Seasonal abundance of trematode cercariae in Bulinus truncatus in a small focus of schistosomiasis in the Nile Delta.

Bulinus truncatus was most numerous from June to August, and echinostome cercariae, present in 19.3% of snails, were the most prevalent of the 4 types of trematode cercariae found. The echinostomes occurred throughout the year, other cercariae only in certain months. Double infections were uncommon. The findings suggest that echinostome cercariae exercise some biological control of Schistosoma haematobium in the area studied.

Cercarial production from Biomphalaria alexandrina infected with Schistosoma mansoni.

The present paper deals with longitudinal and cross-sectional methods of counting cercariae shed from Biomphalaria alexandrina in Egypt, both experimentally and naturally infected with schistosomes. By the longitudinal method, the daily output of cercariae was counted from the first shedding from experimentally infected snails and from the day of collection from naturally infected ones. The results show that the size of the snails at the time of shedding exerts a very large effect on the output of cercariae and that the numbers obtained in the laboratory are not representative of cercarial output in the field. By the cross-sectional method, the cercarial output in the first 24 hours from infected snails collected in different months from the field was counted. The results show that output is size-specific. When the size-specific output is adjusted to the size-composition of infected snails taken from the field, it is estimated that the daily output from infected snails in the field may be 957.7 cercariae. However, this number may vary with the season.

Cercarial transmission seasons of Schistosoma mansoni in the Nile Delta area.

The cercarial transmission seasons of Schistosoma mansoni in the Nile Delta area were determined by 3 methods: (1) monthly cercarial infection rates in Biomphalaria alexandrina, (2) monthly numbers of infected snails collected by means of a uniform sampling method, and (3) monthly cercarial output and monthly numbers of infected snails collected by means of a uniform sampling method. It was found that the third method is more logical and defines more precisely the transmission seasons of S. mansoni in the area. The paper also includes a discussion on the important factors that influence the timing of mollusciciding operations for the control of schistosomiasis in the Nile Delta area.

The ecology of Bulinus truncatus and Biomphalaria alexandrina and its implications for the control of bilharziasis in the Egypt-49 project area.

The respective vectors of the two forms of bilharziasis in Egypt do not have the same ecological distribution. Bulinus truncatus is most abundant in large canals, and decreases in density as the water approaches and flows into drains. Biomphalaria alexandrina is most abundant in drains, and decreases in density upstream from these habitats. Both species are most abundant in the presence of aquatic vegetation, but they differ in their respective associations with the water hyacinth, Eichhornia crassipes. Biomph. alexandrina reaches maximum abundance in the presence of this plant, but Bul. truncatus is as uncommon in the absence of plants as in the presence of E. crassipes.Calculation of life-table parameters from field data shows that, under optimum field conditions, both species can double their populations in 14-16 days. The reproductive rates of both species are greatest in March and the death rates in midsummer. The observed peak densities in May and June give a false impression of optima because of undercollection of young snails, which are most abundant in March and April.Control operations should take advantage of the findings on population parameters. A single area-wide treatment with molluscicide in April is recommended. During the remainder of the year, search for isolated foci of snail breeding and individual treatment of these will effect large savings of chemical and will be effective in controlling the transmission of the parasites.

Large-scale application of Bayluscide and sodium pentachlorophenate in the Egypt-49 project area. Evaluation of relative efficacy and comparative costs.

A study has been undertaken to evaluate the comparative efficacy of two molluscicides-Bayluscide and sodium pentachlorophenate-in controlling the snail population in a complete watershed and to obtain an analysis of the comparative costs of application. Main canals were treated by the drip-feed method, utilizing the water-carriage system, and other watercourses by spraying. Both molluscicides were found to be highly effective, the snail mortality rates exceeding 98%. Treatment involving a first application in May and June and a second one some five months later kept snail densities very low throughout the year.When costs were compared on the basis of unit volume of water treated, for main canals and other watercourses separately, it was found that the chemical costs were very similar; so were the operational costs for the main canals, but in the other watercourses the cost of applying sodium pentachlorophenate was nearly three times that for Bayluscide. For this reason, and because of the toxic properties of the former compound, it is concluded that Bayluscide is the preferred molluscicide in the Egypt-49 project area and probably also in the rest of the Nile Delta area.

Field tests on two new molluscicides (Molucid and WL 8008) in the Egypt-49 project area.

In a continuing search for more effective molluscicides, tests were carried out in the Egypt-49 area on Molucid and WL 8008. Molucid, a 35% w/v solution of isobutyltriphenyl-methylamine (ICI 24223), was applied at 2 ppm for 6 hours; after the treatment, no live Bulinus truncatus, Biomphalaria alexandrina or Lymnaea caillaudi were observed for at least three months and snail densities remained low for at least seven months. Egg-masses were not affected by the treatment, however. WL 8008, N-tritylmorpholine, was applied as a 10% w/v emulsifiable concentrate in carbon tetrachloride (formulation 1), as a 20% w/v emulsifiable concentrate in tetrachlorethylene (formulation 2) or as bait. The two liquid formulations, applied at 0.25 ppm for 6 hours, gave a high initial kill, but snail densities regained their original levels within two months for Bulinus and within four months for Biomphalaria. The bait, applied at 62 lb-150 lb (28 kg-68 kg) per feddan, was ineffective. Like Molucid, WL 8008 is non-ovicidal.