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As of March 22, 1995, an addendum has been appended to this article.

A family returning from vacation stopped at a fast food shop in south central Texas, where the 2 daughters were approached by an especially friendly kitten. It was thin, but alert, and obviously homeless. The girls persuaded their parents to let them "adopt" the kitten, which blissfully accompanied them the remaining few hours of their drive home. The next morning, the kitten was taken to a veterinarian for vaccinations and worming. The veterinarian judged the kitten to be approximately 4 months old and, although thin, in apparent good health. A taeniafuge was administered, and the kitten was vaccinated against feline panleukopenia, rhinotracheitis, and calicivirus. Inactivated rabies vaccine was administered IM in a hind limb.

For the rest of that day, the cat was subdued and somewhat withdrawn, but was docile and tolerant of being handled. The next morning, the kitten could not be found; it was later discovered to be hiding in a closet. It did not resist handling, but it seemed apprehensive and tried to escape and hide whenever it could. It wasn't eating or drinking much, if at all.

On the third day, the kitten was returned to the veterinary clinic. On arrival, it was agitated, but after a few minutes it was calm enough to be examined. Extreme nervousness and a slight hind limb paresis, or weakness without evidence of pain, were the only abnormal findings. After completing the examination, the veterinarian moved a few steps away. When she approached the cat again, it attacked violently, inflicting several bites and scratches to the veterinarian and a veterinary assistant.

Although the locality was essentially a rabies-free area, the kitten was euthanatized, and the head was submitted to the Texas Department of Health Laboratory, where the veterinarian's diagnosis of rabies was confirmed by immunofluorescence. Subsequent testing revealed the viral strain was probably of skunk origin.

The family was apprised of the diagnosis, and was advised to consult their physician to determine whether postexposure rabies prophylaxis was indicated. Both the veterinarian and the veterinary assistant, each of whom had received preexposure rabies vaccination, were given 2 doses each of human diploid cell rabies vaccine,^a(a) with an interval of 3 days between doses.

The family asked the following questions of the veterinarian:

Q: Isn't rabies rare in cats?

A: Rabies is rare in all domestic animals in the United States, but cats are more frequently affected than dogs. During the past decade, the total number of laboratory-confirmed cases of rabies in cats has exceeded the number in dogs by greater than 10%.¹ In this country, rabies is primarily a disease of wild animals; less than 10% of the reported cases are in domestic animals.¹

Q: Is this a typical case?

A: Yes, although the range of signs may be quite extensive, they typically involve behavioral changes and unexplained paralysis. In this instance, a friendly and docile kitten became withdrawn, anorectic, apprehensive, agitated, and vicious; there also was an indication of partial hind limb paresis, which would probably have progressed rapidly had the cat not been euthanatized.

Q: Is rabies in cats different from rabies in other animals?

A: Cats, unlike dogs, skunks, raccoons, foxes, and bats, do not have an independent rabies cycle (ie, cat-to-cat transmission is not known to occur); therefore, rabies in cats is acquired from other species. When rabies develops in a species that does not have an independent rabies cycle, it is called spillover.

Q: Does this mean that cats cannot transmit rabies?

A: Absolutely not! Since 1946, at least 10 cases of rabies in human beings in the United States have been associated with bites of rabid or suspected rabid cats.^2,3

Q: What is the importance of independent rabies cycles?

A: Rabies tends to be compartmentalized or confined principally to a particular species in a particular locality.¹ This species maintains the infection in nature, and fluctuations of population are integrally related to characteristic enzootic-epizootic cycles. Compartmentalization may result naturally from biological characteristics of the viral strain (ecotype), behavioral patterns of the host species, or both. Natural or induced resistance to infection on the part of other potential hosts in the area also may favor compartmentalization by reducing spillover. For example, the increase in the number of cats affected by wildlife-transmitted rabies in relation to the number of dogs affected by wildlife-transmitted rabies may be attributable to greater inherent susceptibility, to behavioral traits that increase exposures, or to a lower vaccination rate in cats.

Q: How does behavior influence compartmentalization?

A: The inherent behavior patterns characteristic of a particular species may affect the likelihood of contracting rabies. For example cats and skunks frequently share the same habitat (such as a barn), and do so in a state of peaceful coexistence, whereas any encounter between a dog and a skunk will invariably be antagonistic. After one such encounter, (which usually does not include physical contact with the skunk's body) most dogs will keep a safe distance from skunks, but most cats have not learned avoidance, and therefore are more vulnerable to attack by a rabid skunk. Furthermore, in the case of infected animals, behavior typical of the species may tend to promote or retard dissemination of the disease. For example, rabid foxes usually travel much farther than do rabid skunks.

Q: How are different strains of rabies virus detected?

