Background. - Currently, the control of urinary schistosomiasis is mostly based on mass treatment. Characterization of Schistosoma haematobium transmission could lead to adding new methods to the control strategy.Methods. - We carried out malacological and cercariometrical surveys in irrigated perimeters of the Niger River valley. A semi-monthly follow-up was performed in three main sites of human-water contact in a village located at the border of the irrigated perimeter. Bulinids were collected during 20 min; after identification based on the shell morphology, their parasites were characterized by isoelectrofocalisation. The cercariometrical technique by differential filtration was used to evaluate the density of cercariae in every site with two 20 litres samples of water collected at various periods of the day (9, 12, 15 and 18 h).Results. - In the arm of the river, the cercariometry never showed cercariae whereas only some Bulinus truncatus, all negative, were collected. In the channels, B. truncatus and B. globosus were observed and highest density was found during the dry season. Only B. truncatus was naturally infested. Homozygote 1313 phenotype (characteristic of S. haematobium) was observed in all positive molluscs. Using cercariometry, more than two third of cercariae were collected at 3:00 PM.Conclusion. - Transmission of S. haematobium was primarily focused in the irrigating channels during the dry hot season at the beginning of afternoon. These results should lead to improve the output of S. haematobium control methods within irrigated areas. (C) 2008 Elsevier Masson SAS. Tous droits reserves.

Malacological monitoring of two irrigated areas in Niger highlighted the natural susceptibility of Bulinus forskalii to infection with Schistosoma haematobium when the parasitic load is high. This first report of S. haematobium infection in B. forskalii, which is an abundant snail in irrigated areas, has been confirmed experimentally.

Intrinsic vector characteristics and environmental factors affect the sporogonic development of P. falciparum in Anopheles mosquitoes. We tested for the presence of the circumsporozoite protein, as a marker of the oocyst to sporozoite transition in naturally infected Anopheles gambiae s.l. and Anopheles funestus. Malaria vectors were collected in a village in the Sahel of Niger during the rainy and dry seasons. ELISA-CSP was carried out on abdomen and head/thorax portions from more than 2,000 samples. No significant difference was found in the overall rates of infection of An. gambiae s.l. (4.13%) and An. funestus (3.58%). Given the differences in duration of the two parasite stages, P. falciparum CSP antigen prevalence was nearly as high in the abdomen as in the head/thorax, and did not differ significantly between An. gambiae s.l. and An. funestus. These preliminary results suggest that development from oocysts to salivary gland sporozoites is similar in the two vectors. However, these developmental indices varied as a function of the season in which samples were collected, particularly for An. gambiae s.l. This simple method may be useful for field studies assessing the effect of environmental and genetic factors on parasite survival.

Urinary and intestinal schistosomiasis are both endemic in Niger with a dominance of Schistosoma haematobium. This study pointed out the appearance of the infection to S. mansoni in the Niger River valley, where it was until then unknown in Niger. In one year the prevalence increased from 5.9 to 19.5%.

