Bat Damage Assessment

Wildlife Damage Management February 15, 2008|Print

Bats | Bat Overview | Bat Damage Assessment | Bat Damage Management | Bat Resources | Bat Acknowledgments | ICWDM | Wildlife Species Information

Contents

Damage and Damage Identification

Bat Presence

Little brown bat, Myotis lucifugus
Little brown bat, Myotis lucifugus


Bats often fly about swimming pools, from which they drink or catch insects. White light (with an ultraviolet component), commonly used for porch lights, building illumination, street and parking lot lights, may attract flying insects, which in turn attract bats. Unfortunately, the mere presence of a bat outdoors is sometimes beyond the tolerance of some uninformed people. Information is a good remedy for such situations.

Figure 7. Common points of entry and roosting sites of house bats.
Figure 7. Common points of entry and roosting sites of house bats.

Bats commonly enter buildings through openings associated with the roof edge and valleys, eaves, apex of the gable, chimney, attic or roof vent, dormers, and siding (see Fig. 7). Other openings may be found under loose-fitting doors, around windows, gaps around various conduits (wiring, plumbing, air conditioning) that pass through walls, and through utility vents.

Bats are able to squeeze through narrow slits and cracks. For purposes of bat management, one should pay attention to any gap of approximately 1/4 x 1 1/2 inches (0.6 x 3.8 cm) or a hole 5/8 x 7/8 inch (1.6 x 2.2 cm). Such openings must be considered potential entries for at least the smaller species, such as the little brown bat. The smaller species require an opening no wider than 3/8 inch (0.95 cm), that is, a hole the diameter of a US 10-cent coin (Greenhall 1982). Openings of these dimensions are not uncommon in older wood frame structures where boards have shrunk, warped, or other-wise become loosened.

The discovery of one or two bats in a house is a frequent problem. In the Northeast, big brown bats probably account for most sudden appearances (see Figs. 3 and 8). Common in urban areas, they often enter homes through open windows or unscreened fireplaces. If unused chimneys are selected for summer roosts, bats may fall or crawl through the open damper into the house. Sometimes bats may appear in a room, then disappear by crawling under a door to another room, hallway, or closet. They may also disappear behind curtains, wall hangings, bookcases, under beds, into waste baskets, and so forth. Locating and removing individual bats from living quarters can be laborious but is important. If all else fails, wait until dusk when the bat may appear once again as it attempts to find an exit. Since big brown bats may hibernate in the cooler recesses of heated buildings, they may suddenly appear (flying indoors or outdoors) in midwinter during a warm spell or a cold snap as they move about to adjust to the temperature shift.

Roosting Sites

Bats use roosting niches that are indoors (human dwellings, out buildings, livestock quarters, warehouses), semi-enclosed (loading docks, entrance foyers), partially sheltered (porches, carports, pavilions, highway underpasses, bridges), and open structural areas (window shutters, signs). Once there, active bats in and on buildings can have several economic and aesthetic effects, often intertwined with public health issues (Frantz, 1988). Unusual roosting areas include wells, sewers, and graveyard crypts. Before considering control measures, verify that bats are actually the cause of the problem.

Rub Marks

Surface areas on walls, under loose woodwork, between bricks and around other bat entryways often have a smooth, polished appearance. The stained area is slightly sticky, may contain a few bat hairs, and is yellow-brown to blackish brown in color. The smooth gloss of these rub marks is due to oils from fur and other bodily secretions mixed with dust, deposited there as many animals pass repeatedly for a long period over the same surface. Openings marked in this way have been used heavily by bats.

Noise

Disturbing sounds may be heard from vocalizations and grooming, scratching, crawling, or climbing in attics, under eaves, behind walls, and between floors. Bats become particularly noisy on hot days in attics, before leaving the roost at dusk, and upon returning at dawn. Note that rustling sounds in chimneys may be caused by birds or raccoons, and scratching and thumping sounds in attics and behind walls may indicate rats, mice, or squirrels.

Guano and Urine

Fecal pellets indicate the presence of animals and are found on attic floors, in wall recesses, and outside the house at its base. Fecal pellets along and inside walls may indicate the presence of mice, rats, or even roaches. Since most house bats north of Mexico are insectivorous, their droppings are easily distinguished from those of small rodents. Bat droppings tend to be segmented, elongated, and friable. When crushed, they become powdery and reveal shiny bits of undigested insect remains. In contrast, mice and rat droppings tend to taper, are unsegmented, are harder and more fibrous, and do not become powdery when crushed (unless extremely aged).

