The complicated strains of rabbit calicivirus

Dr Alex Rosenwax from Waterloo in Sydney, reports that he has had five unrelated unusual cases of rabbit deaths over the past few weeks. All presented with gastrointestinal stasis and were febrile with anorexia. The signs appeared to be consistent with calicivirus and all died within 6-48 hours, despite intensive treatment. All came from one general area of Sydney, and they had been vaccinated in the past.

“I have contacted the DPI and they have confirmed that a new strain of calicivirus has been documented in Sydney by Elizabeth Macarthur Agricultural Institute (EMAI) in the past year. That strain is resistant to the vaccine. Their signs were similar to the ones in the rabbits we have seen,” said Dr Rosenwax.

Rabbit haemorrhagic disease virus (RHDV) is a calicivirus that usually kills 95% of susceptible adult rabbits within 72 hours, but the molecular mechanisms for this virulence are unknown. It has been used in Australia as a biological control agent to reduce rabbit numbers since it was released in 1995. There is also an endemic non-pathogenic Australian rabbit calicivirus, RCV-A1, that is known to provide some cross-protection to lethal infection with RHDV, and pet rabbits are usually vaccinated against the endemic strain to protect them.

Australia’s Chief Veterinary Officer, Dr Mark Schipp, notified OIE of the new strain in January this year.1 The notification related to the outbreak in Sydney where sudden deaths occurred in show rabbits of various ages and both sexes. There were very few clinical signs prior to death. The rabbits had previously been in good health and were vaccinated against the endemic strain of rabbit calicivirus. Of the 80 susceptible rabbits, there were 35 cases, and 30 deaths, equating to an apparent morbidity rate of 43.8%, mortality rate of 37.5%, and case fatality rate of 85.7%.

Gross necropsy findings showed little signs of the heavy haemorrhage usually seen with RHDV. They had more cranial changes. EMAI reported that the normal ELISA test for calicivirus was negative whereas PCR was positive.

Some of the rabbits were presented to Dr Rosenwax because they were apparently having seizures. He noted that although his cases were highly suspicious of calicivirus, as the appropriate samples had not been taken, the cases could not be confirmed. A non-formalin-fixed, frozen 1-g liver sample is required for submission to EMAI to confirm the presence of the virus.

Tracing and surveillance of the new strain is underway to determine how many rabbits have been exposed and how widespread the virus is. This may have implications for pet rabbits, as evidence suggests the current vaccine may not be effective against this new strain.

This new strain also has implications for rabbit control. The Invasive Animal Cooperative Research Centre (IA CRC) research shows that Australian native vegetation is very sensitive to rabbit damage, and as few as 0.5 rabbits per hectare can remove all seedlings of the more palatable native trees and shrubs, so delaying natural regeneration.2 Rabbits are Australian agriculture’s most costly pest animal with the annual cost of over $200 million.

According to the IA CRC, the use of myxomatosis and calicivirus is still limiting rabbit numbers and without them, the annual cost to agriculture from the imported pest would exceed $2 billion.2 However, they report that rabbit numbers are increasing, with research also showing there are a number of rabbit colonies that are immune to the calicivirus. Without this virus, other control techniques would need to be tried, including warren ripping, fumigating, shooting and baiting.

The IA CRC is now involved in studies to evaluate other RHDV strains, mainly because there is increasing genetic resistance in the rabbits to current RHDV strains and young rabbits are acquiring immunity.

Dr Rosenwax reports that in the absence of a definitive diagnosis, his practice has instituted quarantine for all rabbits with suspicious clinical signs.

Anne Jackson
Editor in Chief

  1. Schipp, M. Rabbit haemorrhagic disease. OIE.
  2. Invasive animals Cooperative Research Centre. Landscape control –

This article appeared in the November 2014 issue of the Australian Veterinary Journal

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