Different response to Plasmodium falciparum malaria in west African sympatric ethnic groups

Abstract

The comparison of malaria indicators among populations that have different genetic backgrounds and are uniformly exposed to the same parasite strains is one approach to the study of human heterogeneties in the response to the infection. We report the results of comparative surveys on three sympatric West African ethnic groups, Fulani, Mossi, and Rimaibé, living in the same conditions of hyperendemic transmission in a Sudan savanna area northeast of Ouagadougou, Burkina Faso. The Mossi and Rimaibé are Sudanese negroid populations with a long tradition of sedentary farming, while the Fulani are nomadic pastoralists, partly settled and characterized by non-negroid features of possible caucasoid origin. Parasitological, clinical, and immunological investigations showed consistent interethnic differences in Plasmodium falciparum infection rates, malaria morbidity, and prevalence and levels of antibodies to various P. falciparum antigens. The data point to a remarkably similar response to malaria in the Mossi and Rimaibé, while the Fulani are clearly less parasitized, less affected by the disease, and more responsive to all antigens tested. No difference in the use of malaria protective measures was demonstrated that could account for these findings, and sociocultural or environmental factors do not seem to be involved. Known genetic factors of resistance to malaria did not show higher frequencies in the Fulani. The differences in the immune response were not explained by the entomological observations, which indicated substantially uniform exposure to infective bites. The available data support the existence of unknown genetic factors, possibly related to humoral immune responses, determining interethnic differences in the susceptibility to malaria.

Figure 1

Sketch map of the study area showing the villages of Barkoumbilen and Barkoundouba. Capital letters indicate the position of the compounds of the ethnic groups. M, Mossi; R, Rimaibé; F, Fulani.

Figure 2

Entomological inoculation rates in the compounds of the three ethnic groups (Mossi, Rimaibé, and Fulani) in the Barkoundouba area from August 1994 to October 1995.

Figure 3

Values ± standard errors, by age and ethnic group, of P. falciparum parasite rate (A) and log₁₀ of positive parasite density (B) in the Barkoundouba area, Burkina Faso. The positions of the symbols on the x-axis correspond to the mean age for each of the following age groups: 0–5, 6–10, 11–20, 21–30, and >30 years. Data refer to five cross-sectional surveys (August 1994, October 1994, March 1995, August 1995, and October 1995). To indicate a significant difference between the Fulani and one or both the other two groups, we used a half-filled or filled symbols, respectively. The sample sizes of each survey are shown under the respective month. M, Mossi; R, Rimaibé; and F, Fulani.

Figure 4

Comparative analysis in three sympatric ethnic groups of the percentage (± SE) of negative individuals at the first examination (August 1994) and rate of persistence of their negativity in the successive cross-sectional surveys. Only individuals more than 10 years old and examined in all five surveys of Fig. 1 are considered. No significant difference of mean age was observed among the three subsamples. The comparison does not include the 0–10 year age groups, since their negativity rates were too low for this type of analysis. All the Fulani/Mossi and Fulani/Rimaibé comparisons are statistically significant with P < 0.01.

Figure 5

Anti-P. falciparum CSP antibody prevalences (A) and levels (B) by age and ethnic group. The data refer to the same samples examined in the first three surveys illustrated in Fig. 3 and the same symbols are used. The prevalences are shown as percent positive ± SE, and the levels are shown as log₁₀ immunoenzymatic units ± SE.