In addition to leaving disgusting stains, bat droppings, or guano, can become a serious hazard. Guano not only can spread diseases, but sufficiently heavy amounts can cause structural damage. Extreme care should be taken in removing bat droppings.
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Thoroughly bleaching the area is the best individual action, if not outright removing the stained area. However, bat guano carries histoplasmosis, an extremely dangerous fungus. Attempts to clean bat guano should be done with a filter mask and gloves.
The best way to deal with bat guano is to ensure that bats do not enter your house in the first place. Any openings in the structure, particularly in the overhead of the structure, should be sealed. Poisons are not recommended, as dead bats will also contaminate the area.
Bat guano is extremely infectious, and an imperfect attempt at removal can spread around the contamination. It is highly recommended that you hire a professional from your local health department to completely sanitize the area.