Bats and Bat Exclusion

batBats are among the most unique and fascinating of all animals. No other mammals can fly. Bats use echolocation to find flying insects at night much like sonar helps ships locate objects under water. Bats also have good night vision. They are not blind, as myth would have it.

While Midwestern bats feed exclusively on insects, consuming many pest species, they prefer to expend the least amount of energy to obtain the most food. Thus bats typically capture larger insects, such as night-flying moths, and do not live up to their reputation for controlling mosquitoes.

Correctly considered beneficial animals, in certain situations bats, however, pose a threat to human health. Histoplasmosis (see RESOURCES for more info) is a disease associated with bat guano and bird droppings. When droppings accumulate for years, a fungus (Histoplasma capsulatum) can grow and produce spores that may cause histoplasmosis when inhaled. Where bat or bird droppings accumulate, in an attic for example, care should be taken to avoid contracting this disease. Clean up generally involves wetting the droppings before removal and wearing personal protective equipment, including a HEPA-equipped respirator or self-contained breathing apparatus (SCBA). Removal of large amounts of guano or droppings from structures should be left to experienced professionals familiar with proper removal procedures. 

Perhaps the greatest health risk from bats is rabies. In Illinois, rabies is found in bats more than any other wildlife species. Yet it should be noted that typically less than 5 percent of bats tested for rabies are found to be rabid. In the bat population as a whole, the percentage of rabid bats is much smaller – less than 1 percent. 

Rabies (see RESOURCES for more info) is a viral disease causing encephalitis (brain inflammation) in humans and animals. Humans can become infected when bitten by a rabid bat. Transmission also can occur when an infected bat’s saliva (but not blood, urine or feces unless these are mixed with spinal fluid – as can happen when a bat is beaten or crushed) comes in contact with a person’s eye, nose, mouth, a scratch or wound. Contact with aerosolized bat saliva, especially where large numbers of bats are roosting, also can transmit rabies to humans, although this type of transmission is quite rare. 

Of less importance are parasites associated with bats. Fleas, lice, mites and bat bugs can infest bats, birds and other animals. Some may transmit diseases to humans. If the host animals are killed or leave their nests or roosts, the parasites look for alternate hosts and may wander into the living spaces of structures. They may bite people and domestic animals, but most parasites cannot live long away from their preferred hosts. Control can often be accomplished by simply vacuuming the parasites and carefully discarding the vacuumed material. Sometimes, bat parasites such as bat bugs may have to be eliminated by application of pesticides labeled for this purpose.


Bats flying outside at night pose little risk. However, bats flying outside in daytime, flopping around on the ground, landing on or near someone, or roosting in accessible locations should be avoided, as should any bat found indoors. Bats typically enter structures in spring. Buildings, where bats may be roosting, should be inspected at this time. During daylight hours, inspect attics, rafters, walls, chimneys, porches and cellars for roosting bats, bat guano, crystallized urine or musty ammonia odor. Also inspect for exterior openings that will allow bats to enter the structure.

If a bat is found indoors, the structure should be thoroughly inspected for the presence of roosting bats. Structure-infesting bats pose a health risk and must be prevented from entering occupied rooms. When bats are found roosting inside a structure, the building should be inspected for routes by which the bats might gain access to the living quarters. Such passageways might include ductwork, false ceilings, attic doors, chimney, holes in walls, and gaps around pipes and wiring. Every effort should be made to seal openings large enough for bats to squeeze through (see exclusion procedures below).

Rabid bats may exhibit no obvious abnormalities, so all contact with bats should be avoided. Where there is a likelihood of encountering bats, such as at children’s outdoor camps, people should be instructed not to touch bats. Similarly, people should NOT be allowed to occupy a room in which bats are found, until it is certain that no bats remain in the room and that the room has been sealed to prevent their re-entry.

Any bat suspected of having physical contact with a person should be captured and submitted for rabies testing. Bats can be captured using gloves, by netting, or by covering them with a box or can, then sliding a piece of cardboard or other stiff material under the container to trap the bat inside. It is advisable to wear heavy leather gloves to avoid bites and scratches. Your local health department, animal control office or veterinarian can assist you with submitting the bat to a laboratory for rabies testing. If the bat tests negative, rabies treatment can be avoided.

If a bat bites or has physical contact with a person, the wound or contact area should be washed immediately with soap and water. Unfortunately, b at bites and scratches are small and may go unnoticed. In certain situations it may be impossible to know if contact with a bat has occurred. These situations occur when a bat is found in the same room with a sleeping person, infant or young child, persons with diminished sensory or mental capacity, or persons under the influence of alcohol or drugs. Whenever a bat has physical contact with a person, or is suspected of coming in contact with a person, the bat should be captured, if possible, and the incident should be reported immediately to a physician and local health authority to assess the need for rabies treatment.

