How dangerous are bats?
Bat rabies accounts for approximately one human death per year in the United States. Thus, some people consider bats to be dangerous. Nevertheless, dogs which often are considered "man's best friend," attack and kill more humans annually than die from bat rabies in a decade. Statistically speaking, pets, playground equipment, and sports are far more dangerous than bats. Clearly, bats do not rank very high among mortality threats to humans. Nevertheless, prudence and simple precautions can save lives.
What is rabies, and how is it transmitted?
Rabies is an infectious viral disease that invades the central nervous system of humans and other warm-blooded animals. A wide variety of mammals can contract the disease, but it is most often noticed in dogs, cats, foxes, raccoons, skunks, coyotes, bats, and livestock. Worldwide, more than 30,000 humans die of rabies each year, 99% of these cases resulting from contact with dogs. In the United States, due to highly successful dog vaccination programs, transmission from dogs is now rare, eliminating the vast majority of human cases.
Rabies is nearly always transmitted by a bite, though non-bite exposures can result from contact between infected saliva or nervous tissues and open wounds or the mucous membranes of the eyes, nose, or mouth. Careless handling is the primary source of rabies exposure from bats.
Rabies virus has not been isolated from bat blood, urine or feces, and there is no evidence of air-borne transmission in buildings. Two cases of aerosol transmission were reported in the 1950's in Texas caves that support very unusual environments. However, no similar cases have occurred since, despite the fact that many thousands of people explore bat caves each year. No such transmission has occurred outside or in buildings.
Which bat variances of the rabies virus have been transmitted to humans?
Rabies virus variances associated with six of the 45 bat species living in the continental United States have been transmitted to humans. These include the silver-haired bat (Lasionycteris noctivagans), the Mexican free-tailed bat (Tadarida brasiliensis), the big brown bat (Eptesicus fuscus), the eastern pipistrelle (Pipistrellus subflavus), and two species that were not positively identified. These are suspected of having been western (Myotis ciliolabrum) and eastern (M. leibii) small-footed myotis. Variances of the rabies virus attributed to bats that commonly live in buildings have been associated with eight human fatalities in U.S. history.
Do large bat populations lead to increased incidence of rabies transmission to humans?
The largest urban bat populations consist almost exclusively of colonial species, and there is no evidence linking them to increased transmission to humans. Tens of thousands of people have closely observed the emergences of 1.5 million Mexican free-tailed bats at the Congress Avenue Bridge in Austin, Texas each summer for over 16 years without incident. In fact, though Austin, San Antonio, and several other Texas Hill Country towns likely support the highest bat densities in America, they have recorded no human cases of bat-transmitted rabies.
Are bats likely to cause rabies outbreaks in other wildlife or in domestic animals?
There is no evidence that rabies from bats has ever triggered an outbreak in other animals. It occasionally does spill over into other species, causing individual animals to die, but even this is apparently rare. Despite the fact that numerous carnivores gather to feed on the 20 million Mexican free-tailed bats at Bracken Cave, Texas, no outbreaks of rabies are known from this source. No transmission from bats to dogs is known to have occurred, though rare cases of transmission to cats have been documented. The presence or absence of bats is irrelevant to the fact that all dogs and cats should be vaccinated.
What can be done to prevent rabies transmission to humans?
By far the most important prevention is dog and cat vaccination. Also, children should be especially warned never to handle any unfamiliar animal. Explain that wild animals that can be touched may be rabid and dangerous. Ninety to 95% of sick bats are not rabid, but taking a careless chance on being bitten could prove fatal. Any animal bite should be reported immediately to a family physician or public health professional for evaluation as a possible rabies exposure.
The U.S. Centers for Disease Control and Prevention recommend pre-exposure vaccinations for people who are at high risk of exposure, such as rabies researchers, veterinarians, field biologists, and animal rehabilitators. Vaccines currently available include Imovax (HDCV--a human diploid cell vaccine) and Rabavert (PCEC--a purified chick embryo cell vaccine). Vaccinations are administered on days 0, 7, and 21 or 28. For those at continued risk of exposure to rabies, a booster dose of vaccine or serology may be necessary at intervals of 6 months to 2 years.
What are the symptoms of rabies?
Rabies causes fatal inflammation of the brain or spinal cord. Symptoms most often develop about 10 days to seven months after infection, and death follows 2-12 days after symptoms appear. Early symptoms in humans include pain, burning, and numbness at the site of infection. Victims complain of headaches, inability to sleep, irritability, muscle spasms of the throat and difficulty swallowing. Convulsions may occur, followed by unconsciousness and death.
Rabies is often referred to as hydrophobia because victims fear swallowing. Drinking or eating can bring on muscle spasms of the throat. The fear of swallowing also accounts for saliva accumulation referred to as "foaming" at the mouth. Infected animals may be either agitated and aggressive or paralyzed and passive. Dogs, cats, and other carnivores often become aggressive and try to attack humans and other animals, but bats are typically passive. Bats normally bite only in self-defense if handled, and aggressive behavior is rare even when rabid.