The droppings of some birds and lizards may occasionally be found along with those of bats. However, bat droppings never contain the white chalky material characteristic of the feces of these other animals.

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 powder-like 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 (Frantz and Trimarchi 1984). 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 (Frantz 1988).

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. Area fumigation 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.

Public Health Issues

Rabies—General Epidemiology. Bats are distinct from most vertebrate pests that inhabit human dwellings because of their 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 2 weeks to many months), prompt vaccination following exposure can prevent the disease in humans. Dogs, cats, and livestock also can be protected by periodic vaccinations.

Bats are not asymptomatic carriers of rabies. After an incubation period of 2 weeks to 6 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 (Brass, pers. commun.; Trimarchi et al. 1979). 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.

Figure 8. Profile of bat species submitted to the New York State Rabies Laboratory, 1981-1992.
Figure 8. Profile of bat species submitted to the New York State Rabies Laboratory, 1981-1992.

Significance. Rabies is the most important public health hazard associated with bats. Infection with rabies has been confirmed in all 40 North American species of bats that have been adequately sampled, in all of the contiguous United States and in most provinces of Canada. Figure 8 shows the frequency of bat species submitted for rabies testing in New York State over the last 12 years. While not a nationwide measure of human encounters with bats, Figure 8 illustrates that bat species are not encountered equally. Note that bats submitted for testing are often ill and/or easily captured. The numbers and species encountered will vary with the region of the country. Data are generally available from local and state health authorities.

Random sampling of bats (healthy and ill) indicates an overall infection rate of less than 1%. Finding a rabid bat in a colony does not imply that the remaining animals are rabid. In fact, the probability of immediately finding more than one additional infected bat in that colony is small. Bats rank third (behind raccoons and skunks) in incidence of wildlife rabies in the United States (Krebs et al. 1992).

In the last 20 years, however, there have been more human rabies cases of bat origin in the United States than of any other wildlife group. Furthermore, the disease in bats is more widely distributed (in all 48 contiguous states in 1989) than in any other species. In Canada, bats also rank third (behind foxes and skunks) in the incidence of wildlife rabies. Therefore, every bat bite or contact must be considered a potential exposure to rabies. While aerosol transmission of the rabies virus from bats in caves to humans and some other mammals has been reported, this is not a likely route of infection for humans entering bat roosts in buildings in temperate North America. Note that vampire bats are not a threat north of Mexico.

Histoplasmosis—General Epidemiology.

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.

Significance. When soil or guano containing H. capsulatum is physically disturbed, the spores become airborne. Persons at particular risk of histoplasmosis of bat origin include spelunkers, bat biologists, pest control technicians, people who clean out or work in areas where bats have habitually roosted, and people in contact with guano-enriched soil — such as around the foundation of a building where guano has sifted down through the walls.

Infection occurs upon inhalation of spores and can result in a variety of clinical manifestations. Severity partially depends on the quantity of spores inhaled. The infection may remain localized in the lungs where it may resolve uneventfully. This is the case for about 95% of the 500,000 infections occurring annually in the United States. Such infections are identified only by the presence of a positive histoplasmin skin test and/or calcified lesions on routine radiographs. Other individuals may have chronic or progressive lung disease requiring treatment. Less severe forms of these infections may be accompanied by fever, cough, and generalized symptoms similar to a prolonged influenza. Resolution of the disease confers a degree of immunity to reinfection. In addition, resolution confers varying degrees of hypersensitivity to H. capsulatum; as a consequence, massive reinfection in highly sensitized lungs may result in a fatal acute allergic reaction.

In a small percentage of chronic histoplasmosis cases, the fungus disseminates to involve multiple organ systems and may be fatal. This form is usually seen in young children (1 year or older) and in immunocompromised adults. In recent years, systemic infections have been increasing in frequency globally as an opportunistic infection of AIDS patients.



Bats | Bat Overview | Bat Damage Assessment | Bat Damage Management | Bat Resources | Bat Acknowledgments | ICWDM | Wildlife Species Information

LOCATE

Donate to Wildlife Damage Management

Your donation keeps eXtension growing.

Give Now