What Is Bat Excrement Called? Health Risks and Uses

Bat excrement is called guano, a term that also applies to the droppings of seabirds but has become most closely associated with bats, especially in the context of cave deposits. Guano is far more than waste: it is a nutrient-dense substance with a long history of agricultural, industrial, and even scientific use, but it also carries genuine health risks that anyone encountering it should understand.

Where the Word Comes From and What Guano Actually Is

“Guano” entered English from Spanish, which borrowed it from Quechua (the language family of the Inca Empire), where “wanu” meant fertilizer or dung. Although it originally referred to the massive seabird dropping deposits along the Pacific coast of South America, the word now applies broadly to bat droppings as well. In everyday speech, people say “bat guano” to distinguish it from the seabird variety, though in scientific literature the unqualified word “guano” in a cave context almost always means bat droppings.

Fresh bat guano looks different depending on what the bat eats. Droppings from insect-eating bats tend to be small, dark, dry pellets that crumble easily into a powdery, glittery material (the shimmer comes from undigested insect exoskeleton fragments). Fruit-eating bats produce wetter, more amorphous droppings. Over time, guano accumulates in caves or roost sites and can build up into thick deposits spanning meters in depth, sometimes representing centuries or millennia of continuous accumulation.

The chemical makeup of guano varies with the bat’s diet and the age of the deposit. Guano from insect-eating bats tends to be rich in organic matter, carbon, nitrogen, and phosphorus, along with iron oxide, potassium, calcium, and sulfur.¹ Research comparing fresh guano from fruit-eating, insect-eating, and blood-drinking bats found that animal-based diets produce guano with different chemical signatures than fruit-based diets, with lower carbon-to-nitrogen ratios and distinct sterol profiles.² These chemical differences turn out to matter for every use of guano, from farming to forensic climate science.

The Serious Health Risk You Should Know About

The single biggest health threat from bat guano is histoplasmosis, a lung infection caused by the fungus Histoplasma capsulatum. The fungus thrives in nitrogen-rich soil, and bat droppings create an ideal growth medium for it. When dried guano is disturbed, microscopic fungal spores become airborne and can be inhaled deep into the lungs.

For most healthy people, a mild exposure causes flu-like symptoms that resolve on their own. But heavy exposure or exposure in someone with a weakened immune system can be devastating. In one outbreak among tunnel workers in the Dominican Republic who cleared large amounts of bat guano without respiratory protection over roughly three weeks, 83% of the 36-person crew became ill. Of those who got sick, 93% were hospitalized, 30% needed intensive care, 20% required mechanical ventilation, and three workers died. None of them were immunocompromised beforehand.³ That outbreak underscores that even young, healthy adults can develop severe or fatal histoplasmosis when exposure is heavy enough.

Cave explorers face similar risks. An outbreak in Costa Rica between 1998 and 1999 sickened 51 people from two separate groups who visited a bat-inhabited cave. Researchers isolated H. capsulatum directly from guano collected inside the cave.⁴ The fungus has also turned up in more surprising places: two cannabis growers who used bat guano as fertilizer developed histoplasmosis, a reminder that the risk extends beyond caves into any setting where guano is handled.⁵

Viruses and Bacteria in Bat Droppings

Histoplasmosis gets the most attention, but bat guano harbors other pathogens too. Research across multiple bat species has detected coronaviruses in fecal samples, including alphacoronaviruses found in roughly 2% to 10% of various insectivorous bat species in Colorado, with juvenile bats about twice as likely to test positive as adults.⁶ A study of bat populations in France found alphacoronavirus in about 28% of fecal samples, alongside low rates of rotavirus detection.⁷

For a related SARS-like coronavirus studied in bat feces, contact with the fecal environment caused significant damage to the virus’s structure and a dramatic drop in its ability to infect cells, even though genetic material remained detectable for a time.⁸ In other words, finding viral genetic material in guano does not necessarily mean the droppings are infectious. The same pattern holds for rabies-related viruses (lyssaviruses): researchers who tested bat fecal pellets for lyssavirus detected genetic material in some, but could not grow live virus from any of the samples, suggesting the virus in feces is not infectious.⁹ You cannot get rabies from bat droppings. Rabies transmission requires a bite or direct contact with saliva through a wound.

