Distribution and habitats of Biomphalaria pfeifferi, snail intermediate host of Schistosoma mansoni, in South Africa KN de Kock*, CT Wolmarans and M Bornman School of Environmental Sciences and Development, Zoology, Potchefstroom University for Christian Higher Education, Private Bag X6001, Potchefstroom 2520, South Africa Abstract As intermediate host for Schistosoma mansoni, Biomphalaria pfeifferi plays a major role in the transmission of intestinal bilharziasis in the endemic areas of South Africa. This article focuses on the geographical distribution and habitats of this species, as reflected by the 1 639 samples on record in the database of the National Freshwater Snail Collection of South Africa. This snail is well represented in the Northern Province, Mpumalanga and the coastal areas of KwaZulu-Natal, that represent the bilharzia endemic areas of South Africa. Details of each habitat, as well as mean altitude and mean annual temperature and rainfall of each locality, were processed to determine chi-square and effect size values and to construct an integrated decision tree that makes a selection of those variables that could maximally discriminate between this snail and all the other species in the database. The results indicated that temperature and type of water-body are the major factors determining the distribution of B. pfeifferi in South Africa. These findings support the results of demographic studies reported by several authors that led them to the conclusion that B. pfeifferi does best under warm stable conditions. The importance of four isolated and persistent foci occurring further west than the western arm of its range of distribution and far removed from the bilharzia endemic areas, is discussed. Two of these localities are popular holiday resorts and the fact that specimens from both these localities showed a high compatibility with a local strain of S. mansoni is cause for concern. Keywords: geographical distribution, habitat preferences, experimental infection, epidemiology of schistosomiasis, Biomphalaria pfeifferi Introduction included in the analysis. The majority of these samples were collected during surveys conducted by staff of government and Biomphalaria pfeifferi is the most important intermediate host in local health authorities and then sent to the former Snail Research tropical Africa for Schistosoma mansoni, the parasite causing Unit at the Potchefstroom University for identification and to be intestinal bilharzia in man (Brown,1994). It was first described added to the NFSC. Details of the habitats were documented by from specimens collected in the Umgeni Valley, KwaZulu-Natal. collectors during surveys by selecting the relevant options on forms The oldest sample of this species on record in the National compiled by the staff of the Snail Research Unit. The number of loci Freshwater Snail Collection (NFSC) of South Africa, dates back to in which the collection sites were located was distributed in 1952. This report focuses on the geographical distribution and intervals of mean annual rainfall and air temperature, as well as habitats of this snail as reflected by the samples on record in the intervals of mean altitude, and the results tabled to illustrate the NFSC. The ecological implications of the range of values reported frequency of occurrence in specific intervals. Rainfall, temperature by various authors for the demographic parameter r (intrinsic rate and altitude data were obtained from the Computing Centre for of natural increase) are also discussed. Attention is drawn to four Water Research, University of Natal. A temperature index was persistent populations of this species occurring in isolated foci calculated for all mollusc species in the database from their outside the bilharzia endemic areas in South Africa. Details are frequencies of occurrence in the selected temperature intervals and given of snail habitats as described by the collectors at the time of the results used to rank them in order of association with low to high collection and also of the mean altitude and mean annual rainfall climatic temperatures. This was done by allocating numeric values, 1 and mean annual air temperature of the loci ( /16 square degree) in ranging from one for the coolest to five for the warmest, to five which the collections were made. selected temperature intervals. The proportion of the total number of loci of each species falling in a particular temperature interval Materials and method was then multiplied by the value allocated to that specific tempe- rature interval. This was done for each temperature interval in Data pertaining to the geographical distribution and habitats of which the species was recorded and the sum of these scores was then taken as the temperature