Human intestinal parasites in Subsaharan Africa. I. Eastern Boè and Canhabaque Island (Guinea-Bissau)

A coprological survey was carried out in 1982-1983 in East Boè and in Canhabaque Island, (Bijagòs Islands), where 289 and 288 specimens of stools, respectively, have been collected. The samples were taken from apparently healthy subjects, of both sexes and of different age groups. The tests were made according to the modified Ritchie technique. The following results were obtained: A) East Boè: 1) Protozoa: E. coli 68.9%, E. nana 24.6%, G. intestinalis 8.7%, I. buetschlii 5.9%, E. histolytica 1.7%, E. hartmanni 0.7%, T. intestinalis 0.7%. b) Helminths: Ancylostomatidae 69.2%, T. trichiura 38.4%, S. stercoralis 6.2%, Taenia sp. 1.7%, H. nana 0.7%, S. haematobium 0.7%, S. mansoni 0.7%, S. fuelleborni 0.7%. B) Canhabaque: a) Protozoa: E. coli 85.1%, I. buetschlii 14.9%, E. nana 12.5%, G. intestinalis 8.3%, C. mesnili 7.3%, E. hartmanni 1.4%, E. histolytica 1.0%, T. intestinalis 0.4%. b) Helminths: Ancylostomatidae 87.9%, T. trichiura 9.4%, S. stercoralis 7.9%, S. fuelleborni 2.8%, Trichostrongylus sp. 1.4%, A. lumbricoides 0.7%, H. nana 0.4%. Eggs of Capillaria sp. probably pseudoparasites, were found in 8.7% of samples. The high prevalence of Ancylostomatidae infections appears to be related to the heavy fecal pollution all over the examined territory. The utilization of antihelminthic drug of popular medicine, which has a specific action on A. lumbricoides, could explain the lack and the low prevalence, respectively, of A. lumbricoides infections in both examined areas. In the past, other authors emphasized the low prevalence of this parasite for other Guinea Bissau regions which have a different geomorfological constitution. The cases of S. fuelleborni and H. nana are the first reported for Guinea Bissau.     

Human intestinal parasites in Subsaharan Africa. II. Sao Tomé and Principe

In 1983 the authors carried out a survey in the Democratic Republic of S?o Tomé and Principe, analysing 1050 specimens of stools collected among the population from apparently healthy subjects chosen at random and in a number proportional to the distribution of the population in the regions of the country (about 1% of the population was examined). The examined subjects were divided into 3 age groups (0-3, 4-12, more than 12 years old), to have homogeneous groups in relation principally to modalities of life and nutritional patterns. There were 488 male subjects and 562 females. The survey was preceded by a sensitization of the people to the problem of intestinal parasites and by two preliminary surveys about the number of existing latrines and about people's believes and attitudes in relation to helmintiasis. The tests were made according to the modified Ritchie technique on fecal specimens preserved with 10% formol solution. The following results were found: a) Protozoa: Entamoeba coli, 43.0%; Iodamoeba buetschlii, 9.0%; Giardia intestinalis, 8.8%; Endolimax nana, 7.0%; E. histolytica, 5.5%; E. hartmanni, 2.5%; Chilomastix mesnili, 2.3%; Trichomonas intestinalis, 0.2%; Balantidium coli, 0.1%. b) Helminths: Trichuris trichiura, 87.7%; Ascaris lumbricoides, 64.3%; Ancylostomatidae, 40.5%; Strongyloides stercoralis, 6.8%; Hymenolepis diminuta, 0.3%; H. nana, 0.2%; Schistosoma haematobium, 0.2%. In 28.2% of the specimens (with more than 50% of subjects in some villages) eggs of Heterophyidae were found, very similar to Metagonimus yokogawai, but not yet identified by us, with the following characteristics: elliptical shape, average size 25 mu (22.2-27.7) X 18.5 mu (17-21), thick wall, operculum difficult to see, not sticking out from the outline but visible by focusing being in a different refractiveness, presence of a small polar knob, colour slightly brownish, asymmetric miracidium. Further investigations are necessary to identify the species of this trematode and to understand if it is a true human parasite or a pseudoparasite. The general results show the existence of a heavy fecal pollution all over the territory of the D.R. of S?o Tomé and Principe, connected with unhygienic life conditions, dangerous for health. This suggests to start urgently a sanitation program.     

