Arthur M. Greenhall
Department of Mammalogy
American Museum of Natural History
New York, NY 10024
Stephen C. Frantz
Vertebrate Vector Specialist
Wadsworth Center for Laboratories and Research
New York State Department of Health
Albany, NY 12201-0509
Fig. 1. Little brown bat, Myotis lucifugus
Damage and Damage Identification
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.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 loosefitting
doors, around windows, gaps around various conduits (wiring, plumbing, air
conditioning) that pass through walls, and through utility vents.
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 otherwise become
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.
Bats use roosting niches that are
indoors (human dwellings, outbuildings, 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.
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 yellowbrown 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.
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
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 powderlike coating. With large numbers of bats, thick and
hard stalactites and stalagmites of crystallized bat urine are occasionally
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
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 pyrethrumbased 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 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 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
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.
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.
|Fig.8. Profile of bat species submitted to the New York State
Rabies Laboratory, 1981-1992.
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
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
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 guanoenriched 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.
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.
Economics of Damage and Control
Virtually all bats are of some economic importance; those north of Mexico
are beneficial because of their insectivorous diet which eliminates many
insect pests of humans. The accumulated bat droppings, called guano, is rich
in nitrogen and is a good organic fertilizer. At one time, bat guano was
commercially mined in the Southwest; but its importance has declined due to
reduced bat populations and the development of inorganic fertilizers. Bat
guano is still considered a valuable fertilizer resource in some parts of
the world (such as Thailand and Mexico). No figures are available to
determine the extent of damage caused by nuisance bats or the cost for their
control. The problem is widespread in this and other countries.
Costs for remedial services are highly variable, depending on the nature
of the problem and who will do the work. For example, to fabricate a few
Frantz’ checkvalves on the “average” two-story house would probably require
two workers about one-half day, mostly on stepladders, and less than $50 in
materials. Much more time would be required to seal up all the other active
and potential bat exit/ entry holes. In addition, if a deteriorated roof,
eaves, or other woodwork must be replaced, the costs can increase rapidly.
It is often difficult or expensive for the public to obtain the services
of reliable, licensed pest control operators (PCOs). Many PCOs have limited
knowledge of basic bat biology and are apprehensive to work with bats. They
may want to avoid any liabilities should bathuman contact occur. Select a
qualified professional service that concentrates on the exclusion of live
bats from a structure rather than on use of lethal chemicals.
Scott E. Hygnstrom;
Robert M. Timm; Gary E. Larson
PREVENTION AND CONTROL OF WILDLIFE DAMAGE â€” 1994
Cooperative Extension Division Institute of Agriculture and Natural Resources University of Nebraska -Lincoln
United States Department of Agriculture Animal and Plant Health Inspection Service Animal Damage Control
Great Plains Agricultural Council Wildlife Committee