Relationship between entomological inoculation rate, Plasmodium falciparum prevalence rate, and incidence of malaria attack in rural Gabon.

To assess the relationships between variations of Plasmodium falciparum transmission and those of peripheral parasitaemia prevalence or malaria attack incidence rates in regions with limited fluctuations of transmission, we conducted a follow-up in two Gabonese populations. Entomological surveys were carried out from May 1995 to April 1996 in Dienga, and from May 1998 to April 1999 in Benguia. In Dienga, malaria transmission was seasonal, being not detected during two 3-month periods. Mean entomological inoculation rate (EIR) was 0.28 infective bite/person/night. In Benguia, malaria transmission was perennial with seasonal fluctuations, mean EIR being 0.76 infective bite/person/night. In Dienga, 301 schoolchildren were followed from October 1995 to March 1996. Clinical malaria attack was defined as fever associated with >5000 parasites/microl of blood. P. falciparum prevalence varied from 28 to 42%, and monthly malaria attack incidence from 30 to 169 per thousand. In Benguia, the entire population (122 persons) was followed from November 1998 to April 1999. Prevalence varied from 22 to 50%, and monthly malaria attack incidence from 52 to 179 per thousand. In each area, entomological variations were not related to parasite prevalence, but preceded malaria attack incidence with 1- or 2-month time lag, corresponding to the pre-patency period that differs in the two populations, possibly according to differences in immunity related to parasite transmission.

Malaria transmission in a region of savanna-forest mosaic, Haut-Ogooué, Gabon.

During the 2 years 1993 to 1995, an entomological survey was carried out in the savanna-forest area of Franceville, Gabon, investigating malaria transmission in one suburban district of Franceville (Akou) and in one rural village (Benguia). The biting rates of the Anopheles vectors were 10 times higher in the rural zone compared to the suburban zone. Anopheles funestus Giles was the predominant species in both zones followed by Anopheles gambiae s.l. Giles. The densities of Anopheles nili Theobald and Anopheles moucheti Evans were very low. In the suburban zone, transmission was maintained throughout the year by An. funestus and An. gambiae s.l., whereas in rural zones the secondary vectors An. nili and An. moucheti were also involved in transmission. Humans in a suburban setting received one infective bite per person every 4 days, whereas in the rural area the infective biting rate was 4 times higher. Considering each vector, the observed entomological inoculation rates (EIRs) were one infective bite per person every 6 and 17 days for An. funestus and An. gambiae s.l., respectively, at Akou. At Benguia, the EIRs were one infective bite per person every 2, 3, 6, and 19 days for the 4 An. funestus, An. gambiae s.l., An. nili, and An. moucheti, respectively. The predominance of An. funestus over An. gambiae s.l. and its high EIR make it the most important malaria vector in this region of Haut-Ogooué.