Locally acquired mosquito-transmitted malaria: a guide for investigations in the United States.

Recent outbreaks of locally acquired mosquito-transmitted malaria in the United States demonstrate the continued risk for reintroduction of the disease. Since 1957, when CDC's Malaria Branch started conducting malaria surveillance, 63 outbreaks have occurred, constituting 156 cases (annual range: 1-32) that were a result of locally acquired mosquitoborne transmission. This report describes the steps that should be taken to 1) investigate a case that might have been acquired locally, 2) prevent a small focus of malaria cases from becoming a source of sustained transmission, and 3) inform clinicians regarding the process of an investigation so they can effectively address concerns and questions from patients. Although these locally acquired mosquito-transmitted outbreaks frequently involve only a limited number of infected persons, they frequently raise concerns in the community and require substantial public health resources. For example, as a result of the most recent local outbreak of eight malaria cases in Florida in 2003, reverse 911 telephone calls (a community notification system) were made to approximately 300,000 residents; insect repellent, postcards, flyers, and posters in multiple languages were distributed; public announcements were made through the media and to schools and homeless shelters; and notifications were sent to local hospitals and physicians to inform residents of that community. When a local health department investigates a potential locally acquired mosquito-transmitted case, the systematic inquiry should include epidemiologic, environmental, and laboratory components. Local and state health departments inquiring about the proper approach to investigate and control a potential locally acquired case frequently request urgent assistance and tools from CDC. This report provides a starting point for such investigations to local and state health departments by providing them with the tools necessary to initiate an investigation.