Long-term efficacy of single-dose mass treatment with diethylcarbamazine citrate against diurnally subperiodic Wuchereria bancrofti: eight years' experience in Samoa.

A single-dose of diethylcarbamazine citrate (DEC, 6 mg per kg body weight) was administered in three mass treatment campaigns to > 80% of the estimated total Samoan population (160,000) in 1982, 1983, and 1986. The effect of the drug was evaluated before and after each campaign by conducting four blood surveys covering 9600 to 13,700 people from 26-34 villages on each occasion. The drug reduced the prevalence of Wuchereria bancrofti microfilariae from 5.6% to 2.5% (a 55% reduction), while the transmission potential (the estimated mosquito infection rate if everyone is equally exposed to mosquito bites) dropped from 2.18 to 0.67 (a 70% reduction). The total number of microfilariae in the Samoan population is estimated to have been reduced by 80%. A spaced, single-dose treatment with DEC at a 1-2-year interval therefore seems to be an effective control measure against diurnally subperiodic W. bancrofti.

Vector control of snail-mediated human trematode infections.

Inspite of the recent general acceptance of the use of safe and effective chemotherapeutic drugs in the treatment of major human trematode infections, vector control e.g. control of the snail intermediate host will continue to play an important role in programmes for controlling schistosomiasis, clonorchiasis, opisthorchiasis and paragonimiasis. The main vector control measures are: a) use of chemical pesticides or molluscicides and plants showing molluscicidal properties; b) biological control through introduction of competitor species or natural enemies of snails; c) alteration of the environment either by application of one or a combination of the following; drainage, filling, ponding, clearing of vegetation and regulation of use of water for agricultural purposes.

A schistosomiasis pilot control project in Lindu valley, Central Sulawesi, Indonesia.

This pilot control project was an intervention study, consisting of: 1) treating the positive cases with Niridazole; 2) spraying the foci with Niclosamide; 3) improving the water supply system and construction of public latrines. The intervention not only has lowered the human prevalence rate, but has also lowered the transmission of the disease in that area. Niridazole appeared relatively safe and effective, the cure rate after one year was 80%. Spraying the foci with Niclosamide 20-40 mg per litre did not appear very effective. Using this epidemiological data it was estimated that infected persons would become spontaneously negative after 4.75 years, if there was no reinfection.

Control of Schistosoma japonicum infection in Lindu Valley, Central Sulawesi, Indonesia.

In Indonesia, schistosomiasis is presently confined to two endemic areas in Central Sulawesi; Lindu (1,500) and Napu (5,000 inhabitants) valleys. Because of various development schemes now underway, it is feared that the natural balance which existed for many years between the parasite, the snail intermediate hosts and man, will be upset and will place some 70,000 people of the adjoining Palu and Palolo areas at risk. Measures are being taken to control and prevent the spread of the disease. A pilot control programme is planned for two years in the village of Anca in Lindu with a population of 391 and a prevalence rate of 70.5%. Langko village with a population of 464 and a prevalence rate of 53.7% will be utilized as the comparison area. Baseline data on human, malacological and animal reservoir hosts are being collected and control operations are anticipated to commence in July 1975. Measures to be applied to control schistosomiasis in the order of priorities are: (a) agro-engineering or modification of the environment; (b) environmental sanitation through the provision of adequate water supply and construction of latrines; (c) mollusciciding of remaining snail infested areas; (d) chemotherapy, and (e) health education campaign. At the end of the two-year control programme, the following activities will be carried out to determine the degree of intervention achieved; (a) human data: calculation of prevalence rates, incidence rates, and changes in the intensity of infection based on quantitative egg counts; (b) malacological data: changes in the number and extent of snail foci; snail density, and snail infection rates; (c) reservoir hosts: changes in the prevalence rates among domestic and wild animals. Calculation of reclaimed areas and increase in agricultural production will also be assessed together with the overall cost/benefit analysis of the project undertaking.

Two new field techniques for detection and counting of Schistosoma haematobium eggs in urine samples, with an evaluation of both methods.

Two new techniques for the quantitative determination of Schistosoma haematobium egg densities in urine samples, the first using sedimentation and the second filtration, have been developed and evaluated. In the first method the concentrated sediment is examined in a counting chamber under a low-power microscope and in the second method the final concentration achieved by filtration is also examined microscopically. Both procedures are easily carried out under primitive field conditions, while retaining their reproducibility and reliability. The techniques can be useful in mass surveys of populations suffering from schistosomiasis.

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.