Fungi and Disease

Fungi cause about 100,000 diseases of plants, including about 70 percent of the major crop diseases, resulting in an economic loss of billions of dollars each year. These plant pathogens cause extensive disease to seeds, seedlings, mature plants, and aging plants, resulting in decreased growth and reproduction of crop plants. Fungi also attack forest trees and wooden structures.

A number of fungi cause diseases in humans and other vertebrates. In general, these fungus infections, or mycoses, develop slowly, recur more frequently than bacterial infections, and do not produce a lasting immunity in the body. A mycosis is classified in one of two groups, depending on the part of the body that is infected. A dermatomycosis is an infection of the skin, hair, or nails, such as ringworm or athlete’s foot. These infections rarely progress to the internal organs. Most respond well to medication, although treatment may take several weeks.

Fungi cause a number of human respiratory diseases. Coccidioidomycosis is caused by the yeast Coccidioides immitis. Although typically contracted by the inhalation of dust containing yeast spores, the fungus may also be introduced through the skin from infected soil. Initial symptoms may resemble the flu, with fever, cough, and possibly a skin rash, and the infection usually runs its course without specific treatment. In rare cases, the fungus penetrates internal tissues, such as the bones, joints, and brain, producing tumors that later form abscesses or ulcers. No treatment is available that can halt the course of this form of the disease.

Histoplasmosis is caused by the yeastlike fungus Histoplasma capsulatum, which grows in pigeon, bat, and chicken droppings. Contracted by the inhalation of dust from animal droppings, by ingestion, or through the skin. The fungus causing histoplasmosis lives as a parasite in certain tissue and blood cells of the infected person. An infection in the respiratory system is similar to tuberculosis small spots form in the lungs although these lesions heal on their own. A progressive form typically invades the bone marrow and is rapidly fatal.

Aspergillosis is an infection of the skin, nasal sinuses, and lungs or other internal organs caused by molds of the genus Aspergillus. The disease, contracted by the inhalation of spores, occurs most often among agricultural workers. Itching and pain are frequent symptoms, and if scratching is prolonged, the skin may thicken and become gray or black. A virulent type of pneumonia is caused by the yeastlike fungus Pneumocystis carinii, particularly prevalent in people with compromised immune systems, such as AIDS patients.

Mycotoxins are poisons produced by fungal growth in cereals, nuts, fruits, and vegetables. More than 100 species of fungi produce these toxins. The most common mycotoxin is aflatoxin, produced by Aspergillus flavus and Aspergillus parasiticus. Commonly found on corn, peanuts, and tree nuts, the toxin also can be transmitted to humans through the milk, meat, or eggs of animals fed contaminated grains. Aflatoxin is the most potent carcinogen, or potentially cancer-causing agent, yet discovered. Other mycotoxins include trichothecenes and zearalenone, compounds known to injure the intestines, bone marrow, lymph nodes, spleen, and thymus. They are produced by species of Fusarium that grow on grain, straw, or hay stored while damp. Occasionally, circumstances prevent the harvesting of grains during the autumn, and the grains lie dormant in the damp fields until they are harvested in the spring. These grains are especially vulnerable to trichothecenes and zearalenone contamination. A large outbreak of trichothecenes contamination occurred in Russia in early 1944 among hungry peasants who had been searching the winter fields for unharvested wheat and millet.