Malaria, schistosomiasis, and intestinal helminths in relation to microdams in Tigray, northern Ethiopia.

A survey was undertaken in Tigray, Northern Ethiopia, to assess the prevalence of malaria, schistosomiasis, and intestinal helminths in relation to microdams. The survey took place from March to June 1995, during the dry season, at 41 microdams. At each site the village nearest the dam (within thirty minutes walk) was selected, ten households were randomly chosen, and all family members were examined for malaria and intestinal parasites. The overall study sample was 2271 people, of all age groups. Plasmodium falciparum infection was documented in four villages (at 10% of microdams); prevalence was 1.2% (range 0-20% by village). Larvae of Anopheles gambiae s.l. were found at one microdam. Infection with intestinal schistosomiasis was documented in 20 villages (at 49% of microdams), and one third of those infected had moderate to heavy infections. Biomphalaria species, the intermediate host snails of Schistosoma mansoni, were found at 16 microdams (39%), and snails infected by mammalian cercariae were found in one locality. Infections with soil-transmitted nematodes were prevalent: hookworm was detected in more than two thirds of the villages, and Ascaris lumbricoides and Trichuris trichiura were present in almost half of the villages. Out of 2078 stool examinations, the prevalence of S. mansoni infection was 7.2% (range 0-48% by village), of A. lumbricoides 2.3% (range 0-31%), of T. trichiura 2.4% (range 0-21%), and of hookworm 8.9% (range 0-78%). The prevalence of malaria, S. mansoni and hookworm was higher at altitudes below 2000 metres above sea level. S. mansoni was more prevalent in microdams built more than 5 years before the survey, while T. trichiura was more prevalent at recently constructed microdams. The widespread distribution of schistosomiasis and intestinal helminths, and the presence of malaria infection during the dry season confirm that microdams create favourable conditions for the transmission of these parasitic diseases. Health safeguards must be incorporated into the planning, construction, and operation of microdams and irrigation systems in order to prevent or reduce these diseases. In areas with high prevalence, control activities should be intensified.     

Plasmodium/intestinal helminth co-infections among pregnant Nigerian women.

Hospital based studies were conducted to investigate the occurrence of Plasmodium/intestinal helminth co-infections among pregnant Nigerian women, and their effects on birthweights, anaemia and spleen size. From 2,104 near-term pregnant women examined, 816 (38.8%) were found to be infected with malaria parasites. Among the 816 parasitaemic subjects, 394 (48.3%) were also infected with intestinal helminths, 102 (12.5%) having mixed helminth infections. The prevalence of the helminth species found in stool samples of parasitaemic subjects examined was, Ascaris lumbricoides (19.1%), hookworm (14.2%), Trichuris trichiura (7%) Schistosoma mansoni (3.4%), Enterobius vermicularis (2%), Hymenolepis sp. (1.6%) and Taenia sp. (1%). Mothers with Plasmodium infection but without intestinal helminth infection had neonates of higher mean birthweights than those presenting both Plasmodium and intestinal helminth infections and this effect was more pronounced in primigravids. The mean haemoglobin values of malarial mothers with intestinal helminth infections were lower than those with Plasmodium infection but without intestinal helminth infections but these were not statistically significant. Severe splenomegaly was predominant among parasitaemic gravidae who also harboured S. mansoni infection in two of the hospitals studied.     

Geographical distribution of schistosomiasis and soil-transmitted helminths among school children in informal settlements in Kisumu City, Western Kenya

This cross-sectional study determined the prevalence and distribution of schistosome and soil-transmitted helminth (STH) infections among 1,308 children aged 10-18 years in 34 primary schools in 8 informal urban settlements in Kisumu City, western Kenya. Stool samples were collected and examined for eggs of Schistosoma mansoni and STH (Hookworms, Ascaris lumbricoides and Trichuris trichiura) using the Kato-Katz technique. Haematuria was used as a proxy indicator of urinary schistosomiasis. Schools and water bodies were mapped using a geographical information system. Overall, 34% of children were infected with one or more helminth species whereas 16·2% of children were infected with one or more STH species. Schools in closest proximity to Lake Victoria and River Nyamasaria had the highest S. mansoni prevalence while schools with STH were more homogenously distributed. Mean school prevalence of S. mansoni infection was 21% (range=0-69·7%), S. haematobium 3·6% (range=0-12%), hookworms 6·1% (range=0-20%), A. lumbricoides 4·9% (range=0-18·4%), and T. trichiura 7·7% (range=0-18·6%). Helminth-related morbidities were not associated with infection. Our study demonstrates that schistosomiasis and STH are important health priorities among schools in informal settlements of Kisumu City, and highlights the need for routine deworming in similar settings.     

