Epidemiology and control of schistosomiasis mansoni in communities living on the Cuando River floodplain of East Caprivi, Namibia.

The Cuando River area of eastern Caprivi, Namibia, is highly endemic for Schistosoma mansoni whereas S. haematobium transmission, due to the scarcity of its intermediate host snail, Bulinus africanus, does not occur. Chemotherapy (6-monthly blanket treatments with praziquantel) combined with focal mollusciciding (monthly application of niclosamide) was used in a project in the area to control the disease. Although as many adults and pre-school children as possible were tested and treated, the project concentrated largely on school-age children. It took 3 years for prevalence to decline from > 80% to 20% because of a lack of proper sanitary facilities and piped water supplies and high rates of absenteeism and re-infection. However, intensity of infection decreased more rapidly, from an arithmetic mean of > 200 to < 5 eggs/g faeces. Hepatomegaly was common among school children when the project started but could be seen in only a small percentage of them after 3 years of control. Neither the bovine schistosome, S. mattheei, nor the lechwe schistosomes, S. margrebowiei and S. leiperi, were observed in the excreta of humans living in the area.

Laboratory evaluation of B-2 as a molluscicide in the control of the snail intermediate hosts of schistosomiasis in South Africa.

There is a continuous need for the development and evaluation of new, inexpensive but highly effective molluscides for the control of freshwater snails acting as intermediate hosts of schistomiasis. For this reason B-2 (Hokko Chemical Industry Co. Ltd, Japan), also called Phebrol (sodium 2,5-dichloro-4-bromophenol), was evaluated in our laboratory as a candidate molluscicide for the control of freshwater snails in South Africa. Bulinus africanus and Biomphalaria pfeifferi, intermediate hosts of Schistosoma haematobium and Schistosoma mansoni respectively, were exposed to different B-2 concentrations for 24 and 48 hours. An indigenous fish species, Oreochromis mossambicus, which is common in local schistosomiasis endemic areas, was also exposed to the molluscicide. The calculated values obtained from a probit analysis (LC50, 24 hours: B. africanus = 2.6 mg 1(-1) and B. pfeifferi = 2.9 mg 1(-1), indicated that these species from southern Africa are less sensitive to B-2 than are B. truncatus and B. pfeifferi from northern Africa, which in turn are less sensitive than the Oncomelania spp. from Japan, China and the Philippines. It is expected that molluscicidal levels of B-2 would be harmful to O. mossambicus populations. Although B-2 has a marked potential for snail control in South Africa, niclosamide (Bayluscide) remains the molluscicide of choice.

The molluscicidal properties of Apodytes dimidiata subsp. dimidiata (Icacinaceae).

The molluscicidal and piscicidal activity of extracts from the leaves, berries and bark of the tree Apodytes dimidiata were evaluated experimentally. The leaves were highly toxic to both Bulinus africanus and Biomphalaria pfeifferi, the intermediate host snails of Schistosoma spp. B. pfeifferi profusely exuded mucus which appeared to render some protection against the toxic substance(s) of the plant but did not prevent their eventual demise. The LC10 after one-hour exposures of fish (Oreochromis mossambicus) was within molluscicidal levels.

Further studies on the susceptibility of Bulinus africanus to infection with Schistosoma haematobium.

Genetic manipulation of the intermediate host snails of schistosomiasis has been proposed as a possible method of reducing the rate at which the parasite is transmitted to the final host. This technique is based on the finding that snail-schistosome compatibility is variable, and that refractory snails could be introduced into natural habitats in an attempt to change existing highly-susceptible populations into non-susceptible ones. In our search for such a refractory snail population, offspring from eight different Bulinus africanus populations were infected with Schistosoma haematobium, isolated from school children in the Nelspruit district. A great variation in minimum prepatent period was recorded (33-55 days), while the infected snails surviving that period ranged from 22-89%. A significant difference was found between the infection rates of B. africanus from Newlands in Natal and those of the other seven populations from the Eastern Transvaal Lowveld. The former population could be regarded as partially refractory, and none of the other populations proved to be completely refractory, to infection.

Susceptibility of South African Bulinus africanus populations to infection with Schistosoma haematobium.

The use of intermediate host snails of Schistosoma spp. which are insusceptible to infection has been proposed as a possible method of controlling schistosomiasis. The objective of this approach is to change the susceptibility of natural snail populations from being predominantly susceptible to a non-susceptible state, through the release of refractory snails into natural habitats. In an attempt to determine whether or not such refractory Bulinus africanus populations occur in eastern South Africa, F1 generation snails of populations from 8 different areas were exposed to miracidia hatched from eggs excreted in the urine of schoolchildren infected with Schistosoma haematobium in the Nelspruit district. The proportion of snails successfully infected ranged from 27 to 100%, which revealed considerable genetic heterogeneity amongst populations of the same snail species. One population from Natal could be regarded as partially refractory, while a laboratory population from Durban proved to be 100% susceptible. A completely refractory strain of B. africanus has not yet been identified.

Geographical distribution and habitat preferences of the invader freshwater snail species Lymnaea columella (Mollusca: Gastropoda) in South Africa.

The present geographical distribution of Lymnaea columella, as recorded in the National Freshwater Snail Collection, is described and discussed. It appears that L. columella is the most successful colonist of all the freshwater snail species in South Africa, and, together with Bulinus tropicus and Lymnaea natalensis, it forms the most widely distributed freshwater snail species in the region. Data regarding the 2,341 discovery sites of L. columella recorded by collectors during surveys are analysed. L. columella cluster mainly in rivers or streams with perennial, slow flowing, or stagnant freshwater with many plants and a mud substrate. L. columella is regarded as a more effective intermediate host for the transmission of fascioliasis than the indigenous snail species. The economical implications of this are discussed briefly in the light of its success as an invader species.

Bilharzia risk in Lichtenburg, Tvl.

The discovery of Biomphalaria pfeifferi, the intermediate host of Schistosoma mansoni, in the RSA in the recreational area of Lichtenburg prompted an investigation to assess the bilharzia risk posed by their presence. Localized populations of B. pfeifferi were found in the recreational area and in the adjoining game breeding farm. The population of downstream areas by the snails appears to be prevented by the water quality. It is proposed that the large populations of aquatic birds in the game breeding farm be lured to the snail-infested areas as a possible means of eradication. The susceptibility of these snails to S. mansoni was experimentally confirmed.

False-positive reactions in the serological diagnosis of schistosomiasis.

Stool and urine samples from children living in an area in the Western Transvaal in which human schistosomiasis was not endemic were examined for parasites and the indirect fluorescent antibody test was performed on their sera. Since none of these children passed any schistosome ova in their excreta but approximately half of them had a positive serological reaction they must have been infected with either Schistosoma mattheei, which is common in snails and cattle in the area, or avian schistosomes. In view of the occurrence of such 'false-positive' results, general practitioners are advised not to rely too heavily on serological tests in the diagnosis of schistosomiasis.

[The Tugela-Vaal state water scheme as a bilharzia risk]. / Die Tugela-Vaal-Staatswaterskema as 'n bilharziarisiko

The Tugela-Vaal River water scheme constitutes a possible source for the introduction of bilharziasis into an area where the disease is at present not endemic. A survey was made of the snail species in the area encompassed by the scheme. Although Bulinus (Physopsis) africanus (a host of Schistosoma species responsible for urinary bilharziasis) was found in the proposed source area of the water scheme, it is not possible at this stage to determine with certainty whether this host will be introduced over the Drakensberg escarpment into the Orange Free State. However, this is a distinct possibility, and it is discussed in relation to ecological factors.