Estimated risks and benefits from introducing Marisa cornuarietis into the Sudan.

As it has been proposed that Marisa cornuarietis, the ampullarid snail from the tropical Americas be used in the Sudan as a biological control agent against schistosomiasis, an estimate was made of the potential ecological hazards which might result from its widespread distribution in the Nile River Valley, and also of the potential benefits. The only foreseeable hazard would result from colonization of rice fields by the ampullarid snail and subsequent crop damage. However calculations on population dynamics indicated that Marisa cornuarietis would not be able to establish significant populations in rice fields in the Nile Valley as the flooded periods are too short. Thus the expected benefits of schistosomiasis control far outweigh any expected risks, and the ampullarid should be used in expanded field trials in the Gezira Irrigated Area as a control method for schistosomiasis.

Field study of different oxamniquine dose for Schistosoma mansoni in Gezira, Sudan.

Two hundred and ninety-six individuals of whom 126 were children and 170 adults with positive stool for Schistosoma mansoni were randomly given oxamniquine treatment in a dose of either 20, 40 or 60 mg/kg body wt. They were followed in the field for ova excretion at 1, 3 and 6 months for adults and at 1, 3, 6 and 8 months for children. The cure rates in adults at 6 months are 93%, 87% and 73% for the 60, 40, 20 mg/kg-1 treatments in order. For children the cure rates at 6 months are 36%, 58% and 63% and at 8 months are 16%, 21% and 26% for corresponding doses. There is no difference in egg reduction for those not cured between the three treatments at 6 months. It is concluded that oxamniquine need not be given in the previously recommended 60 mg/kg body wt. A choice between 40 and 20 mg/kg body wt is suggested with the 40 mg/kg body wt to be reserved for individuals leaving the endemic area, and the 20 mg/kg body wt for those continuing to stay there with substantial exposure for reinfection. The overall proper cure rates for children with a possible advantage of the low treatment in the long term for this age group is noted.

Survival, growth and reproduction of the imported ampullarid snail Marisa cornuarietis in Central Sudan.

The ampullarid snail Marisa cornuarietis was imported to the Sudan in 1981 for evaluation as a biological control agent against the planorbid snails which transmit human schistosomes. In initial field studies in small protected ponds the generation time of M. cornuarietis was 4 months, as in Puerto Rico. The snails reached 4 cm in diameter after 1 year, compared to 3 cm in Puerto Rico. Their population density varied from 60 to 175 snails per metre of shore-line, compared to a similar pond in Puerto Rico where the stable density was about 115 snails per metre. The proportion surviving after 1 year was 0.03, less than the annual proportion surviving of 0.10 in Puerto Rico. Thus the preliminary results indicated that the ampullarid snails could establish strong populations in permanent habitats in central Sudan where there was adequate food, although it might take longer than it does in Puerto Rico.

Malaria control in the Gezira-Managil Irrigated Scheme of the Sudan.

The development of malaria control in the Gezira-Managil Irrigated Scheme of Central Sudan has gone through several phases. As a result of agricultural and irrigation practices in the Gezira, falciparum malaria transmission became perennial instead of seasonal and the mosquito vector developed resistance to several insecticides. Subsequent failure to maintain control led to serious epidemics after 1971. By 1975 malaria was again put under control through an annual round of house spraying with malathion from 1975 to 1980, and with fenitrothion since 1981. A proposal is outlined for a rational strategy for malaria control in the future.

The impact of the exotic fish Gambusia affinis (Baird and Girard) on some natural predators of immature mosquitoes.

The effects of Gambusia affinis on some suspected predators of mosquitoes were studied for a period of 12 months. A list of such predators was given. The most abundant invertebrate predators were Odonata, followed by Hemiptera and Ephemeroptera and the least common were Coleoptera. The population density of mosquito predators, with exception of Notonectidae and the fish Oreochromis niloticus, was markedly reduced in the presence of G. affinis. Application of aerial pesticides reduced the population of both G. affinis and O. niloticus fish. However it had no effect on the numbers of all invertebrate predators.

The environmental impacts of four insecticides on non-target organisms in the Gezira Irrigation Scheme canals of Sudan.

The toxic effects of the insecticides endosulfan, decamethrin, dimethoate and dursban on the aquatic fauna of some of the Gezira irrigation canals were studied. The animal groups selected were fishes, arthropods and snails. Endosulfan was found to be highly toxic, particularly to Gambusia affinis, under field and laboratory conditions. All animals proved to be less sensitive to decamethrin in the field probably because this insecticide reacted with the organic matter and hence its toxicity was reduced. However in the laboratory it proved to be the most toxic chemical to all organisms probably because of the presence of the solvent in the emulsifiable concentrate formulation. The third insecticide, dimethoate had a very low toxicity to all organisms in the field or in the laboratory. Similarly dursban had a lower toxicity to fish than endosulfan under laboratory conditions, but higher toxicity to arthropods and snails. In the field the highest mortality among fish was obtained when dursban was mixed with endosulfan. For all insecticides, fish were the least tolerant of all animals tested, followed by arthropods and then molluscs.

The food of the larvivorous fish Gambusia affinis (Baird and Girard) and Oreochromis (formerly Tilapia) niloticus (Linnaeus) in Gezira irrigation canals.

The food of both Gambusia affinis and Oreochromis niloticus was studied. Organisms eaten by both species of fish are tabulated, together with the amounts eaten during the various months of the year. G. affinis larger than 25 mm are carnivorous and become more so with age. Food selection by G. affinis depends on the availability of food items rather than choice. It showed a marked preference for mosquito larvae. O. niloticus smaller than 150 mm were markedly carnivorous, but this decreased with age. Only small fish of this species are useful for the biological control of mosquitoes. Fish larger than 150 mm showed a marked preference for higher aquatic macrophytes.