Plasmodium falciparum transmission blocking immunity under conditions of low and high endemicity in Cameroon.

Transmission blocking immunity (TBI) was studied in relation to age, gametocyte density and transmission intensity. subjects with high gametocytaemias were selected in a hypo-endemic urban district and a hyper-endemic rural area in South Cameroon. TBI was determined in blood from gametocyte carriers in a bioassay (Direct Membrane Feeding Assay), with either autologous plasma (OWN) or control serum (AB). Mosquito infection rates (IR) were compared. infection rates correlated positively with gametocyte and oocyst densities. Three TBI indicators were analysed: the proportion of transmission reducers (IRAB > IROWN, P < 0.01), the mean intensity of TBI (IRAB - IROWN), and the contribution of TBI to total inhibition [(IRAB-IROWN)/(100-IROWN)]. we could not discriminate between areas with regard to either the proportion of transmission reducers (urban 15% and rural 29%) or the mean levels of TBI (urban 10% and rural 9%), or contribution of TBI to total inhibition (urban 10% and rural 13%). there was no relationship between TBI indicators and age, but a trend of increasing values was observed with rising gametocytaemia, which was considered as a confusing factor. a multivariable analysis showed that the probability of being a reducer was 4.6 fold higher in the rural area than in the urban district.

Estimation of malaria transmission from humans to mosquitoes in two neighbouring villages in south Cameroon: evaluation and comparison of several indices.

Malaria transmission from humans to mosquitoes was assessed in two neighbouring villages in a rural area near Yaoundé, Cameroon during high and low transmission seasons during 1998-2000, using several indices previously evaluated in different areas endemic for malaria but never directly compared. These indices were estimated from human parasitological data and mosquito infection rates and, for each individual, thick blood films were prepared at the same time as experimental infection of laboratory-bred mosquitoes. Among the 685 volunteers examined, the prevalence of Plasmodium falciparum gametocyte carriers was 16%, and 8% of individuals were able to infect mosquitoes. The percentage of mosquitoes that became infected by feeding on the infectious individuals was 21%. Children aged < 10 years contributed to about 75% of the infectious reservoir, although they constituted only 35% of the total population. Differences were found between the transmission seasons and the villages, and varied according to the index examined. Although there were more infectious individuals in one of the two villages, they were less infectious than those in the other village during the high transmission season. Comparative analysis of the transmission indices suggests the existence of functioning transmission-blocking immunity in one of the villages, which until now has been only hypothetically considered to play a role in malaria transmission in a natural setting. The epidemiological value of all the indices used and their accuracy in estimating the human infectious reservoir and its natural or induced variations are discussed.

Level and dynamics of malaria transmission and morbidity in an equatorial area of South Cameroon.

We conducted parasitological and entomological malaria surveys among the population of Mengang district in southern Cameroon to analyse the relationship between malaria transmission intensity and malaria morbidity. We investigated two adjacent areas which differ 10-fold in transmission intensity [annual entomological inoculation rate (EIR) 17 vs. 170], but have very similar Plasmodium falciparum malariometric profiles with parasite prevalences of 58 vs. 64%, high parasitaemia prevalences (> 1000 parasites/microl) of 15 vs. 16% and the same morbidity of 0.17-0.5 attacks/person/year. Plasmodium malariae prevalence was 14 vs. 16%. One possible explanation is that the similarity of the duration of the short and high transmission seasons in both areas is equally, if not more, significant for parasitological and clinical profiles as the annual EIR. We discuss the relationships between variations in transmission levels, parasitaemia and clinical incidence, and draw parallels to similar situations elsewhere.

Comparison of artificial membrane feeding with direct skin feeding to estimate infectiousness of Plasmodium falciparum gametocyte carriers to mosquitoes.

Human infectiousness to mosquitoes can be estimated by 2 tests: direct feeding on the skin and membrane feeding on venous blood. To validate the membrane feeding assay, the infectiousness of Plasmodium falciparum gametocyte carriers to Anopheles gambiae was estimated by these 2 methods in the same individuals in a rural area of Cameroon. Results from 37 experiments showed that direct feeding gave significantly higher infection rates than membrane feeding. We observed an average of 19.4% infected mosquitoes by direct feeding compared with 12.1% by membrane feeding, and a mean oocyst load of 5.63 by direct feeding compared with 2.65 by membrane feeding. However, there was a very good concordance between the 2 tests: 84.3% with the Kappa test on percentages of infected mosquitoes and 98.7% with the interclass correlation coefficient on oocyst loads. In addition, we found a good linear correlation between the 2 methods.