Intestinal helminthiases and schistosomiasis among school children in an urban center and some rural communities in southwest Nigeria

Intestinal helminths and schistosomiasis among school children were investigated in an urban and some rural communities of Ogun State, southwest Nigeria. Fecal samples of 1,059 subjects (524 males, 535 females) aged 3-18 years were examined using direct smear and brine concentration methods between June 2005 and November 2006. The pooled prevalence of infection was 66.2%. Ascaris lumbricoides showed the highest prevalence (53.4%) (P < 0.001) followed by hookworms (17.8%), Trichuris trichiura (10.4%), Taenia sp. (9.6%), Schistosoma mansoni (2.3%), Strongyloides stercoralis (0.7%), Schistosoma haematobium (0.6%), and Enterobius vermicularis (0.3%). The prevalences of A. lumbricoides, hookworms, Taenia sp., S. mansoni, and S. stercoralis in the urban centre were similar (P > 0.05) to those in the rural communities. The fertile and infertile egg ratios of A. lumbricoides in the urban centre and the rural communities were 13: 1 and 3.7: 1, respectively. Each helminth had similar prevalences among both genders (P > 0.05). The prevalence of A. lumbricoides increased significantly with age (P < 0.001). The commonest double infections were Ascaris and hookworms, while the commonest triple infections were Ascaris, hookworms, and Trichuris. The study demonstrates the need for urgent intervention programmes against intestinal helminthiases and schistosomiasis in the study area.     

Prevalence of intestinal parasitic infections in nursery schools from Ciego de Avila Province, Cuba. 1989-1993]

The prevalence of intestinal parasitsm in nursery schools from Ciego de Avila province was annually avaluated during 1989-1993. The percentage of infected children ranged from 16.6% to 31.6%. An increase of frequence of infection was observed from 1991. Protozoa were more predominant than helminths. In the whole five-year period studied Giardia intestinalis had a higher prevalence (19.6%) followed by Entamoeba histolytica (3.0%). The nematode most frequently found was Trichuris trichiura (0.5%).     

Genotyping natural infections of Schistosoma mansoni in Biomphalaria alexandrina from Damietta, Egypt, with comparisons to natural snail infections from Kenya

The distribution of Schistosoma genotypes among individuals in snail populations provides insights regarding the dynamics of transmission and compatibility between schistosome and snail hosts. A survey of Biomphalaria alexandrina from Damietta (Nile Delta, Egypt), an area subjected to persistent schistosomiasis control efforts, provided only 17 snails infected with Schistosoma mansoni (6.1% overall prevalence), each shown by microsatellite analysis to have a single genotype infection. By contrast, recent studies of uncontrolled S. mansoni transmission foci in Kenya revealed that 4.3% Biomphalaria pfeifferi and 20-25% Biomphalaria sudanica snails had multiple genotype infections. Compared with the 3 Kenyan populations, the Egyptian population of S. mansoni also showed a lesser degree of genetic variability and was genetically differentiated from them. We suggest that tracking of genotype diversity in infected snails could be further developed to serve as an additional and valuable independent indicator of efficacy of schistosomiasis control in Egypt and elsewhere.     

Intestinal parasites in Iaualapiti Indians from Xingu Park, Mato Grosso, Brazil.

Brine flotation and gravity sedimentation coproscopical examinations were performed in stool samples from 69 of the 147 Iaualapiti Indians of the Xingu Park, Mato Grosso State, Brazil. Intestinal parasites were present in 89.9% of the population examined. High rates of prevalence were found for some parasite species: Ancylostomidae, 82.6%; Enterobius vermicularis, 26.1%; Ascaris lumbricoides, 20.3%; and Entamoeba coli, 68.1%. Infection by Trichuris trichiura, Schistosoma mansoni, Taenia spp. and Hymenolepis nana was not detected. Helminth's prevalence in children aged one year or less was comparatively low (33.3%). Quantitative coproscopy was done in positive samples for Ascaris and Ancylostomidae and the results expressed in eggs per gram of feces (EPG). Quantitative results revealed that worm burdens are very low and overdispersed in this Indian tribe, a previously unreported fact.     

