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Malaria Surveillance in the United States
Abstract & Commentary
By Brian G. Blackburn, MD, Clinical Assistant Professor, Division of Infectious and Geographic Medicine, Stanford University School of Medicine, CA, is Associate Editor for Infectious Disease Alert.
Dr. Blackburn reports no financial relationship relevant to this field of study.
Synopsis: In 2008, a five-year low of 1,298 malaria cases (9% severe and only two fatal) in the United States was reported to the Centers for Disease Control and Prevention (CDC), a 14% decrease from 2007. Plasmodium falciparum (41% of cases) was the most commonly identified infecting species. About 72% of case patients reported noncompliance with recommended malaria chemoprophylaxis measures.
Source: Mali S, et.al. Malaria surveillance United States, 2008. MMWR Surveill Summ. 2010;59:1-15.
Malaria continues to be a global scourge, with nearly half the world's population living in malaria-endemic areas, 200-500 million annual clinical cases, and nearly a million annual deaths (almost all due to P. falciparum).1 However, in the United States, malaria now occurs almost exclusively in the setting of imported cases in travelers returning from endemic areas. Malaria is a nationally notifiable disease in the United States, and cases are reported to the CDC through two passive reporting systems, the National Malaria Surveillance System and the National Notifiable Diseases Surveillance System.
This most recent Malaria Surveillance Summary compiled U.S. malaria cases from 2008. Overall, 1,298 cases were reported, a 14% decrease compared to 2007, and the lowest case number since 2003. The infecting malaria species was P. falciparum in 41% of cases, P. vivax in 15%, and P. malariae or P. ovale in 3%. For the first time in the United States, a human infection with P. knowlesi was confirmed. The infecting species remained undetermined in 41% of cases.
Of patients for whom the region of acquisition was known, 71% were acquired in Africa (7/8 of these in West Africa), 22% in Asia (2/3 of these in India), and 6% in the Americas. Relative case rates were highest among travelers to West Africa (e.g., Guinea, Togo, Côte d'Ivoire, Sierra Leone, and Nigeria) compared to other regions. Of the malaria cases for which travel information was known and the infecting species was identified, 91% of travelers to Africa had P. falciparum and 6% had P. vivax/ovale; 11% of travelers to Asia/Oceania had P. falciparum and 84% had P. vivax/ovale; and 15% of travelers to the Americas (excluding Haiti, where all malaria is due to P. falciparum) had P. falciparum and 81% had P. vivax/ovale.
Illness onset occurred < 1 month after arrival in the United States (including those ill before returning to the United States) in 97% of those infected with P. falciparum, and in 57% of those infected with P. vivax. The most common reason for travel among case patients was to visit friends and relatives ("VFR travelers," 65% of patients). Overall, 117 (9%) patients had severe malaria and two patients died; 72% of case-patients did not take malaria chemoprophylaxis appropriately.
The number of reported malaria cases in the United States has been relatively stable for over a decade, although 14% decrease from the previous year was seen in 2008. This decrease in the malaria case number could have resulted from changing travel patterns among U.S. residents, improved chemoprophylaxis measures, or a global decrease in malaria burden. Recent evidence suggests that these are unlikely; in fact, noncompliance with chemoprophylaxis remains a prominent contributing factor to most U.S. cases of malaria.1 Despite the 14% decrease being statistically significant, small fluctuations in the annual case number have been seen year-to-year in past Malaria Surveillance Summaries, and a true difference would be better confirmed by seeing a trend sustained over multiple years. The passive nature of the malaria reporting system in the United States renders it less robust and more susceptible to fluctuation.
Several aspects of these data are worth noting. Death due to malaria remains extremely rare in the United States, with only two fatal cases in 2008. Notably, one death was due to P. vivax, a malaria species generally thought of as more benign than P. falciparum. However, the number of severe malaria cases nearly doubled compared to 2007 (117 vs. 57), although a modification in the case report form may have resulted in reporting bias that resulted in this apparent change. Also of note, the first case of P. knowlesi infection was confirmed in the United States during 2008. Although easily confused with P. malariae morphologically, P. knowlesi can cause severe malaria (in contrast to the more indolent illness usually caused by P. malariae). This emerging pathogen, endemic to Southeast Asia/Oceania, should be considered in patients with travel histories that include this region.
As noted in previous Malaria Surveillance Summaries, VFR travelers represent a group at higher risk for acquiring malaria than tourists or other casual travelers. A growing body of literature supports this, and though VFR travelers represent a small proportion of U.S. international travelers, they are far over-represented in the number of malaria cases in the United States.2,3 Reasons for this include a lower propensity to seek pre-travel heath care and take malaria chemoprophylaxis, assumption of partial immunity, and undertaking riskier behaviors while abroad.2 This represents an important opportunity for prevention efforts among primary care physicians that see these patients, as they may be the only point of contact within the medical system prior to departure.
In summary, fever in a returned traveler should prompt consideration of the diagnosis of malaria; travel histories should also be obtained from all febrile patients to determine if this is a consideration. Among U.S. travelers, the risk of acquiring malaria appears to be highest in West Africa, and among travelers to Africa, and the risk of P. falciparum is much higher than the more benign forms of malaria.