Academic journal article Bulletin of the World Health Organization

Plasmodium Falciparum and P. Malariae Epidemiology in a West African Village

Academic journal article Bulletin of the World Health Organization

Plasmodium Falciparum and P. Malariae Epidemiology in a West African Village

Article excerpt

INTRODUCTION

After Plasmodium falciparum, P. malariae is the most important cause of human malaria in West Africa (6). The geographical distribution of both species is widespread, but P. malariae is localized in foci (7). The periods during which these two species are transmitted are different (12). P. malariae is responsible for some malaria morbidity and chronic infections in endemic areas, and can sometimes induce a nephrotic syndrome (9). In some foci it can therefore be important to follow P. malarieae transmission in order to select specific control measures against this parasite species.

By microscopy, it is virtually impossible to differentiate P. malariae sporozoites and P. falciparum in the salivary glands of infected mosquitos. Recently, however, a two-site enzyme-linked immunosorbent assay (ELISA) that uses a monoclonal antibody against a surface P. malariae sporozoite antigen has been employed to detect this parasite in infected mosquitos (4). Specific vectors for P. malariae have already been characterized in Kenya (1), but there has been no investigation of the transmission of this species in West Africa.

Using the above-mentioned ELISA with monoclonal antibodies against either P. falciparum or P. malariae, as well as entomological and parasitological surveys, we carried out a longitudinal investigation of malaria transmission in a savanna area of Burkina Faso.

Materials and methods

Characteristics of the study area

The study was conducted in Karangasso, Burkina Faso, near the city of Bobo-Dioulasso. Karangasso is a typical savanna village with a semipermanent river but is sufficiently distant from Bobo-Dioulasso to preclude significant use of chloroquine by the local population. The area is characterized by a hot and rainy season from June to October (average rainfall, 1000 mm per year; mean temperature, >25 [degrees]C); a cold and dry season from November to February (minimum temperature, 15 [degrees]C); and a hot and dry season from March to May.

Entomological investigations

Consecutive surveys were carried out from January 1985 to February 1986. Eight indoor human night catches were performed in the village twice a month. The species of mosquitos caught were identified and each batch of Anopheles gambiae, A. funestus, or A. nili, the main malaria vectors in the area (13, 14) during the rainy season, was randomly divided into two lots: one for microscopy of salivary glands and ovaries and the other for immunological testing.

The proportion of parous females (parity rate, PR) was calculated for each survey (5). The percentage of infected glands (parasitological sporozoite rate, s) was also determined. The human biting rate (ma) was estimated by determining the mean number of bites per person per night, and reflects the dangerous anopheline density for humans. The inoculation rate (h), which reflects the intensity of transmission was calculated from the relationship h = ma X s, and is expressed in infected bites per person per night (11).

Enzyme-linked immunosorbent assay

Mosquitos from the second lot were tested with a two-site ELISA, using the monoclonal antibodies (MAb) 3SP2 and 6B10-1F2 against P. falciparum and P. malariae sporozoites, respectively. Details of the ELISA have been described previously (17, 18), and are presented here only in outline. Head-thoraxes and abdomens were separately ground in Nonidet P-40 with an anti-enzyme solution. After incubation, the supernatant was tested on microtitration plates sensitized with each of the two MAbs. The plates were incubated for 2 hours at room temperature and then washed three times. Biotinylated MAb solution was then added and incubated for 1.5 hours at room temperature. After rinsing the plates three times, 50 [mu]l of a pre-formed streptavidin-biotin-peroxidase complex was added and the mixture incubated for 30 minutes. …

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