Mémoire

Sur quelques particularités écologiques de la faune malacologique vectrice de trématodes dans les Dallols nigériens

Remarks on the ecological adaptation of the snail aquatic fauna in saline medium of the Dallol ponds. (République du Niger)

Ecole des Assistants d’Elevage de Niamey, République du Niger.

Résumé

Le réseau hydrographique fossile des Dallols en République du Niger est réduit pendant toute la période sèche de l’année à une série de mares alimentées par une nappe affleurante très légèrement salée ou par des sources d’eau douce.

La plupart de ces points d’eau ont des constantes bio-physico-chimiques propices à la vie et à la prolifération de Gastéropodes vecteurs de bilharzioses, paramphistomoses et distomatose.

Tous les ans, par suite d’une évaporation intense durant les mois de février, mars, avril et mai, non entièrement compensée par un apport d’eau suffisant, la concentration en sels dissous du milieu (chlorure, sulfate et bicarbonate de sodium), augmente pour atteindre, au bout d’un certain temps, une valeur létale, variable suivant chaque espèce de Mollusque vecteur.

En reproduisant expérimentalement des conditions analogues en laboratoire, il a été possible de déterminer les courbes de mortalité de Biomphalaria pfeifferi, Bulinus truncatus, Bulinus forskalii et Afrogyrus coretus, en fonction du taux de salinité du milieu. La DL 50 étant pour chacun d’entre eux de : 9,6 ; 9,00 ; 1,05 et 2,35 g/litre.

Abstract

Human vesical and intestinal bilharziasis, bovine fasciolosis and paramphistomosis, equine gastrodiscosis and ovine carmyeriosis, are frequent in the Dallols’region, (12° - 13° 30 N. lat. ; 3° E. long.), République du Niger, Africa.

Dallols are fossil valleys pouring water from late Saharian lakes. They are also tributaries of the Niger River.

During the dry season, they become dry and many residual ponds of varied dimensions: from tens feet to one or two miles long, are lying along the valley bottom. The water is sometimes fresh but more frequently salt, (sodium, calcium and potassium, chlorides, sulfates, carbonates and bicarbonate), are in solution of variable proportions.

From november to april, the total salt concentration is increasing by high evaporation and the medium becomes non likely to live for aquatic vector snails, Bulinus truncatus, Biomphalaria pfeifferi, Bulinus forskalii, Lymnaea natalensis and Afrogyrus coretus.

They dead and desappear more or less early. B. pfeifferi and B. truncatus are the more resistant, (10 to 11 g/salt/liter), L. natalensis beeing very sensible, (1 to 2 g/salt/liter).

Laboratory tests in aquarium, to realise as much as possible the natural conditions, (pH = 6 to 6,5; temperature: 25°-27°C, with good oxygenation, air-bull), with adding daily 150 mg/liter of saline compound extract of the “sel du Fogha” (mixed saline of the soil of the Dallol Fogha), give the following results:

For each species, the salt concentration L.D. 30-35, stops the snail nutrition, when the eggs are more numerous but no able to hatch. The young snails is enlarged inside the mucilaginous wrapper. The salt concentration L.D. 50, kills all young snails.

Mortality rate, L.D. 50 and L.D. 100, for each species are indicated fig. 1; (semi logarithmical curve).

Consequently, snail population disappears yearly in the salted ponds, but lateral ponds of the valley banks supplied by fresh springs, allow the preservation of snail stocks.

The snail invasion of the dallol reappears as soon as the first rains of the raining season. Molluscs are coming out from these “estivation” lodges and are invading all the points of the river valley when the water becomes fresh.

Prophylactic programs by molluscicide products will be very easy. It is possible to obtain snail eradication in the rare permanent freshwater ponds during the dry season.