Novel calicivirus identified in rabbits, Michigan, USA

15 December 2009

We report a disease outbreak in a Michigan rabbitry of a rabbit calicivirus distinct from the foreign animal disease agent, rabbit hemorrhagic disease virus (RHDV). The novel virus has been designated Michigan rabbit calicivirus (MRCV). Caliciviruses of the Lagovirus genus other than RHDV have not been described in US rabbit populations. The case-fatality rate was 32.5% (65/200). Clinical signs included hemorrhage and sudden death, with hepatic necrosis. Analysis of viral RNA sequence from >95% of the viral genome showed an average similarity of 79% with RHDV. Similarity of the predicted MRCV capsid amino acid sequence ranged from 89.8% to 91.3%, much lower than the 98% amino acid similarity between RHDV strains. Experimentally infected rabbits lacked clinical disease, but MRCV was detected in tissues by PCR. We propose that MRCV primarily causes subclinical infection but may induce overt RHD-like disease under certain field conditions.

Rabbit hemorrhagic disease (RHD) is caused by a calicivirus and is associated with illness and death in up to 90%–100% of susceptible rabbit populations. Domestic rabbits (Oryctolagus cuniculi) and free-ranging European rabbits (O. cunicuusi) are highly susceptible; cottontail rabbits (Sylvilagus floridanus) and hares (Lepus spp.) are unaffected. Rabbit hemorrhagic disease virus (RHDV) was first detected in China in 1984 and subsequently has been described in eastern and western Europe, Asia, South America, Australia, Mexico, and the United Kingdom. In the United States, it is considered a foreign animal disease, and outbreaks are of considerable economic concern to the US rabbit industry. In the United States 4 outbreaks of RHD have occurred in domestic, captive rabbits (O. cuniculi) since 2000. These cases were confirmed by inoculation study and reverse transcription–PCR (RT-PCR) because the virus is not cultivable in vitro. Subsequent genomic analyses suggested that these incidences resulted from separate viral introductions; however, confirmed points of origin (e.g., imported animal or animal product) were never identified.

Emerging Infectious Diseases
December 15, 2009

Original web page at Emerging Infectious Diseases