In a comment to the first post in this series on feline vaccines, I mentioned an especially virulent form of calicivirus. Stephanie from Raleigh brought up some excellent points in response, and after reading her comments I wanted to clarify a few things.

The calicivirus to which I was referring in the main post was “regular” calicivirus, which typically causes fever, sneezing, discharge from the eyes and nose, red and swollen eyes, drooling, ulcers in the mouth, and loss of appetite … most often in kittens. Animals that become infected with this type of calicivirus generally recover with symptomatic and supportive treatment, although some cats can suffer flare-ups of the disease throughout their lives. I consider vaccination against “regular” calicivirus essential.

The virulent systemic feline calicivirus (VS-FCV) that both Stephanie and I were talking about in the comment section is very different. This form of the virus develops as a result of a mutation of the “regular” calicivirus and causes much more serious disease, sometimes leading to death. However, I do not recommend the VS-FCV vaccine for client-owned animals. The disease is quite rare (and seemingly becoming rarer) and is almost always diagnosed in discreet outbreaks associated with shelter animals. Also since each outbreak appears to be associated with a unique mutation to the circulating “regular” calicivirus, the vaccine that is available is not effective.

Hope that clears up any confusion.

Stephanie also asked that I talk about the concept of sterile versus non-sterile immunity as it applies to vaccination. When a vaccine produces what is called “sterile” immunity, the animal should not be able to be infected with that microorganism. In contrast, a vaccine that produced non-sterile immunity often does not prevent infection but can significantly reduce the severity of any symptoms that do develop.

The rabies vaccine is an example of a “shot” that produces sterile immunity. An animal current on its rabies vaccine that is exposed to the virus should not come down with the disease, period. This is not to say that vaccine failures don’t occur; they do. Nothing in medicine, including vaccines, is 100% effective. But when the vaccine works (which is in the vast majority of times), it acts as a barrier to infection.

Non-sterile immunity is different. Even if an individual responds well to the vaccine, he or she may still become infected. Think flu vaccines for people and dogs, or herpes virus and calicivirus vaccines for cats. The goal of vaccination is not necessarily to prevent infection (though that can occur) but to lessen the effect of the disease if it does occur.

Think of it this way. I’m a vet and am therefore vaccinated against rabies. I expect that vaccine to protect me from infection, not simply to let me get off with a “mild case of rabies.” That’s sterile immunity. I also get vaccinated against the flu every year. If I develop some coughing and sneezing but my coworkers and friends are laid up in bed for a week, I’m still pretty happy I got vaccinated. That’s non-sterile immunity.

Dr. Jennifer Coates

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