Bat colonization in buildings is often undesired in America because (1) one bat among hundreds or thousands may develop paralytic rabies, fall down, and bite people who handle it, (2) in the Southeastern United States and in tropical areas of the world, bat guano may contain infective spores of the fungus Histoplasma capsulatum, which if inhaled in sufficient quantity can produce histoplasmosis in man and animals, or (3) the bats are considered a nuisance. The fear of rabies is the greatest problem.
An estimated 75% of the rabies-infected bats found in California live in trees or other natural harborages. However, a significant quantity find shelter as colonies in buildings. Pest control operators and public health and animal control agencies used to destroy bat colonies in buildings using pesticides or other toxicants, an approach now known to be counterproductive. Killing the bats or repelling them chemically is a waste of time and other resources, because other bats soon colonize the open roost. Moreover, toxicants (and chemical repellents, often misapplied on the bats instead of the roost) scatter sick bats that bite inquisitive persons and pets, increasing manifoldly the numbers of antirabies treatments given people and the numbers of pets that must be destroyed or quarantined and a "treated" roost can produce downed bats for four years or longer.
It has long been known that the only permanent way to rid buildings of bat colonies is to stop up their exit holes after the bats leave to migrate elsewhere for the winter or after they fly out at night to feed on flying insects or in select situations by installing physical repellents such as lights or other disturbing factors. Exterminators claim the displaced bats which then move to a new or alternate roost take with them an impending rabies outbreak, despite abundant evidence that rabies outbreaks do not occur in insectivorous bats. It can be suspected that batproofing eliminates a renewable source of income for exterminators, who would be called to destroy successive groups of bats that move into unsealed roosts.
It is frequently difficult to find someone to do the batproofing work for many reasons including the necessity to work at night, frequently requiring ladder work at night, the mistaken fear of retaliatory attacks by either rabid or nonrabid bats, the fear of acquiring rabies infection through inhalation, and the fear of taking antirabies inoculations either after exposure or the abbreviated preexposure series.