Bat Diseases and Parasites
Last Update/Review: June 17, 2002
Contrary to popular belief, few parasites and diseases of insectivorous bats pose any threat to human health. Rabies, an acute infectious disease of the central nervous system, is an exception. It has been identified in a variety of bats throughout Canada and the United States; however, many of the facts about rabies have been sensationalized and misrepresented. The following information summarizes what is known about rabies in insectivorous bats and indicates clearly that although individuals should be handled cautiously, widespread fear and persecution of bats is unwarranted.
Rabies probably has been present in insectivorous bats in North America for a long time, but it was first identified in 1953 in Florida. The virus is maintained within bat populations and is transmitted between individuals of the same or different species. It overwinters in a dormant or latent state in hibernating bats and is re-activated by stress or by a return to the normal metabolic condition of the bat after hibernation. This mechanism gives rise to rabid bats in early summer and fall. Bats which undertake long rigorous migrations are subject to added stress which may affect development of the virus. The pattern of rabies infections in insectivorous bats is consistent across North America and has not changed over the last 30-40 years. Data from Alberta were reviewed in 1986 and are discussed below.
Rabies has been diagnosed in little brown, big brown, silver-haired, hoary, and one western small-footed bat from Alberta. Most rabid bats are collected in August and September. The number of infected individuals differs from year to year and among species, but is consistently low. Bats collected randomly from nursery colonies by Alberta Environment personnel rarely are infected with rabies (1 of 1500 tested since 1979). Prior to 1979 infection in bats collected randomly was approximately one per cent.
Most of the bats collected by the public and submitted for rabies testing are collected because they behave abnormally and/or appear sick (suspect bats). In spite of this, few rabid bats are found. However, 13 rabid bats were collected from one roost in an occupied house near Penhold in 1973. Between January 1979 and December 1983, rabies was diagnosed in one of 467 (0.4%) little brown bats, 23 of 223 (10%) big brown bats, 12 of 203 (6%) silver-haired bats, and 4 of 23 (17%) hoary bats submitted by the public in Alberta.
The prevalence of rabies in little brown bats decreased significantly between 1973 and 1978 and the virus has all but disappeared in this species since then. The prevalence in suspect big brown bats has remained relatively constant at 5 to 10 per cent throughout the documented time period. The prevalence in suspect silver-haired bats has been erratic, ranging from 0 to 18 per cent in 1979 and 1981, respectively. The number of hoary bats submitted for testing was too low to establish the rate of infection.
Rabid
bats were distributed widely throughout the province and generally
reflect the presence of people (Figure 9). Sick or dead bats in uninhabited
areas would go unnoticed. The higher number of bats collected in southern
Alberta is associated with increased public awareness and concern about
rabies in this area.
When bats inhabit a building, they pose little threat to the safety of people or other animals within the area. Their nocturnal habit of feeding exclusively on insects means they seldom interact with humans. There are few valid accounts of bats attacking people, even rabid bats rarely become aggressive. The major problem comes from directly handling bats since like most wild animals, biting is one of their defensive reactions. Any bat found on the ground should be collected and tested for rabies (contact the District Federal Veterinarian). If the bat is handled, gloves should be worn. If a person is bitten, the wound should be washed immediately and medical assistance should be sought. The animal should be collected if possible and submitted for rabies testing.
Information concerning the potential danger from rabies in forest-dwelling bats is incomplete. In July 1985, one man apparently contracted rabies from an unknown forest-dwelling bat in northern Alberta. He did not notify health officials and did not receive rabies preventative vaccine before or after the exposure. In December, the man died of the disease. This is the only recorded human fatality due to exposure to rabies in Alberta. Although the prevalence of rabies in bats is considered low in bats in Alberta, any incident resulting in direct contact and bite from an unknown bat should be reported immediately to health officials. Appropriate follow-up procedures can then be considered. A new fast, effective, and relatively painless anti-rabies vaccine is available to prevent the disease in humans. Pre-exposure immunization should be considered in any occupation with a high potential for contacting wild animals.
Gone are the days when contact with a rabid animal leads to a long series of painful inoculations often with unpleasant side-effects. The current vaccine, human diploid cell vaccine, requires a maximum of five inoculations administered into the muscles of the upper arm to give full protection against infection. Because the vaccine is of human origin, there is little or no reaction against it. However, the protection provided develops rapidly and lasts a long time.
