Problem/Condition: Malaria in humans is caused by intraerythrocytic protozoa of the genus Plasmodium. These parasites are transmitted by the bite of an infective female Anopheles mosquito. The majority of malaria infections in the United States occur among persons who have traveled to regions with ongoing malaria transmission. However, malaria is also occasionally acquired by persons who have not traveled out of the country, through exposure to infected blood products, congenital transmission, laboratory exposure, or local mosquitoborne transmission. Malaria surveillance in the United States is conducted to identify episodes of local transmission and to guide prevention recommendations for travelers.

Period Covered: This report summarizes cases in persons with onset of illness in 2011 and summarizes trends during previous years.

Description of System: Malaria cases diagnosed by blood film, polymerase chain reaction, or rapid diagnostic tests are mandated to be reported to local and state health departments by health-care providers or laboratory staff. Case investigations are conducted by local and state health departments, and reports are transmitted to CDC through the National Malaria Surveillance System, National Notifiable Diseases Surveillance System, or direct CDC consults. Data from these reporting systems serve as the basis for this report.

Results: CDC received 1,925 reported cases of malaria with an onset of symptoms in 2011 among persons in the United States, including 1,920 cases classified as imported, one laboratory-acquired case, one transfusion-related case, two congenital cases, and one cryptic case. The total number of cases represents an increase of 14% from the 1,691 cases reported for 2010 and the largest number of reported cases since 1971. Plasmodium falciparum, P. vivax, P. malariae, and P. ovale were identified in 49%, 22%, 3%, and 3% of cases, respectively. Twenty-one (1%) patients were infected by two species. The infecting species was unreported or undetermined in 23% of cases, an increase of 5 percentage points from 2010. Of the 871 patients who reported purpose of travel, 607 (70%) were visiting friends or relatives (VFR). Among the 929 cases in U.S. civilians for whom information on chemoprophylaxis use and travel region was known, 57 (6%) patients reported that they had followed and adhered to a chemoprophylactic drug regimen recommended by CDC for the regions to which they had traveled. Thirty-seven cases were reported in pregnant women, among whom only one adhered to chemoprophylaxis. Among all reported cases, significantly more cases (n=275 [14%]) were classified as severe infections in 2011 compared with 2010 (n=183 [11%]; p=0.0018; chi square). Five persons with malaria died in 2011. After 2 years of improvement in completion of data elements on the malaria case form, higher percentages of incomplete data in 2011 for residential status (from 11% in 2010 to 19% in 2011) and species (from 18% in 2010 to 22% in 2011) were noted.

Interpretation: The number of cases reported in 2011 marked the largest number of cases since 1971 (N = 3,180). Despite progress in reducing the global burden of malaria, the disease remains endemic in many regions, and the use of appropriate prevention measures by travelers is still inadequate.

Public Health Actions: Completion of data elements on the malaria case report form decreased in 2011 compared with 2010. This incomplete reporting compromises efforts to examine trends in malaria cases and prevent infections. VFR travelers continue to be a difficult population to reach with effective malaria prevention strategies. Evidence-based prevention strategies that effectively target VFR travelers need to be developed and implemented to have a substantial impact on the numbers of imported malaria cases in the United States. Although more persons with cases reported taking chemoprophylaxis to prevent malaria, the majority reported not taking it, and adherence was poor among those who did take chemoprophylaxis. Proper use of malaria chemoprophylaxis will prevent the majority of malaria illness and reduce the risk for severe disease (http://www.cdc.gov/malaria/travelers/drugs.html). Malaria infections can be fatal if not diagnosed and treated promptly with antimalarial medications appropriate for the patient’s age and medical history, the likely country of malaria acquisition, and previous use of antimalarial chemoprophylaxis. Clinicians should consult the CDC Guidelines for Treatment of Malaria and contact the CDC’s Malaria Hotline for case management advice, when needed. Malaria treatment recommendations can be obtained online (http://www.cdc.gov/malaria/diagnosis_treatment) or by calling the Malaria Hotline (770-488-7788 or toll-free at 855-856-4713).

Stephen B. Thacker CDC Library collection