Malaria is no longer transmitted in the United States, but many hundreds of cases continue to be reported each year in this country as a result of infections acquired by U.S. civilians in areas with endemic malaria. The epidemiology of imported malaria reflects the trends in the travel habits of U.S. civilians and in patterns of migration of foreigners to the United States. This review examines the trends of malaria imported into the United States in the period 1978-1982 from the four areas responsible for most imported cases. These include Central America and Mexico, Haiti, Southeast Asia, and India.
Information on malariais obtained through two surveillance systems. The states and territories report to the CDC the numbers of cases of malaria classified by date of report, county of occurrence, and age of patient; these data appear in the weekly Morbidity and Mortality Weekly Report (MMWR) and the MMWR Annual Summary. More complete epidemiologic and laboratory data are obtained from a separate, voluntary state reporting system operated by the CDC's Division of Parasitic Diseases, Center for Infectious Diseases. These data are reported on a malaria case-surveillance-report form, which provides personal, clinical, and epidemiologic information, including whether the patient is a U.S. citizen or not. Because reporting through this system is not complete, these data on imported malaria should be interpreted to reflect the trends in disease occurrence rather than to measure the precise incidence of disease.
Report Classifications and Malaria Import
A case is defined as:
1) An individual's first attack of malaria in the United States, regardless of whether s/he has had other attacks of malaria while outside the country, and
2) The presence of a positive peripheral blood smear examined in the local or state health department laboratory.
Blood smears associated with doubtful cases were referred to the CDC's National Malaria Repository for confirmation of the diagnosis. A subsequent attack experienced by the same person but caused by a species of Plasmodium other than the one that caused the initial attack is counted as an additional case; however, a subsequent attack of malaria caused by the same species of Plasmodium that caused the initial attack is not considered an additional case in this reporting system. While autochthonous, induced, relapsing, congenital, and cryptic malaria may occur in the United States, this report is limited to disease acquired in areas other than the United States, Puerto Rico, and Guam.
In the period 1978-1982, 5,204 cases of malaria were imported into the United States. Of these cases, 26% were imported by U.S. citizens, 33% occurred in Southeast Asian refugees, and 41% were imported by other foreigners.
Cases by Jurisdictional Classifications
The cases by jurisdictions are explained below.
Central America and Mexico: In the 5-year reporting period, numbers of imported malaria cases from Mexico and Central America (Belize, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua, and Panama) increased markedly. Malaria in U.S. civilians accounted for 245 (33%) of the total of 743 cases imported from that region during the same period. The number of malaria cases imported by U.S. civilians varied slightly--going from 44 in 1978 to 65 in 1982. However, the number of cases imported by foreigners during the same period increased from 34 to 152.
Plasmodium vivax accounted for 86% of all imported cases from Central America and Mexico. The increase in malaria cases from Central America was mainly due to the rise in number of cases imported from El Salvador. Malaria in foreigners accounted for 250 (92%) of the 272 cases imported from El Salvador.
In 1978, 21% of the cases from Central America originated in El Salvador, but in 1982, 43% of such cases were from El Salvador. The number of cases imported from Mexico also increased markedly from 15 cases in 1978 to 55 cases in 1982. In the same period, malaria in foreigners accounted for 82 (62%) of the 132 cases imported from Mexico. In 1978, 19% of the cases from Central America came from Mexico, as did 25% of the cases in 1982. In contrast with the other Central American countries, most malaria infections imported into the United States from Honduras and Belize were in U.S. civilians (107 cases) rather than foreigners (27 cases).
Haiti: The number of malaria cases imported from Haiti rose from 10 in 1978 to 22 in 1982; most of these cases were in U.S. civilians. All cases were caused by P. falciparum. Since there are no chloroquine-resistant strains of P. falciparum in Haiti, all these cases in U.S. citizens could have been prevented by chemoprophylaxis.
Southeast Asia: Of the 1,930 cases imported from Southeast Asia, 96 (5%) were in U.S. civilians and 1,834 (95%) were in foreigners or Southeast Asian refugees. Malaria in refugees accounted for 1,709 (89%) of all cases imported from Southeast Asia. The 5-year profile of imported malaria in Southeast Asian refugees was dominated by the rapid increase in the number of malaria cases, which peaked at 1,034 cases in 1980 and subsequently declined to 134 cases in 1982. P. vivax accounted for 80% of all infections imported from Southeast Asia, while only 20% were P. falciparum infections.
India: In each of the 5 years studied, the largest number of imported cases from a single country came from India. Excluding infections in Southeast Asian refugees, malaria infections acquired in India accounted for 30%-40% of all imported cases in the United States. The vast majority (92%) of these infections were imported from India by foreigners. In the reporting period, 87% of all infections imported from India were caused by P. vivax.
In the period 1978-1982, an increased number of imported cases of malaria were reported in the United States. The number of U.S. civilians with reported malaria peaked in 1982, whereas the numbers of reported cases in both foreigners and refugees peaked in 1980. Excluding Southeast Asian refugees, the most malaria cases from a single country were reported for persons from India. There has been a steady increase in number of cases of imported malaria from Central America and Mexico, especially El Salvador. Most cases reported from these countries were caused by P. vivax. Numbers of imported malaria cases from Haiti are also increasing, but all the cases reported as being imported from that country were caused by chloroquine-sensitive P. falciparum. Although more than 5,000 cases of malaria are known to have been imported into the United States in the period 1978-1982, this has not resulted in documented domestic mosquito-borne transmission of malaria.
One isolated case of P. vivax malaria occurred in California in 1980 and another in 1981, affecting individuals who had no history of travel outside the country, blood transfusion, or drug abuse. The source of infection of these cases could not be established. No secondary cases were associated with either of these two cases. In the reporting period, 16 persons died from malaria infections acquired abroad. Health providers in the United States need to intensify efforts to advise travelers to malaria-endemic countries about malaria prophylaxis. The risk of fatal malaria can be reduced greatly if travelers use adequate chemoprophylaxis, and if health professionals are alert to the possibility of malaria in foreign nationals and other travelers from malaria-endemic countries who develop fever, irrespective of their history of malaria prophylaxis.
In particular, while P. falciparum infections constituted only 20% of imported cases from Southeast Asia, such infections merit special attention because of the resistance to multiple drugs of strains originating in Southeast Asia. The interpretation of malaria surveillance data in the United States is limited by the lack of knowledge about the completeness of case reporting and about the exposure to malaria among foreign visitors and U.S. travelers from malaria-endemic countries. However, the available information identifies certain high-risk groups who fail to be protected, e.g., travelers from India, Central America, and Haiti and refugees from Southeast Asia. Through publications such as the MMWR (1), CDC disseminates recommendations for chemoprophylaxis to prevent malaria in travelers.
1. CDC. Prevention of malaria in travelers. MMWR 1982;31 (1S).