On the bacterial side, several genera known to cause disease in humans have been isolated from bats or their droppings, including Salmonella, Escherichia, Pasteurella, Bartonella, Leptospira, and Yersinia.¹⁰ Research in Indonesia found antibiotic-resistant Salmonella in cave-dwelling bats, raising the possibility of drug-resistant bacteria spreading through guano-contaminated water and food.¹¹ The practical risk from bacterial contamination is mostly indirect, through contaminated water sources or food rather than airborne exposure, but it reinforces the case for treating bat guano as a biohazard when you encounter it in quantity.

How to Handle Guano Safely

If you discover bat guano in an attic, church tower, or other enclosed space, the worst thing you can do is start shoveling or sweeping it dry. That launches a cloud of potentially spore-laden dust directly into your breathing zone. The National Institute for Occupational Safety and Health (NIOSH) recommends wetting guano with water before and during removal, using a NIOSH-approved full-facepiece powered air-purifying respirator, and wearing protective clothing.¹² For small residential cleanups, a well-fitted N95 respirator and disposable coveralls are the minimum. For large accumulations, especially in enclosed spaces like tunnels, attics, or caves, professional remediation is strongly advisable.

Timing matters too. In many regions, bats are legally protected and cannot be disturbed during maternity season (typically late spring through mid-summer in temperate climates). In the United Kingdom, bats and their roosts are protected by law year-round, making it illegal to deliberately disturb or exclude them without a license.¹³ Similar protections exist in the United States, the European Union, and Australia, so cleaning up guano often requires coordination with wildlife authorities. Research into non-lethal deterrents and alternative roost sites is ongoing precisely because simple exclusion can harm bat populations if done clumsily.

Guano as Fertilizer

The agricultural value of bat guano goes back centuries. High levels of nitrogen, phosphorus, and potassium, the three macronutrients plants need most, make it an effective organic fertilizer.¹⁴ Modern field trials bear out the traditional practice. In one study comparing bat guano to farmyard manure and synthetic fertilizer on tomato plants, the highest dose of bat guano produced the best soil organic carbon, nitrogen, phosphorus, and manganese levels, while also improving crop growth.¹⁵

Gardeners and cannabis growers use commercial bat guano products widely, typically as a soil amendment or diluted into “guano tea.” Most commercial products are heat-treated and processed enough to reduce pathogen loads, but the histoplasmosis cases among cannabis growers mentioned earlier are a reminder that not all products are sterile. If you use bat guano as fertilizer, wear a dust mask when handling the dry product, mix it outdoors or in a well-ventilated space, and wash your hands afterward. The risk is much lower than working with raw cave guano, but it is not zero.

Biotechnological Uses

Guano from insect-eating bats is packed with chitin fragments, since insect exoskeletons are mostly chitin. This creates a habitat rich in chitin-degrading bacteria that researchers are now mining for useful enzymes. In one study, 70% of bacterial isolates from Brazilian free-tailed bat guano showed chitinolytic (chitin-breaking) activity, and several of the most active strains also demonstrated antifungal properties against crop-damaging fungi.¹⁶ Separately, researchers isolated a chitin-degrading bacterium from the gut of an insectivorous bat species in India, identifying it as a potential source for industrial chitinase production.¹⁷

Why does anyone care about chitinase enzymes? Chitin is the second most abundant natural polymer on Earth (after cellulose), and enzymes that break it down have applications in biocontrol of crop pests and fungal diseases, processing of seafood waste, and development of biodegradable materials. Bat guano turns out to be one of the richest natural environments for finding bacteria that are already experts at this chemistry.

