Fernex, M.

Mittelholzer, M.L.

Naef, U.

Reber, R.M.

Steffen, R.

Stürchler, D.

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780 TRANSACTIONS OPTHE ROYAI. Socrar~ OF TRO~CAL MEDICINE AND HYGIENE (1990) 84, 780 [ Short Report 1 Malaria and mosquitoes: how often for how long? D. Stiirchler, U. Naef, M. Femex, M. L. Mittelhol- zer, R. M. Reber and R. Steffen F. HofffmannLa Roche AG, CHdOO2 Basel, Switzerland and University Institute for Social and Preventive Medicine, Sumatra- strasse 30, CH-8006 Ziirich, Switzerland Conceptually, exposure to Anapheles mosquitoes can be divided into an intensity and a duration component. Entomological surveys have characte- rized the influence of intensity (HII et al., 1988; BURKOT et al., 1988; TRAPE & ZOULANI, 1987; DEL GIUDICE et al., 1987; DRUILHE et al., 1986): infective bites range from <O*l to 3/person/day and vary with Atwpheles species, season, and malaria endemicity. Few studies have addressed the influence of duration of exposure. In Peace Corps volunteers working in Africa, fluctuations of quarterly Plasmodium falcipar- urn malaria incidence rates have been observed (BERNARD et al., 1989). In British travellers, the relative risk of malaria was 80 times higher for travels of 27 to 52 weeks’ duration compared to travels of 1 week duration (PHILLIPS-HOWARD et al., 1990). In endemic populations, age is usually taken as a surrogate measure of exposure time (DEL GIUDICE et al., 1987; MOLINEAUX, 1988), assuming stable malar- ia transmission and absence of population move- ments. We have attempted to determine the effect of exposure time on travellers from Switzerland to malarious areas in East and West Africa (STEFFEN et al., 1990). In this population, exposure ranged from cl week to > 12 weeks, a time frame which makes confounding by acquired immunity in this non- immune population unlikely. The confirmed malaria attack rate was <l/1000 travellers for exposures of < 1 week, and 12/1000 travellers for exposures of 4-12 weeks, a significant lbfold increase of relative risk (Table). Table. Parasitologically coafirmcd malaria casts ia haVetieR from Swimrlpnd to E&t and West Africa, classilied by length of stay abroad No. of weeks abroad <l 2-3 4-12 >12 hIal&a cases per no. of travellers Without prophylaxis O/l83 o/371 5161 216 With prophylaxis any 111210 15113670 Y797 o/12 Overall 111393 15/14041 10/858 2/18 IncidenccNlOO travekrs 0’7 1.1 11.7 111.1 Relative risk 1 1.5 16.0 139 95% cotidence limits - 0.1-5.1 2-1-125 EL1476 Corresponding author: PD Dr D. Stiirchler, PKFITI, F, ~-I~fmann-La Roche AG, Postfach, CH4002 Basel, Swazer- These attack rates are probably confounded by variations in intensity of exposure and protective measures in a study population which includes mainly businessmen and tourists. However, our data confirm the correlation between duration of exposure and malaria risk. This finding is expected and plausible, in fact exposure time is shown to be one of the more relevant malaria risk factors for visitors to endemic areas. References Bernard, K. W., Graitcer, P. L., Van der Vlugt, T., Moran, J. S. & Pulley, K. M. (1989). Epidemiological surveili- ante in Peace Corps volunteers: a model for monitoring health in temporary residents of developing countries. International 3oumal of Epidemiology, 18, 22C226. Burkot, T. R., Graves, P. M., Paru, R., Wirtz, R. A. & Heywood, P. F. (1988). Human malaria transmission studies in the Anapheles punctula~~~ complex in Papua New Guinea: sporozoite rates, inoculation rates, and sporozoite densities. American Journal of Tropical Medi- cine and Hygiew, 39, 135-144. Del Giudice, G., Engers, H. D., Tougne, C., Biro, S. S., Weiss, N., Verdini, A. S., Pessi, A., Degremont, A. A., Freyvogel, T. A., Lambert, P. H. & Tanner, M. (1987). Malaria antibodies fo the repetitive epitope of Plasmo- dium falciparum circumsporozoite protein in a rural Tanzanian communitv: a loneitudinal studv of 132 children. American 3&zal of- Tropical Medicine and Hygiene, 36, 2OS212. Druilhe, P., Pradier, O., Marc, J. P., Miltgen, F., Mazier, D. & Parent, G. (1986). Levels of antibodies to Plawwdium faiciparuh spo;ozoite surface antigens reflect malaria transmission rates and are persistent in the absence of reinfection. Infection and Immunity, 53, 393-397. Hii, J. L. K., Kan, S., Vun, Y. S., Chin, K. F., Tambakau, S. & Ghan, M. K. C. (1988). Transmission dynamics and estimates of malaria vectorial capacity for Anopheks balabacensis and Anopheles flavirostris (Diptera: Culici- dae) on Banggi island, Sabah, Malaysia. Annals of Tropical Medicine and Parasitology, 82, 91-101. Molineaux, L. (1988). The epidemiology of human malaria as an explanation of its distribution, including some implications for its control. In: Malaria. Principles and Practice of.Malariolqy, Wemsdorfer, W, v. & McGre- Frk9edltors). Edmburgh: Churchill L~vmgslone, pp, Phillips-Howard, P. A., Radalowicz, A., Mitchell, J. & Bradley, D. J. (1990). Risk of malaria m British residents returning from malarious areas. British MedicalJournal, 300, 499-503. Steffen, R., Heusser, R., Miichler, R., Bruppacher, R., Naef, U., Chen, D., Hofmann, A. M. & Somaini, B. (1990). Malaria chemoprophylaxis n European tourists visiting tropical Africa: use, adverse drug reactions, and ef6cacy. Bulletin of the World Health Organization, in Tra’zf: F. & Zoulani, A. (1987). Malaria and urbanization in Central Africa: the example of Brazxaville. Part II: results of entomological surveys and epidemiological analysis. Transactions of the Royal Sociew of Tropical Medicine and Hygiene, 81, supplement no. 1, 10-18. Received 24 April 1990; revised 1 I June 1990; accepted fm publication 12 June 1990 

Malaria and mosquitoes: how often for how long?

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