Systemic Mycoses

The pathogens of the five systemic mycoses to be considered all show specialization for parasitism. All are dimorphic - that is, the saprobic state is mycelial (filamentous) and the pathogenic phase is unicellular and yeastlike. It is possible to effect the mycelial-to-yeast conversion in vitro. For Blastomyces dermatitidis (blastomycosis), the transformation is temperature dependent; for the others, nutritional adjustments are also necessary. On first description there was a tendency to assign these pathogens to the Protozoa.

Coccidioidomycosis

The first case of this disease was described from Argentina by Alejandro Posadas in 1892. About the same time a case was also studied in California by E. Rixford and T. C. Gilchrist, who attributed the cause to a protozoan, which, in 1896, they named Coccidioides immitis. In 1905, however, a fellow American, W. Opuls, established its mycotic nature. It was known only as a rather rare, acute, or chronic disseminating and often fatal disease. In 1938, Myrnie A. Grifford and E. Dickson, independently, and then in collaboration, established that "valley fever," prevalent in the San Joaquin Valley of California, was a mild form of coccidioidomycosis. Further investigations showed mild, often subclinical, respiratory Coccidioides infection to be widespread in arid parts of California and neighboring states and to induce lifelong immunity to subsequent at tack. This immunity is demonstrable by a positive skin test with an antigen (coccidioidin) prepared from cultures of the pathogen.

C. immitis was isolated from soil, and when Emmons showed desert rodents to be infected, they were at first thought perhaps to constitute an animal reservoir of infection. But it soon became clear that these rodents, like humans, were subject to infection by this soil-inhabiting fungus. It also became clear that the dry, airborne (or dustborne) spores of the pathogen were extremely infectious (there have been many accidental laboratory infections) and that coccidioidomycosis could be contracted, for example, by servicing automobiles that had been driven through areas where the mycosis was endemic. In addition, dust storms were found to increase the rate of conversion from negative to positive skin tests with coccidioidin in local inhabitants and their domestic animals. It may be noted, that although light- and dark-skinned peoples appear to be equally susceptible to C. immitis infection, the disease is more likely to be systemic in those with pigmented skin. Filipinos and blacks, along with the Portuguese, are those chiefly employed in agriculture in the districts where coccidioidomycosis is endemic.

Coccidioidomycosis is endemic and of high incidence in warm dry regions (the Lower Sonoran Life Zone) of the United States and Mexico and also in parts of Central and South America where the climatic conditions are similar. Records from other parts of the world are of doubtful validity.

Histoplasmosis

This disease (Histoplasma capsulatum) shows many parallels with coccidioidomycosis and may be viewed as the humid region equivalent. At first considered a rare protozoan disease, it has been shown to be mycotic, and to have a mild form affecting millions of the inhabitants of the midwestern United States. The pathogen shows a predilection for bird (chicken, starling) and bat droppings.

Blastomycosis and Paracoccidioidomycosis The former (North American blastomycosis), caused by B. dermatitidis, and the latter (South American blastomycosis), caused by Paracoccidioides brasilien-sis, are both chronic granulomatous diseases of the skin and internal organs, characterized by budding cells of the pathogen in the infected tissues. The two were at first confused both with each other and with cryptococcosis (called "European blastomycosis" or "torulosis" and caused by Cryptococcus neoformans), but these three mycoses are now well differentiated.

Blastomycosis was first described by Gilchrist in the United States in 1894, and paracoccidioidomycosis, by Adolfo Lutz in Brazil during 1908. Neither has a mild form such as that characteristic of coccidioidomycosis and histoplasmosis, and though there have been a few records of both pathogens from soil, the natural habitats of both these fungi have not been established with certainty.

Paracoccidioidomycosis is confined to Central and South America, where it has been the subject of intensive study. Blastomycosis is endemic to the western and southeastern states of the United States, where epidemics occur; there are also records of the disease from a number of tropical African countries.

Sporotrichosis

This disease shows certain parallels with mycetoma but is included here because the causal pathogen is dimorphic. It is a cutaneous and subcutaneous infection characterized by the development of nodular lesions, often in a series affecting successive lymph nodes. Infection is frequently initiated by a lesion, often of the hand. It is caused by Sporothrix schenckii and was first described in the United States by B. R. Schenck in 1898. Subsequently many cases were reported from Europe, especially France, where the disease was the subject of a massive monograph, Les Sporotrichoses, by C. L. de Beurmann and H. Gougerot in 1912. Sporotrichosis, which is sporadic in north temperate regions, has also been recorded in Central and South America. In Uruguay, J. E. Mackinnon (1949) attempted to correlate incidence of the disease with the weather, and obtained evidence that infection occurred during periods of moist warm weather, which he suggested encouraged growth of the pathogen on plant material from which humans are infected. S. schenckii is one of the rare fungus pathogens of humans that has been shown experimentally to cause disease in plants (carnations). The largest outbreak of sporotrichosis ever recorded was in the Witwatersrand gold mines in South Africa during 1941-3, when approximately 3,000 miners were infected. The epidemic was brought under control by potassium iodide therapy for the men affected and fungicidal treatment of the mine timbers from which the infection was contracted.

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