Heterogeneity of Schistosoma haematobium transmission in irrigated fields. Although irrigated areas exist since a long time in the Niger Valley the distribution of the urinary schistosomiasis does not appear homogeneous, testifying to the existence of limiting or favourable factors. The identification of these factors could lead to a better definition of the distribution of the schistosomiasis risks and to optimise control programmes. The population of five villages about 1,900 inhabitants living in the same irrigated area (Sébéri) was examined at the end of 1999 before treatment and surveyed two and ten months after treatment by praziquantel in order to investigate re-infections. In parallel, the transmission sites were subject to a semi-monthly malacological follow-up from 1998 to 2001 and the water contacts were quantified in the sites of the main village during 2000. Before treatment, schistosomiasis risks appeared related to the proximity between habitat and lateral canals: the infections concerning youngers were all the more intense that the dwellings were close to the canal. The parasitological indices were the highest in the village lacking of other water sources. The morbidity indices followed a similar distribution with maximum values in the children of the 3 villages located to less than 1 km from the canal; however, morbidity was mainly observed in the adult population, in particular male, of the 2 villages which were the most distant from the canal. After treatment, the incidence of the re-infection between 2 and 10 months was comparable in the 3 villages close to the canal (28%) but was significantly weaker in the 2 villages far from the transmission sites (5%). In the villages bordering the canal, the incidence in the children was all the more high since the habitat was close to the canal. Between 1999 and 2000, the collected number of Bulinus truncatus decreased from 1.4 to 0.6 individuals per survey; moreover, no mollusc harbouring parasites was found, representing the decrease of the parasite burden. The abnormal weakness of re-infection, regarding this type of focus, could be explained by the repeated stop of water supply inducing a complete drying out of the canal for 2 months during the year preceding the study. These repeated drying out also resulted in a reduction of the exposure. Whereas the average frequentation of the sites of the canal remained rather comparable between January (cold dry season) and May (dry hot season), it decreased dramatically in September (rainy season but canals were not irrigated this year) from 99 to 11 daily contacts. The use of the lateral canal when filled represented 80% of the contacts. In the event of drying out, 80% of the contacts were transferred in the ponds but not in the river (5% of the contacts whatever the season). These results confirmed (1) that the presence of canals reduced the use of natural sites and (2) that the drying out of the canals induced a total reduction of the contacts.

En depit de l'anciennete des perimetres irrigues de la vallee du Niger, la distribution de la schistosomose urinaire n'y apparait pas homogene. L'identification de facteurs limitants ou favorables peut permettre de mieux definir la distribution du risque bilharzien et d'optimiser les programmes de lutte. Afin de preciser ces facteurs, la population de cinq villages, environ 1900 habitants, exploitant le meme perimetre a ete examinee avant et apres traitement par praziquantel afin d'analyser les reinfections. Parallelement, les sites de contact homme-eau ont fait l'objet d'un suivi malacologique bimensuel et les contacts ont ete quantifies. Avant traitement, la distribution du risque bilharzien apparaissait etroitement liee a la proximite de l'habitat par rapport aux canaux secondaires. Les indices de morbidite suivaient une distribution comparable. Elevee chez les enfants, la morbidite etait maximale chez les adultes masculins. Apres traitement, l'incidence des infections etait comparable dans les 3 villages proches du canal (28 %) et significativement plus faible dans les 2 autres (5 %). Dans les villages distants de moins de 1000 metres du canal, l'incidence chez les enfants etait d'autant plus elevee que l'habitat etait proche du canal. En revanche, l'impact de la distance habitat-canal paraissait moins marque chez les adultes. Entre 1999 et 2000, le nombre de Bulinus truncatus recoltes a diminue et aucun mollusque n'a ete trouve parasite. L'anormale faiblesse des reinfections pourrait s'expliquer par l'arret repete de l'alimentation en eau du perimetre. Ces resultats illustrent le benefice d'assechements repetes sur la transmission. Ces assechements repetes se traduisent egalement par une diminution de l'exposition en deplacant et en reduisant les contacts homme-eau. Ces resultats confirment, d'une part, que la presence de canaux tend a marginaliser l'utilisation des sites naturels et, d'autre part, que l'assechement des canaux entraine une reduction globale des contacts.

Located in an area where Schistosoma haematobium is hyperendemic, the town of Niamey is a focus of hypoendemicity. The aim of the malacological follow-up studies undertaken over a one year period was the determination of the mollusc species present in the water bodies, the appreciation of their space-time distribution of the snail and to find out the potential seasons of transmission. The investigation applied to 22 sites of transmission in the urban community. Four molluscs species (Bulinus truncatus, B. forskalii, B. globosus and B. senegalensis) are collected. The first three species are present all the year long, with a peak of density in the dry season. The last species (B. senegalensis) is found only during the rainy season. B truncatus and B. senegalensis are the two naturally infested species. The main site of transmission is the river Niger. The ponds play a secondary role in the peripheral districts. The main intermediate host of S. haematobium is B. truncatus.