A new threat to pet rabbits
20 November 2018
Rabbit Haemorrhagic Disease
1984 saw the outbreak of a lethal rabbit disease in China. Cases were then seen in Korea, presumed to have been spread with exported rabbit fur. The disease was later found throughout Europe and then worldwide, probably introduced through rabbit meat.
This new disease characterised by bleeding (haemorrhaging), was named ‘viral haemorrhagic disease’ (VHD) or ‘rabbit haemorrhagic disease’ (RHD). Other clinical signs, including fever and lethargy, were rarely seen as rabbits died too quickly.
Investigations proved a calicivirus, now known as ‘rabbit haemorrhagic disease virus’ (RHDV) to be responsible. Such caliciviruses rapidly mutate and many variants of RHDV have been found since the original strain, named RHDV1. A non-pathogenic strain is simply called ‘rabbit calicivirus’ (RCV), there is also a highly pathogenic strain called RHDV-K5.
Rather unusually, rabbits are born resistant to RHDV1. Mortality rates of 90% were seen of rabbits over 6 weeks which have lost this ‘age immunity’. Such was the threat of RHD to the UK rabbit population that the government insisted that cases were reported to DEFRA (Department for Environment, Food and Rural Affairs). The disease has been largely controlled through good husbandry, early diagnosis, isolation of cases and vaccination. RHD has subsequently re-classified as nonreportable.
Non-pathogenic rabbit calicivirus (RCV)
During the worldwide spread of RHD, rabbits in some areas were found to be already immune to this RHDV1. By a process quite different to the age immunity seen in very young rabbits, it is thought that prior exposure to the non-pathogenic RVC protects against RHDV1. This is similar to harmless cowpox exposure, protecting people from the dangerous disease smallpox. An observation which led Edward Jenner to pioneer the world’s first vaccination over 200 years ago.
RHDV-K5 is a more lethal variant of RHD which was deliberately released in Australia to kill wild rabbits. RCV exposure does not provide immunity to this strain. However, vaccination against RHDV1 does protect pet rabbits from RHDV-K5.
New variant (RHDV2)
In 2010, many vaccinated rabbits died after an outbreak of an atypical RHD in France. Genetic analysis identified a new, related calicivirus which was named RHDV2. Unlike the original strain of RHDV, RHDV2 can kill even very young rabbits, it also appeared to affect hares.
RHDV2 is thought to have been brought into the UK by show rabbits. Infections may still spread at shows or in pet shops, rescue centres or when breeders import new stock. RHDV2 infection is easily transmitted between infected rabbits by the oral, nasal or conjunctival routes. Live virus can persist on hutches and food bowls or on the clothing and shoes of owners for many months. Wild rabbits can act as a reservoir of infection and spread the virus to pet rabbits. This occurs directly by wild rabbits entering gardens or indirectly by wild rabbits contaminating grass, hay where people walk or vegetables and wild flowers which are touched. RHDV can survive in the digestive tract of animals that feed on carcasses of rabbits that have died and scavenging animals such as crows and foxes can transfer the virus. Flies can travel long distances and be carried along by the wind, spreading the disease far and wide. Faeces from infected flying infects, such as blowflies and mosquitoes and fleas can contaminate pasture. Biting insects may mechanically transmit the virus in viremia blood from one animal to another.
When compared to RVHD1 infection, rabbits die more slowly and less frequently from RVHD2. RHCV2 is more difficult to diagnose and signs of infection are less obvious, making quarantine more problematic. Diagnosis of RHDV2 infection requires post mortem, with additional genetic testing.
RHDV2 is resistant to temperatures of up to 50oC and is not inactivated by freezing. RHDV2 is now very common in mainland Europe and has totally replaced RHDV1 in France.
Hands should be washed before handling pet rabbits, indoor rabbits should be housed away from outdoor foot wear. Outdoor accommodation should be secured from wild rabbits, flies and other insects. New rabbits should be vaccinated completely quarantined for five days. Sudden death in rabbits should be discussed with a vet. Rabbits that survive RHD should be kept separate to prevent possible spread of residual virus
RHDV on hutches and bowls can be destroyed by a 10-minute soak in a 10% bleach solution. Carpets and outdoor areas can be difficult to disinfect.
After the death of millions of rabbits in the China outbreak, RHDV1 vaccines were developed. A combined myxomatosis and RHDV1 vaccine (Nobivac Myxo-RHD) replaced the original RHDV1 products. However, there was no protection against RHDV2. Vaccines against RHDV2 are now available, Filavac is most commonly used in the UK. Although Filavac offers protection against RHDV1 and RHDV2, it does not protect rabbits against myxomatosis (which continues to be a threat in the UK). It is recommended that both Nobivac Myxo-RHD and Filavac are given annually.
Rabbits infected with RHDV2 are more likely to be brought into vet practices as they survive longer than those with RHDV1, thus increasing the risk of transmission to other pet rabbits.
RHD has been found in most parts of the UK, there is currently no cure and the only way to protect rabbits is by vaccination. The vaccine for RHDV2 needs to be given at least two weeks after the combined myxomatosis / RHDV1 vaccination. Annual booster vaccines of both are recommended.
Please discuss this or any other aspect of your rabbits’ care with your vet.