Molecular Surveillance for Markers Associated with Antimalarial Drug Resistance Among Imported Plasmodium Falciparum Cases Diagnosed in the United States, 2012-2013

BACKGROUND: The prevalence of antimalarial drug resistance among imported malaria cases in the United States (US) is unknown. CDC conducts surveillance to confirm species identification and detect molecular markers associated with drug resistance among Plasmodium falciparum parasites imported into the US. This data will be useful to determine trends in the importation of drug-resistant parasites and to adapt prevention and treatment guidelines. We sought to determine the prevalence of molecular markers associated with antimalarial drug resistance among imported P. falciparum cases in the U.S. in 2012-2013.

METHODS: Blood samples from malaria cases diagnosed in different parts of the U.S. were submitted to CDC for species confirmation and molecular characterization. P. falciparum-positive specimens were genotyped for genetic polymorphisms associated with resistance to chloroquine (pfcrt), sulfadoxine (pfdhps), pyrimethamine (pfdhfr), mefloquine (pfmdr1 and copy number), and atovaquone (pfcyt b) using Sanger sequencing and real-time PCR methods. Sequencing data were linked to epidemiologic data.

RESULTS: In 2012-2013, 239 specimens were submitted to CDC for malaria reference diagnosis and genetic characterization. CDC corrected/determined the species for 129 (54%) samples. P. falciparum was confirmed in 164 (69%) samples (including 2 mixed infections), P. vivax in 29 (12%), P. ovale in 26 (11%), and P. malariae in 15 (6%). Travel history was reported for 193 cases (83%), with 106 having recently traveled to West Africa. Among 43 cases who reported some malaria chemoprophylaxis use, 21 (49%) reported doxycycline, 11 (26%) atovaquone/proguanil, 5 (12%) mefloquine, 4 (9%) chloroquine, and one (2%) artemether-lumefantrine. Among 121 cases with information on treatment, 52 (43%) received atovaquone/proguanil, 39 (32%) doxycycline, 31 (26%) quinine, 10 (8%) primaquine, and 9 (7%) chloroquine, including combination therapies. Among the P. falciparum-positive samples, 140 (89%) had genetic polymorphisms associated with resistance to pyrimethamine, 133 (83%) to sulfadoxine and 82 (51%) to chloroquine. Samples from West Africa were more likely to show chloroquine-associated resistance markers than samples from East Africa (54% vs. 33%). One P. falciparum sample (<1%) had multiple pfmdr1 copies, which has been associated with mefloquine resistance; the patient had experienced mefloquine prophylaxis failure. No pre-treatment samples had pfcyt b mutations. However, pfcyt b mutations were found post-treatment in two travelers returning from Nigeria.

CONCLUSIONS: These findings highlight the need for continued surveillance to detect drug-resistant parasites and the importance of PCR confirmation of species. This information will also help in refining prevention guidelines for US travelers and treatment recommendations for imported malaria in the US.