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Malaria

Malarial Transmission

Once thought to be on its way to eradication, malaria has made a troubling comeback worldwide since the failed eradication programs of the 1950s and 60s. Now considered the world’s most prevalent vector-borne disease, malaria exists in endemic proportions in at least 92 countries.

“Approximately 41% of the world’s population is at risk, and each year 300 million to 500 million clinical cases of malaria are reported. Worldwide, approximately 2 million deaths per year can be attributed to malaria, half of these in children under 5 years of age,” according to Pim Martens and Lisbeth Hall in an article published in Centers for Disease Control’s Emerging Infectious Diseases.

Of increasing concern is malaria’s reemergence through human migration. And, for the United States and other developed counties, the part travel and immigration have played in that reemergence. The failure to understand how human migration impacts the spread of malaria played a significant role in the failure of eradication programs in the mid 20th century. And, according to Martens and Hall, “identifying and understanding the influence of…population movement can improve prevention measures and malaria control programs.”

Rapid urbanization, when accompanied by adequate housing and sanitation, can lead to decreases in malaria through reduced human-mosquito contact and eradication of mosquito breeding sites. However, in developing countries, unregulated urban growth can lead to an increase in malaria transmission because of poor housing and sanitation, improper drainage of surface water and use of unprotected water reservoirs that increase human-mosquito contact and mosquito breeding.

The alteration of the environment by humans plays a significant role in the spread of malaria, as demonstrated by the dramatic resurgence of the disease in Brazil since the 1960s. Massive population movement, facilitated by the construction of highways into the Amazon rainforest has meant that 98% of Brazil’s malaria cases in the 1990s were recorded in the Amazon region. With the settlement of agricultural regions, the construction of hydroelectric projects and the discovery of gold in the Amazon, some regions saw nearly ten-fold population increases in 20 years. Meanwhile, Brazil as a whole witnessed a more than 100-fold increase in cases of malaria over the same time period (50,000 cases in 1970 compared to 577,520 cases in 1990).

For developed countries, like the United States, a sense of safety may be leading to complacency. According to an article by Ellen Ruppel Shell in the August 1997 issue of The Atlantic Montly, “the United States has shown little interest in the problem. Malaria is transferable in blood, yet it is not screened for in the American blood supply. The country’s Anopheles mosquito population has gone unmonitored for more than fifty years…. Most Americans seem to think the disease has been eradicated or, at worst, is confined to the tropics. In fact there are few places on earth that cannot sustain a malaria epidemic.” Complacency, in the time of modern travel, can be disastrous.

Air travel has increased by almost 7% a year in the last 20 years and is predicted to increase by more than 5% a year during the next 20 years (1). As a result, developed countries have seen new means of malarial transmission in “airport malaria”. This transmission of the disease is defined by its spread from a tropical Anopheline mosquito to persons whose geographical history precludes exposure to this mosquito’s natural habitat. Generally, the mosquito is introduced to a nonendemic country via international air travel.

Martens and Hall noted “random searches of airplanes at Gatwick Airport (London) found that 12 of 67 airplanes from tropical countries contained mosquitoes (2).” In their article for the CDC, the authors cited the following cases of airport malaria:

“During a hot summer in 1994, six cases of airport malaria were identified in and around Roissy-Charles-de-Gaulle Airport (3). Four of the patients were airport workers, and the others lived in Villeparisis, approximately 7.5 km away. Anopheline mosquitoes were thought to have traveled in the cars of airport workers who lived next door to two of the patients. In 1989, two cases of P. falciparum malaria were identified in Italy in two persons who lived in Geneva (4). Another five cases of airport malaria were reported in Geneva in the summer of 1989 (5). High minimum temperatures were thought to have allowed the survival of infected anophelines introduced by aircraft. In Britain, two cases of P. falciparum malaria were observed in persons living 10 km and 15 km from Gatwick Airport (6). Hot, humid weather in Britain may have facilitated the survival of an imported mosquito.”

Another area of concern for developed countries is “import malaria”. In this case, local, uninfected mosquitoes are exposed to the disease via contact with infected immigrants from malaria-endemic areas. The local mosquitoes then spread malaria to individuals who would not otherwise be exposed to the disease.

Increases in this form of malarial transmission have been reported in the United Kingdom (where import from Africa is thought to be the culprit), Italy (with the endemic area being India) and the United States (from Mexican migrant workers, among others).

In the case of the United States, a 1986 outbreak in California involved 28 cases (26 in Mexican migrant workers) during a 3-month period (7) and, because the epidemic curve indicated secondary spread, health authorities we able to confirm local mosquito-borne transmission.

While developed countries can count on good financial resources and established healthcare procedures to quickly and effectively quell malaria outbreaks, they should keep an eye to the increasing urbanization of underdeveloped countries. Encouragement should be made for adequate sanitation and regular malaria testing and treatment. And, there should be discouragement of unmonitored deforestation and sub-standard housing near mosquito breed sites. With immigration becoming global at an unprecedented rate, transmission factors must be closely monitored to ensure the health and well-being of all members of the global community.

References

1. World Health Organization. The World Health Report 1996: fighting disease, fostering development. Geneva: The Organization; 1996.
2. Curtis CF, White GB. Plasmodium falciparum mission in England: entomological and epidemiological data relative to cases in 1983. Journal of Tropical Medicine and Hygiene 1984;87:101-14.
3. Giacomini T, Mouchet J, Mathieu P, Petithory JC. Study of 6 cases of malaria acquired near Roissy-Charles-de-Gaulle in 1994. Bull Acad Natl Med 1995;179:335-51.
4. Majori G, Gradoni L, Gianzi FP, Carboni P, Cioppi A, Aureli G. Two imported malaria cases from Switzerland. Tropical Medicine and Parasitology 1990;41:439-40.
5. Bouvier M, Pittet D, Loutan L, Starobinski M. Airport malaria: mini epidemic in Switzerland. Schweiz Med Wochenschr 1990;120:1217-22.
6. Whitfield D, Curtis CF, White GD, Targett GA, Warhurst DC, Bradley DJ. Two cases of falciparum malaria acquired in Britain. BMJ 1984;289:1607-9.
G, Romi R, Severini C, Cuccagna G, et al. Malaria in Maremma, Italy. Lancet 1998;351:1246-7.

7. Maldonado YA, Nahlen BL, Roberto RR, Ginsberg M, Orellana E, Mizrahi M, etMalaria on the Move: Human Population Movement and Malaria Transmission