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7. TRYPANOSOMIASIS (Chagas' disease)

Trypanosomes among blood cells

American trypanosomiasis is a serious, progressive and often life-threatening disease. The cause is a protozoan parasite, Trypanosoma cruzi, that lives in the blood. Its range extends from South America to the south and southwestern United States. Several dozen kinds of wild mammals are naturally infected; in California they tend to be rodents. Blood sucking bugs (known as "cone-nose bugs" or "kissing bugs") carry the parasite from host to host.

Trypanosoma cruzi's threat comes from its method of reproduction. Most trypanosomes divide while circulating in the blood, but this one multiplies inside smooth muscle cells - especially those of the heart. Infected heart muscle becomes thin and weak, and can eventually stop working altogether.

Trypanosome strains vary in their tendency to invade heart muscle. Infections in northern Mexico and the US are less severe than those in Central and South America. Also, different kissing bug species vary in their efficiency as vectors. The most dangerous, like Rhodnius prolixus of Latin America, often live in human dwellings and bite the residents while they sleep. In contrast, the main carrier insect in California, Triatoma protacta, is most often found in the tangled mounds of sticks that make up nests of packrats Neotoma fuscipes.

Nevertheless, health departments in southern California see enough potential danger to maintain an active surveillance program. When surveys show a high kissing bug population density, or a high frequency of infection in wildlife, they may close nearby campgrounds. T. cruzi infections in northern California mammals are less common, and except for occasional surveillance, have not been considered a serious public health threat.