The Dielmo Project: a Longitudinal Study of Natural Malaria Infection and the Mechanisms of Protective Immunity in a Community Living in a Holoendemic Area of Senegal

Abstract

The Dielmo project, initiated in 1990, consisted of long-term investigations on host-parasite relationships and the mechanisms of protective immunity in the 247 residents of a Senegalese village in which malaria is holoendemic. Anopheles gambiae s.1. and An. funestus constituted more than 98% of 11,685 anophelines collected and were present all year round. Inoculation rates of Plasmodium falciparum, P. malariae, and P. ovale averaged respectively 0.51, 0.10, and 0.04 infective bites per person per night. During a four-month period of intensive parasitologic and clinical monitoring, Plasmodium falciparum, P. malariae, and P. ovale were observed in 72.0%, 21.1% and 6.0%, respectively, of the 8,539 thick smears examined. Individual longitudinal data revealed that 98.6% of the villagers harbored trophozoites of P. falciparum at least once during the period of the study. Infections by P. malariae and P. ovale were both observed in individuals of all age groups and their cumulative prevalences reached 50.5% and 40.3%, Î-espectively. Malaria was responsible for 162 (60.9%) of 266 febrile episodes; 159 of these attacks were due to P. falciparum, three to P. ovale, and none to P. malariae. The incidence of malaria attacks was 40 times higher in children 0-4 years of age than in adults more than 40 years old. Our findings suggest that sterile immunity and clinical protection are never fully achieved in humans continuously exposed since birth to intense transmission.

Plasmodium malariae and Plasmodium ovale – the ‘bashful’ malaria parasites

“…P. malariae infections are most common in sub-Saharan Africa and the southwest Pacific, where age-specific prevalence in mass blood surveys have exceeded 15-30% [2][3][4][5][6][7][8]. By contrast, when P. malariae has been detected in malaria-endemic regions of Asia [9][10][11][12], the Middle East [13], South America [14] and Central America [15], it is observed as an infrequent infection, with blood-smear light microscopy (LM) prevalence rarely exceeding 1-2%.…”

“…Infections with P. ovale, however, have also been reported in the Middle East [13], the Indian subcontinent [21] and different parts of Southeast Asia [11,22,23]. In West Africa (and to a lesser extent Central Africa), age specific LM prevalence of >10% have been observed [3,6] places where P. ovale is observed, it is relatively uncommon and its prevalence (as detected by LM) rarely exceeds 3-5% [22,[24][25][26].Indepth descriptions of the epidemiology of both infections based upon LM data are, thus, restricted almost exclusively to highly endemic areas in Africa and the Southwest Pacific. Detailed epidemiological studies from South America and Asia are lacking.…”

“…in children under ten years of age) [3,6,8]. In children under four years, in addition to adults, most LM-diagnosed infections were observed to be of low density (<500 parasites per µl) and, at least in children, usually of long duration [6]. Coinfections of P. malariae and P. falciparum were reported more commonly than would be expected by chance [3,27,28].…”

“…Apart from differences in a wide range of ecological factors (such as climate and Anopheline vector species), these contrasting observations in P. malariae epidemiology could be linked to the virtual absence of Plasmodium vivax infections in many African populations because of the lack of the Duffy blood-group-antigen, which P. vivax requires to invade red blood cells [33]. Interestingly, Smith et al reported that in Wosera, PNG, P. vivax infections might be associated with an antagonistic effect on P. malariae infections[34].Like P. malariae, P. ovale infections in West African populations tend to be most common in children under ten years of age [3,6,35]. The low numbers of LM detectable infections in most PNG and Southeast Asian surveys precludes a firm assessment of age distribution of infections in these regions.…”

“…<800 parasites per µl) [6,35]. However, the incidence of infections was high, with 40% of people of all ages experiencing at least one P. ovale infection during the four-month study period [6,35]. The rarity of reported P. ovale infections in cross-sectional surveys might be heavily influenced by the low species-specific parasitemia and short duration of patent infections.…”