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

Abstract

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/μl of blood. P. falciparum prevalence varied from 28 to 42%, and monthly malaria attack incidence from 30 to 169‰. 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‰. 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.

Introduction

Gabon is an endemic area for malaria where little work related to the dynamics of neither Plasmodium species nor vector species has been done. Given the lack of data from malaria control and survey program, the impact of malaria in the population remains difficult to assess. However, in this country, in which P. falciparum resistance to chloroquine may be as high as 90% in some areas (Kremsner et al., 1994; Philipps et al., 1998), malaria and its complications in early childhood are likely to cause many casualties (Kremsner et al., 1995). In some epidemiological situations, malaria transmission intensity, as assessed by entomological parameters, has been associated with parasite prevalence, malaria-associated symptoms, and death in children (Trape and Greenwood, 1994; Beier et al., 1999; Pinto et al., 2000). In African countries where transmission is highly seasonal, with clearly distinct high and low (or no) transmission seasons, such as Senegal and Kenya, the number of subjects presenting with malaria-associated symptoms has been correlated with the rate of transmission (Beadle et al., 1995)., In equatorial African areas, previous reports indicated that malaria morbidity also varies throughout the year in parallel with the transmission intensity at a given site (Carme, 1996; Sylla et al., 2001), but no differences in morbidity are observed between two distinct locations, eventually with very different global EIR. For example, in peripheral and central Brazzaville, Popular Republic of Congo, the malaria transmission level appeared not to be related to the incidence of malaria attacks (Trape et al., 1987; Carme et al., 1992). Similar observations were recently made in Cameroon (Bonnet et al., 2002). Most of equatorial areas are characterized by rainy seasons interrupted by short dry seasons, which may reduce, but to only a limited extent, the anopheline density and the level of malaria transmission. Consequently, malaria transmission is usually perennial, eventually with some seasonal variations in intensity. There is a critical need to understand the relationship between EIR and malaria disease. Therefore, two areas with distinct transmission dynamics and distinct age population were chosen to conduct the study. The objective of this study was not to compare the two populations nor the two villages, but to investigate simultaneously, in two epidemiological settings, parasite prevalence, malaria attack incidence and transmission intensity.