RHDV1 K5 in Victoria
Rabbit biocontrol programs have existed in Australia since the early 1950’s with the release of the Myxoma virus. Myxoma virus causes myxomatosis which is a generalised viral disease that kills European rabbits. Spread by biting insects, myxomatosis causes florid skin lesions, acute blepharo-conjunctivitis and oedematous swelling of the genital area. Myxomatosis ultimately compromises the rabbit’s immune system which allows for respiratory infections that often lead to death. When myxoma virus was released, it produced knockdowns of over 90 per cent of some rabbit populations in Australia. But, as with most viruses, it became less effective over time due to rabbits developing genetic resistance. Today Myxoma Virus affects an estimated 40 to 50 per cent of the rabbit population and it is no-longer being released to manage wild rabbits.
In 1996, a Czech strain of Rabbit Haemorrhagic Disease Virus (RHDV1) was released in Australia in another attempt to curb the growing rabbit population. RHDV1 is a highly infectious and contagious disease of the European rabbit that is spread by flies and close contact with other infected rabbits. Rabbits infected with RHDV1 become feverish and display flu-like symptoms. Death often results from disseminated intravascular coagulation and necrotic hepatitis. While initially effective when it was released, many rabbits began to develop a genetic resistance to the Czech strain. An endemic non-lethal strain of RHDV (RCV-A1) was also found to provide rabbits in cooler climates with protection to the Czech strain.
RHD Boost was a national biological control project that sought to identify new (RHDV) variants to boost the effectiveness of Australia’s rabbit biocontrol as existing variants gradually become less virulent over time.
The RHD Boost project involved identifying and evaluating 38 genetically and anti-genetically different RHDV variants. Six of these were selected for further testing. A Korean variant of rabbit calicivirus called Rabbit Haemorrhagic Disease Virus K5 (RHDV1 K5) was identified as the favourite.
What is RHDV1 K5?
RHDV1 K5 is a Korean variant of Rabbit Haemorrhagic Disease Virus (RHDV1) that causes a fatal haemorrhagic disease in European rabbits (Oryctolagus cuniculus). It was selected as part of the RHD Boost program for its ability to overcome some of the limitations that begun to emerge with the original Czech strain of RHDV1. The national release of RHDV1 K5 occurred in March 2017, but not before it was rigorously tested, assessed and registered as part of a strategic collaboration between research providers, government and industry bodies.
RHDV1 K5 was released at 323 community monitored sites and 18 intensively monitored sites. RHDV1 K5 produced average rabbit population knockdowns of 34%, but the results were variable. It was also discovered that RHDV1 K5 worked more as a biocide, than a biocontrol agent. That is, it generally did not spread beyond the original release site like a self-disseminating biocontrol agent would.
Consequently, farmers and other land managers should continue to use an integrated and coordinated approach to manage rabbits, which includes a well-designed warren ripping component that is complemented by other control techniques.
How RHDV1 spreads naturally
RHDV1 K5 and the Czech strain of RHDV1 are spread by insect vectors, such as bushflies and blowflies. Direct contact between a rabbit and a rabbit carcass infected with RHDV is also an avenue of spread. Animals that consume rabbit carcasses such as foxes, dogs and cats may also excrete the virus in their faeces without being impacted by the virus themselves.
Where RHDV1 K5 works
RHDV1 K5 works in a range of different habitats, although the best results were seen in cooler/wetter regions of the state, where the benign strain of the virus (RCV-A1) was previously providing the rabbit population with some protection.
RHDV1 K5 is more humane
RHDV1 K5 is one of the more humane methods of controlling wild rabbits. Once contracting the virus, rabbits develop 'cold-like' symptoms, become lethargic and then quickly die.
Post-infection, there is a rise in body temperature lasting up to 24 hours, often followed by death that occurs around 48 hours after the fever. The overall welfare impact prior to death has been assessed as low using the relative humaneness model developed under the Australian Animal Welfare Strategy.
Safety of RHDV1 K5
RHDV1 (K5 and the Czech strain) have only ever been found to cause infection in European rabbits. Even predatory animals that eat a rabbit that have died from RHDV1 K5 do not develop an infection. RHDV1 (K5 and the Czech strain) is considered safe for people, wildlife, livestock and pets (except domestic rabbits).
Pet rabbits can be infected by both RHDV1 strains, as they are descendants of wild rabbits. Owners of pet rabbits are advised to vaccinate their animals against the virus.
Cylap® is a vaccine available for the prevention of RHDV1 K5 and the Czech strain of RHDV1. This vaccine is effective against both strains, provided the correct vaccination protocols are followed. Talk to your vet for further information on vaccinating pet rabbits against RHDV1 K5 virus and other measures to prevent disease.
When to vaccinate
The Australian Veterinary Association recommends vaccinating rabbits against RHDV1 at 10 to 12 weeks of age. An annual booster dose and health check is advised thereafter. Occasionally, in the face of an outbreak, rabbits may be vaccinated earlier than 10 weeks of age in which case a booster dose is recommended four weeks later.
How effective is the vaccine?
Before registering RHDV1 K5 for use in wild rabbit control, the NSW Department of Primary Industries, in partnership with the Invasive Animals CRC, examined the vaccine for suitability in protecting domestic and production rabbits from RHDV1 K5. This experiment compared the mortality of a small number of vaccinated and unvaccinated rabbits that were subsequently infected with a high dose of K5.
All rabbits vaccinated with the currently available vaccine survived the infection with RHDV1 K5, while none of the unvaccinated rabbits survived.
This experiment indicates that the currently registered vaccine will protect pet rabbits against RHDV1 K5.
More information is available at Pestsmart.
Purchasing RHDV1 K5 to control rabbits
Authorised users include those holding a Commercial Operators Licence with vermin destroyers’ endorsement or Pest Control Licences authorising the use of pesticides formulated for the control of pest animals. Authorised users for aerial application are those holding a Pilot (Chemical Rating) Licence. In other situations, the use of RHDV1 K5 is not restricted, meaning it may be used and supplied to any person within Victoria.
Landholders can contact the Elizabeth Macarthur Agricultural Institute (EMAI) directly on (02) 4640 6337 or firstname.lastname@example.org to purchase RHDV1 K5.
An exotic RHDV, called RHDV2, is also circulating within the Australian landscape. It was first reported in wild rabbits in May 2015. This virus has spread throughout most of Australia and is currently the dominant strain of RHDV in the country.
RHDV2 is the first ever form of RHDV that is not 100 per cent species specific, as it has been detected in hares. It is currently unclear if these were rare, spill-over infections from rabbits to hares, or if RHDV2 actually spreads effectively between hares, like it does for rabbits.
In contrast to RHDV1 K5 and the Czech strain of RHDV1, RHDV2 can cause death in young rabbit kittens (3 to 4 weeks) and vaccinated adults. Importantly, the existing vaccine that is effective against RHDV1 K5 and the Czech strain of RHDV1 may only provide partial protection against RHDV2.
Figure 1 courtesy of W Mules