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Virulent Systemic Feline Calicivirus: A Case Study

Researchers trace origins of the disease in one California community and recommend effective prevention strategies

Researchers trace origins of the disease in one California community and recommend effective prevention strategies

A recent case study of virulent systemic feline calicivirus warns that outbreaks of the disease, while still rare, are increasingly common and most often seem to originate with a shelter or rescue cat.

Feline calicivirus infection is very common in shelters. Most often it causes signs of upper respiratory infection, oral ulceration, fever, and sometimes limping. The disease is usually self-limiting and rarely fatal. Thousands or even tens of thousands of shelter cats infected with FCV are probably adopted out when still shedding the virus, but usually it does not wreak the kind of havoc seen in this study, says veterinarian Kate Hurley.

In addition to the signs associated with the more common form of the disease, “virulent systemic” strains cause swelling or severe sores and hair loss on the face and limbs, tend to affect adult cats more severely than kittens, and are commonly fatal. Strains of intermediate severity are also reported with increasing frequency and may cause severe oral ulceration.

But the news isn’t as scary as it may sound—mainly because the disease can be contained through isolation of infected animals and stringent cleaning procedures that reduce the risk of transmission through hands, food bowls, and other fomites. The disease is also self-limiting in nature, appearing to peter out within two months after hitting a community, say the study’s investigators.

The studied virus was also not the kind often seen in shelters; it was a special “quasi strain” that’s particularly virulent. While common forms of feline calicivirus infection are rarely fatal, “virulent systemic” strains are a different story—and much more rare.

Written by Maddie’s Shelter Medicine Program director Kate Hurley and her colleagues at the University of California, Davis*, the study examined outbreaks at three neighboring veterinary clinics and a rescue organization in the west Los Angeles area. By examining medical records and conducting interviews, researchers were able to trace the source of the outbreaks back to a mother cat and three kittens who’d undergone spay/neuter surgeries at one of the practices.

Owned by the rescue group, the cats appeared healthy when they returned to their foster homes, and they were brought to an adoption event six days later. The next day, all four had difficulty walking and were feverish; they also suffered a loss of appetite and oral ulcerations.

While these cats apparently made a full recovery, the infection spread wildly among most of the cats who’d been directly exposed to the initial four or indirectly exposed via infected surfaces. The veterinary clinic that had spayed and neutered the four rescue cats experienced an outbreak, as did another veterinary clinic that later treated the four cats when they became sick. Yet another clinic became involved when the feline housemates of a cat who’d undergone intestinal surgery at the first clinic were brought in for treatment at the third practice. The researchers surmise that in this case, since the sick cat had not had contact with his feline kin post-infection, the owners acted as accidental vectors—first visiting their hospitalized cat post-surgery, then going home and exposing their other cats before eventually bringing them in for treatment at a different, previously unexposed facility.

Hurley and her fellow researchers reported other interesting findings that attested to the varying degrees of viral strength and helped characterize susceptible populations:

© Marianne Skoczek
Even vaccinated adult cats are susceptible to infection by the virulent systemic feline calicivirus. In fact, adults in the study were significantly more likely than kittens to develop severe disease and die—a finding similar to those reported in cases of the highly virulent rabbit hemorrhagic disease, which also develops from a calicivirus.
? In one case, a four-year-old spayed female cat who boarded at one of the practices never developed signs of feline calicivirus (FCV) infection but apparently carried the disease home and passed it to her housemate, who died of it 19 days later.
? But in another case, one cat who had tested positive for FCV and was released a week later did not infect his housemates, suggesting that some infected cats are “low-level shedders” and pose less of a threat to other cats.
? Cats of all ages were susceptible to infection by the calicivirus strain, but adults were significantly more likely than kittens to develop severe disease and die—a finding similar to those reported in cases of the highly virulent rabbit hemorrhagic disease, which also develops from a calicivirus.
? All adult cats in the study were either vaccinated or had unknown vaccination status. Vaccinated cats have been prominently affected in all known outbreaks of virulent systemic FCV—something that could be related to immune system response, mutated strains, or a bias toward detection of strains that are vaccine-resistant. (Hurley notes that up to 50 percent of calicivirus strains are now resistant to the commonly used vaccine, which was formulated about 40 years ago.)
? Cats tested positive soon after exposure, even when they were not yet showing clinical signs. But as the disease progressed, it was harder to isolate the virus, and negative results could not necessarily be trusted—cats may still have been excreting the virus at low levels.

The study’s investigators described six outbreaks of FCV disease in Pennsylvania, Massachusetts, Tennessee, and Nevada, noting several similarities to the west Los Angeles cases. In the five outbreaks that were traceable back to a known source or “index case,” all the index cases were shelter or rescue animals. Affected cats in all six outbreaks included those who were otherwise healthy and vaccinated, and the disease spread readily to cats belonging to employees or clients.

Interestingly, although the virus seemed virulent and did considerable damage in these cases, its spread was limited to affected practices or shelters and seemed to disappear within two months. Its origin in the shelter environment is not a surprise. There is more opportunity for persistent infection that would allow for changes in the nature of the virus or the disease; the multiple-cat environment and high population turnover of susceptible animals can be a breeding ground for spread, replication, and mutation.

Disease Control Recommendations

Certain characteristics of the west Los Angeles case have significant implications for control of the disease. Mildly infected animals can play an important role in transmission, and FCV can persist in a dried state at room temperature for up to 28 days. Aerosol transmission probably does not contribute to the spread of infection over distances of more than four feet; rather, client and technician traffic among practices apparently plays a prominent role in outbreaks. With this data in mind, the researchers made several recommendations for prevention of FCV spread:

? All cats exposed to virulent systemic FCV during an outbreak should be considered potentially contagious even if they are not showing symptoms. Before cats can be considered negative for the virus, results of at least two oral swab specimens should be obtained.
? Potentially exposed cats should be isolated from unexposed populations.
? Veterinary practices and other animal care organizations should communicate quickly when an outbreak is suspected so workers at all potential sites of infection in the community can implement more stringent cleaning and handling procedures.
? Contaminated areas should be thoroughly cleaned and then disinfected with a 5% sodium hypochlorite solution in a 1:32 dilution, as the study found that cats admitted to areas that had been thoroughly disinfected with bleach were not infected. Other research supports the notion that bleach is more effective than quaternary ammonium products in disabling the virus.
? Employees facing an outbreak should not rely on hand sanitizers but should instead wash their hands thoroughly between the handling of cats.
? Contaminated clothing may be an important means of spread. Infected cats should be handled by employees wearing gowns, gloves, protective caps, and shoe covers.

“In the absence of a protective vaccine ... rapid recognition and implementation of control measures is the best defense against a severe outbreak, and clinicians should be alert for signs of the disease, especially in cats with a recent history of exposure to a multiple-cat environment,” wrote the researchers. “Even mildly ill cats from high-risk environments should be handled with suitable precautions, and contaminated fomites should be cleaned with a disinfectant proven effective against caliciviruses. The self-limiting nature of this and previous outbreaks provide hope but should not allow for complacency. A future strain may not be so confined and, like RHD, [may] pose a serious risk for the general cat population.”

*“An outbreak of virulent systemic feline calicivirus disease” was written by Kate F. Hurley, DVM, MPVM; Patricia A. Pesavento, DVM, PhD; Niels C. Pederson, DVM, PhD; Amy M. Poland, B.S.; Erin Wilson, DVM; and Janet E. Foley, DVM, PhD. It appeared in the Journal of the American Veterinary Medical Association, Vol. 224, No. 2, January 15, 2004. To view an abstract, visit

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