Edited Oct 14, 2014: Two updates since I posted this. Most importantly: Merial has begun producing a Purevax nonadjuvanted rabies vaccine with a 3-year label (it was originally labeled for only 1 year). Also, the Purevax shortage that I mentioned in this post was over within a few weeks, as predicted.
If your cat has needed a rabies vaccine in the last few weeks, you might have heard that one of the feline rabies vaccines is temporarily out of production. That’s Purevax from Merial, the only nonadjuvanted rabies vaccine on the US market. Vets and cat owners have had to decide whether to use an adjuvanted rabies vaccine (with a potentially higher risk of injection-site tumor) or to let the cat’s rabies coverage lapse until Purevax is available again. The shortage of Purevax is temporary, but it gives me a good opportunity to discuss rabies vaccines and injection-site tumors in cats.
Traditionally, rabies vaccines have been made using killed (inactivated) virus, which is considered safer than modified-live virus because it cannot revert to its virulent form. Because killed vaccines do not provoke as strong an immune response as modified-live vaccines do, vaccines made using killed virus include adjuvants: substances that enhance the immune response to the vaccine.
Now, one quirk of cats as a species is that they are more susceptible than other animals to oxidative injury to their cells (this is the reason that onions and Tylenol are so toxic to cats).1 Oxidative injury is damage caused by the release of free radicals. Adjuvants in vaccines induce inflammation at the injection site; this is part of the necessary immune response. However, this inflammation also leads to the release of free radicals and thus can cause oxidative damage. In some cats, inflammation and oxidative injury can lead to malignant transformation of cells at the injection site.2,3
This is the reason that cats occasionally develop malignant tumors at injection sites. These tumors tend to be very invasive sarcomas that have a high rate of recurrence even after surgical removal.4 Luckily they are not common; they are estimated to occur in about 1:10,000 cases.5 Other species can get injection-site tumors too, but they are much more rare than in cats. Adjuvanted vaccines are not the only things that can cause sarcomas. Any injection that causes local inflammation is a potential trigger for tumor formation in susceptible cats. Long-acting steroids and microchips, for instance, have been associated with injection-site sarcoma — though the incidence, again, is quite low.
Some veterinarians prefer to use nonadjuvanted vaccines for cats because they cause less inflammation and therefore less oxidative damage at the injection site than adjuvanted vaccines do. Purevax rabies is not a killed vaccine; it’s a recombinant vaccine that uses a canarypox vector (this means that some of the genetic material of the rabies virus is attached to a harmless carrier virus). The drawbacks are that it’s more expensive than killed vaccines and must be repeated every year. Killed vaccines can be given every 3 years, depending on local laws.
There is no blanket recommendation about vaccine type that can be made for all cats. Although nonadjuvanted vaccines should be associated with a lower risk of injection-site sarcoma in individual cats, the published literature has not shown that the incidence of sarcoma in the population as a whole has decreased since they came on the market. It’s difficult to assess cause and effect in many cases because of the time it takes a sarcoma to form (months to years) and because other kinds of injections may have been given in the same location over time. For some cats, especially hard-to-catch strays, it’s better to give a rabies vaccine that is licensed for use every three years. The American Academy of Feline Practitioners’ advisory panel on feline vaccinations concluded in 2013 that “at the current time, there is insufficient information to make definitive recommendations to use particular vaccine types to reduce the risk” of injection-site sarcoma.5
So what should you do about your own cat’s rabies vaccine? Talk to your veterinarian. The question of whether to wait for the Purevax rabies vaccine or to use an adjuvanted one is not straightforward. Vaccinating against rabies is not negotiable; it’s the law in most places, for very good reason. Luckily this particular dilemma is likely to be short-lived, but knowing a little bit about the reasoning behind your vet’s recommendation might help you make vaccine decisions in the future.
For more information:
2013 AAFP Feline Vaccination Advisory Panel Report
1. Tarigo-Martinie J and Krimer P. Heinz Body Anemia in Cats; courtesy of UGa Coll Vet Med. Available at: http://www.vetmed.vt.edu/education/Curriculum/VM8304/vet%20pathology/CASES/Heinz%20Body%20Anemia%20In%20Cats.htm . Accessed May 23, 2014.
2. Woodward KN. Origins of Injection-Site Sarcomas in Cats: The Possible Role of Chronic Inflammation — A Review. ISRN Vet Sci. Apr 2011.
3. Martin M. Vaccine-associated fibrosarcoma in a cat. Can Vet J. 2003;44(8):660–663.
4. Wilcock B, Wilcock A, and Bottoms K. Feline postvaccinal sarcoma: 20 years later. Can Vet J. 2012;53(4):430–434.
5. Scherk MA et al. 2013 AAFP Feline Vaccination Advisory Panel Report. J Feline Med Surg 2013;15(9):785-808.
© Laurie Anne Walden, 2014