How should potential exposures to rabid bats be evaluated and treated?
Any bat that bites a human should be tested for rabies as soon as possible, and post-exposure treatment should begin immediately unless the bat is confirmed negative. Bat bites are typically felt and detected at the time. Visual examination for bite marks is unreliable. If visible at all, bites may appear only as a single tiny puncture or scratch. Most punctures are a millimeter or less in diameter, and most bat inflicted scratch marks are less than a centimeter long. Extenuating circumstances can make detection difficult. If a lost or sick bat hides in bedding, it could be inadvertently pinched during one's sleep, bite, and leave without detection. Also, people hauling in firewood or moving outdoor lumber piles may accidentally poke and be bitten by a bat without noticing. These are obviously remote possibilities, though wearing gloves when moving wood piles could provide protection. If a young child or a mentally incapacitated person is found alone with a bat in the same room and the possibility of a bite cannot be eliminated, post-exposure treatment should be considered unless prompt testing of the bat can rule out infection. When questioning about possible exposure, it is essential first to calm fears of painful shots. For the majority of patients, the post-exposure shots are less painful than tetanus vaccinations. Also, persons who wake up with a bat in the same room where they have been sleeping are advised to submit it for testing, especially if the bat is unable to fly or seems weak.
What is the recommended treatment for a known or suspected rabies exposure?
Modern rabies treatment is highly effective and relatively painless. Post-exposure rabies prophylaxis should begin as soon after exposure as possible. According the Centers for Disease Control and Prevention, exposed humans who have not previously been vaccinated against rabies should receive an initial IM injection of Human Rabies Immune Globulin (HRIG), twenty international units per kilogram body weight or nine international units per pound of body weight in total. If anatomically feasible, the full dose of HRIG should be thoroughly infiltrated in the area around and into the wound(s). Any remaining volume should be administered intramuscularly at a site distant from vaccine inoculation. The HRIG is followed by a series of five 1.0 ml of either Imovax (HDCV--a Human Diploid Cell Vaccine) or Rabavert (PCEC--a purified chick embryo cell vaccine). The vaccination series is given on days 0, 3, 7, 14, and 28. Vaccines are administered intramuscularly in the deltoid region. Persons who have previously received rabies vaccination should receive two 1.0 ml IM doses of either of the two vaccines given above, one on day 0, the second on day 3.
Where can the vaccine be obtained?
Rabies post-exposure vaccinations can be obtained from hospitals, emergency clinics, and doctors. If unavailable locally, vaccines and human rabies immunoglobulin (HRIG) can be obtained as follows: 1) Imovax (HDCV) and Imogan (HRIG) from Aventis Pasteur at (800) 822-2463; 2) Rabavert (PCEC) from Chiron at (800) 244-7668; 3) Bayrab (HRIG) from Bayer at (800) 288-8370. Additional information is available from the Division of Viral and Rickettsial Diseases, U.S. Centers for Disease Control and Prevention at (404) 639-1075 during working hours, or at (404) 639-2888 on nights, weekends, or holidays.
Are there other diseases to be concerned about from bats?
The only other disease of public health concern in the United States is Histoplasmosis, which is caused by a fungus, Histoplasma capsulatum. This fungus lives in soil enriched by bird or bat droppings. Human infection is common in and adjacent to the Ohio and Mississippi River drainages where warm, humid climates favor fungal development. The fungus is rare in dry western and cool northern climates. It can be present, but is uncommon in dry, hot attics of buildings. Infection is caused by inhalation of air-borne spores in dust enriched by animal droppings. The vast majority of histoplasmosis cases in humans are asymptomatic or involve no more than flu-like symptoms, though a few individuals may become seriously ill, especially if exposed to large quantities of spore-laden dust. The disease can be avoided by not breathing dust suspected of being enriched by animal feces. Risks from bats are no different from those of birds.
What about West Nile virus?
Despite what you may have heard, bats do not transmit the West Nile Virus to people or other animals. The virus is transmitted to people by infected mosquitoes. Members of only two bat species have been found to be infected with the virus and, like most other mammals, bats are "dead-end hosts" - they do not transmit the virus back to mosquitoes as birds do.
Let nature help
Natural predators that feed on mosquitoes - including bats, birds, fish, and other insects - can play an important role in reducing insect-borne diseases. Although no single approach to mosquito control is appropriate for all locations, encouraging these natural defenses should be an important part of long-term control whenever possible. Quick-fix options, such as wide-scale chemical applications and expensive mosquito-killing machines, are not practical. They often endanger human health and threaten non-target insects and animals. They also, over time, kill far more predators of mosquitoes than mosquitoes, actually aiding mosquito population growth and increasing demands for stronger and more dangerous pesticides.
Consider bat houses
Under the right circumstances, bat houses (artificial structures for bats) can aid our efforts to control mosquitoes. Bats eat huge quantities of insect pests, and some species are fond of mosquitoes. But, like bluebirds and purple martins, bat populations are declining for lack of places to live. You can help bats and reduce the number of insects in your yard by installing bat houses.