A: Rabies virus strains are detected by immunofluorescent microscopic examination, in much the same manner as routine diagnosis, using monoclonal antibody analysis, a sophisticated new technique.⁴ The development of monoclonal antibodies, which are specific for one antigenic focus on the viral particle, has made it possible to identify antigenically different strains of rabies virus. The identifiable strains correlate well with the species and geographic distributions observed epizootiologically, and are stable over long periods.⁵ Because this technique allows identification of the original source of infection, it is a valuable epidemiologic tool.

Q: How many strains are in this country?

A: At least 5 antigenically distinct strains have been isolated from terrestrial animals in the United States (2 from skunks, 1 from raccoons, and 1 each from gray and red foxes). Bat strains seem to be more variable, but it appears that at least 5 distinct strains can be matched with a like number of bat species.⁵

Q: Are these strains separated geographically?

A: Yes, for the most part. The red fox strain is found in Canada and New York; the gray fox strain is in Arizona; one skunk strain is in the north central states and California; the other skunk strain is in the south central states; the raccoon strain is enzootic in the southeastern states and epizootic in the mid-Atlantic states. An epidemiologically distinct focus of gray fox rabies exists concurrently with skunk rabies in southwest Texas but, thus far, the viral isolates are indistinguishable by monoclonal antibody analysis from the northern skunk and urban Mexican dog strains. The northern skunk and Texas skunk strains coexist in Kansas, at the limits of their respective extensions.^b

Q: What roles do insectivorous bats play in transmitting rabies to terrestrial animals?

A: Bats infected with rabies may shed the virus in their saliva, and several species, including cats, cattle, and man have developed rabies after being bitten by rabid bats. However, monoclonal antibody analysis indicates that few rabid terrestrial animals are infected with bat strains.

Q: What about aerosol transmission?

A: Aerosol transmission of rabies has occurred, but only under specialized conditions in which the air contains a high concentration of suspended particles or droplets carrying viral particles. Such conditions are unusual, but have occurred in laboratory settings, caves with extremely high populations of colonial bats, and perhaps once in animal quarters where rabies-infected wild animals were housed near uninfected ones.⁶ There is no danger of aerosol rabies transmission under most circumstances, and rabies virus has not been isolated from skunk musk (spray).⁷

Q: How long does rabies virus persist in the environment?

A: It depends on the ambient conditions, but rabies virus is not especially persistent under most normal conditions. It is destroyed rapidly at temperatures greater than 50 C,⁸ and generally is believed to survive no more than a few hours at room temperature however, it will persist for years in frozen tissues.

Q: What tissues and secretions are infective?

A: The virus may be located in the central and peripheral nervous systems, the saliva, and sometimes in the urine of infected bats.⁹

Q: Is there a reliable method of rabies diagnosis applicable to live animals?

A: No. Although several techniques, including skin biopsy, corneal smears, and serologic tests have been promoted, an unacceptable level of false-negative results severely limits their usefulness.^c

Q: What role do rodents play in rabies transmission?

A: Most rodents are highly susceptible to artificial infection with rabies, but are rarely found to be naturally infected, possibly because they rarely survive the bite of a rabid carnivore. They are not important in rabies transmission, and there has never been a report of human rabies transmitted by a rodent.

Q: How can wildlife rabies be controlled?

A: Disease control in wild populations is much more difficult than in domestic animals. Because wildlife rabies is a disease of overpopulation, population reduction programs have been advocated; however, these programs generally have proven to be impractical or not cost-effective, principally because a population explosion usually follows the reduction program unless it is sustained.

Oral vaccination of susceptible reservoir species appears to hold the most promise for the future. Foxes can be successfully vaccinated by the oral route, using modified live virus rabies vaccine.^10,11 Fox rabies has been controlled in Switzerland by aerial distribution of chicken necks injected with modified live virus rabies vaccine. A recombinant vaccine with potential usefulness in several species, made by linking a highly immunogenic portion of the rabies virus to vaccinia virus, appears especially promising in preliminary studies.¹²

Q: If 90% of rabies in the United States is in wild animals, why is vaccination of dogs and cats so important?

A: Pets frequently are the targets of attacks by rabid wild animals, and pets with rabies are a much greater hazard to human health than are rabid wild animals. Data accumulated by the Texas Department of Health indicates that the average rabid domestic animal exposes 5 times as many people to rabies as the average rabid wild animal. Furthermore, a vaccinated pet population affords a protective barrier between rabid wildlife and man.

Q: Do the vaccines available currently protect against all known indigenous strains of rabies?

A: Yes.

Q: Did our cat have feline rabies?

A: The preferred terminology would be that this was a case of skunk rabies in a cat. The species name used as an adjective should be restricted to indicate the host that maintains the cycle, if known. The term canine rabies should be reserved to describe rabies that is transmitted from dog to dog; raccoon rabies should mean rabies transmitted by raccoons. Conversely, if rabies spills over into the dog population from a non-canine source, it should be called wildlife rabies in a dog, or fox (skunk, raccoon, etc) rabies in a dog if the viral strain is determined. This convention is more accurate and more informative for those involved in control measures.

Footnotes

(a) Rabies Vaccine, Institut Merieux, Miami, Fla.