index for that particular species B. pfeifferi were extracted from the database of the NFSC. Only 5 those samples for which the collection sites could be pinpointed on ( ) and the results presented in a table. This analysis was Iip= ∑ i the 1:250 000 topo-cadastral map series of South Africa, were i=1 recommended by Brown (2002). Chi-square values were calculated to determine the significance of the difference between the frequency of occurrence in, on, or at the different * To whom all correspondence should be addressed. +2718 299 2380; fax: +2718 299 2370; options for each variable, such as type of water-body, type of e-mail: [email protected] substratum and temperature interval. Furthermore, an effect size Received 7 February 2003; accepted in revised form 6 November 2003. was calculated for all the different variables discussed in this paper. Available on website http://www.wrc.org.za ISSN 0378-4738 = Water SA Vol. 30 No. 1 January 2004 29 Figure 1 (left) The geographical distribution of 1 Biomphalaria pfeifferi in /16 square degree loci and mean annual air temperature in South Africa. Loci of the four persistent populations: 1 = source of the Kuruman River, 2 = Blue Pool at Buxton, district of Taung, 3 = source of the Harts River at Lichtenburg and 4 = source of the Molopo River. The effect size is an index which measures the degree of discrep- ancy between the frequency distri- bution of a given species in the set of alternatives of a given variable such as water-bodies, as compared to the frequency distribution of all other mollusc species in the data- base in the set of alternatives of the same variable (Cohen, 1977). Ac- cording to this author values for this index in the order of 0.1 and 0.3 indicate small and medium ef- fects respectively, while values of 0.5 and higher indicate practical significantly large effects (Cohen, 1977). A value for this index in the order of 0.5, calculated for the frequency distribution of a given TABLE 1 mollusc species in the different types of water-body, for instance, Types of water-bodies in which Biomphalaria pfeifferi would indicate that this factor played an important role in determin- was found in 1 639 collection sites recorded during ing the geographical distribution of this particular species as surveys reflected by the data in the database. The data in the database were also adapted and processed to Water-bodies A B C D construct an integrated decision tree (Breiman et al., 1984). This is a statistical model that enables the selection and ranking of those Channel 6 0.4% 169 3.6% variables which can maximally discriminate between the frequency Concrete dam 53 3.2% 221 24.0% of occurrence of a given species under specific conditions as Dam 451 27.5% 8400 5.4% compared to all other species in the database. This was accom- Ditch 21 1.3% 636 3.3% plished by making use of the SAS Enterprise Miner for Windows Irrigation furrow 5 0.3% 113 4.4% NT Release 4.0, April 19, 2000 programme and Decision Tree Pan 20 1.2% 306 6.5% Modelling Course Notes (Potts, 1999). Pond 63 3.8% 1566 4.0% Quarry 2 0.1% 122 1.6% Results River 445 27.2% 7507 5.9% Spring 8 0.5% 301 2.7% The 1 639 samples of B. pfeifferi which could be pinpointed on our Stream 212 12.9% 7211 2.9% maps, were collected from 251 different loci (Fig. 1). Swamp 32 2.0% 2076 1.5% This species was present in a wide variety of water-bodies, but Vlei 1 0.1% 103 0.8% the highest percentages were recovered from dams (27.5%) and Pool 3 0.2% 225 1.3% rivers (27.2%). The frequency of occurrence in these two water- bodies differed significantly from streams (X2 = 56.3, df = 1; Effect size w = 0.5 (large effect) p < 0.05; X2 = 76.9, df = 1; p < 0.05, respectively), i.e. streams yielded the third highest percentage of samples (12.93%) (Table 1). A Number of times collected in a specific water-body The largest number of samples (71.5%) was collected in habitats B % of the total number of collections (1 639) on record for with perennial water and this differed significantly (X2 = 130.7, thisspecies df = 1; p < 0.05) from the number of samples from habitats with C Number of times any mollusc was collected in a specific seasonal water (Table 2). The highest percentage of samples was water-body recovered from habitats with standing water, clear water and D % occurrence of this species in the total number of freshwater (Table 2) and the chi-square values showed that they collections in a specific water-body differed significantly, in each case, from their respective alternative options. The highest percentage of samples (28.6%) came from 30 ISSN 0378-4738 = Water SA Vol. 30 No. 1 January 2004 Available on website http://www.wrc.org.za TABLE 2 Water conditions in the habitats of Biomphalaria pfeifferi as described by collectors during