UNSTABLE, LOW-LEVEL TRANSMISSION OF MALARIA ON THE COLOMBIAN PACIFIC COAST

The development of immune responses to malarial infection in inhabitan ts of endemic areas differs according to the level of exposure to the parasite. Adults living in a region where the level of malaria transmi ssion is low (Colombia) have been shown to exhibit a similar response to each of the three regions of the circumsporozoite protein (the cent ral repeated NANP region, and the flanking N- and C-termini). Converse ly, donors exposed to a frequent sporozoite challenge in areas of high malaria transmission (Mali) exhibit antibodies predominantly to the N ANP repeated domain. Malaria in the people of Zacarias, a community on the Pacific Coast of Colombia where malaria transmission is low and u nstable, was the subject of the present study. Within a 9-year period, a negative correlation between rainfall and documented malaria cases was recorded for this area. Thick smears of blood samples of 319 indiv iduals revealed that 8.5% had malarial infections. As most (67%) of th e smear-positive cases were asymptomatic, it seems that, despite the l ow prevalence of malaria in this area, the establishment of clinical s ymptoms is attenuated, probably because of the acquisition of premunit ion. Within this region, the most commonly found Anopheles species (re presenting 61.1% of the mosquitoes caught) and that giving the highest monthly biting rate (4.0 bites/man) was An. neivai. Most (90%) of the human sera tested possessed antibodies to blood-stage forms of Plasmo dium falciparum, and 18% had antibodies to sporozoites. More than half (58%) of the adults had been in contact with hepatitis B virus, 7.2% carried hepatitis B surface antigen, and syphilis was common but no su bject was found to be seropositive for HIV. A better understanding of the dynamics of the different elements influencing malaria in areas of low, unstable transmission, such as the one described here, is essent ial for the design of new malaria-control strategies.