At a glance
CDC aims to provide health departments with a set of best practices to consider when investigating suspected or confirmed malaria cases as part of their standard activities.
Purpose
This document provides best practices for health departments to consider as part of their usual activities to investigate malaria cases.
If you have questions about malaria investigation or surveillance, please reach out to the state health department (if applicable) or the CDC Malaria Branch (malaria@cdc.gov or 770-488-7788) to discuss.
Additional guidance from CDC is available for jurisdictions with concerns for a possible case of locally acquired malaria. Please request this information from the CDC malaria surveillance epidemiologist or the CDC Malaria Branch (malaria@cdc.gov or 770-488-7788).
Background
Plasmodium parasites that cause malaria are transmitted through the bite of an infectious Anopheles mosquito. For details about how parasites are transmitted, see Appendix A: Malaria Lifecycle. Five species of Plasmodium cause illness in humans: P. falciparum, P. vivax, P. ovale, P. malariae, and P. knowlesi. Because malaria was eliminated from the United States (U.S.) in the early 1950s, it is assumed that U.S. residents do not have protective immunity to malaria and are susceptible to severe illness and death. Around 2,000 people are diagnosed with malaria in the U.S. annually, and most of them acquired malaria while in countries with ongoing mosquito-borne transmission (imported malaria). Potential for malaria transmission from an imported case to a non-traveler within the U.S. is possible (but rare) since Anopheles mosquitoes that can transmit malaria are widespread across the majority of states.
Malaria is transmitted from the bite of an infectious Anopheles mosquito and not from person to person, although rarely malaria can be transmitted via blood exposures (induced malaria) or to a neonate during pregnancy or birth (congenital malaria).
If someone in the U.S. is infected with malaria and is bitten by an Anopheles mosquito, then the mosquito may become infectious and could transmit the parasite to other people in the area. Between 1957 and 2003, CDC documented 63 incidents of locally acquired malaria that resulted in 156 total cases. While no such events were documented between 2003 and 2022, in the summer and fall of 2023 ten locally acquired mosquito-transmitted malaria cases in four states were reported. These cases demonstrate the potential for onward transmission of Plasmodium parasites from imported malaria cases to persons who have not traveled outside of the U.S. and the importance of rapid diagnosis, treatment, timely investigation, and control of malaria in the U.S.