Histoplasmosis is one of the most common systemic mycoses in the American continent. In Mexico, the predominating clinical form is primary pulmonary histoplasmosis (PPH), which manifests itself, in the beginning, as an infection compromising the lung and, in general, with a benign course. However, PPH depicts a range of clinical manifestations going from a moderate to a severe phase, depending on the degree of the infection or on the immunosuppression condition of the individual exposed to the infective propagules of the etiologic agent. Disseminated histoplasmosis is a clinical disorder predominating in immune compromised hosts, hence, its association to AIDS. In Mexico, this association continues to be important, despite its worldwide decrease with anti-retrovirus therapy. ^1-5

Figure 1 (Photograph of mycelial-phase cultures of H. capsulatum)

Figure 2 (Photograph of finger-like projections of H. capsulatum macroconidia)

The pathogenic dimorphic fungus, Histoplasma capsulatum var. capsulatum (Darling 1906) is the etiologic agent of histoplasmosis that also causes disease in several mammal species. This fungus lives in special ecologic niches that favor its growth and dissemination in nature. In Mexico, the pathogen has been found in both enclosed and open places, where bat and bird droppings are found. In these environments, the fungus develops its mycelial and infectious phase, primarily represented by small spores (micronidia) and hyphal fragments, besides finger-like projections macronidia (Figures 1 and 2). These propagules can cause infection when inhaled by susceptible hosts⁶.

The mechanisms involved in H. capsulatum dispersion are not well established yet. It has been proposed that the wind can disseminate the spores in open spaces, whereas in enclosed spaces, infected mammals are considered as dispersers of the fungus in nature. The colony habits of bats in caverns and their flying capacity are important parameters to explain fungus dispersion ^7-11.

INFECTION RISK

Handling of droppings from domestic (hens) and/or of ornate birds (Australian parakeets) besides the accidental contact with birds droppings (blackbirds, oilbirds) in their natural environment and, in particular, with bat guano (flying mammals) expose individuals to infection risks. The risk is greater in those places where nutritional conditions are favorable for fungus growth, characterized by dropping accumulations and favorable temperature and humidity that, together with low environmental light, foster fungal sporulation.⁷ Constant inhalation accompanied by greater effort produces an infection due to high doses, which, in Mexico, is represented by a severe form of PPH. Mostly, these severe cases are related to occupational risk factors due to an association with working activities in enclosed spaces, such as: guano collecting; ecotourism in caves; work and research closely related to mining; biology; geology; anthropology; and archaeology; among others⁵.

REFERENCES

1. Pedroza-Serés M, Quiroz-Mercado H, Granados J. Taylor ML: The syndrome of presumed ocular histoplasmosis in Mexico: A Preliminary study. J Med Vet Mycol, 1994; 32:83 - 92.
2. Taylor ML, Granados J, Toriello C. Biological and sociocultural approaches of histoplasmosis, in the State of Guerrero, Mexico. Mycoses, 1996; 39; 375-9.
3. Taylor ML, Perez-Mejia A, Yamamoto-Furusho JK, Granados J. Immunologic, genetic and social human risk factors associated to histoplasmosis: Studies in the State of Guerrero, Mexico. Mycopathology, 1997; 138: 137-41.
4. Vaca-Martin MA, Martinez-Rivera MA, Flores-Estrada JJ. Histoplamosis en Mexico, aspectos históricos y epidemiológicos. Rev Inst Nal Enf Resp Mex, 1998; 11:208-15.
   
5. Velasco-Castrejón O. La histoplasmosis pulmonary primaria en Mexico, Rev Inst Nal Enf Resp Mex, 1998; 11:221-5.
   
6. Kwon-Chung KJ, Bennett JE. Medical Mycology. Philadelphia: Lea and Febiger, 1992.
   
7. Taylor ML, Reyes - Montes MR, Chávez-Tapia CB, Curiel-Quesada E, Duarte-Escalante E, Rodriguez- Arellanes G, Peña-Sandoval GR, Valenzuela-Tovar F. Ecology and molecular epidemiology findings of Histoplasma capsulatum in Mexico. In: Research Advance in Microbiology, (eds. Mojan RM, Benedik M). Kerala: Global Research Network, 2000; pp 29-35.
   
8. Taylor ML, Toriello C, Pérez-Mejia A, Martinez MA, Reyes - Montes MR, Espinosa-Avila L, Chávez-Tapia C. Histoplasmosis in the State of Guerrero, Mexico: A biological approach. Rev. Mex Mic, 1994; 10:49-62.
   
9. Taylor ML, Reyes - Montes MR, Martinez-Rivera MA, Rodriguez-Arellanes G, Duarte-Escalante E, Flores-Estrada JJ. Histoplasmosis en Mexico. Aportaciones inmunológicas y moleculares sobre su epidemiología. Ciencia y Desarrollo, 1997;23:58-63.
   
10. Taylor ML, Chávez-Tapia C, Vargas-Yañez R, Rodriguez-Arellanes G, Peña-Sandoval GR, Toriello C, Reyes - Montes MR. Environmental conditions favoring bat infection with Histoplasma capsulatum in Mexican shelters. Am J Trop Med Hyg, 1999; 61:914-9.
    
11. Taylor ML, Chávez-Tapia C, Reyes-Montes MR. Molecular typing of Histoplasma capsulatum isolated from infected bats, captures in Mexico. Fung Genet Biol, 2000;30:207-12.