Basic human health considerations and bats

Health considerations

Rabies and bats
Rabies is the only serious public health hazard associated with bats, but its impact has been vastly exaggerated. Florida was the first state to report a case of bat rabies in 1953. By 1978, rabies had been reported in 30 of the approximately 40 bat species normally found in the contiguous United States. No increase in the rate of infection has been detected since that time. In the past 55 years, there have been only 44 human fatalities in the United States and Canada attributed to actual bites of rabid bats. Far more people die every year from dog attacks, bee stings, power mower accidents, or even from being struck by lightening. Unfortunately, newspaper reports and television coverage of bat bites are often sensational, exaggerated, and grossly inaccurate, perpetuating misleading information. Such misleading accounts often elicit intense public reactions that generate vociferous demands for complete bat destruction.

The truth is, there are only six species of bats known to have transmitted the rabies virus to humans. The most common bat house bat throughout most of the U.S., the little brown bat (Myotis lucifugus), has never been implicated in a human rabies case, and the next most common, the big brown bat (Eptesicus fuscus), has been implicated in only one case. Most other cases stem from non-colonial, tree roosting species unlikely to ever use bat houses. Airborne transmission of rabies was suspected during the 1950's in two biologists exposed to millions of Mexican free-tailed bats (Tadarida brasiliensis) in Frio cave in Texas. As the biologists were shown to have been exposed to the rabies virus through other bodily fluids, airborne transmission between bats and humans could not be proven. Aerosol rabies transmission between bats and other animals has been found to occur in very extreme conditions known to exist in only one cave environment in the world. Aerosol rabies transmission is not a public health hazard with house bats.

Nevertheless, any bite from a wild mammal should always be considered as a potential for rabies exposure. Any bite or scratch wounds should be immediately and thoroughly washed with soap and water. Any bat that has bitten a person or pet should be captured, without destroying the head, and placed in a cloth or plastic bag. Bats should be transported under refrigeration (not frozen) to the nearest health laboratory for examination. Any time a bat bite is suspected (or if a bat is found in a room with a infant or impaired individual who cannot deny a bite exposure) a doctor or public health department should be contacted in order to obtain the post-exposure rabies series immediately.

Prevention of exposure to rabies
Most rabies exposures could be avoided if people simply refrained from handling any wild or unfamiliar mammals, including bats, but also, foxes, coyotes, skunks, raccoons or domestic cats and dogs. Because rabies is almost always fatal in humans, bitten persons need to be immediately treated with post-exposure rabies vaccines. The treatment is 100% effective if received prior to the onset of symptoms. Unprovoked bat attacks on humans are extremely rare, despite exaggerated stories. Bat bites are usually defensive, occurring when people handle sick or moribund individuals. Effective ways to minimize potential human-bat contact include: (1) cautioning the general public not to handle wildlife, (2) exercising care in handling suspected sick wildlife, and (3) supporting mandatory dog and cat rabies vaccinations. Pets that have been bitten by a rabid animal and have not received the rabies pre-exposure vaccination are either quarantined or humanely euthanized.

Histoplasmosis and bats
Histoplasmosis is an airborne disease caused by the microscopic spores of soil fungus, Histoplasma capsulatum, which affects the lungs of humans. It can masquerade as influenza, or with more severe symptoms, be misdiagnosed as tuberculosis. Many infections in humans do not produce symptoms or cause distress. Skin and blood tests reveal the presence of an infection; however, a positive histoplasmin reaction may only be evidence of a previous exposure. When soil containing the fungus is disturbed, the spores, and possibly hyphal fragments, become airborne and may be inhaled by people who enter bat roosts.
At left: safety percautions are taken when dealing with huge maturnity colonies, this one totals over 12,000 little browns accompanied by a few Indiana bats. Typical colonies found in private homes usually number less than 200.

Prevention and protection

Fortunately, attics that have harbored bats for many years and contain sizable accumulations of guano are not generally located where the fungus can survive and grow. Most bat-related transmissions of histoplasmosis occur in tropical or sub-tropical caves or other large bat roosts. Relatively few people, even among those actually exposed, become seriously ill. However, there is a small potential risk of infection to anyone intending to remove bat guano, due to spores released by the disturbance. Pest control operators and others proposing to undertake these tasks on a regular basis should be healthy persons with positive histoplasmin skin tests and clear chest x-rays. Some protection is possible by wearing respirators that fit properly and are capable of filtering out particles as small as 2µ in diameter or by using a self-contained breathing apparatus. Respirators should be approved for nuisance dusts by the National Institute for Occupational Safety and Health. Dry guano can be dampened with water before its removal to further reduce the hazard of dust inhalation.

Guano, urine, odor, and ectoparasites
Bat guano and urine accumulating in attics and wall spaces can attract arthropods such as roaches as well as other pests. The accompanying odor from a large bat roost can be pungent but not dangerous. Bat ectoparasites, such as ticks, mites, fleas, and bugs, rarely parasitize humans. They are most likely to cause a nuisance after a house has been bat-proofed, thereby ridding the home of bats, but leaving the parasites. Parasite problems are unlikely except in very large, well-established bat colonies where fumigation may be appropriate. Ectoparasites quickly die without their bat hosts.

Source material for this section has been condensed, adapted, and updated from:
Greenhall, Arthur, M. 1982. House bat management. U.S. Fish
and Wildlife Service, Resource Publication 143.
Northern Prairie Wildlife Research Center Home Page.
http://www.npwrc.usgs.gov/resource/1998/housebat/housebat.htm
(Version 15MAY98).