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Malaria Branch
Division of Parasitic Diseases
Center for Infectious Diseases
Introduction
Malaria is no longer transmitted in the United States, but many
hundreds of cases continue to be reported each year in this
country as the result of infections acquired by U.S. civilians in
areas with endemic malaria. The epidemiology of imported malaria
reflects 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.
Methods
Information on malaria is obtained through two surveillance
systems. The states and territories report to CDC 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 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. Because reporting through this system is not
complete, these data on imported malaria should be interpreted to
reflect trends in disease occurrence rather than to measure the
precise incidence of disease.
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 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.
Results
In the period 1978-1982, 5,204 cases of malaria were imported
into the United States (Table 1). Of these cases, 26% were
imported by U.S. citizens, 33% occurred in Southeast Asian
refugees, and 41% were imported by other foreigners.
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 (Table
2). 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 (Table 3). 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 (Table 4). 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 (Table 5).
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.
Discussion
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 mosquitoborne 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 malarious 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 malarious 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 malarious 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.
References
1.CDC. Prevention of malaria in travelers. MMWR 1982;31(1S).
Table 1.Reported cases of malaria imported into the United
States, 1978-1982
Table 2. Reported cases of malaria imported into the United
States from Central America and Mexico, 1978-1982
Table 3.Reported cases of malaria imported into the United States
from Haiti, 1978-1982
Table 4.Reported cases of malaria imported into the United States
from Southeast Asia, 1978-1982
Table 5.Reported cases of malaria imported into the United States
from India, 1978-1982
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