Geophagy as a therapeutic mediator of endoparasitism in a free-ranging group of rhesus macaques (Macaca mulatta)

Parasite levels were determined for 141 members of a naturally formed social group of rhesus macaques living under free-ranging conditions. Results indicate that group members harbor Trichuris trichiura, Balantidium coli, and large numbers of Strongyloides fuelleborni. Parasite counts decrease significantly with age in this population. Females have significantly fewer parasites than males and a significantly lower prevalence of multiple infections. There were no rank effects regarding parasitosis. While 89% of the animals examined are infected with one or more species of enteric parasite, the prevalence of diarrhea is negligible (2%). The low prevalence of diarrhea in the presence of high parasite loads may be due to the practice of geophagy, which is engaged in by 76% of group members. Soil eaten by these monkeys contains large amounts of kaolinitic clays. Kaolin-based pharmaceuticals (i.e. Kaopectate^®) are commonly used in human populations to treat diarrhea and intestinal upsets. The mechanical and pharmaceutical properties of the natural kaolinite-based clays may counteract the effects of parasitosis in this free-ranging population. Am. J. Primatol. 44:71-82, 1998. © 1998 Wiley-Liss, Inc.     

Multiple infection diagnosis of intestinal helminthiasis in the assessment of health and environmental effect of development projects in Nigeria

Patterns of intestinal helminth infections among school-aged children have been assessed in Eko-ende and Ore, as part of an overall assessment of the public health impact of Erinle Dam Reservoir in Osun State, Nigeria. The investigation was carried out between January and May 2005 using the Kato Katz thick smear technique and simple questionnaire for information on the bio-data, knowledge, attitude and practice of individuals towards disease transmission and control. Ascaris lumbricoides, hookworm, Trichuris trichiura, Taenia spp., Strongyloides stercoralis and Schistosoma mansoni were recovered at an overall prevalence of 78.3% among 309 children examined. Ascaris lumbricoides, hookworm, T. trichiura and S. mansoni were the most common infections while S. stercoralis and Taenia spp. were found only among a few children. Infection patterns of the common diseases were age-specific with peaks in the 11-15 age bracket. Children not attending school were significantly (P < 0.05) more at risk of infection than those attending school. Multiple infections were pronounced with over 54% double infections and 6% four parasites or more infections. The need for urgent intervention to arrest the obviously serious public health situation attributable to Erinle Dam is emphasized.     

Assessment of schistosomiasis, through school surveys, in the Forest Zone of Pernambuco, Brazil

This work had the objective of assessing the present epidemiological situation regarding schistosomiasis through performing Kato-Katz coproscopic tests on representative samples of schoolchildren from each of the 43 municipality of endemic area of the state of Pernambuco, Brazil. The methodology is recommended by the World Health Organization to conduct sampled surveys among children at elementary school levels, ideal target group for baseline surveys: (i) schools are accessible; (ii) the greatest prevalence of schistosomiasis is found within this group; (iii) the data gathered from this age group can be used for intervention within the community as a whole. The following infection indicators were utilized: positivity (percentage of individuals examined with eggs of Schistosoma mansoni in the feces) and severity (geometric mean number of eggs per gram of feces, epg). These indicators allowed the area in general and the municipalities in particular to be categorized into prevalence and severity classes for S. mansoni. The prevalence classes were: low (<10%), medium (> 10 and < 50%), and high (> 50%); the severity classes were: low (1-99 epg), moderate (100-399 epg), and severe (> 400 epg). For the geohelminthic diseases, the following indicators were used: positivity for each geohelminth (percentage of individuals examined with eggs of geohelminths), and cumulative positivity (percentage of individuals examined with eggs of at least one geohelminth). The municipalities were categorized by means of their cumulative positivity into the following geohelminth prevalence classes (WHO 2002): low (< 50%), medium (> 50 and < 70%), and high (> 70%). The study covered 271 schools in 179 different localities, thus giving a total of 11,234 examinations performed. The overall positivity for S. mansoni was 14.4% and the egg count for this parasite in the feces gave a geometric mean of 67.9 epg which suggests a low general state of infection. These results allow this mesoregion to be categorized as presenting medium prevalence and low severity of schistosomiasis. The overall positivity rates for the geohelminths, Ascaris lumbricoides, Ancylostomidae, and Trichuris trichiura were, respectively, 30.4, 10.1, and 27.8%; the cumulative positivity was 45.4%. These results allow this mesoregion to be categorized as presenting low prevalence of geohelminthic diseases. The data show some municipalities in Pernambuco with prevalence greater than 20%, while others presented parasite loads greater than 100 epg. These indicators attest to the significant morbidity due to schistosomiasis regarding to the severity of infections established in young populations.     