Are there reasons for conserving bats?
Most bats are valuable allies, well worth protecting. Worldwide, they are primary predators of vast numbers of insect pests that cost farmers and foresters billions of dollars annually and spread human disease. In the United States, little brown bats often eat mosquitos and can catch up to 1,200 tiny insects in an hour. An average-sized colony of big brown bats can eat enough cucumber beetles to protect farmers from tens of millions of the beetle's rootworm larva each summer. Large colonies of Mexican free-tailed bats eat hundreds of tons of moth pests weekly. Bats play key roles in keeping a wide variety of insect populations in balance. Yet, they rank as North America's most rapidly declining and endangered land mammals. The largest known cause of decline is exaggerated human fear and persecution.
Is it safe to put up bat houses?
Statistically, it's safer than owning a dog or planting flowers. Flowers attract bees whose stings account for far more human fatality than bats. Just banning bicycles or swimming pools would be hundreds of times more effective in saving lives, but how safe do we really want to be?
Which kinds of bats are attracted to bat houses, and what are the risks and benefits?
In the northern United States and Canada, little brown and big brown bats are the most frequent bat house users. No one has been known to have contracted a disease from little brown bats, while only three have been known to have contracted rabies from big brown bats in all of U.S. history. In southern areas, the two most frequent bat house users are twilight bats (Nycticeius humeralis) and Mexican free-tailed bats. The twilight bat has a perfect safety record. Only five humans have been known to have contracted rabies from free-tailed bats, making them far safer than having dogs in a neighborhood. Children should be warned to leave bats alone, just as they learn to leave bees and unfamiliar dogs alone. Bats that live in our yards, in addition to eating pests, serve as natural insect repellents. Many yard pests, especially moths that attack gardens, lawns, and shrubs, can hear bats from over 100 feet away and attempt to avoid them by leaving the area.
Should bats be tolerated or encouraged in our neighborhoods?
There are clear benefits to sharing our neighborhoods with bats, but as with any wild animal, they never should be tolerated inside our living quarters. Most bats that enter living areas are lost youngsters with no greater interest than a safe escape. They can be chased out through an open door or window or caught in a butterfly net, a leather gloved hand, or a coffee can slowly placed over them while a piece of cardboard is slid between the bat and wall. Rabies testing is expensive and unnecessary unless a possible rabies exposure has occurred.
In the vast majority of cases, exclusion of bats from human living quarters is simple, inexpensive, and can be accomplished by the homeowner with minimal instruction. Exclusion of bats from an entire building is also feasible in most cases, though professional advice may be needed. More than 80% of bat colonies living in buildings go undetected by human occupants, but large colonies can cause odor or noise problems that justify exclusion. Many people simply exclude bats from entering living quarters while permitting them to remain in outer walls or in unused attics.
When bats must be entirely excluded from a building, providing an adequate-sized bat house nearby can resolve a nuisance without sending it to a neighbor. Without such an alternative, evicted bats will attempt to move into a neighbor's home, or sicken and die, increasing the probability of being picked up by children or pets.
Are there risks of people overreacting to news of rabies in bats?
Rabies incidents involving bats are often distorted during media reporting. When risks are not kept in perspective, panicked people overreact in ways that increase rather than decrease the risk of rabies. Attempts to poison or exclude bats from buildings by inappropriate methods can dramatically increase human contact, as sick or homeless bats scatter to exposed positions throughout an entire neighborhood.
Efforts to kill or evict bats invariably center on colonial species. Silver-haired bats and eastern pipistrelles, the two bat variances of the rabies virus most implicated in transmission to humans, overlap big and little brown bats in both roosting and feeding behavior. In urban settings, silver-haired bats are apparently less able to compete with the more colonial species and are scarce. When frightened humans declare war on bats, they may actually help these species by reducing their primary competitors.
The public needs to recognize the inescapability and desirability of coexisting with bats, as well as how to minimize contact and associated risks. Collaboration between bat researchers, conservationists, public health and animal control officials is essential to progress.
Additional References
Bats and Rabies: A Public Health Guide, A Special Publication of the U.S. Department of Health and Human Services. 1998.
Brass, D. 1994. Rabies in Bats: Natural History and Public Health Implications. Livia Press, Ridgefield, CT, 352 pp
Constantine, D.G. 1988. "Health Precautions for Bat Researchers". Pp. 491-528, Ecological and Behavioral Methods for the Study of Bats, (T.H. Kunz, ed). Washington D.C.:Smithsonian Institution Press, 533 pp.
Tuttle, M.D. 1995. "Saving North America's Beleaguered Bats". National Geographic, 88(2):37-57.
Tuttle, M.D. 1997. America's Neighborhood Bats. Rev. ed. Austin: Univ. of Texas Press, 98 pp.
Link:
http://www.batcon.org/home/index.asp...1&idSubPage=62
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