Malaria Risk in Travelers

Abstract & Commentary

Synopsis: The GeoSentinel surveillance database was examined to identify patient and travel characteristics associated with the acquisition of malaria. Travel to sub-Saharan Africa and Oceania had the highest relative risk for acquisition of malaria infections. The most common reason for travel among malaria patients was to visit friends and relatives (VFR).

Source: Leder K, et al. Malaria in Travelers: A Review of the GeoSentinel Surveillance Network. Clin Inf Dis. 2004;39:1104-1112.

Between november 1997 and december 2002, 1140 malaria cases were reported to and recorded within the GeoSentinel database. Sixty percent of reported malaria cases were caused by Plasmodium falciparum, and 24% were due to Plasmodium vivax. Twenty-one cases (2%) had mixed species infections. Sixty-nine percent of patients were male, and the median age was 33 years. Fever was a presenting symptom in 93% of patients with P. falciparum malaria, 95 % of patients with P. vivax malaria, 83% of those with P. ovale malaria, and 95% of patients with P. malariae malaria. Severe and/or complicated malaria occurred in 33 cases, with 3 deaths. Two of the 3 deaths were associated with malaria acquired in sub-Saharan Africa.

Most patients with malaria were travelers (39%) or immigrants/refugees (38%), followed by expatriates (12%), foreign visitors (7%), students (3%), and military personnel (0.4%). The most common reason for travel was for visiting friends and relatives (35%: VFRs), followed by tourism (26%), business (14%), immigration (10%), missionary/volunteer (9%), and research/education (5%). Only 37 % of all malaria cases had had a pretravel visit with a health care provider. Of those who were in the immigrant and VFR groups, 85% had not been seen by a health care provider prior to travel, making it unlikely that they had taken antimalarial prophylaxis.

Using a subset of patients in the GeoSentinel database who had traveled and acquired malaria during 2000-2002 and World Travel Organization data as estimates of all visitors to a region of malaria risk during the same time period, the risk per 10 million travelers of presenting to a GeoSentinel clinic with malaria was calculated. The relative risk of malaria in travelers to different regions was then calculated by comparison to the rates observed after travel to very low low-risk area such as Europe and North America. Sub-Saharan Africa (RR, 208) and Oceania (RR, 77) were associated with the greatest risk of acquiring malaria, followed by south Asia (RR, 54), Central America (RR, 38), Southeast Asia (RR, 11.5), South America (RR, 8),North Africa (RR, 7), and the Caribbean (RR, 4).

Species information was available for 1035 cases (91%). The majority (89%) of all reported P. falciparum cases were acquired in sub-Saharan Africa, whereas acquisition of P. vivax was more geographically diversified with ~30% acquired in sub-Saharan Africa, 30% from Asia, 15% from Central/South America, and 15% from Oceania.

Comment by Mary-Louise Scully, MD

A potential wealth of information regarding travelers and their risk of disease acquisition is now available through the GeoSentinel database. GeoSentinel is a global sentinel surveillance network that was established in 1995 through efforts of the International Society for Travel Medicine and the Centers for Disease Control and Prevention (CDC) for collaboration in the surveillance and monitoring of travel related illnesses. At least 27 travel clinic sites on 6 continents participate in this organization. The GeoSentinel surveillance data is derived from sites and network members in both the Northern and Southern Hemispheres, and therefore, may provide new insights into global trends.

Specific information on relative risk of malaria acquisition by geographic region showed that the relative risk of malaria acquisition was greatest in the traveler to sub-Saharan Africa. Previous studies have demonstrated similar findings. In the CDC 2002 malaria surveillance data, 72.1 % of all cases were acquired in Africa, in particular, countries in West Africa.1 Additional data from the TropNetEurop, a European surveillance network, identified similar trends with the greatest number of their infected patients acquiring their malaria in West Africa.2

The next highest area of relative risk in the GeoSentinel data was Oceania. The CDC lists Papua New Guinea, the Solomon Islands, and Vanuatu as areas of malaria risk in Oceania.3 The GeoSentinel data did show a greater relative risk estimate for Central America than has been previously reported. Although Leder and colleagues acknowledge that there are some limitations to the epidemiologic methods they used to calculate relative risk, the data none-the-less provide information obtained from a global surveillance network. Certainly any data on malaria risk should be interpreted with the understanding that malaria risk can often vary within designated geographic areas (ie rural vs cities, lowlands vs high altitudes, etc.), as well as with a traveler’s specific itinerary (backpacking vs air-conditioned hotels), hence malaria pretravel advice should always be individualized.

Some additional insights into the types of travelers were notable. Previous studies have also shown that VFRs often return to their country of origin without seeking pretravel advice or taking chemoprophylaxis; the unfortunate result being the acquisition of malaria.4,5

However, 73% of the GeoSentinel group of missionary/volunteer group did have pretravel advice, yet still acquired malaria. The need for a long duration of prophylaxis, misconceptions about risk, and development of side effects to chemoprophylaxis may have contributed to these findings in the missionary/volunteer group. Perhaps devoting extra time during pretravel consultation to reinforce the need for long term adherence to prophylaxis, and even providing alternative regimens in the event of an adverse reaction, might reduce the incidence of malaria among missionary/volunteer patients.

Lastly, we are reminded that even trips of short duration in malaria risk regions can result in disease; 37% of GeoSentinel malaria cases had travel duration of < 4 weeks, and 5% had trips with durations of £ 1 week. In addition, 3% of patients became clinically ill while still abroad. Whereas 80% of patients with P. falciparum malaria presented within 4 weeks after return, only 40% of patients with P. vivax malaria presented within the first month. Also, patients with P. vivax who had seen a health care provider presented much later (median, 58 days), compared to those who did not (median, 15.5 days). Similar findings were reported using malaria surveillance data from the United States and Israel, where more than one-third of patients with P. vivax or P. ovale who had received malaria chemoprophylaxis became ill more than 2 months after their return.6 At that point, since their return time has lengthened, patients may be less likely to attribute their symptoms of illness to their previous travel. These patients may present to local walk-in or emergency facilities where they are not known and there may be language/communication issues as well. Only a careful history by the physician to identify the possibility of previous travel to a malaria risk region can lead to the correct, potentially life saving diagnosis in these ill patients.

References

1. Shah S, et al. Malaria Surveillance-United States, 2002. MMWR. 2004;53(No. SS-1):21-34.

2. Jelinek T, et al. Imported Malaria in Europe: Sentinel Surveillance Data From the European Network on Surveillance of Imported Infectious Diseases. Clin Inf Dis.2002;34:572-576.

3. http://www.cdc.gov/travel/regionmalaria/austspac.htm

4. Ryan ET, et al. Illness After International Travel. N Eng J Med. 2002;347:505-516.

5. Freedman DO, et al. Emerging Infectious Diseases and Risk to the Traveler. Med Clinic North Am. 1999;83:865-883.

6. Schwartz E, et al. Delayed Onset of Malaria-Implications for Chemoprophylaxis in Travelers. N Eng J Med. 2003;349:1510-1516.

Mary-Louise Scully, MD, Sansum-Santa Barbara Medical Foundation Clinic, Santa Barbara, Calif., is Associate Editor for Travel Medicine Advisor.