Inmnarional Journalfor Printed in Great Britain

Parasirology

Vol. 20. ic’o. 3,pp.

325-327.

0

ON THE DISTRIBUTION

fxx?o-7519190 !f.3.w + 0.00 Pergamon Press plc Socieryfor Parosirology

1990

OF SCHISTOSOME HOST SNAILS

1990 Australian

INFECTIONS

AMONG

M. E. J. WOOLHOUSE,*$ S. K. CHANDIWANA~ and M. BRADLEY? * Department of Pure and Applied Biology, Imperial College, Prince Consort Road, London SW7 2BB, U.K. tBlair Research Laboratory, P.O. Box 8105, Causeway, Harare, Zimbabwe (Received 21 August 1989; accepted 14 December 1989)

AbStraCt-WooLHOUSEM. E. J., CHANDIWANA S. K. and BRADLEY M. 1990. On the distribution of schistosome infections among host snaifs. International Journa! for Parasitology 20: 325-327. The distribution of Schistosoma haematobium infections in a population of Bulinus globosus snails collected from a field site in Zimbabwe is described. Cercariae of both sexes emerged from a proportion of patent infected snails, as was demonstrated by the detection of paired schistosomes in hamsters each exposed to cercariae from a single snail. These snails must have been infected by two or more miracidia. Anatysis showed that the proportion of mixed-sex infections was higher than expected if infections were distributed at random among snails. An index of overdispersion was calculated. Overdispersion of infections results from heterogeneities in the exposure and/or susceptibility of snails to infection. The implications for the epidemiology and control of schistosomiasis haematobium are discussed. INDEX KEY WORDS: ~ulinus globosus; epidemiology; negative binomial distribution; Schistosoma haematobium.

INTRODUCTION

THE distribution of parasites among hosts is typically overdispersed; that is, more hosts support few or no parasites and more hosts support very many parasites than would be expected if the parasites were distributed at random among hosts. For the schistosomes overdispersion of parasites among human hosts has been widely reported (for examples see Anderson, 1982; Anderson & May, 1985). In contrast, the distribution of infections among the intermediate host snails has received little attention. Nevertheless, this distribution is of epidemiological importance because the number of miracidia developing within the host snail has little or no effect on the number of cercariae subsequently released (Christie & Prentice, 1978; Touassen & Theron, 1989). This means that the relationship between the rate of input of miracidia and the rate of release of cercariae is significantly affected by the d~st~bution of infections among the host snails. Estimates of the degree of overdispersion of infections of intermediate host populations have been obtained by Anderson (1982) for Wuchereriu bancrofti (data from Pichon, Prod’hon & Riviere, 1980) and Onchocercu spp. (data from Cheke, Garms & Kerner, 1982). No such estimates have been made for schistosomes due to the lack of suitable field data. In

$ Present address and address for correspondence: Department of Zoology, Oxford University, South Parks Road, Oxford OX1 3PS, U.K.

overdispersion;

this paper such an estimate is made using data on the proportion of single-sex and mixed-sex infections of Schistosoma haematobium found in Bulinus globosus snails collected in Zimbabwe. MATERIALS AND METHODS The study site was the Nyamakari river in eastern Zimbabwe. The river is used extensively by the local population and S. h~emutobi~z is endemic in the area. B. globosus were collected on four occasions between late October and early December 1987. Snails were collected along a 580 m stretch of river using mesh scoops as described by Woolhouse & Chandiwana (1989). Only snails of at least 5 mm shell length are collected by this method. Snails were individually exposed to artificial light in a 1h light-l h darkI h light sequence between 1100 and 1400 h. Two to three hundred cercariae were collected from each of a subsample of snails with patent infections. One hamster was exposed to each batch ofcercariae. The hamsters were perfused 12 weeks after infection. The resultant infections were classified as single-sex (no paired schistosomes) or mixed-sex (paired schistosomes found). Infections of a subsample (eight) of snails were identified to species using starch gel electrophoresis (see Mahon & Shiff, 1978) for adult flukes and by inspection of egg morphology. RESULTS A total of 255 B. globosus were collected. Of these 28

(I 1.O%) supported patent schistosome infections. There was no significant difference in prevalence among the four samples making up this total (x’ < 1,3 d.f., NS). Adult schistosomes were obtained from 19 325

M. E. J.

326

WOOLHOUSE,S. K. CHANDIWANAand M. BRADLEY

exposed hamsters. (Two exposed hamsters failed to develop an infection and were subsequently ignored.) Fourteen (74%) developed single-sex infections and five (26%) developed mixed-sex infections. All identified infections were S. haematobium. The overall fractions of the snail population in each of the three categories of infection are therefore: no patent infection, single-sex infection, mixed-sex infection,

P0 P+ P *r

= 0.890; = 0.081; = 0.029.

The snail population can also be categorized by the number of mature infections supported, n, where n 2 0. The fraction of snails with n infections is denotedf,. If the sex ratio of mature infections is assumed to be 1: I (discussed below) then the fraction of snails with no infection, single-sex infections and mixed-sex infections is given by: P” = .fo; P,

(la)

=fi+f,/2+f,/4+f,/8+...

P ++ =,f;/2+3f,/4+7fk/8+...

= c”;_,.f;/2”-‘I;

(lb)

=

UC)

l-P,-P,.

Using equations (1) the expected values of PO, P, and P,, can be calculated for any distribution off, values. The important feature of these equations is that some multiple infections (n> 1) are. through chance, either all male or all female. If infections are distributed at random among snails then f” values are given by the Poisson distribution with the characteristic parameter, m, the mean number of infections per snail. m can be estimated from the equation for the zero term of the Poisson distribution: ,fi = emm.

(2)

Substitution into equation (la) with known POgives m = 0.12 and derived estimates for P, = 0.107 and P, _ = 0.003. The expected frequencies of single-sex and mixed-sex infections from 19 exposed hamsters are thus 18.5 and 0.5, respectively. Expected values differ significantly from observed values (x’ (with Yates correction) = 16.9, 1 d.f., P

On the distribution of schistosome infections among host snails.

The distribution of Schistosoma haematobium infections in a population of Bulinus globosus snails collected from a field site in Zimbabwe is described...
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