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Indiana State Department of Health

Epidemiology Resource Center Home > Surveillance and Investigation > Vector Control Information > Histoplasmosis: A Hoosier Concern Histoplasmosis: A Hoosier Concern

The Disease
The Fungus
The Hoosier Experience
Handling Suspected Infestations
Reporting Possible Problems

The Disease

Histoplasmosis is an infection caused by a fungus, Histplasma capsulatum. This infection usually results in a mild, self-limiting upper respiratory illness that may be unnoticed or ignored. The severity of the illness depends upon two primary factors:

  • The immune status of the patient.
  • The dose of spores that are inhaled.

The very young, the very old, those individuals who are weakened by a chronic illness, and the immunocompromised or immunosuppressed will be more susceptible to developing severe illness. People, therefore, who suffer from diseases such as HIV infection, which can compromise the immune system, or cancer patients undergoing chemotherapy which may suppress the system, are at particular risk for severe clinical illness. This severe illness, while occurring in a minority of patients, can lead to permanent damage to the lungs, or to blindness or death. While previous exposure to spores may result in some degree of immunity it is not absolutely protective because it can be over-ridden with exposure to large numbers of spores.

Signs of the disease include chronic cough, chest pain, fever, lack of appetite and weight loss. These are symptoms that can be caused by other illnesses; therefore, a physician must conduct specific diagnostic tests to make the distinction between this and other respiratory infections.

The Fungus

Histoplasma capsulatum is a common soil fungus. It is widespread throughout the midwestern and southern parts of the United States. It has become associated with accumulations of bird and bat droppings because an accumulation of these droppings over time will result in a change in environmental conditions that allows the fungus to multiply. This accumulation normally has to take place over a period of at least three to five years.

H. capsulatum is also a dimorphic fungus. This means that it occurs in two radically different forms, depending on the environment in which it is growing.

  • In the soil, it is found in the mycelial or mold phase. In this phase, it is composed of branches, or mycelia, onto which conidia, or spores, are produced. These spores are very small (2 microns), and readily dislodge and become airborne when they are disturbed. This is the dispersal stage of the fungus, and also the stage through which most people become infected.
  • The second phase of the fungus is the yeast phase, which grows in human tissue at normal body temperature. In this stage, the fungus is not passed from one person to another. When people become infected, it is through exposure to the spores.

The spores are so resistant to environmental stress that they can remain viable in the environment for many years. Spores are not normally found in bird droppings. Thus, the conditions that promote fungal growth pose a threat result solely from an accumulation of droppings on the soil where the spores may already be found. Bat droppings, however, are different. Spores can be found directly in them. Under proper conditions, and when many droppings are deposited over a length of time, the fungus literally explodes as a population. Many millions of spores can be formed in a relatively small area, ready to become airborne when disturbed.

The Hoosier Experience

Histoplasma capsulatum is endemic to the state of Indiana. This means that it is constantly present. Indiana happens to lie in a section of the country drained by the Mississippi River Basin. Areas in this basin experience an optimum combination of temperature and humidity under which the fungus can grow very well.

Studies conducted by the U. S. Navy on recruits in the 1950's showed that Hoosiers were positive at a rate of 90 percent for antibodies to the fungus. This indicates that most people who were born in Indiana will have been infected by Histoplasma spores by the time they reach adulthood.

Unfortunately, the fact that the fungus is both common and ubiquitous in the state means that the potential exists for outbreaks of different sizes. The largest recorded outbreak of histoplasmosis in the country occurred in Indianapolis in September 1978 and August 1979. Four hundred and thirty five (435) people were diagnosed with clinical cases of histoplasmosis. Subsequent studies of antibodies in the population indicated that 100,000 residents were exposed to spores during that episode. A smaller outbreak occurred in 1980. Since several construction activities occurred in the area during this time period, the exact source was not at first established. Serological tests of high school students indicated that the sources most likely were wooded areas that were being cleared for construction. These wooded areas had served for years as major fall and winter roosts for starlings and other blackbirds.

