Are there any potential health risks associated with allowing bats to permanently stay in my building?

Bats are distinct from most vertebrate pests that inhabit human dwellings because of the potential for transmitting rabies—a viral infection of mammals that is usually transmitted via the bite of an infected animal. Rabies does not respond to antibiotic therapy and is nearly always fatal once symptoms occur. However, because of the long incubation period (from two weeks to many months), prompt vaccination following exposure can prevent the disease in humans. Dogs, cats, and livestock also can be protected by periodic pre-exposure vaccinations. Bats are not asymptomatic carriers of rabies. After an incubation period of two weeks to six months, they become ill with the disease for as long as 10 days. During this latter period, a rabid bat’s behavior is generally not normal—it may be found active during the daytime or on the ground incapable of flying. Most human exposures are the result of accidental or careless handling of grounded bats. Even less frequently, bats in this stage of illness may be involved in unprovoked attacks on people or pets. It is during this stage that the rabid bat is capable of transmitting the disease by biting another mammal. As the disease progresses, the bat becomes increasingly paralyzed and dies as a result of the infection. The virus in the carcass is reported to remain infectious until decomposition is well advanced.

Histoplasmosis is a very common lung disease of worldwide distribution caused by a microscopic fungus, Histoplasma capsulatum. Histoplasma exists in nature as a saprophytic mold that grows in soil with high nitrogen content, generally associated with the guano and debris of birds (particularly starlings, Sturnus vulgaris, and chickens) and bats. Wind is probably the main agent of dispersal, but the fungus can survive and be transmitted from one site to another in the intestinal contents of bats and also in the dermal appendages of both bats and birds. The disease can be acquired by the casual inhalation of windblown spores, but infections are more likely to result from visits to point sources of growth of the fungus. Relative to bats, such sources include bat roosts in caves, barns, attics, and belfries and soil enriched with bat guano.

Numerous wild and domestic animals are susceptible to histoplasmosis, but bats (and perhaps the armadillo) are the only important animal vectors. Unlike bats, birds do not appear to become infected with the fungus. Both the presence of guano and particular environmental conditions are necessary for H. capsulatum to proliferate. In avian habitats, the organism apparently grows best where the guano is in large deposits, rotting and mixed with soil rather than in nests or in fresh deposits. Specific requirements regarding bats have not been described, though bat roosts with long-term infestation are often mentioned in the literature.

While histoplasmosis in the United States is particularly endemic to the Ohio-Mississippi Valley region (which is also an area with the greatest starling concentration) and areas along the Appalachian Mountains, it is also found in the lake and river valleys of other states. Outside areas with “appropriate” environmental conditions, there also occur scattered foci with high infection rates usually associated with caves inhabited by bats or birds.

Guano and Urine
Bat excrement produces an unpleasant odor as it decomposes in attics, wall spaces, and other voids. The pungent, musty, acrid odor can often be detected from outside a building containing a large or long-term colony. Similar odor problems occur when animals die in inaccessible locations. The odor also attracts arthropods which may later invade other areas of a building.

Bat guano may provide a growth medium for microorganisms, some of which are pathogenic (histoplasmosis, for example) to humans. Guano accumulations may fill spaces between walls, floors, and ceilings. It may create a safety hazard on floors, steps, and ladders and may even collapse ceilings. Accumulations also result in the staining of ceilings, soffits, and siding, producing unsightly and unsanitary conditions.

Bats also urinate and defecate in flight, causing multiple spotting and staining on sides of buildings, windows, patio furniture, automobiles, and other objects at and near entry/exit holes or beneath roosts.

Bat excrement may also contaminate stored food, commercial products, and work surfaces.

Bat urine readily crystallizes at room temperature. In warm conditions under roofs exposed to sun and on chimney walls, the urine evaporates so quickly that it crystallizes in great accumulations. Boards and beams saturated with urine acquire a whitish powderlike coating. With large numbers of bats, thick and hard stalactites and stalagmites of crystallized bat urine are occasionally formed.

Although the fresh urine of a single bat is relatively odorless, that of any moderate-sized colony is obvious, and the odor increases during damp weather. Over a long period of time, urine may cause mild wood deterioration. As the urine saturates the surfaces of dry wood beams and crystallizes, the wood fibers expand and separate. These fibers then are torn loose by the bats crawling over such surfaces, resulting in wood fibers being mixed with guano accumulations underneath.

The close proximity of bat roosts to human living quarters can result in excreta, animal dander, fragments of arthropods, and various microorganisms entering air ducts as well as falling onto the unfortunate residents below. Such contaminants can result in airborne particles of public health significance.

Ectoparasites and other Arthropods
Several arthropods (fungivores, detritivores, predators, and bat ectoparasites) are often associated with colonies of bats in buildings. Their diversity depends on the number of bats, age and quantity of excreta deposits, and season. Arthropods such as dermestid beetles (Attagenus megatoma) contribute to the decomposition of guano and insect remnants but may also become a pest of stored goods and/or a nuisance within the living quarters.

Cockroaches (for example, Blatta orientalis) attracted to guano may invade other parts of a building. Bat bugs (Cimex spp.) are sometimes found crawling on the surface of beams or around holes leading to secluded recesses used by bats. Bat ectoparasites (ticks, mites, fleas, and bugs) rarely attack humans or pets and quickly die in the absence of bats. Ectoparasites may become a nuisance, however, following exclusion of large numbers of bats from a well-established roost site. Aerosol dispensing with a total-release pyrethrum-based aerosol may be an appropriate solution for arthropod knockdown within an enclosed space but only after bats have departed. For long-term arthropod control, lightly dust appropriate surfaces (affected attic beams, soffits) with boric acid powder or diatomaceous earth; carefully read all product labels before using any pesticide.

Note that neither rabies nor Lyme disease is transmitted by any arthropods associated with bats.