There are three different types of vaccines: Modified Live Virus, Killed Virus and Recombinant. What do these mean?
Modified Live Virus (MLV) vaccines use a live but weakened strain of a virus. When the virus is injected into the body, it multiplies many-fold and stimulates the immune system’s production of antibodies, creating an immune response that protects the body against future exposure to the disease. This produces a stronger and longer-lasting immunity with fewer doses of vaccine.
The most common MLV vaccines for dogs provide protection against distemper and parvo, also known as the DP vaccine, and many clinics use a DP that also includes hepatitis and parainfluenza, commonly known as a “DHPP” vaccine. The common MLV vaccines for cats protect against feline viral rhinotracheitis, calicivirus and panleukopenia, also known as feline distemper. These are the “FVRCP” vaccines.
MLV vaccines have been associated with the development of temporary seizures in both puppies and adult dogs of breeds or crossbreeds susceptible to immune-mediated diseases. The introduction of MLV vaccines more than 20 years ago is linked to increasing allergic problems and immunological disease in companion animals. Dogs with pre-existing inhalant allergies (atopy) to pollens display an increased risk of vaccinosis (an adverse vaccine response).
Killed Vaccines use an inactivated “dead” form of the virus (previously live microorganisms that have been killed with chemicals or heat), along with an adjuvant, a substance added to a vaccine to enhance its effectiveness without itself causing an immune response. Because of the deadly nature of rabies, rabies vaccines for dogs are from a killed virus.
As with MLV vaccines, killed vaccines can trigger both immediate and delayed adverse reactions. Of highest concern are the vaccine injection-site sarcomas most commonly seen after rabies vaccination in cats, but also seen occasionally in dogs. Genetic predisposition to these disorders in humans has been linked to the leucocyte antigen D-related gene locus of the major histocompatibility complex, and is likely to have parallel associations in domestic animals. Killed vaccines can at worst aggravate an already existing autoimmune disease and may prove ineffective.
Recombinant Vaccines utilize an extremely complex process of recombinant DNA engineering instead of an adjuvant. In cats, adjuvants have been linked to vaccine-related fibro sarcomas, cancerous tumors at the site of the vaccine. Although rare, it is a major concern since this type cancer is extremely malignant, fast-growing, and will spread quickly to other parts of the body. For that reason we use only Merial PureVax rabies vaccine for cats, a recombinant vaccine. The complex process involved makes the feline rabies vaccine more expensive than canine rabies vaccine. While less expensive adjuvanted feline rabies vaccines are still available and widely used, we never use them at our clinic and we would strongly urge you not to allow them to be used on your cat.