Key result
Oral GI.1d strain inoculation in rabbits induces ~89% mortality at high viral doses.
Why the study?
Early stages of RHDV infection and the minimum infective dose of the GI.1d/RHDV strain were not previously characterized.
Absolute Event Rate: 89% vs 0%
p-value: p=<0.0001
The GI.1d/00-21 RHDV strain requires a minimum infective dose of ≤10^4 gRNA copies to infect rabbits, with higher doses needed to induce mortality.
Defines RHDV dose thresholds in rabbits; leaves open translation to natural outbreaks or control strategies.
Background Rabbit haemorrhagic disease virus Lagovirus europaeus/GI.1d variant (GI.1d/RHDV) was identified in 1990 in France, and until the emergence of the new genotype GI.2, it was the main variant circulating in the country. The early stages of RHDV infection have been described in a few studies of rabbits experimentally infected with earlier strains, but no information was given on the minimum infective dose. We report the genomic and phenotypic characterisation of a GI.1d/RHDV strain collected in 2000 in France (GI.1d/00–21). Results We performed in vivo assays in rabbits to study virus replication kinetics in several tissues at the early stage of infection, and to estimate the minimum infective dose. Four tested doses, negligible (10− 1 viral genome copies), low (104), high (107) and very high (1011) were quantified using a method combining density gradient centrifugation of the viral particles and an RT-qPCR technique developed to quantify genomic RNA (gRNA). The GI.1d/00–21 genome showed the same genomic organisation as other lagoviruses; however, a substitution in the 5′ untranslated region and a change in the potential p23/2C-like helicase cleavage site were observed. We showed that the liver of one of the two rabbits inoculated via the oral route was infected at 16 h post-infection and all tissues at 39 h post-infection. GI.1d/00–21 induced classical RHD signs (depression) and lesions (haemorrhage and splenomegaly). Although infective dose estimation should be interpreted with caution, the minimum infective dose that infected an inoculated rabbit was lower or equal to 104 gRNA copies, whereas between 104 and 107 gRNA copies were required to also induce mortality. Conclusions These results provide a better understanding of GI.1d/RHDV infection in rabbits. The genome analysis showed a newly observed mutation in the 5′ untranslated region of a lagovirus, whose role remains unknown. The phenotypic analysis showed that the pathogenicity of GI.1d/00–21 and the replication kinetics in infected organs were close to those reported for the original GI.1 strains, and could not alone explain the observed selective advantage of the GI.1d strains. Determining the minimum dose of viral particles required to cause mortality in rabbits is an important input for in vivo studies.
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Droillard et al. (2021) studied Rabbit haemorrhagic disease (n=68). GI.1d/00-21 strain of RHDV vs. Uninfected controls and lower doses was evaluated on Mortality (case fatality rate) (p=<0.0001). Oral inoculation of the GI.1d/00-21 strain in rabbits showed a minimum infective dose of ≤10^4 gRNA copies, while doses between 10^4 and 10^7 gRNA copies were required to induce mortality.
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