National surveillance and the epidemiology of human Q fever in the United States, 1978-2004.

Although Q fever is considered enzootic in the United States, surveillance for human Q fever has been historically limited. From 1978 through 1999, 436 cases (average = 20 per year) of human Q fever were reported. After Q fever became nationally reportable in 1999, 255 human Q fever cases (average = 51 per year) were reported with illness onset during 2000 through 2004. The median age of cases was 51 years, and most cases were male (77%). The average annual incidence of Q fever was 0.28 cases per million persons, and was highest in persons 50-59 years of age (0.39 cases per million). State-specific incidence ranged from a high of 2.40 cases per million persons in Wyoming, to 0 cases in some states. Since Q fever became reportable, case reports have increased by more than 250%. Surveillance for Q fever is essential to establish the distribution and magnitude of disease and to complement U.S. bioterrorism preparedness activities.