Guano’s Role in Cave Ecosystems

In caves, guano is often the primary energy source that sustains the entire food web. Sunlight does not penetrate caves, so the organic matter bats bring in from the surface, packaged as droppings, is what feeds everything else. Fungi, bacteria, beetles, mites, and fly larvae colonize guano piles, and cave-adapted predators in turn feed on those invertebrates.

Some cave species have evolved to eat the guano directly. Cave-adapted salamanders in the genus Eurycea, for instance, have been observed consuming bat guano. Researchers found through stable isotope analysis that guano was being assimilated into the salamanders’ muscle tissue, and nutritional analysis confirmed that guano is comparable as a food source to the invertebrate prey these salamanders would normally eat.¹⁸ In nutrient-poor cave environments, the ability to digest guano directly is a meaningful survival strategy. This dynamic also means that removing bat populations from caves, whether through exclusion, disease, or disturbance, can collapse the entire ecosystem downstream.

Reading Climate History in Ancient Guano

One of the more unexpected uses of bat guano is as a paleoclimate archive. Because guano accumulates layer by layer over centuries, and its chemical composition reflects what the bats were eating, which in turn reflects what plants and insects were available, researchers can read environmental change from guano cores much as they read it from ice cores or lake sediments.

A 1.5-meter core from a bat guano deposit in a cave in western Romania provided a 900-year record of environmental change. By analyzing carbon isotope values in the guano alongside pollen and microscopic charcoal fragments trapped in the deposit, researchers tracked shifts in local vegetation and fire activity over nearly a millennium.¹⁹ In Iran, radiocarbon-dated guano deposits in the Karaftu Cave revealed three distinct paleoclimate phases, including shifts between dry and wet conditions, based on the distribution of rare earth elements in the guano layers.²⁰ And that 4,300-year dietary history reconstructed from a guano deposit used chemical ratios in guano to show whether local bats had been eating mostly insects or mostly fruit across millennia, reflecting broader shifts in the surrounding landscape.²

These guano archives are particularly valuable in tropical and subtropical regions where traditional paleoclimate records like ice cores are unavailable. A cave with a long history of bat occupation can fill gaps in the climate record for areas where other proxy data simply do not exist.

Guano and the History of Gunpowder

Before synthetic chemistry made it obsolete, bat guano was a strategic military resource. Potassium nitrate (saltpeter), an essential ingredient in black powder, was historically manufactured by extracting nitrates from guano-enriched cave earth. The process involved dissolving nitrogen-rich cave deposits in water, adding wood ash to convert the native calcium nitrates into potassium nitrate, then boiling and crystallizing the solution.²¹ This technique was practiced across Southeast Asia, Europe, and the Americas, and access to guano-rich caves was sometimes a matter of geopolitical significance. During the American Civil War, Confederate forces mined caves in the southern United States for saltpeter production when Union blockades cut off other supply lines. The connection between bat colonies and military capability persisted until the Haber-Bosch process made industrial nitrogen fixation possible in the early twentieth century, severing the link between cave guano and the ammunition supply chain for good.

When Guano Shows Up in Your Home

Most people encounter bat guano not in caves or on farms but in their own attics, wall voids, or soffits. A small bat colony roosting in a residential attic can produce noticeable guano accumulation within a single season. The droppings are often confused with mouse droppings, but bat guano typically crumbles when pressed (because of the insect exoskeleton fragments) and tends to accumulate in piles directly below roosting spots, whereas mouse droppings are scattered along travel paths.

Beyond the histoplasmosis risk, bat guano in a home can cause a persistent ammonia-like odor, stain ceilings and walls, and attract secondary pests such as carpet beetles and dermestid beetles that feed on the droppings. If the accumulation is heavy enough, the weight and moisture can damage drywall and insulation. Homeowners who discover guano should avoid disturbing it until they have appropriate respiratory protection and have confirmed that the bats have left or been excluded legally. In many jurisdictions, a licensed wildlife control operator is required to handle both the exclusion and the cleanup. Insurance coverage for bat guano remediation varies widely, and some policies explicitly exclude it, so checking your homeowner’s policy before calling a contractor can save a painful surprise.