POSTERPRESENTATIONS 623 tropical ambient temperatures resulted in profuse algal growth and the reservoir stratified. The concentration of manganese in the hypolimnion was 0.47 mg/L. The concentration of taste and odour producing algae in the epilimnion was 90 000 species per millilitre. The domi- nant species were Anabaena and Oscillatoria (APHA, 1980: Standard Methods for Examination of Water and Wastewater, 15th edition). This resulted in serious treatment problems at the works, with taste and odour passing into the treated water. Use of algicides proved expensive. It was also difficult to regulate the applica- tion (Chipofya, V. H. & Makwakwa, K. B., Blantyre Water Board, unpublished report). With the introduc- tion of compressed air, the reservoir destratified (Chi- pofya, V. H. & Matapa, E. J., 3rd W,4RFSA/WaterNet Symposium). There was a reduction of 20% in algal numbers. The reduced numbers were uniformly dis- tributed at different depths in the reservoir. The con- centration of manganese dropped to a uniform 0.1 mg/L at different depths in the reservoir. There was an overall reduction of 39% in water treatment costs as a result of the improvement in the raw water quality. Treated water from the works did not have any taste and odour. In conclusion, compressed air is re- commended for the control of algae and other chemical pollutants in Mudi reservoir rather than the use of algicides. Compressed air was also more environmen- tally friendly than algicides. Dynamics of the immune response in mixed genotype malarial infections compared with sin- gle infections L. E. Cuthbertson and R. S. Phillips Division of Infection and Immunity, Institute of Biomedical and Life Sciences, Joseph Black Building, University of Glasgow, Glasgow G12 8QQ, UK Malarious individuals are often infected with mixed genotypes of a malaria parasite such as Plasmodium falciparum. It is unclear how the immune response to one genotype population in a mixed infection affects the response to others present or introduced later. Our study uses an animal model to investigate how the immune response to individual different strains of P. chabaudi compare when present as mixed or as single infections. Protective immunity to P. chabaudi AS in- fection in resistant mice has been shown to be initially mediated by CD4 ÷ T helper type 1 (Thl) cells fol- lowed by a switch to CD4 + Th2 cells later in infection (Langhorne, J., 1989: Parasitology Today, 5, 362-364). In susceptible mice infected with AS parasites, there is a more rapid switch to CD4 ÷ Th2 mechanisms (Ste- venson, M. M. et al., 1993: Clinical and Experimental Immunology, 92, 77-84). We are examining the nature of the immune response to different strains of P. chabaudi which vary in their virulence in NIH mice, either as single or as mixed infections. Semi-quantitative reverse transcriptase- polymerase chain reaction experiments are being used to measure mRNA levels of both Thl-type (interferon- ~, tumour necrosis factor-a and interleukin [IL]-2) and Th2-type (IL-4 and IL-10) cytokines from spleens of mice infected either singly with AS, ER, CB lines or in a mixed infection of AS and ER lines. Evolutionary relationships between the Plasmo- dium falciparum rif gene family and related multigene families in five species of malaria C. S. Janssen, R. S. Phillips and C. M. R. Turner Division of Infection and Immunity, Institute of Biomedical and Life Sciences, Joseph Black Building, University of Glasgow, Glasgow G12 8QQ, UK Several large multigene families have been described in various malaria species, including rif (Cheng, Q. et al., 1998: Molecular and Biochemical Parasitology, 97, 161-176) in Plasmodium falciparum, vir (Del Portillo, H. A. et al., 2001: Nature, 410, 839-842) in P. vivax, and cir (Janssen, C. S. et al., 2002: Proceedings of the Royal Society of London, Series B, 269, 431-436) in rodent malarias. Here we describe observations on the evolutionary relationships among these gene families. Our data indicates that all these gene families share common ancestry. The conservation of key sequence motifs and of variant domain distribution within the protein families is indicative that the molecules may perform the same function in all species. This is further supported by the conservation of predicted secondary structure among the proteins. Variable and conserved regions are found in the same topographical positions in all cases, and the hypervariable region is positionally conserved. The sequence variation pattern is also topo- graphically conserved with regard to predicted second- ary structure. These relationships in the structures, possible functions, and evolution of these families have important implications for comparative immunological studies. Schistosome seropositivity in Scotland, 1998-2002 D. Lawrie, G. Spence and H. V. Smith Scottish Parasite Diagnostic Laboratory, Stobhill Hospital, Spring- burn, Glasgow G2I 3UW, UK At the Scottish Parasite Diagnostic Laboratory, schistosome serology accounts for 41-51% of all serol- ogy requests from 1998 onwards (/> 500 annually, with 25-31% seropositivity). Schistosoma mansoni egg anti- gen (SEA) is used in an immunoglobulin (Ig) G isotype enzyme-linked immunosorbent assay (ELISA). A history of foreign travel was available for 58-84% of positive cases: most (> 95%) identified travel to Africa, with travel on other continents < 10%. Infec- tions acquired in Saharan Africa accounted for < 1% of cases, while > 70% of cases reported visiting sub- Saharan countries. Many requests are from visitors to Malawi, where schistosomiasis is endemic, with be- tween 36 and 44 cases (33-38% seropositivity) identi- fied annually. Analysis of post-chemotherapy specimens (19-32 annually) showed the majority (58-78% of cases) had similar IgG antibody titres to SEA to when initially diagnosed. Where differences were noted between ini- tial and repeat samples the trend was towards falling IgG titres, although in some cases rising titres were noted. IgG antibody titres decline slowly in this low- exposure, praziquantel-treated group and their useful- ness in determining successful treatment outcomes must be assessed. Other isotype/subclass assays, possi- bly coupled with a different choice of antigen(s), may offer better outcomes for determining the effectiveness of drug treatment. Incidence of Cryptosporidium species in paedia- tric patients in Malawi T. D. Morse 1, R. A. B. Nichols 2, A. M. Grimason ~, K. C. Tembo 3 and H. V. Smith 2 1Divisionof Envir- onmental Health, Department of Civil Engineering, Uni- versity of Strathclyde, Glasgow G40NG, UK; 2Scottish Parasite Diagnostic Laboratory, Stobhill Hospital, Spring- burn, Glasgow G21 3UW, UK; 3Department of Environ- mental Health, Polytechnic, Universi,y of Malawi, Private Bag 303, Blan~yre, Chichiri, Malawi An urban and rural Malawian hospital-based study was undertaken to determine the incidence of cryptos- poridiosis paediatric patients presenting with diarrhoea. Stool samples were collected over a period of 22