Distribution and Incidence

The distribution of acute Q fever is worldwide, but chronic Q fever appears to be more limited, most cases being reported from the United Kingdom, Australia, and Ireland. In the United Kingdom, Coxiella infection accounted for 3 percent of all cases of endocarditis reported by laboratories during 1975-81. In other countries, it is still rare enough for individual cases to be reported in the medical literature. This anomaly may be due to underdiagnosis in most countries or perhaps to differences in the virulence of prevalent strains. Acute Q fever affects predominantly adult men, and there is a well-recognized association with certain occupational groups such as farmers, abattoir workers, and veterinarians. Q fever often occurs in seasonal spring and autumn peaks associated with lambing and calving.

It is certain that Q fever is very considerably underdiagnosed so that accurate national incidence figures are not available. In the United States by 1960, Q fever was recognized to be endemic in dairy herds throughout the country. Reports of human cases were uncommon but increased steadily from 1948 to 1977. Over this period, 1,164 cases were reported to the Centers for Disease Control. Although most states reported cases, fully 67 percent of them were from California, where there is a very high prevalence of infection in cattle and consequently a high proportion of cattle excreting organisms in milk. Reported infection in humans, however, is relatively rare, with only 148 being reported from 1970 to 1978. Seropreva-lence surveys of cattle in the United Kingdom suggest that about 3 to 5 percent of dairy cattle and 2 to 6 percent of sheep have been infected. About 100 to 200 cases are reported by laboratories each year, and 10 percent of these are chronic infections.

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