The Epidemiology of Rabies

THE EPIDEMIOLOGY OF RABIES

In this page:
Animal Rabies Outside of the United States | Animal Rabies Within the United States
Animal Rabies in Washington State | Rabies in Humans

Rabies is primarily a disease of non-human animals. It appears that any mammal species can develop rabies. The prevalence of rabies in specific animal species varies greatly by geographical region. Knowing which animals are most likely to be rabid in a given location is essential to implementing appropriate preventive and postexposure measures.

Animal Rabies Outside of the United States

Rabies has traditionally been associated with dogs more than any other animal, and in parts of the world where domestic animal control and vaccination programs are limited, dogs remain the most important reservoir of the disease. Other domestic and farm animals can be rabid, too, though, and rabies occurs in a variety of wild animal species found in other countries. Consequently, persons who have been bitten by any animal in another country should be fully evaluated as soon as possible by health authorities in that country and by their personal physician in the United States. Local health departments can assist Washington physicians with this evaluation.

"Rabies-free" Locations: Rabies does not occur in all parts of the world. The CDC's "Health Information for International Travel" (The Yellow Book) contains information about the worldwide distribution of rabies. Since these conditions are constantly subject to change, providers evaluating animal encounters that occur outside the United States should contact their local health department or i the CDC Division of Quarantine (404-639-3311) for the most up-to-date information.

Figure 1 shows a Mexican poster warning of the risks of rabies from dog bites. Courtesy of the Ministry of Health, Mexico City.

Animal Rabies Within the United States

While dog rabies is a major problem in much of the world, in the United States, animal control and vaccination programs assure that rabies remains rare in dogs, cats, and other domestic animals. In this country, over 90% of animal rabies cases occur in wildlife¹. Rabies has been detected in many different wild animal species. However, certain geographically distinct reservoirs of terrestrial rabies exist (Figure 2), each with its own variants of the virus. The boundaries of these reservoirs shift constantly. Within each area, rabies transmission occurs predominantly within the dominant reservoir species -- with occasional "spillover infection" to other species¹. There are currently four terrestrial reservoir species in the U.S.: raccoons, skunks, foxes and coyotes. In addition to these reservoirs of terrestrial rabies, indigenous rabid bats have been found in every state except Hawaii.

Figure 2, Distribution of major terrestrial reservoirs of rabies in the United States, 1999 (reproduced with permission from the Journal of the AVMA, 1999¹) .

Animal Rabies in Washington State

Currently, Washington has no terrestrial animal reservoirs of rabies. Bats are the most commonly rabid animal in this state. Of the 5175 Washington bats examined from 1960-2000, 433 (8%) were rabid. Rabid bats have been found in almost every county in the state. The virus has been identified in the bat species common here, and likely occurs in all of the 16-18 bat species present in Washington.

Other than bats, only eight individual animals from this state have tested positive for rabies since 1960. In 1976, an unvaccinated cat from Thurston County was found to be rabid. The following year, a five year old unvaccinated dachshund tested positive for rabies. These were the last confirmed rabid cat and dog in the state.

Two other domestic animals have been diagnosed with rabies since 1960: a horse from Benton County in 1992; and a llama from King County in 1994. The llama was found to be infected with a bat variant of the rabies virus. Because the specimens from the horse were fixed in formalin, the virus strain infecting that animal could not be further characterized at the time.

The only other animals to be identified as rabid in Washington since 1960 were four pet skunks. Two of the skunks were inappropriately imported into Washington and were likely infected out-of-state. The remaining two skunks were pets improperly given live attenuated rabies vaccine.

Table 1 lists the results of animal testing for rabies from 1960 - 2000.

Rabies in Humans

Worldwide: At least 50,000 humans develop rabies each year¹⁰. The overwhelming majority of cases occur in areas where dog rabies is common. Most have a history of having been bitten by a dog. In this country, human rabies is very rare; furthermore, most of the recent human rabies infections in the United States have been caused by variants of the rabies virus associated with bats.

U.S.: From 1980 through December 2000, there were 42 human rabies cases reported in the United States^{3, 16, 17}. Of those, 13 appeared to have been exposed in other countries. Of the remaining 29 people, two were infected with the canine strain of rabies present along the Texas/Mexico border and one was infected by a rabid skunk. The other 26 cases were caused by bat variants of the virus. A definite history of a bat bite was documented for only two of these 26 cases. Twelve of the remaining 24 cases had known contact with a bat.

Airborne transmission: Questions about airborne transmission of rabies are common. Airborne transmission has been demonstrated experimentally in some animal species subjected to prolonged exposures to high concentrations of virus. This means of transmission is of little importance in human rabies. Airborne transmission of rabies was implicated in the deaths of two men who had worked in a Texas cave that harbored millions of bats. Other routes of transmission could not be ruled out in either of these cases, though: one man was noted to have a bleeding scratch after leaving the cave; the other, though gloved, handled numerous bats and had an active case of dermatitis at the time he was exposed⁴. In addition, two laboratory workers developed rabies after accidental exposures to high concentrations of aerosolized virus, although other routes of exposure could not be ruled out in those cases, either^{5, 6}.

Washington state: Eight human rabies cases have been reported in Washington. The first six occurred from 1933-1939. All appeared to have been contracted from rabid dogs. Two recent cases were caused by bat variants of the virus. The first occurred in 1995 in a four-year-old child from Lewis County. A bat found in her bedroom approximately two weeks prior to onset of symptoms later tested positive for rabies [MMWR 1995; 44(34):625-7] The second case was in a 64-year-old man from Mason County who died in January 1997. The case-patient lived in a heavily wooded rural area. He engaged in several outdoor activities, sometimes after dark when bats are active, but had no known exposures to bats or other wild animals in the months prior to his death [MMWR 1997; 46(33):770-3].

Human-to-human transmission: Despite the theoretical risk--and the thousands of human rabies cases worldwide each year--the only documented human-to-human transmission of rabies virus occurred among eight recipients of transplanted corneas obtained from individuals who had died with undiagnosed rabies². Since that time, more stringent guidelines have been established to prevent similar problems from occurring in the future.

This monograph was produced for the World Wide Web by the Northwest Center for Public Health Practice in cooperation with the Washington State Department of Health. See references for further acknowledgements.