Should You Vaccinate Your Cat Against Feline Calicivirus?


One day, Jeanne Prins’ six-month old kitten Paris was playing actively; the next she was like a limp rag. “I took her to the veterinarian,” recounts Prins. “We assumed it was an abscess. He gave her a shot and said to bring her back on Monday if she wasn’t better. I took her home and she couldn’t stand up. On Sunday morning, my 20-year-old cat Sanibel couldn’t walk. I assumed it was coincidental. But then on Monday, my one-year-old cat, Higgins, also couldn’t walk. My 10-year-old cat, Kitten, stopped eating and I took him in, too. Then A.J. couldn’t climb the stairs. The virus hit us like a tornado.”

Prins, a veterinary supplies salesperson from Reisterstown, Md., discovered last November that she ultimately lost three cats to what was confirmed, upon autopsy, to have been a virulent systemic strain of the feline calicivirus. Two of her cats, Paris and Higgins, survived the illness, while the five others never showed a single symptom. Prins is now an active advocate in her community for vaccinating against the illness.

Tough to Target
The sickness is called virulent systemic feline calicivirus (VS-FCV). Thanks to the efforts of Prins and other concerned pet owners and researchers, a new vaccine is available against VS-FCV, a potentially fatal mutation of the feline equivalent of the common cold. Similar to preventing human flues and viruses, however, targeting this illness can prove difficult.

It is not unusual for your feline to feel under the weather every so often. Upper respiratory infections, oral ulceration, limping and lethargy occur fairly frequently in cats, and may be symptoms of the very common feline calicivirus (FCV). According to the Center for Companion Animal Health at the University of California’s Davis School of Veterinary Medicine, up to eight percent of house cats and 25 percent of cats from multiple cat environments like shelters are calicivirus carriers at any given time. The virus is most often fairly harmless. A few days of rest and your cat should be back to its old self.

More Dangerous Forms of Calicivirus
Some strains of the constantly mutating calicivirus cause the symptoms mentioned above. As in the case of Jeanne Prins and her kitten, certain types of the virus cause no symptoms at all, while other more infrequently occurring strains become highly virulent and dangerous. The virulent and non-virulent forms of calicivirus can both begin with the same symptoms — including the aforementioned oral ulceration, limping and fever. However, unlike the more common strains of FCV, VS-FCV can progress in some cats to more severe problems, including limb swelling, hair loss, ulceration and oozing of the skin, and even death. Outbreaks of VS-FCV in any cat community are very rare. More commonly, cats experience this severity of symptoms due to common FCV combined with panleukopenia or another respiratory issue.

Documenting numbers of cases has proven difficult, since over 65 feline caliciviruses exist worldwide. Recently outbreaks of the deadly version of the virus, however, have been reported in Northern California and New England. One strain appears to be particularly fatal to cats housed in animal shelters.

An insidious feature of FCV, including the virulent forms, is that it spreads easily. Cats may shed the virus through their saliva, so a single sneeze could blast other felines with it. Even asymptomatic cats could harbor the virus and then pass it on to others. If your cat is ever diagnosed with any type of calicivirus, be sure to quarantine it from other animals. Other species, like raccoons, can get it too.

To Vaccinate or Not to Vaccinate?
As with any vaccine, however, there are pros and cons when it comes to administering it to your cat. The major con arises largely from the fact that this vaccine is a “killed vaccine,” which means that additional chemicals are needed to stimulate the vaccine’s immune response. There is some theoretical but to date unproven concern that these chemicals might predispose cats to injection site tumors. The major pro, of course, is that the vaccine may protect your cat against a potentially life-threatening disease.

Veterinarian Kate Hurley, an assistant clinical professor at California’s UC Davis School of Veterinary Medicine, studies all forms of the calicivirus. She offers the following thoughts:

  • VS-FCV is a rare mutation of feline calicivirus that is a serious threat when it emerges. However, it is uncommon. VS-FCV emerges and resolves spontaneously. This is because the calicivirus is an unstable virus, which means it mutates every time it is passed from one cat to the next. So every calicivirus that mutates into a virulent calicivirus will, as it is passed on to the next cat, ultimately once again become a non-virulent calicivirus.
  • Because the virus is always mutating, there is no single virulent strain to vaccinate against. While the new vaccine may provide broader protection against calicivirus in general, it is not certain that it will protect against another virulent strain.

The Bottom Line
“If I were already getting my cat vaccinated with a killed vaccine, I would add the new strain. There might be benefits, so why not?” says Dr. Hurley. “But I wouldn’t switch from a vaccine I was already happy with, [such as] a nasal spray or modified live vaccine, to get the benefit of the protection which there may or may not be. I would not panic in the face of a reported outbreak and rush to get my cat vaccinated. The place your cat is most likely to pick up the virus is at the vet. Wait for the crisis to pass, and have your cat vaccinated in its usual series, adding the new vaccine strain if you choose.”

Protecting your cat’s health always involves a series of choices. The same is true when dealing with concern about virulent systemic feline calicivirus. Consult with your own veterinarian about the issue to decide what could be best for your cat.

Darcy Lockman is a Brooklyn, N.Y.-based freelance writer and frequent contributor to The Daily Cat. Her work has appeared in publications such as the New York Times and Rolling Stone.

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