The 434(G>C) polymorphism within the coding sequence of Eosinophil Cationic Protein (ECP) correlates with the natural course of Schistosoma mansoni infection

Schistosomiasis is a chronic parasitic infection with over 200 million people infected worldwide. In Schistosoma mansoni infections, parasite-derived eggs get trapped in the liver, causing the formation of granulomas, which may develop into periportal fibrosis and portal hypertension, and thus severe morbidity. Eosinophil cationic protein (ECP) is a secretory protein of eosinophil granulocytes that efficiently kills the larval stage of S. mansoni, but also affects fibroblast functions. We have investigated the prevalence of the ECP gene polymorphism 434(G>C) in two African populations, from an S. mansoni endemic area in Uganda (n=297) and from a non-endemic area in Sudan (n=78), and also compared these with a Swedish population (n=209). The genotype frequencies in the Ugandan population differed significantly from both the Sudanese and Swedish populations (P<0.001). In the Ugandan population there was a significant association between genotype and prevalence of infection (P=0.03), with lower prevalence in subjects with the GG genotype compared with GC (P=0.02) and CC (P=0.03). There was also a trend towards an association with periportal fibrosis (P=0.08) in the Ugandan population. This suggested association was confirmed when the predominant tribe (n=212) was analysed separately (P=0.004). Our results suggest that ECP may be an important protein, both in the immune response against S. mansoni and in the development of periportal fibrosis. The results also suggest genetic selection towards the ECP 434CC genotype in populations living in S. mansoni endemic areas.     

High levels of Schistosoma mansoni infections among schoolchildren in central Sudan one year after treatment with praziquantel

A longitudinal study was conducted to evaluate the impact of praziquantel (PZQ) for the treatment of Schistosoma mansoni infection among schoolchildren in Al Gunaid in Central Sudan. A cohort of schoolchildren (6-15 years of age) was investigated before and 1 year after treatment with a single dose of PZQ 40?mg/kg. Parasitological examinations for S. mansoni were performed before and after treatment, and prevalence and intensity of infection were analysed. Of 2741 schoolchildren recruited from six elementary schools at baseline, 2521 were successfully traced and re-examined at follow-up, with two complete sets of longitudinal parasitological data on S. mansoni. Boys showed significantly higher prevalence of S. mansoni infection than girls. A single dose of PZQ reduced the overall prevalence of S. mansoni infection by 36.7% (from 59.1 to 37.4%) and the intensity of infection by 41.1% (from 116.7 to 68.7 eggs per gram of stool) 1 year after treatment. The reduction in prevalence was significantly higher among the group of children with heavy infections (by 76.1%, from 6.7 to 1.6%) and among girls (by 54.1%, 42.3 to 19.4%) at 1 year after treatment. Thus, in spite of a significant reduction in the prevalence and intensity of S. mansoni infection 1 year after PZQ treatment, the prevalence of the disease was still high and further research is needed on this topic.     

Regional differences in the infection of wild Japanese macaques by gastrointestinal helminth parasites

A coprological survey of gastrointestinal parasites in wild Japanese macaques (Macaca fuscata), from 14 natural habitats was done. Ova of five nematode species (Trichuris trichiura, Strongyloides fuelleborni, Streptopharagus pigmentatus, Oesophagostomum aculeatum, andGongylonema species) and a worm of the cestode species,Bertiella studieri, were detected. Some differences found in rates of infection by sex were observed in the 14 areas. Differences based on age were found in only troops in which the infection rates ofStrongyloides fuelleborni, Streptopharagus pigmentatus, andTrichuris trichiura were higher in the juvenile monkeys. The number of parasite species was related to the geographical and climatic conditions of the habitat of the troop: troops living in areas cold in winter were infected by fewer parasite species than those in southern warmer areas.     