(b) Smith JS, Rabies Laboratory, Centers for Disease Control, Lawrenceville, Ga: Personal communication, 1986.

(c) Baer GM, Rabies Laboratory, Centers for Disease Control, Lawrenceville, Ga: Personal communication, 1985.

References

1. Rabies surveillance annual summary 1986. Atlanta: Centers for Disease Control, 1987.

2. The natural history of rabies. Baer GM, ed. Vol 2. New York: Academic Press Inc, 141.

3. Veterinary public health notes. January 1975. Atlanta: Centers for Disease Control, 1975.

4. Wiktor TJ, Flamand A, Koprowski H. Use of monoclonal antibodies in diagnosis of rabies virus infection and differentiation of rabies and rabies-related viruses. J Virol Methods 1980; 1:33-46.

5. Smith JS, Reid-Sanden FL, Roumillat LF, et al. Demonstration of antigenic variation among rabies virus isolates by using monoclonal antibodies to nucleocapsid proteins. J Clin Microbiol 1986; 24:573-580.

6. Winkler WG, Baker EF, Hopkins CC. An outbreak of non-bite transmitted rabies in a laboratory animal colony Am J Epidemiol 1972; 95:267-277.

7. Beauregard M, Casey GA. Studies on the scent glands and musk of rabid skunks (Mephitis mephitis) Can J Comp Med 1973; 37:103-104.

8. Fargeaud D, Bugand M, Precausta P, et al. Thermostability of the rabies virion. Comp Immunol Microbiol Infect Dis 1982; 5:39-47.

9. Girard KF, Hitchcock HB, Edsall G, et al. Rabies in bats in southern New England. N Engl J Med 1965; 272:75-80.

10. Baer GM, Abelseth MK, Debbie JG. Oral vaccination of foxes against rabies. Am J Epidemol 1971; 93:487-490.

11. Steck F, Wandeler A, Bichsel P, et al. Oral immunization of foxes against rabies. Comp Immunol Microbiol Infect Dis 1982; 5:165-171.

12. Rupprecht CE, Wiktor TJ, Johnston DH, et al. Oral immunization and protection of raccoons (Procyon lotor) with a vaccinia-rabies glycoprotein recombinant virus vaccine. Proc Natl Acad Sci USA 1986; 83:7947-7950.

Addendum (1995)

Monoclonal antibody analysis has been used to identify 5 major antigenic variants of rabies virus from terrestrial animals in the United States. One variant is associated with raccoons in northeastern and southeastern states; 2 variants have been isolated from skunks, 1 geographically located in north-central states and California and 1 in south-central states; and 3 variants have been isolated from foxes. Two of these variants are unique to foxes: 1 in gray foxes in Arizona and 1 in Arctic foxes in Canada and red foxes along the New York-Canada border.¹ Prior to the advent of polymerase chain reaction (PCR) technology, a process of genetic typing that uses nucleotide sequencing, it was not possible to distinguish antigenic variants found in gray foxes, coyotes, and dogs located in Texas. However, PCR techniques have differentiated between the Texas fox variant (found in southwest Texas in gray foxes and animals infected by gray foxes) and the urban Mexican dog variant (UMD; associated with rabid dogs and coyotes along the US-Mexico border). A closely related variant to the UMD was determined in specimens from dogs in adjoining areas of Mexico.²

Fox rabies has been controlled, and possibly eradicated, in Switzerland through the distribution of chicken necks containing modified-live oral rabies vaccine (ORV). Belgium and France have used a recombinant ORV to markedly reduce rabies in red foxes; this was accompanied by a great decrease in human exposures to rabies. Thirteen other countries in western and eastern Europe have successfully used the modified-live or recombinant ORV for rabies control in red foxes; Canada has used the modified-live ORV. A raccoon rabies-control program, using the recombinant ORV in a fish meal bait, has recently been implemented in eastern United States.^a Oral rabies vaccination is being evaluated for control of the canine rabies epizootic in south Texas through use of the vaccine in coyotes and for control of the gray fox rabies epizootic in west-central Texas, with gray foxes as the vaccine target species. The 2 Texas epizootics were declared a state health emergency in July 1994.

Canine rabies is especially troublesome because it is readily transmitted from coyotes to domestic dogs and, subsequently, between domestic dogs. Spillover to other domestic animals, including cats, cows, horses, and goats, also has been documented. Rabid pets can bring the disease into the home, which greatly increases the chance for human exposure to the rabies virus.

Footnote

(a) Van Kampen KR, Van Kampen Group, Ogden, Utah: Personal communication, 1994.

References

1. Krebs JW, Strine, TW, Smith, JS, et al. Rabies surveillance in the United States during 1993. J Am Vet Med Assoc 1994;205:1695-1709.

2. Clark KA, Neill SU, Smith JS, et al. Epizootic canine rabies transmitted by coyotes in south Texas. J Am Vet Med Assoc 1994;204:536-540.