The incubation period (time from exposure to appearance of symptoms) varies from days to years, but is usually one to three months. The initial symptoms of rabies in humans may be flu-like and progress to anxiety, confusion, agitation, insomnia, hallucinations, delirium and other abnormal behavior. To be effective, treatment should begin as soon as possible after exposure. Once symptoms appear, rabies is almost always fatal, although a recent experimental treatment appears promising.


Like most birds and other wildlife, all 12 species of bats inhabiting Illinois are protected by law. Species most commonly found in structures include the little brown bat ( Myotis lucifugus ) and big brown bat (Eptesicus fuscus ). These species have a wingspan of less than 12 inches and weigh ½ ounce or less. Four other species are classified as endangered species. It is unlawful to harm or kill a bat. Only under special circumstances are permits to kill bats granted by the Illinois Department of Natural Resources.

Popular “home remedies” for eliminating bats are temporary, ineffective, and/or illegal. No pesticides are registered for bat control. Moth crystals (naphthalene) can be temporarily effective, but the typical attic requires three to five pounds to be used and changed every few weeks. Other types of repellents may not be registered for use as bat repellents and therefore cannot legally be used to harm or repel bats. Bright lights, as well as fans and air-conditioners (used to cool down the roosting area), may be effective but temporary controls. In addition, ultrasonic and electromagnetic devices do not effectively repel bats, rodents or insect pests, despite advertising claims.

Exclusion remains the best way to prevent and control bats in a structure. Bats can be excluded by sealing exterior openings larger than ½-inch, using caulk, expandable foam, plywood, mortar, metal flashing, steel wool or ¼-inch mesh screen or netting. Make sure doors, windows and vents have screens and are securely framed; chimneys are capped; and gaps around utility lines are plugged.

In May and June, one or two “pups” are born to pregnant bats in Illinois. By the end of July, the young bats have taken wing, though they will continue to nurse until able to feed themselves exclusively on insects. Most bats, especially those in northern Illinois, leave their roosting places in September and early October to migrate south where they will overwinter in caves, rocky ledges and cliffs, and occasionally accessible walls and attics. Therefore, bat entry points in structures are best sealed during the months of September through April, when no bats are present. Proper exclusion at this time will prevent bats from entering the structure in spring. Only at certain times can exclusion be performed while bats are roosting within the structure. This involves sealing openings after the young bats are old enough to fly (August or later in Illinois).

Some skill is required to identify all entry points and to apply exclusion materials to openings. Openings through which bats are entering and exiting a structure may be identified from inside the structure by entering the roosting area, an attic for example, on a sunny day when light can be seen through the openings. Another method is to turn on a bright light in the attic at night and look for light escaping through the openings on the building’s exterior. Dark stains may be seen around and beneath openings used by bats. These result from bat guano and from “rub marks” where oils and dirt accumulate as bats pass through the openings. Yet another method of finding bat entry points is to watch for bats leaving the structure at dusk to make their evening feeding flights.

When all openings are identified, a “one-way valve” can be applied to each opening. One-way devices are those that allow bats to leave the structure, and prevent them from reentering. These can be as simple as a sheet of plastic or plastic bag attached above the opening and allowed to hang flush against the building’s exterior. The plastic should be wider than the opening and long enough to hang at least one foot below it. The sides (but not the bottom) of the plastic can be attached to the building by staples or duct tape, to prevent wind from lifting the flap. At dusk, the bats will find their way out beneath the plastic flap, but will not be able to lift the flap to reenter the structure.

Similar devices can be constructed from screening or polypropylene netting of ¼-inch mesh, or a short length of PVC pipe can be positioned in the opening. A tube sock should be fitted around the outside end of the pipe and allowed to hang down with the toe cut out. Bats will exit the pipe and crawl through the sock to get out but will not be able to reenter through the collapsed opening in the toe of the sock. Once all resident bats have exited the structure, the one-way devices can be removed and the openings immediately sealed as described above. Again, this type of exclusion should NOT be performed when young bats, incapable of flying, are present (typically May-July).

Although exclusion is the best way to rid structures of bats, knowledge and timing are critical for effective “bat proofing.” Especially in older construction, there may be several bat entry points that can be difficult to discover. If all openings are not found and sealed, bat problems will continue. Installing sealing materials and one-way devices can also be difficult because bat entry points are often several feet off the ground, requiring the use of ladders (note that falling is a much more common accident than being bitten by a rabid bat). For these reasons, bat exclusion may be best left to professionals. A list of wildlife control specialists, who may be familiar with bat exclusion procedures, can be obtained from the University of Illinois Extension's Web page Living with Wildlife (see RESOURCES for more info).

Illustrations courtesy of Penn State University and the University of Missouri.

For more information, contact the Illinois Department of Public Health, Division of Environmental Health, 525 W. Jefferson St., Springfield, IL 62761, 217-782-5830, TTY (hearing-impaired use only) 800-547-0466.