Investigating non-pathogenic rabbit caliciviruses in New Zealand

Abstract

Rabbit haemorrhagic disease virus (RHDV) is a pathogenic member of the Caliciviridae family. The Czech v351 isolate of RHDV is used in Australia and New Zealand as a biological control agent for rabbits, which are an important and damaging introduced vertebrate pest in these countries. However, non-pathogenic rabbit caliciviruses (RCVs) can provide partial immunological cross-protection to lethal RHDV infection and thus interfere with effective rabbit biocontrol. Antibodies that cross reacted against RHDV antigens were found in serum from wild rabbits sampled before the release of RHDV in New Zealand in 1997, suggesting that non-pathogenic RCVs were already present in New Zealand. The aim of this study was to confirm the presence of non-pathogenic RCV in New Zealand and describe its geographical distribution. RCV and RHDV antibody assays were used to screen serum samples from 350 wild rabbits from 13 locations (14 populations; one location was sampled twice) in New Zealand. The serological survey indicated that both RCV and RHDV are widespread in New Zealand wild rabbits, with antibodies detected in 10 out of 14 and 13 out of 14 populations, respectively. Two closely related RCV strains were identified in the duodenal tissue from a New Zealand wild rabbit (RCV Gore-425A and RCV Gore-425B). Both variants are most closely related to Australian RCV strains, but with 88% nucleotide identity they are genetically distinct. Phylogenetic analysis revealed that the New Zealand RCV variants fall within the genetic diversity of the Australian RCV isolates, indicating a relatively recent movement of RCVs between Australia and New Zealand. To confirm the pathogenicity of RCV Gore-425, 21 rabbits were orally dosed with the gut homogenate from a rabbit infected with the original RCV Gore-425 isolate and closely monitored for up to 28 days post-infection. Consistent with other known RCVs, RCV Gore-425 caused a non-pathogenic infection of the duodenum and was shed in the faeces for at least 28 days. RCV was detected in many tissues, including, but not limited to the jejunum, ileum, bone marrow and the mesenteric lymph nodes. RCV was also detected in the bile for up to 28 days post-infection, possibly suggesting a persistent infection. The possible cross-protective nature of RCV Gore-425 was examined by infecting 10 rabbits with RCV and orally challenging with 1.5 × 107 copies of RHDV Czech v351 28 days post- RCV infection. A further six control rabbits were orally dosed with phosphate buffer solution (PBS) and were challenged with 1.5 × 107 copies of RHDV Czech v351 28 days later. All 16 animals died from RHDV infection, illustrating that under these conditions RCV Gore-425 does not provide protection from lethal RHDV infection. Despite no animals surviving RHDV challenge, RCV infected animals appeared to have a longer time to death and had a lower RHDV viral liver titre compared to the control animals, suggesting RCV Gore-425 can influence RHD progression.