Feline rabies: Diagnosis and prevention methods
Rabies is widely distributed across the globe, with certain exceptions. According to WHO data, more than 55,000 people die of rabies each year. About 95% of human deaths occur in Asia and Africa; the highest numbers are seen in India and China.
The United States maintains one of the world’s most advanced rabies eradication programs, but rabies is still present. In 2008, 49 states and Puerto Rico reported 6,841 cases of rabies in animals and two cases in humans, representing a 3.1% decrease from the 7,060 cases in animals and one case in a human reported in 2007. Approximately 93% of the cases were in wildlife and 7% were in domestic animals, namely cats (294, 4.3%), dogs (75, 1.1%) and cattle (59, 0.9%). The fourfold higher number of rabid cats, compared with dogs, is related predominantly to those cats allowed to roam freely outside. While dog rabies decreased in 2008 compared with 2007, the number of rabid cats showed a 12% increase.
Since the beginning of 2009, feline rabies was diagnosed in domestic or semi-feral cats in Illinois, Montana, Virginia, Michigan, New Jersey and Maine. The latter event presented a typical scenario: A stray kitten that was taken in by an Oxford County family in late August was suspected to be rabid in early September, when it bit three family members and their dog. The cat was euthanized and was confirmed positive for rabies at the state’s health laboratory. The three bitten people underwent the prescribed post-exposure treatment during 28 days; the dog was current on its rabies vaccination, thus not euthanized but placed in a 45-day quarantine.
Rabies in cats
The cat is considered in some European countries to be high-risk species for transmission to human beings. Of more than 20,000 inhabitants in Switzerland that had to be vaccinated after exposure to rabies in the years from the late 1960s until the early 1990s, about 70% had been either bitten or in close contact with cats. Behavioral characteristics of cats and clinical aspects of the disease in this species render it important for public health reasons. In fact, despite a lower number of post-exposure prophylaxis treatment for people after cat bites compared with dog bites, treatment is justified more often.
The average incubation period of rabies in cats is two months but may vary from two weeks to several months or even more, depending on the dose of virus transmitted and the severity and site of the wound. Very long incubation periods have been described in some experimental cases; this must be taken into account when evaluating wound history, especially in free-roaming cats exhibiting sudden behavioral change and/or signs of motor neuron dysfunction that may initiate the clinical phase.
The virus replicates in striated muscle and in connective tissue at the site of inoculation and then enters the peripheral nerves through the neuromuscular junction. Alternatively, it can infect directly the peripheral nerves, spreading to the central nervous system via the axonal route. The virus can then travel to the salivary glands by the retrograde axonal route. At this time, the animal becomes infectious (ie, about three days before the first clinical signs appear). By the time clinical signs appear, the virus is widely disseminated throughout the organs. In most cases, death occurs within five days so that a cat or a dog will be shedding the virus in saliva for about eight days in total.
Aggressive behavior toward humans is unusual in healthy cats, so any unjustified aggressive behavior in cats must be considered highly suspicious.
Rabies should be suspected not only when there has been a recent history of a bite by or exposure to a rabid animal but also when an unvaccinated cat may have been in contact with potentially infected wildlife, such as bats. Indeed, in November 2007, a cat in France died of rabies as a result of infection with bat lyssavirus; however, the risk of cats becoming infected with rabies from bats may be low.
Two disease forms can be identified in cats: the furious and the paralytic one. The furious form of rabies has three clinical phases (prodromal, furious or psychotic and paralytic or dumb), but they are not always clearly distinct in cats. The other form has only two phases: prodromal and paralytic.
During the very short prodromal phase (12 to 48 hours) of both forms, a wide range of quite non-specific clinical signs (including fever, anorexia, vomiting, diarrhea) may occur, sometimes accompanied by neurological signs. Marked behavioral changes may be noticed at first, such as unusually friendly or shy or irritated behavior or increased vocalization. Altered behavior depends on forebrain involvement and may be associated with other neurological signs reflecting the inoculation site.
The tendency to bite may be the consequence of the loss of inhibitory control by cortical neurons over the subcortical bite reflex; if this is the case, the animal snaps without warning or showing any emotion when doing it. Pruritus at the bite site can be observed.
Differential diagnosis of CNS diseases characterized by sudden onset and rapidly evolving clinical signs always includes rabies for free roaming, unvaccinated cats living in endemic areas or traveling there. The clinical approach must make safety the priority because the manipulation and restraint of the cat easily may provoke biting at the time when salivary glands are usually already infected and rabies virus is shed in saliva.
Because clinical diagnosis of rabies is not reliable, a definitive diagnosis can only be obtained by laboratory investigations post-mortem.
Samples (animal heads, brain tissues or other organs) should be sent, and care should be taken to avoid potential human contamination. Brain tissue (especially thalamus, pons and medulla) is the preferred sample for post-mortem diagnosis, but other organs such as salivary glands can also be used.
Routine laboratory diagnosis should be undertaken using only the techniques specified by the OIE (Terrestrial Manual) and WHO (Laboratory Techniques in Rabies):
- The FAT (Fluorescent antibody test) is the primary method recommended.
- The confirmation test should use rabbit tissue culture inoculation test (RTCIT). The mouse inoculation test can be used only if rabbit tissue culture is not available.
- PCR is currently not recommended for routine diagnosis but may be useful for epidemiological studies or for confirmatory diagnosis only in reference laboratories.
Rabies control in cats
Domestic cats in endemic regions should be kept indoors and be vaccinated according to the prevailing local regulations. Keeping cats as semi-ferals is risky and highly contraindicated.
The post-exposure management of cats depends on the national public health regulations, but is forbidden in many countries. Usually, it is not authorized in case of clinical suspicion. No supportive or specific treatment has proved to be effective in rabid cats, so treatment is not recommended.
Rabies in cats is usually controlled by traditional inactivated vaccines, and currently, several very efficient inactivated rabies vaccines are available commercially. These products have been shown to induce protective immune responses after a single vaccination. Cats respond better to rabies vaccination than dogs. The peak of rabies neutralizing antibodies is generally reached four to six weeks after the first immunization. Cats and dogs with a neutralization titre more than 0.5 IU/ml, regardless of time elapsed since vaccination, have a very high probability of survival after a rabies infection.
FIV-positive cats should be kept confined indoors to avoid transmission to other cats, to protect them from other infections and to slow the progression of FIV infection itself. This is an efficient preventive measure for rabies in areas at risk, but follow national or regional legislation. In outdoor cats with risk of exposure to rabies, vaccination is strongly advised.
In endemic areas, stray cats should always be considered an exposure risk, and handling and nursing of rescued animals should be considered dangerous, even if they are asymptomatic.
Risk exposure is generally almost nil in breeding catteries because, usually, pedigree cats are kept strictly indoors, but their vaccination is under local or state regulation.
T. Frymus, D. Addie, S. Belák, et al. Feline rabies: ABCD guidelines on prevention. J Feline Med Surg. 2009;7:585-593.
Arnon Shimshony, DVM, is Associate Professor at the Koret School of Veterinary Medicine Hebrew University of Jerusalem, Rehovot, and is the ProMED-mail Animal Diseases Zoonoses Moderator. Dr. Shimshony was Chief Veterinary Officer, State of Israel, from 1974 to 1999.