Histoplasmosis

Histoplasmosis is a lung infection caused by the fungus Histoplasma capsulatum. It is the most commonly cited disease associated with bat and bird droppings in residential and commercial buildings, but it is not limited to animal infestations. According to the CDC’s page on Histoplasma in the environment, the fungus lives in soil and environmental material, particularly where large amounts of bird or bat droppings have accumulated. Texas is a confirmed endemic state for histoplasmosis, and Houston’s warm, humid climate creates favorable conditions for the fungus to grow in soil, attic spaces, wall cavities, and vent ducts.

People become infected by breathing in microscopic fungal spores that become airborne when contaminated material is disturbed. Most healthy people who are exposed either show no symptoms or develop a mild illness that resolves on its own. But for immunocompromised individuals, young children, and older adults, histoplasmosis can become severe, chronic, or fatal without treatment.

Quick Reference

Sources of Histoplasma Exposure

Histoplasmosis is not exclusively a bat or bird problem. The fungus lives in soil and thrives wherever nitrogen-rich organic material has accumulated. Animal droppings are the most common enrichment source, but construction activity, landscaping, and even natural soil disturbance can release spores. Understanding the full range of exposure sources matters because homeowners often assume they are only at risk if they can see droppings, when the reality is that the fungus can be present in soil that was contaminated years ago.

Exposure Sources and Risk Levels

The CDC’s NIOSH division notes that even in the absence of large accumulations of bat or bird droppings, Histoplasma spores can be aerosolized during construction, excavation, or demolition. Once airborne, spores can be carried by wind currents over long distances and infect people who were not at the original disturbance site.

Histoplasmosis and Bats

Bat guano is one of the most concentrated sources of Histoplasma in residential settings. When bats colonize an attic, they defecate in the same area repeatedly, building up dense accumulations of guano over weeks, months, or years. Unlike bird droppings, which are scattered across a surface, bat guano piles up in concentrated deposits directly below roosting sites.

A bat colony that has occupied an attic through even a single season can produce enough guano to create a significant histoplasmosis risk. Colonies that have been present for multiple years can produce accumulations measured in inches or feet rather than scattered droppings. The enclosed nature of an attic space means the fungal spores have nowhere to disperse and remain concentrated in the air within the cavity.

Houston is home to several bat species that commonly roost in residential attics, including Mexican free-tailed bats, evening bats, and big brown bats. Professional bat removal addresses the colony through humane exclusion methods and includes guano cleanup as part of the restoration process. Because bats are protected during maternity season (May 1 through August 15 in Texas), the timing of exclusion work directly affects how long guano continues to accumulate.

Histoplasmosis and Birds

Bird droppings function differently from bat guano in histoplasmosis transmission. Birds do not actually carry the fungus themselves – their body temperature is too high for Histoplasma to grow on or in them. Instead, bird droppings act as a nitrogen-rich nutrient source that promotes the growth of Histoplasma spores already present in the surrounding soil or building material.

In residential settings, the species most commonly associated with droppings accumulation in structures are house sparrows and European starlings, both of which nest in exhaust vents, attic spaces, and soffit cavities. On commercial properties, great-tailed grackle roosts can deposit large volumes of droppings on and around buildings, contaminating soil underneath roost trees and on building surfaces.

The risk pathway is the same regardless of species: droppings accumulate in or near an enclosed space, the fungus grows in the contaminated material, and spores become airborne when the material is disturbed. Professional bird control removes the source and prevents further accumulation.

How Exposure Happens Inside a Home

The most common misconception about histoplasmosis risk in homes is that attic contamination stays in the attic. It does not. There are multiple pathways for contaminated air to reach the living space:

• Vent ducts – a contaminated bathroom or dryer vent pushes air through contaminated material every time the fan runs
• Attic access doors – approximately seven out of ten attic ladders and access panels do not seal completely, leaving a gap for continuous air exchange
• Can light penetrations – recessed ceiling fixtures create unsealed openings between the attic and the rooms below
• Soffit vents – designed to ventilate the attic, they also circulate air that has passed over contaminated material
• HVAC systems – air handlers and ductwork in the attic can pull contaminated air into the home’s circulation
• Wind-driven pressure differentials – on windy days, attics create an over-pressure system that forces air down through gaps and into the living space

Even an attic that nobody enters can continuously expose the household to low levels of contaminated air through these pathways.

