Novel calicivirus identified in rabbits, Michigan, USA

Abstract

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.

Figure 1

Conjunctival erythema in affected doe.

Figure 2

Liver of affected rabbit with granular texture, accentuated lobular pattern, and multifocal capsular petechiae.

Figure 3

Multifocal gastric hemorrhage in affected rabbit.

Figure 4

Multifocal periportal and midzonal heptic necrosis in affected rabbit. Hematoxylin and eosin stain. Original magnification ×200.

Figure 5

Liver of affected rabbit with positive cytoplasmic immunohistochemical labeling in hepatocytes against rabbit hemorrhagic disease virus capsid. Original magnification ×400.

Figure 6

Liver of affected rabbit showing in situ hybridization of a Michigan rabbit calicivirus-specific oligonucleotide probe within scattered hepatocytes. Original magnification ×400.

Figure 7

Schematic genomic organization of Michigan rabbit calicivirus consistent with a Lagovirus in the family Caliciviridae. Lagoviruses contain an initial large open reading frame (ORF), ORF-1 encoding a polypeptide that overlaps with a smaller ORF, ORF-2. Numbering indicates the corresponding amino acid codons predicated from the genomic sequence. hel, helicase; Vpg, virion protein, linked to genome; pro, protease; pol, polymerase; capsid (VP60), capsid protein VP60.

Figure 8

Dendrogram constructed for comparison of capsid (VP60) amino acid sequences. The geographically and numerically named species are strains of rabbit hemorrhagic disease virus. MRCV, Michigan rabbit calicivirus; RCV, rabbit calicivirus.

Figure 9

Dendrogram constructed for comparison of open reading frame 1 polypeptide genomic sequence minus the capsid sequence. The geographically and numerically named species are strains of rabbit hemorrhagic disease virus. MRCV, Michigan rabbit calicivirus; EBHSV, European brown hare syndrome virus.

Figure 10

Alignment of open reading frame 2 sequences showing that Michigan rabbit calicivirus (MRCV) follows the pattern of European brown hare syndrome virus (EBHSV) and rabbit calicirus (RCV) in having 1 initiation codon (ATG, in boldface) in comparison with the 2 present in rabbit hemorrhagic disease virus (RHDV).