Saturday, September 19, 2009

Rabies vaccine fears and relative risk.

People are often not very good at assessing relative risks. This can be expressed as a fear of flying, even though the relative risk of dying in a car accident on the way to the airport is probably much higher than the risk of a commercial airline crash. Two papers published in the September 15th issue of JAVMA (Rabies surveillance in the United States during 2008, Blanton, et. al. and Rabies in vaccinated dogs and cats in the United States, 1997-2001, Murray, et. al.) and a post and discussion on Dolittler concerning these papers illustrates how the relative risk of side effects of a vaccine versus the risk of the disease and related consequences. The comments related to the post are particularly revealing, and I believe show how veterinarians and their clients sometimes approach an issue from different angles and can end up talking past each other, especially when a client may be mistaking the relative risks involved.

So what are the risks of vaccination versus the risk of disease as illustrated by these two papers and the discussion on Dolittler?

Risks related to vaccination mostly involve adverse effects of vaccination. Adverse effects include allergic reactions, delayed reactions such as granulomas at the injection site and then the well-known and well-publicized vaccine associated sarcomas which occur mostly in cats, but can happen very rarely in dogs.
Most of these adverse effects are treatable and/or preventable (allergic reactions) and may happen a few times in 1,000 vaccinations. More serious life threatening reactions to vaccines are very rare, and many vets may see very few in their career. Vaccine associated sarcomas in cats may happen in one in 1000 cat to 1 in 10,000 cats over their lifetimes, depending on which study you would like to read. These sarcomas can be very difficult to remove depending on the location they form in, and can be caused by inflammation not related to vaccination. There are newer vaccines for cats that do not have the adjuvants suspected of causing sarcomas to form.
Some other serious diseases and problems such as generalized allergies, autoimmune diseases and "vaccinosis" (a general term for ill health blamed on vaccines that does not have any real meaning or definition) have never been clearly linked to vaccines and are probably not directly or specifically caused by vaccines.

What is the risk of any given pet being exposed to Rabies? This risk may be higher than most people think. The first Rabies surveillance paper shows that Rabies exists in wildlife in every state except Hawaii, and that various bat strains are the most widespread type. There are also regional variations-Raccoon rabies along the east coast, several skunk variants in the midwest, southwest and California, and fox variants in the southwest. A new strain of bat rabies has recently made the jump to skunks and foxes in Arizona. Canine rabies strains have been eradicated in the U.S. due to vaccination and animal control efforts, so all cases of rabies in domestic animals come from wildlife. Any animal that goes outside at all is at risk of being exposed to rabies. Since bat rabies is so widespread, it is possible for a pet to be exposed anywhere bats occur. Rabid bats have an annoying habit of turning up in strange places, including inside homes, so keeping cats indoors is no guarantee that they will never be exposed.

Another risk of having a pet that is not vaccinated against rabies or that is not current on rabies boosters is what could happen if the pet happens to bite someone or is exposed to a rabid animal.
Animal bites treated by doctors legally have to be reported to local health departments, so that appropriate follow up and rabies preventative treatment can be done in a timely manner. In the second JAVMA paper, reports from 20 states on dogs and cats tested for rabies between 1997 and 2001 are listed. There is no way to test a live animal for rabies infection, so all of these animals were euthanised because of a bite or died of neurologic disease that raised a suspicion of rabies. During that period these stated tested 78,669 dogs and 92,318 cats for rabies. 248 dogs (0.32%) and 685 cats (0.74%) tested positive. many of these animals may have been strays, but some of them were pets that were not vaccinated. 13 dogs and 22 cats had a history of rabies vaccination, but only 2 dogs and 3 cats were classified as currently vaccinated. As with any vaccine, failures can happen, but are very rare. This also indicates that one vaccine or an extended schedule of rabies booster does increase the risk for rabies infection.
If an unvaccinated pet bites someone, the local health department can have the pet euthanised and tested for rabies if they think they need to. This is a risk to the pet that is unrelated to the actual risk of rabies infection, as shown by this study-over 99% of dogs and cats tested for rabies were negative. Other states undoubtedly tested many animals as well, but did not agree to participate in this paper, so these numbers are probably even more dramatic across the entire country.

Some of the comments on Dollittler were common arguments many veterinarians hear. "My cats never go outside, they are always in the yard, etc." These papers show that rabies is common and widespread in wildlife, and that hundreds of unvaccinated and even a few vaccinated pets do get rabies, and tens of thousands of unvaccinated pets were killed for rabies testing over a five year period. This is the perspective veterinarians are coming from when they recommend rabies vaccines. The risk to pets and humans from rabies is real and the consequences of infection are severe. Some people who do not want to vaccinate their pets consider information like this fear-mongering, but failing to inform pet owners of these risks, along with a discussion of the rare but serious side effects of vaccines would constitute negligence on the veterinarians part.
Failing to keep a pet current on it's rabies vaccination can result in the death of the pet even if it does not get rabies. This is a result of the need to ensure that humans receive appropriate treatment in a timely manner when exposed to a potentially rabid animal and is a risk that most pet owners are not aware of. Just because a client thinks their pet will never bite anyone or will never get outside, does not mean that it will never happen. It does not mean that your vet thinks you are lying, but we understand the relative risks of vaccinating versus not vaccinating in a way that the pet owner may not.


  1. Nice, concise summary! Because people rare see or hear about rabies cases in companion animals, and because they frequently here stories (true and bogus) about the dangers of vaccines, they may naturally have the perception through the availability bias that vaccines are the greater risk, when actually the opposite is true.

  2. And the fact that the average pet owner has never seen the horrible effects rabies has on a loved pet. There is nothing worse than having a pet in quarantine, having high suspicion and watching that pet die a horrid death, which could have been prevented for a measly $20 vaccine.