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House Bat Management

Public Health Hazards

Rabies in Bats

Rabies is the most important public health hazard associated with bats, but its impact has been vastly exaggerated. The first State to report rabies in a bat was Florida in 1953 (Venters et al. 1954). By 1978, rabies had been reported in 30 of the 40 bat species normally found in the United States and in all 48 contiguous United States, but no increase in the rate of infection has been detected (Constantine 1979b). In 26 years, there have been only eight human fatalities in the United States and Canada attributed to actual bites of rabid bats and two human deaths probably due to nonbite aerosol transmission (Table 1). Tuttle (1979b) noted that "Far more people die every year from dog attacks, bee stings, power mower accidents, or even from being struck by lightning." Unfortunately, newspaper reports and television coverage of bat bites are often sensational, exaggerated and grossly inaccurate, perpetuating misleading information. The Washington Post, 20 September 1979 carried this headline "WARNING: SICK BIRD MAY BE RABID BAT: 12 Million of the Winged Mammals to Pass Through Md. on Way South." On 17 August 1980, the Washington Post headlined a news item "Thousands of Bats, Some Rabid, Infest Hagerstown Homes." Such misleading accounts usually elicit intense public reactions that generate vociferous demands for complete bat destruction (National Academy of Sciences 1973; Mohr 1976). In addition, the following sequence of events usually occurs (D. G. Constantine, personal communication): application of some chemical (DDT or anticoagulant) to kill the bats which results in increased numbers of grounded bats, increasing the probability of human contact and anti-rabies treatment. If a dog or cat is involved, the pet may have to be either quarantined or destroyed (Constantine 1979a).

Table 1.  Summary of 10 human cases of rabies attributed to exposure to insectivorous bats, United States and Canada, 1950-1979 (prepared by D.G. Constantine).
Year of exposure Locality Bat species Circumstances of exposure
1951 Big Spring, Texas Unknown Bitten while handling moribund bat. Bat not tested.
1955 Frio Cave, Texas Free-tailed bats (Tadarida brasiliensis) Airborne infection probable; 0.5% infected among millions of bats present.
1958 Magalia, California Silver-haired bat (Lasionycteris noctivagans) Bitten while handling moribund bat. Bat rabies positive.
1959 Frio Cave, Texas Free-tailed bat Airborne infection probable; from 0.5% infected among millions of bats present
1959 Blue River, Wisconsin Unknown Sleeping person bitten on ear. Bat not tested.
1970 Wilshire, Ohio Big brown bat (Eptesicus fuscus) Bitten on the thumb while asleep. Bat rabies positive. Patient survived.
1970 Saskatchewan, Canada Unknown Bitten on face while in bed. Bat not tested.
1971 Sussex County, New Jersey Unknown Person bitten on lower lip. Bat not captured.
1973 Lexington, Kentucky Unknown Bitten on ear while asleep. Bat escaped.
1976 Cecil County, Maryland Big brown bat Bitten while brushing bat off shoulder. Bat rabies positive.

Rabies in insectivorous bats is generally similar to rabies in carnivores, differing in that bats fly, are rarely aggressive, and some species frequent buildings. Constantine (1979a) stated that about 10% of suspect bats submitted to health departments have rabies and less than 0.5% of seemingly normal bats have been found to have the virus.

In California, D. G. Constantine (personal communication) found, in 1979, that about 1 bat in 1,000 was rabid. He cautioned that figures cited about rabies occurrence in bats are frequently misleadingly high because they do not reflect random samples of wild populations.

Aerosol rabies transmission has been shown to be a hazard to humans and other mammals (World Health Organization 1973). It is not a public health hazard with house bats. The two probable airborne rabies cases were acquired in one cave in Texas occupied by millions of Mexican free-tailed bats (Constantine 1967a, Table 1). To demonstrate that airborne rabies occurred, foxes and coyotes died of rabies within the cave even though they had been placed in screened cages to protect them from bat bites. Although rate of infection among the bats was low the exceptional environmental conditions in the cave were conducive to aerosol transmission of rabies. Such transmission likely occurs in only a very few caves (Constantine 1967).

Post-exposure Prophylaxis

A safer and more effective vaccine against human rabies than Duck Embryo Vaccine (DEV) was approved by the Food and Drug Administration on 9 June 1980 (Centers for Disease Control [CDC] 1980). The new vaccine, human diploid cell rabies vaccine (HDCV), is an inactivated virus vaccine grown in cultures of human diploid cell tissues. HDCV induces active immune responses in about 7 to 10 days after just five injections. DEV is less efficient in producing immunity and requires 23 injections. In addition, adverse reactions with HDCV are less common than with DEV. HDCV is administered in the arm and is given along with rabies immune globulin, human (RIG). RIG provides rapid, temporary protection rarely causing adverse reactions. It replaces antirabies serum, equine, which causes serum sickness in over 40% of the adult recipients. Studies conducted in late 1979 in Germany and Iran confirmed the safety and effectiveness of the new vaccines (CDC 1980).

Bat bites should always be considered as potential rabies exposures. Immediate and thorough washing of all bite wounds and scratches with soap and water is probably the most effective measure for preventing rabies. Tetanus prophylaxis and measures to control bacterial infection should be given as indicated (Anderson and Winkler 1979; CDC 1980).

Post-exposure antirabies immunization should always include both passively administered antibody (preferably RIG) and vaccine (preferably HDCV). RIG is administered only once, at the beginning of antirabies prophylaxis to provide antibodies until the patient responds to vaccination. If RIG inadvertently was not given when vaccination was begun, it can still be given up to the eighth day after the first dose of vaccine is given. HDCV should be administered in conjunction with RIG and consists of five 1-mL doses given intramuscularly (in the deltoid regions). The first dose should be given as soon as possible after the exposure; an additional dose should be given on each of days 3, 7, 14, and 28 after the first dose. The World Health Organization (WHO) currently recommends a sixth dose 90 days after the first dose (CDC 1980).

