About Vaccines: Rabies

The rabies vaccine is probably one of the greatest successes in veterinary medicine and the public health field. It has saved countless lives and proves to be a very safe and effective vaccine, greatly reducing the number of cases seen in our pet populations. Rabies is rare these days in pets (the number of reported cases of rabies in the U.S. for 2012 was 81 dogs, 300 cats according to the Centers for Disease Control), but is still prevalent in wildlife and the disease is endemic in all states except Hawaii. A few countries are rabies-free, but all continents except Antarctica have rabies. Most U.S. states have laws requiring dogs and cats to receive regular rabies vaccination; some will also include regulations for ferret vaccination.

Rabies is caused by a number of different species of Lyssavirus, and can cause infection and clinical signs in any mammalian animal. (Experimentally it could infect birds but doesn’t seem to cause any clinical signs.) The virus is transmitted by exposure to the nerve tissue or saliva of an infected animal. The typical route of infection is by a bite from an infected animal, though there are other possible ways to be infected (e.g. organ transplant). After infection by a bite, the virus enters the peripheral nervous system and travels along the nerves to the central nervous system. Once the virus reaches the brain, it causes encephalitis and clinical signs begin to show.

In the first phase of clinical signs, called the “Prodromal phase,” behavioral changes like anxiety, withdrawal or isolation may be noted, fever can be present, and itching can occur at the site of the exposure (bite wound). The prodromal phase may last 2 – 3 days. The disease can then progress to a “paralytic” form or a “furious” form. A majority of dogs show the paralytic form, whereas the majority of cat cases show the furious form. In the paralytic form, animals are lethargic or even paralyzed, can have difficulty swallowing, drool excessively, can have a dropped jaw, and voice or bark changes may be noted. This may last 1 – 7 days, from onset of signs to death. In the furious form, animals may be aggressive and biting, seem hyper-sensitive to visual or auditory stimuli, and can also have altered voice, ataxia, paralysis or seizures. This phase may last 2 – 4 days.

Rabies vaccine is given to cats and dogs around 4 months of age. The vaccine is a killed virus vaccine, meaning that the virus is not infectious when injected into the patient. The vaccine also includes “adjuvants,” which are additives that will stimulate or enhance the immune response. The initial vaccine is followed by a repeat dose the following year, then repeated every 3 years unless using the feline non-adjuvanted rabies vaccine (e.g. the Merial Purevax feline rabies vaccine). The latter requires annual re-vaccination because it does not have the adjuvants that may produce a greater immune response, thus duration of immunity may be less.

Cat owners must decide whether they would like to use the non-adjuvanted rabies vaccine or not. In theory, the non-adjuvanted vaccine reduces the risk of chronic inflammation at the site of injection, which has been associated with soft tissue fibrosarcomas. These are often referred to as “vaccine-induced fibrosarcomas” but in some studies the link is not so certain. The adjuvants or even potentially just the action of puncturing the skin with hypodermic needles has been suggested as the inciting cause of these fibrosarcoma formations. In addition, some cats may be more predisposed to the formation of fibrosarcomas than others. Other studies show some strong statistical evidence in favor of the non-adjuvanted 1-year vaccine. These vaccine will hopefully soon get approved labeling for a 3-year duration, which may effectively end the debate… but for now, this is still a hotly debated topic within veterinary medicine, and when consulting your vet about which vaccine to choose you may get a myriad of answers. Both options are valid and the most important thing will be to follow the vaccine schedule closely for best results.

Rabies vaccine has very effectively reduced the number of rabies cases of domesticated animals in the past few decades, but the disease cannot be eradicated because of the “reservoir” of wild animal hosts. Public health efforts have extended to vaccination of wild animals through dropped oral vaccine in food bait. Though this is also very beneficial, there will probably always be rabies (just think about bats!). Because there is very little that can be done to treat rabies and survival is very rare, vaccination should still be a mainstay of preventive care. If your pet is unvaccinated or overdue for a re-vaccination, and is bitten by an unvaccinated animal, it should be quarantined for 6 months. This is due to how long the incubation period of the virus is before clinical signs develop. Testing for rabies is unfortunately only possible on brain tissue, therefore is only done post-mortem. During the quarantine, there should be minimal to no contact to reduce risk of transmission to others. If your pet is unvaccinated or overdue for re-vaccination and bites another animal or human, it should be quarantined for at least 10 days. This is because if no signs develop in 10 days, the animal was likely NOT shedding virus through its saliva at the time it bit the victim. However, the victim should be treated immediately (if a human victim, most likely will be treated with rabies immunoglobulin therapy and possibly given a vaccine; animals are typically just revaccinated). Any bite wound should be immediately decontaminated as best as possible by thorough flushing with cold water and antibacterial soap. Report any animal bite to a physician or the proper authorities.