July, 2002: Focal outbreak of an unusually virulent strain of feline calicivirus: We are investigating an outbreak of hemorrhagic calicivirus in cats in Los Angeles. The virus appears to be a variant of the common feline calicivirus which causes upper respiratory tract infection (URI) in many cats, but cats in the current outbreak suffer extraordinarily high mortality and morbidity.
Cats present variably, (and numbers given here are preliminary), with approximately 50 percent having facial and paw edema (often markedly so), 90 percent febrile (as high as 106 F), 50 percent with classical signs of URI (ocular and nasal discharge, conjunctivitis, and vesicular or ulcerative stomatitis), 20 percent with icterus, and 30-40 percent with hemorrhage (from nose, feces, et al.).
Necropsy findings also are variable, including lung consolidation and pneumonia (80 percent), hepatomegaly (50 percent), pancreatitis (10 percent), and pericarditis (10 percent).
We are working with a case definition assigning cats as suspect or probable cases. Edema and hemorrhagic discharge occur in probable cases, regardless of concurrent signs of URI. Cats with unusually severe URI, icterus, pericarditis, or other associated problems but historical exposure are regarded as suspect. Since 7/2/02 when we first were contacted with the unusual clinical description, we have observed 30 probable cases in 3 veterinary practices and one foster care network in west L.A. 30-50 percent of cases died or were euthanized.
All cases appear to have originated from a single source and spread via traffic within and between hospitals and hospital clients. In two cases, well cats in homes have acquired the infection and died after sick cats returned home. As a precaution, all area animal shelters have been alerted to this disease; however, no cases from animal shelters have been reported. There was an unfortunate delay in informing local public health officials who had earlier informed local veterinarians to contact them if there were suspect zoonotic or potentially biological warfare agent outbreaks (which this fortunately is NOT).
The incubation period of feline calicivirus (including this hemorrhagic strain) is between 1-5 days. Cats of all ages, including fully vaccinated cats, have been affected, although some affected cats have been immunosuppressed (with neoplasia and chemotherapy). No other species is known to be affected by this strain of calicivirus. Based on several previous outbreaks, the only good news is that the virus is so virulent and infected cats die so quickly that it appears difficult for an epidemic to propagate and the incidence rapidly declines within a few weeks.
Based on previous cases, recovering cats may shed infectious virus for weeks after infection. Infected cats become viremic (as do cats with calicivirus URI). Diagnosis is based on culture of virus from blood, spleen, lungs, nasal or ocular discharge, clinical signs, and pathology.
The virulence determinants associated with this virus are not known but the pathogenesis involves vasculitis. Further investigation of the pathogenesis of disease is being pursued. All isolates of the virus will be sequenced to confirm that all are related to the virus from the index case and an epidemiological/statistical summary will be completed as soon as possible.
A summary of the clinical disease was published previously: Pedersen, Elliot, Glasgow, Poland, and Keel. 2000. An isolated epizootic of hemorrhagic-like fever in cats caused by a novel and highly virulent strain of feline calicivirus. Vet Micro 73:281-300.