Age and Infectious Dose Significantly Affect Disease Progression after RHDV2 Infection in Naïve Domestic Rabbits

Abstract

Rabbit haemorrhagic disease virus 2 (RHDV2 or GI.2, referring to any virus with lagovirus GI.2 structural genes) is a recently emerged calicivirus that causes generalised hepatic necrosis and disseminated intravascular coagulation leading to death in susceptible lagomorphs (rabbits and hares). Previous studies investigating the virulence of RHDV2 have reported conflicting results, with case fatality rates ranging from 0% to 100% even within a single study. Lagoviruses are of particular importance in Australia and New Zealand where they are used as biocontrol agents to manage wild rabbit populations, which threaten over 300 native species and result in economic impacts in excess of $200 million AUD annually to Australian agricultural industries. It is critically important that any pest control method is both highly effective (i.e., virulent, in the context of viral biocontrols) and has minimal animal welfare impacts. To determine whether RHDV2 might be a suitable candidate biocontrol agent, we investigated the virulence and disease progression of a naturally occurring Australian recombinant RHDV2 in both 5-week-old and 11-week-old New Zealand White laboratory rabbits after either high or low dose oral infection. Objective measures of disease progression were recorded through continuous body temperature monitoring collars, continuous activity monitors, and twice daily observations. We observed a 100% case fatality rate in both infected kittens and adult rabbits after either high dose or low dose infection. Clinical signs of disease, such as pyrexia, weight loss, and reduced activity, were evident in the late stages of infection. Clinical disease, i.e., welfare impacts, were limited to the period after the onset of pyrexia, lasting on average 12 h and increasing in severity as disease progressed. These findings confirm the high virulence of this RHDV2 variant in naïve rabbits. While age and infectious dose significantly affected disease progression, the case fatality rate was consistently 100% under all conditions tested.

Passive Immunisation against RHDV2 Induces Protection against Disease but Not Infection

“…The RHDV IgG ELISA is highly cross-reactive between all lagoviruses and does not distinguish between RHDV1 and RHDV2. Twenty-four hours after passive immunisation, rabbits were challenged orally with 50 times the 50% rabbit infectious dose (RID 50 as determined by in vivo titration; equivalent to 3 × 10 5 –6 × 10 6 capsid copies [ 37 ]) of a genotype GI.1bP-GI.2 RHDV2, MEN-1 (GenBank accession number MW467791) [ 9 ]. This variant is homologous to that used for the preparation of the RHDV2 IgG, described above, sharing 98.3% nucleotide identity across the genome.…”

“…Rabbits were monitored twice daily and those with terminal rabbit haemorrhagic disease (RHD), indicated by a rectal temperature less than 38 °C with concurrent weight loss and lethargy, were humanely killed by IV barbiturate overdose after sedation with IM xylazine (5 mg/kg) or medetomidine (130 µg/kg), and ketamine (30 mg/kg). To continuously monitor activity levels and external body temperature profiles, rabbits were fitted with collars comprising a FitBark2 activity monitor (FitBark Inc., Kansas City, MO, USA) and SubCue-Mini temperature datalogger (Canadian Analytical Technologies Inc., Calgary, Canada), as previously described [ 9 ]. Trials were terminated once humane endpoints were reached or at 10 days post infection (dpi), allowing sufficient time for seroconversion to occur [ 35 ].…”

“…However, unlike RHDV1, RHDV2 can also infect several Lepus (hares and jackrabbits) and Sylvilagus (cottontails) species [ 2 , 3 , 4 , 5 , 6 , 7 , 8 ]. RHDV2 also causes disease in young rabbits, which show an age-dependent innate resistance to lethal disease caused by RHDV1 despite being permissive to infection [ 9 ]. RHDV2 is antigenically distinct from RHDV1, overcoming both infection-induced and vaccinal immunity [ 10 , 11 ].…”

Rabbit Haemorrhagic Disease Virus 2 (RHDV2; GI.2) in Ireland Focusing on Wild Irish Hares (Lepus timidus hibernicus): An Overview of the First Outbreaks and Contextual Review

“…The Lagoviruses RHDV1 and RHDV2 are not believed to have any implications for humans (i.e., not zoonotic). They are highly pathogenic viruses of lagomorphs, with high RHD-associated mortality rates [ 20 ]. Historically, RHD was first discovered in China in 1984, in Angora rabbits imported from Germany infected with the classical RHDV1 [ 21 ].…”

“…There are few detailed pathology studies specifically on RHDV2 [ 13 , 20 , 27 , 28 , 29 ], but in many aspects, disease progression resembles that induced by RHDV1 (in rabbits; [ 13 ]) and EBHSV (in hares; [ 8 , 11 , 27 , 30 ]. Infected animals develop fever (pyrexia), and antemortem clinical signs can also include anorexia, collapse, lethargy, seizures, icterus, bleeding from the mouth, dyspnoea, hypothermia, bradycardia, or poor blood clotting [ 28 ].…”