Symptoms by Severity

The CDC’s NIOSH histoplasmosis page reports that most people exposed to Histoplasma never develop symptoms. When illness does occur, the severity depends on the amount and duration of exposure and the individual’s immune status.

In areas where Histoplasma is common, 80 percent or more of the population may have been exposed at some point without ever knowing it. Symptoms typically appear 3 to 17 days after exposure, with an average onset around 10 days.

Who Is at Higher Risk

• People with HIV/AIDS – disseminated histoplasmosis is one of the defining opportunistic infections in AIDS patients
• Organ transplant recipients – immunosuppressive medications increase susceptibility
• Cancer patients on chemotherapy – treatment-related immune suppression
• People on long-term corticosteroids or TNF inhibitors – chronic immune suppression for autoimmune conditions
• Infants and young children – immature immune systems
• Adults over 55 – age-related immune decline
• People with chronic lung disease – existing respiratory compromise increases the likelihood of chronic pulmonary histoplasmosis

For households where an immunocompromised person lives, bat guano or accumulated bird droppings in an attic or vent should be treated as an urgent health concern rather than a nuisance to address at some point.

High-Risk Activities

The CDC lists the following activities as increasing the risk of histoplasmosis exposure:

• Cleaning chicken coops
• Exploring caves
• Cleaning, remodeling, or tearing down old buildings
• Landscaping and farming
• Disturbing soil that contains bird or bat droppings

A study published in Occupational and Environmental Medicine found that among histoplasmosis patients in nine states, 48 percent reported gardening or landscaping, 37 percent reported digging soil, 28 percent reported being near construction or demolition, and 24 percent reported handling bird or bat droppings. Nearly a quarter of patients did not recall any of these specific exposures.

Why Professional Cleanup Matters

The biggest mistake a homeowner can make with accumulated bat guano or bird droppings is disturbing the material without proper precautions. Sweeping, vacuuming with a household vacuum, or pulling nesting material out of a vent by hand turns a contained problem into an airborne exposure event. The CDC recommends that large accumulations of bird or bat droppings be cleaned up by professional companies rather than homeowners.

Professional cleanup involves:

• Respiratory protection (N-95 masks or higher for workers)
• Wetting the material with a low-velocity spray before disturbance to suppress dust
• Containment of the work area to prevent spores from spreading into living spaces
• Careful removal and proper disposal of contaminated material
• Sanitization of the affected area after removal

Cleanup should always be part of a complete wildlife removal project that also includes bird exclusion or bat exclusion and any necessary bird damage repair or attic restoration to prevent recontamination.

Frequently Asked Questions

Can I get histoplasmosis from a small amount of droppings?

The risk from casual contact with a small amount of droppings on a sidewalk or roof is very low. The concern is with accumulated droppings in enclosed spaces where the fungus can grow undisturbed and where disturbance puts a concentrated dose of spores into the air.

Is histoplasmosis contagious?

No. Histoplasmosis cannot spread from person to person or from animals to people. It is only contracted by inhaling fungal spores from the environment.

Does past infection make me immune?

No. Previous histoplasmosis infection does not provide reliable immunity. People who have had histoplasmosis should still take precautions to limit future exposure to the fungus.

How do I know if droppings in my attic are a risk?

There is no way to tell visually whether Histoplasma is present in a particular accumulation of droppings. The CDC’s recommendation is to treat any large accumulation of bird or bat droppings in an enclosed space as a potential risk and have it cleaned up professionally.

Can construction near my home put me at risk?

Yes. The CDC confirms that Histoplasma spores can be aerosolized during construction, excavation, or demolition even without visible droppings, and wind can carry those spores beyond the work site. Construction workers are among the highest-risk occupational groups for histoplasmosis exposure.