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Malaria in the United States

Malaria parasites of the Plasmodium genus are transmitted through the bite of infective female Anopheles mosquitoes. Four Plasmodium species commonly cause illness in humans: P. falciparum, P. vivax, P. ovale, and P. malariae. Mixed infections with multiple species might occur in areas where more than one species is in circulation. Rarely, humans can be infected with P. knowlesi, which is a simian malaria found in Southeast Asia. 

P. falciparum has the highest prevalence in sub-Saharan Africa. It is the most pathogenic malaria species and is most commonly associated with severe illness and death. P. vivax is less prevalent in sub-Saharan Africa because much of the population lacks the Duffy antigen required for P. vivax invasion of red blood cells. Because of its ability to survive at lower temperatures and higher elevations, P. vivax has a broader geographic range than P. falciparum. It accounts for 41% of malaria infections occurring outside the African continent.

Malaria relapses are common with P. vivax and P. ovale parasites, which have dormant liver stages (hypnozoites) that can reactivate months or years after the acute infection. P. malariae parasites mature slowly in human and mosquito hosts and, although they do not typically cause severe symptoms, they can result in persistent low-density infections that can last for years or even a lifetime 

Clinical illness results from the asexual intraerythrocytic stage of the parasite. Malaria symptoms vary, but the majority of patients have fever. Symptoms associated with uncomplicated malaria include chills, sweating, headache, fatigue, myalgia, cough, and nausea. If not treated promptly, malaria can affect multiple organ systems and result in altered consciousness, renal and liver failure, respiratory distress, coma, permanent disability, and death. 

In 2013, malaria was endemic in a total of 97 countries and territories in the tropics and subtropics. An estimated 198 to 214 million cases of malaria occurred worldwide, resulting in approximately 500,000 deaths. The majority of malaria infections in the United States occur among persons who have traveled to regions with ongoing malaria transmission. Occasionally, malaria is acquired by persons who have not traveled out of the country through transfusion with infected blood products, congenital transmission, laboratory exposure, or local mosquito-borne transmission.

In 2014, CDC received reports of 1,724 confirmed malaria cases, including one congenital case and two cryptic cases. Plasmodium falciparum, P. vivax, P. ovale, and P. malariae were identified in 66.1%, 13.3%, 5.2%, and 2.7% of cases, respectively. Less than 1.0% of patients were infected with two species. Among all reported cases, 17.0% were classified as severe illness, and five persons with malaria died.

CDC received 137 P. falciparum-positive samples for the detection of antimalarial resistance markers. Of the 137 samples tested, 131 (95.6%) had genetic polymorphisms associated with pyrimethamine drug resistance, 96 (70.0%) with sulfadoxine resistance, 77 (57.5%) with chloroquine resistance, three (2.3%) with mefloquine drug resistance, one (<1.0%) with atovaquone resistance, and two (1.4%) with artemisinin resistance.

The overall trend of malaria cases has been increasing since 1973; the number of cases reported in 2014 is the fourth highest annual total since then. Despite progress in reducing global prevalence of malaria, the disease remains endemic in many regions and use of appropriate prevention measures by travelers is still inadequate.

Malaria should be included in the differential diagnosis for every patient with fever who has traveled to an area where malaria is endemic. If malaria is suspected, both thick and thin Giemsa-stained blood smears should be examined by microscopy for parasites as soon as possible. Microscopy can quickly detect the presence of malaria parasites and determine the species and percentage of red blood cells that are infected. This information is essential to guiding treatment.

The BinaxNOW malaria rapid diagnostic test (RDT, Inverness Medical Professional Diagnostics, Scarborough, Maine, USA) detects circulating malaria-specific antigens and is approved for use by hospital and commercial laboratories. RDTs can decrease the amount of time required to determine whether a patient is infected with malaria but does not eliminate the need for standard blood smear examination. RDTs are not able to fully speciate or quantify malaria parasites. Positive and negative RDT results must be confirmed by microscopy. If microscopy is not performed, PCR can be performed to confirm an RDT result and determine the species.

PCR cannot be performed quickly enough to be of use in the initial diagnosis and treatment of acute malaria, but is useful to confirm the species and to guide treatment. PCR is available in reference and health department laboratories. CDC recommends that PCR be performed for all cases of malaria to confirm the infecting species.

Reference

CDC. Malaria Surveillance-United States, 2014. MMWR Surveillance Summaries, May 26, 2017;66(12)1-24.

 

 

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