A serum specimen for rabies antibody testing should be collected on day 28 (at the time the last dose is given) or 2-3 weeks after the last dose. If an adequate antibody titer is not detected, a booster dose is given, and another serum specimen for rabies antibody testing is collected 2-3 weeks later. Testing for rabies antibody can be arranged by the State health department.

Pre-exposure Immunization

The WHO Committee on Rabies-Sixth Report (1973) has recommended pre-exposure immunization against rabies for persons who run a high risk of exposure: bat biologists, veterinarians, pest control operators, and others whose pursuits might bring them in contact with potentially rabid bats should consider this precaution (Trimarchi 1978). Although it does not eliminate the need for additional therapy after a rabies exposure, it simplifies the therapy by eliminating the need for globulin and decreases the number of doses of vaccine needed. Three 1-mL injections of HDCV should be given intramuscularly (in the deltoid area), one on each of days 0, 7, and 21 or 28. All who receive pre-exposure immunization should have serum for rabies antibody testing collected 2-3 weeks after the last injection. If the antibody response is not adequate, a booster should be given and serum collected for antibody testing 2-3 weeks later. Persons with continuing risk of exposure should receive a booster dose (1 mL) every 2 years or have their serum tested for rabies antibody every 2 years and, if the titer is inadequate, have a booster dose.

For assistance on problems or questions about rabies prophylaxis, pre-exposure immunization, or inadequate titer, call your local or State health department, or the Viral Disease Division, Bureau of Epidemiology, Centers for Disease Control (404) 329-3727 during working hours, or (404) 329-3644 nights, weekends, and holidays.

Prevention of Exposure to Bat Rabies

Most rabies exposures could be avoided if people simply refrained from handling bats. Because there have been a few confirmed bat-transmitted rabies deaths in humans, bitten persons are routinely treated to prevent rabies. Treatment is discontinued only if the bat can be shown to be rabies free. Pets that have been bitten and not received antirabies vaccination are either quarantined or destroyed. Unprovoked bat attacks on humans are extremely rare. Bites usually are defensive, occurring when people handle sick or moribund bats. Effective ways to minimize potential human-bat contact are (1) cautioning the general public not to handle bats and (2) exercising care in handling bats. Inexperienced people should never touch bats with bare hands. Bats can be picked up with gloves, forceps, or a stick. If a bat has bitten a person or pet, it should be captured, without destroying the head, and placed in a cloth or plastic bag. Dead bats should be shipped under refrigeration to the nearest health laboratory for examination.

Bat Rabies Control Policy

Since rabies has been detected in the majority of bat species and reported from the 48 contiguous United States, a few States have developed bat rabies control policies to assist county and city governments in coordinating their efforts with state public health departments. The State of California (1977) has a "Bat Rabies Control Policy" which can serve as a model. Requests for information should be addressed to the California Dept. of Health Services DCDC/Veterinary Public Health Section 601 N. 7th Street, MS 486 P. O. Box 942732 Sacramento, CA 94234-7320.

Histoplasmosis in Bats

Histoplasmosis is an airborne disease caused by a microscopic soil fungus, Histoplasma capsulatum, that affects the lungs of humans. It can masquerade as influenza, or with more severe symptoms, be misdiagnosed as tuberculosis. X-rays may show pulmonary lesions. The disease does not respond to tuberculosis treatment. 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 infection (Tosh and Weeks, n.d.).

Nature and Distribution

Histoplasma association with house bats was first described by Emmons (1958) when several cases, including one human fatality, occurred among the occupants of a house in Maryland that contained a colony of big brown bats. In the United States and elsewhere, ecological studies have shown that the fungus is most frequently recovered from soils enriched by the excreta from bats and birds. In the United States almost 90% of all reported cases of human histoplasmosis occur in the central part, particularly the Ohio River and Mississippi River valleys, extending eastward into Virginia and Maryland (Hoff and Biglar 1981.) As yet, there are no records of the recovery of H. capsulatum from Canadian soil (L. Ajello, personal communication) or from bats or bat roosts elsewhere in the United States. The fungus is not found in all bat and bird habitats so that the geographic distribution of the fungus does not seem to be correlated with that of bats (Ajello 1969). In contrast to birds, bats are susceptible to histoplasmosis and the organism has been isolated from some species of bats in the United States. However, Ajello (1969) points out, "This has led to the ill-founded speculation that bats are active in the epidemiology of histoplasmosis." There is no evidence of transmission of the disease from bats to man. 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.

Prevention and Protection

Fortunately, attics that have harbored bats for many years and contain sizable accumulations of guano are not generally located where human occupants are easily exposed. Relatively few people, even among those actually exposed to H. capsulatum, become seriously ill. However, there is potential risk of infection to anyone intending to remove the guano, due to spores released by the disturbance. Pest control operators and others proposing to undertake these tasks 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. The respirators should be approved for nuisance dusts by the National Institute for Occupational Safety and Health (NIOSH; Tosh and Weeks, n.d.). Dry guano should be dampened with water before its removal to further reduce the hazard of dust inhalation.

Soil Decontamination

Formalin is the most effective chemical for the outdoor soil decontamination of H. capsulaturn. Because formaldehyde gas and liquid formalin are dangerous to humans, they are used primarily outdoors. A 3% solution of formalin could be used to decontaminate bat guano in an attic, but due to its hazardous qualities, extreme caution must be exercised (L. Ajello, personal communication).

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