Increased virulence of rabbit haemorrhagic disease virus associated with genetic resistance in wild Australian rabbits ( Oryctolagus cuniculus )

Abstract

The release of myxoma virus (MYXV) and Rabbit Haemorrhagic Disease Virus (RHDV) in Australia with the aim of controlling overabundant rabbits has provided a unique opportunity to study the initial spread and establishment of emerging pathogens, as well as their co-evolution with their mammalian hosts. In contrast to MYXV, which attenuated shortly after its introduction, rapid attenuation of RHDV has not been observed. By studying the change in virulence of recent field isolates at a single field site we show, for the first time, that RHDV virulence has increased through time, likely because of selection to overcome developing genetic resistance in Australian wild rabbits. High virulence also appears to be favoured as rabbit carcasses, rather than diseased animals, are the likely source of mechanical insect transmission. These findings not only help elucidate the co-evolutionary interaction between rabbits and RHDV, but reveal some of the key factors shaping virulence evolution.

Comparative Phylodynamics of Rabbit Hemorrhagic Disease Virus in Australia and New Zealand

“…First, because the rereleases of RHDV often took place in smaller and less dense rabbit populations, it is possible that their extinction was simply a chance occurrence. Alternatively, their extinction could be selectively mediated, possibly driven by herd immunity against the original inoculum strain that is dampening the spread of RHDV351-INOC-like viruses, along with rising levels of host resistance (19,33). In a recent experimental study, RHDV351-INOC replicated slower to lower titers, and killed fewer rabbits with overall prolonged survival times, than recently sampled field strains (19).…”

“…Alternatively, their extinction could be selectively mediated, possibly driven by herd immunity against the original inoculum strain that is dampening the spread of RHDV351-INOC-like viruses, along with rising levels of host resistance (19,33). In a recent experimental study, RHDV351-INOC replicated slower to lower titers, and killed fewer rabbits with overall prolonged survival times, than recently sampled field strains (19). The dead rabbit has been suggested as a source of virus transmission, both by direct contact and, more importantly, via mechanical transmission through flies feeding on carcasses (16,18,37).…”

“…The dead rabbit has been suggested as a source of virus transmission, both by direct contact and, more importantly, via mechanical transmission through flies feeding on carcasses (16,18,37). In addition, virus infectivity in carcasses is likely to be highest in rabbits that died within 3 days postinfection, before the onset of adaptive immune responses (19). Therefore, a lower-virulence strain like RHDV351-INOC would be at a disadvantage for sustained virus transmission and would be outcompeted by better-adapted field strains that have coevolved with wild rabbits in Australia.…”

“…ORF1 codons 134 and 142 (p16 gene) and ORF2 codons 102 and 117 (VP10 gene) also were found to have evidence of positive selection, although it was weaker. P16 has been proposed as a virulence determinant in RHDV, with the accumulation of amino acid substitutions in field isolates with higher virulence than that of the Czech strain (19). However, as the function of VP10 has yet to be determined, the consequence of adaptation in this gene is unknown.…”

“…Reverse transcription then was performed using the purified viral RNA, oligo(dT) primers, and the SuperScript III first-strand synthesis system (Life Technologies). The viral genome was amplified in three overlapping fragments (each approximately 2.9 kb in length) from cDNA using Platinum Taq DNA polymerase high fidelity (Life Technologies) and the previously published primers listed in Table 1 (19). All PCRs were performed per the manufacturer's instructions, with an annealing temperature of 58°C and an extension time of 5 min.…”