Malaria Surveillance - United States, 2016.

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 species 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 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 provide information on its occurrence (e.g., temporal, geographic, and demographic), guide prevention and treatment recommendations for travelers and patients, and facilitate transmission control measures if locally acquired cases are identified.

Period covered: This report summarizes confirmed malaria cases in persons with onset of illness in 2016 and summarizes trends in previous years.

Description of system: Malaria cases diagnosed by blood film microscopy, polymerase chain reaction, or rapid diagnostic tests are reported to local and state health departments by health care providers or laboratory staff members. Case investigations are conducted by local and state health departments, and reports are transmitted to CDC through the National Malaria Surveillance System (NMSS), the National Notifiable Diseases Surveillance System (NNDSS), or direct CDC consultations. CDC reference laboratories provide diagnostic assistance and conduct antimalarial drug resistance marker testing on blood samples submitted by health care providers or local or state health departments. This report summarizes data from the integration of all NMSS and NNDSS cases, CDC reference laboratory reports, and CDC clinical consultations.

Results: CDC received reports of 2,078 confirmed malaria cases with onset of symptoms in 2016, including two congenital cases, three cryptic cases, and one case acquired through blood transfusion. The number of malaria cases diagnosed in the United States has been increasing since the mid-1970s. However, in 2015 a decrease occurred in the number of cases, specifically from the region of West Africa, likely due to altered travel related to the Ebola virus disease outbreak. The number of confirmed malaria cases in 2016 represents a 36% increase compared with 2015, and the 2016 total is 153 more cases than in 2011, which previously had the highest number of cases (1,925 cases). In 2016, a total of 1,729 cases originated from Africa, and 1,061 (61.4%) of these came from West Africa. P. falciparum accounted for the majority of the infections (1,419 [68.2%]), followed by P. vivax (251 [12.1%]). Fewer than 2% of patients were infected by two species (23 [1.1%]). The infecting species was not reported or was undetermined in 10.8% of cases. CDC provided diagnostic assistance for 12.1% of confirmed cases and tested 10.8% of specimens with P. falciparum infections for antimalarial resistance markers. Of the U.S. resident patients who reported reason for travel, 69.4% were travelers who were visiting friends and relatives. The proportion of U.S. residents with malaria who reported taking any chemoprophylaxis in 2016 (26.3%) was similar to that in 2015 (26.6%), and adherence was poor among those who took chemoprophylaxis. Among the 964 U.S. residents with malaria for whom information on chemoprophylaxis use and travel region were known, 94.0% of patients with malaria did not adhere to or did not take a CDC-recommended chemoprophylaxis regimen. Among 795 women with malaria, 50 were pregnant, and one had adhered to mefloquine chemoprophylaxis. Forty-one (2.0%) malaria cases occurred among U.S. military personnel in 2016, a comparable proportion to that in 2015 (23 cases [1.5%]). Among all reported cases in 2016, a total of 306 (14.7%) were classified as severe illnesses, and seven persons died. In 2016, CDC analyzed 144 P. falciparum-positive and nine P. falciparum mixed species samples for surveillance of antimalarial resistance markers (although certain loci were untestable in some samples); genetic polymorphisms associated with resistance to pyrimethamine were identified in 142 (97.9%), to sulfadoxine in 98 (70.5%), to chloroquine in 67 (44.7%), to mefloquine in six (4.3%), and to atovaquone in one (<1.0%). The completeness of key variables (e.g., species, country of acquisition, and resident status) was 79.4% in 2016 and 75.7% in 2015.

Interpretation: The number of reported malaria cases in 2016 continued a decades-long increasing trend and is the highest since 1972. The importation of malaria reflects the overall increase in global travel trends to and from areas where malaria is endemic; a transient decrease in the acquisition of cases, predominantly from West Africa, occurred in 2015. In 2016, more cases (absolute number) originated from regions of the world with widespread malaria transmission. Since the early 2000s, worldwide interventions to reduce malaria have been successful; however, progress has plateaued in recent years, the disease remains endemic in many regions, and the use of appropriate prevention measures by travelers remains inadequate.

Public health actions: The best way to prevent malaria is to take chemoprophylaxis medication during travel to a country where malaria is endemic. 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. In 2018, two tafenoquine-based antimalarials were approved by the Food and Drug Administration (FDA) for use in the United States. Arakoda was approved for use by adults for chemoprophylaxis and is available as a weekly dosage that is convenient during travel, which might improve adherence and also can prevent relapses from P. vivax and P. ovale infections. Krintafel was approved for radical cure of P. vivax infections in those >16 years old. In April 2019, intravenous artesunate became the first-line medication for treatment of severe malaria in the United States. Because intravenous artesunate is not FDA approved, it is available from CDC under an investigational new drug protocol. Detailed recommendations for preventing malaria are available to the general public at the CDC website (https://www.cdc.gov/malaria/travelers/drugs.html). Health care providers should consult the CDC Guidelines for Treatment of Malaria in the United States and contact the CDC's Malaria Hotline for case management advice when needed. Malaria treatment recommendations are available online (https://www.cdc.gov/malaria/diagnosis_treatment) and from the Malaria Hotline (770-488-7788 or toll-free at 855-856-4713). Persons submitting malaria case reports (care providers, laboratories, and state and local public health officials) should provide complete information because incomplete reporting compromises case investigations and efforts to prevent infections and examine trends in malaria cases. Adherence to recommended malaria prevention strategies is low among U.S. travelers; reasons for nonadherence include prematurely stopping after leaving the area where malaria was endemic, forgetting to take the medication, and experiencing a side effect. Molecular surveillance of antimalarial drug resistance markers (https://www.cdc.gov/malaria/features/ars.html) enables CDC to track, guide treatment, and manage drug resistance in malaria parasites both domestically and internationally. More samples are needed to improve the completeness of antimalarial drug resistance analysis; therefore, CDC requests that blood specimens be submitted for all cases of malaria diagnosed in the United States.