver samples from the 37 rabbits and the four small mammals were analyzed using duplex real-time PCR (qPCR) to detect RNA from GI.1b/RHDV and GI.2/RHDV2/b lagoviruses in a single analysis (Calvete et al 2018). Each sample was amplified in duplicate in the same run, and the quantification cycle (Cq) value was calculated as an average of both amplifications.…”

“…A subset of the same tissue samples was fixed in 10% buffered formalin and processed using standard procedures for histological examination. The entire VP60 gene of the virus was sequenced (Calvete et al 2018), from the liver samples of 37 wild rabbits, as well as from the laboratory rabbits that died from RHD. Therefore, the full capsid VP60 sequence was used to infer phylogenetic relationships in this study.…”

Retrospective serological analysis reveals presence of the emerging lagovirus RHDV2 in Australian wild rabbits at least six months prior to its first detection

“…A large 362 proportion of rabbits present in every shot sample were >500 days old, and may therefore confound 363 the analysis of recent virus activity. In addition, if some level of immunological cross protection 364 exists between RHDV and RHDV2 (Calvete et al, 2018), the removal of susceptible rabbits by an 365 RHDV2 outbreak from the population could result in an apparent increase of RHDV seroprevalence, 366 due to the resulting increased proportion of surviving old RHDV seropositive animals in the sample. 367…”