The use of anti-Pfs 25 monoclonal antibody for early determination of Plasmodium falciparum oocyst infections in Anopheles gambiae: comparison with the current technique of direct microscopic diagnosis.

Experimental infections of laboratory-reared anopheline mosquitoes were carried out with 57 Plasmodium falciparum gametocyte carriers from Cameroon. Prevalence of infected mosquitoes and oocyst intensity were determined by two independent methods. Young P. falciparum oocysts were detected on day 2 after feeding using an immunofluorescent assay, and the results were compared with direct microscopic examination of midgut oocysts on day 7 postinfection. The immunofluorescent assay was based on a FITC-labeled anti-25-kDa monoclonal antibody, while the direct microscopy was performed on midguts stained with 2% mercurochrome. Young oocysts were easily detected by their typical and bright green-fluorescing Pfs25 positive coat and their characteristic pattern of pigment granules under transmitted white light examination. The agreement between the results of the two methods was assessed using the Kappa coefficient on prevalences of positive infections and the interclass correlation coefficient on arithmetic mean oocyst load per infected midgut. The results indicated a low agreement between the two methods for the comparison of prevalences of infected mosquitoes. However, this agreement was near perfect for the comparison of mean oocyst intensities. Prevalences of positive infections and the overall number of parasites per positive gut were significantly correlated for both methods. Thus, the immunofluorescent test could be an appropriate tool for early determination of malaria infection in mosquitoes, particularly under laboratory conditions. The possible applications of this immuno-fluorescent technique are discussed.

[Malaria transmission in an area of future vaccination in equatorial forest of south Cameroon]. / Etude de la transmission du paludisme dans une future zone d'essai vaccinal en forêt équatoriale du sud Cameroun.

In order to describe malaria transmission in a future antigamete vaccine trial area, a longitudinal entomological study was conducted, together with parasitological and immunological surveys, from June 1997 to May 1998 in two nearby villages in a tropical rain forest area 100 km east of Yaoundé. Koundou is located along the main road in an open and degraded environment combining cultivated lands and forests; Ebolakounou is located 5 km from the road in forest surroundings. Indoor mosquito night catches no human volunteers were performed twice a month, in ten houses. We determined the entomological infection rate as 176 infected bites per human per year in Koundou (47.7% for An. moucheti, 47.3% for An. gambiae and 5% for An. funestus) and only 17.7 infected bites/human/year in Ebolakounou, with An. gambiae only. Transmission appears to be ten times higher in the village situated in a degraded environment than in the village situated in the rainforest.

Comparaison de la gametocytogenese in vivo chez les souches de plasmodium falciparum cloroquino-sensibles ou resistants

Lorsque Plasmodium falciparum souffre; il a tendance a devier son cycle parasitaire vers la gametocytogenese. Toute souffrance entrainee par l'utilisation d'un medicament partiellement efficace sur des souches resistantes pourrait donc potentialiser le pouvoir gametocytogene du parasite qui n'a pas ete elimine par la cure therapeutique. Cette hypothese a ete etudiee en suivant la gametocytogenese apres acces palustres traites a la Chloroquine en zone de forte chloroquino-resistance. Des sujets de tous les ages; atteints d'acces palustre ou presentant une forte densite parasitaire apparemment asymptomatique; mais sans gametocyte a JO; ont ete individualises en 2 groupes (sensible et resistant) au vu de l'evolution de la parasitemie apres traitement controle par la Chloroquine. Au total; 144 sujets ont ete suivis. Une poussee gametocytaire atteignant son maximum de densite ou de prevalence au 7eme jour apres l'echec therapeutique; a ete observee chez les sujets resistants. Cette poussee n'a pas ete significative chez les sujets sensibles. L'hypothese emise au debut de l'etude semble donc se confirmer. La neogametocytogenese semble d'autant plus marquee que l'individu est jeune (3 ans). Ni le niveau de resistance in vivo; ni le fait d'etre symptomatique ou non n'a semble intervenir sur le declenchement de cette poussee gametocytaire