During the past two decades, individual cases had also occurred. In most situations, since the fungus is so easily found, it is difficult to attribute infection to a particular site. There have been instances, however, such as infection of children playing in the debris at the base of a hollow tree in which bats had roosted and infection of adults as a result of cleaning a fireplace below chimneys in which bats and chimney swifts had been roosting. The diversity of circumstances under which people have been exposed illustrates the importance of recognizing those unique conditions under which the fungal spores can be found in abundance.

High-risk factors that may result in exposure of groups of people to Histoplasma spores include:

  • Excavation associated with construction, demolition, and uprooting of trees in areas that have experienced major bird roosts for at least three-to-five years, particularly when it is dry.
  • Disturbing dry areas where droppings have accumulated in chicken coops, attics where pigeons or bats had roosted, or ledges where pigeons had roosted can also be hazardous.

Handling Suspected Infestations

Any area that has historically harbored large numbers of birds or bats, even if the harborage is not current, should be assumed to be an appropriate site for Histoplasma infestation. The ideal situation for identification would be testing of the area to determine whether or not the fungus is present. Unfortunately, current technology does not include a rapid diagnostic test of the environment. In fact, testing usually takes at least eight weeks and is laden with sampling errors, resulting in both a great deal of expense, plus the inability to say that an area is truly negative if the samples do not show fungus. In cases such as this, the area should be treated as if it is known to be infested.

The identification of a potential risk site involves the use of a great deal of common sense, plus asking questions of people living in the area who may have a long-term knowledge of inhabitation of the site by large numbers of birds or bats.

Inspectors should approach such areas cautiously, particularly in an enclosed situation such as an attic or another part of a structure. If upon approach large accumulations of dried droppings are observed, do not enter the area and run the risk of disturbing it and raising spores.

Situations such as this should be entered only with proper clothing and breathing equipment to prevent inhalation of spores into the lungs. Since the spores are extremely small, the use of a respirator with HEPA filters is necessary to keep the spores from entering air passage ways. Respirators should fit properly and be worn only on a clean-shaven face to be effective. Dust masks or surgical masks are not effective in excluding spores. If full-face respirators are not used, tight-fitting goggles should be worn. Remember, the eyes can also serve as a portal of entry into the body. Temporary, throw-away clothing such as coveralls, hats, gloves and shoe covers should be worn and carefully removed as soon as workers leave the environment. A quick shower with soap and water is also helpful. Throw-away items of clothing should be double-bagged and disposed of in a sanitary landfill.

In an indoor situation, prior to the clean-up of accumulated droppings, the area should be wet down. Use a gentle spray rather than a stream of water under high pressure, which can raise spores into the air. Once the material is wet, it can be cleaned up using shovels or other appropriate tools and placed into doubled plastic bags, which can then be disposed of in a sanitary landfill. Do not use a vacuum cleaner. Spores will pass right through the cleaner bag and become airborne. Do not shovel dry debris down a chute, into an open bin. Dust with spores will become airborne and spread over the community. Never attempt to clean up dry material.

Outdoor situations require the area likewise to be wet. In the case of a woodlot, it is prudent to wait to excavate soil or remove trees until after a rainfall. If it is impractical to wait for rain, dousing the area through the use of fire hoses will usually keep the spores down and prevent airborne dissemination over the community.

Decontamination at this time is not a viable option. Formalin was used in large volumes under extremely limited circumstances for decontamination. With the current knowledge of formalin as a powerful carcinogen, it is no longer used for this purpose.

Reporting Possible Problems

It is easy to tell if an area in which construction is scheduled falls into a potentially hazardous situation for Histoplasma. Three-to-five years worth of droppings are hard to miss. The accumulation will usually be several inches thick. Isolated droppings, spread here and there around a floor, will not be a potential hazard.

If you happen to run into a suspected area of concern, please report it to your local health department. The health department can then give advice to contractors or developers on how to proceed with construction in a manner that will insure an outbreak will not occur among construction workers or in the community.