Familial aggregation of human susceptibility to co- and multiple helminth infections in a population from the Poyang Lake region, China

Human helminthiases are common in China, especially in rural areas where sanitation conditions are poor. Co- and multiple infections with helminths are strikingly frequent. A cross-sectional parasitological and questionnaire survey was carried out in a population of 3205 individuals belonging to 498 families from five villages in the Poyang Lake region, Jiangxi Province, China, to assess their helminth infection status and to collect information on risk factors for infection. The prevalences for Ascaris lumbricoides, Schistosoma japonicum and Trichuris trichiura were 30.9%, 15.7% and 47%, respectively. Hookworm infection prevalence was low (0.7%). A significant association was observed between A. lumbricoides and T. trichiura infection, and also between S. japonicum and T. trichiura infection. Variance components analysis was undertaken to investigate the aggregation of S. japonicum and the soil-transmitted helminths, A. lumbricoides and T. trichiura. While A. lumbricoides was found to aggregate only at a household level, T. trichiura was shown to cluster predominantly in families. Both genetic and household effects were found to be important in determining the risk of infection with S. japonicum. Variance components analysis for A. lumbricoides/T. trichiura co-infections indicated a significant domestic environmental effect, attributable for 32.7% of the co-infection risk. Aggregation of S. japonicum/T. trichiura co-infection was also observed at a household level. The risk of infection with multiple helminth species, although mainly environmentally influenced, was also shown to have significant involvement of genetic and household components. The results of this study indicate that a shared household is a major contributing risk factor for helminth co-infections and emphasises the need for increased standards of sanitation and hygiene to prevent parasite transmission. Further, the results suggest that susceptibility to one helminth infection is not completely independent of another, and that there exist common genetic factors underlying infection with multiple helminth species.     

Distribution of schistosome infections in molluscan hosts at different levels of parasite prevalence

Biomphalaria glabrata snails infected with Schistosoma mansoni were collected during consecutive seasons from a site in Brazil known to have a very high percentage of infected snails. Schistosoma mansoni cercariae from single snails were used to infect individual mice, and the recovered adult worms were genetically assessed using a mtVNTR marker. The number of unique parasite genotypes found per snail was compared to expected abundance values, based on the infection prevalence at the site, to determine the distribution of S. mansoni infections within the snail population. The observed distributions and those from previous studies were used to examine the relationship between schistosome prevalence and aggregation across a wide range of prevalence values. Our analysis showed that prevalence was inversely related to the degree of parasite overdispersion, and at high prevalence, S. mansoni infections were randomly distributed among snails.     

Species-specific treatment effects of helminth/HIV-1 co-infection: a systematic review and meta-analysis

In sub-Saharan Africa, over 22 million people are estimated to be co-infected with both helminths and HIV-1. Several studies have suggested that de-worming individuals with HIV-1 may delay HIV-1 disease progression, and that the benefit of de-worming may vary by individual helminth species. We conducted a systematic review and meta-analysis of the published literature to determine the effect of treatment of individual helminth infections on markers of HIV-1 progression (CD4 count and HIV viral load). There was a trend towards an association between treatment for Schistosoma mansoni and a decrease in HIV viral load (Weighted mean difference (WMD)=-0·10; 95% Confidence interval (CI): -0·24, 0·03), although this association was not seen for Ascaris lumbricoides, hookworm or Trichuris trichiura. Treatment of A. lumbricoides, S. mansoni, hookworm or T. trichiura was not associated with a change in CD4 count. While pooled data from randomized trials suggested clinical benefit of de-worming for individual helminth species, these effects decreased when observational data were included in the pooled analysis. While further trials are needed to confirm the role of anthelmintic treatment in HIV-1 co-infected individuals, providing anthelmintics to individuals with HIV-1 may be a safe, inexpensive and practical intervention to slow progression of HIV-1.     

Irrigation and infection: the immunoepidemiology of schistosomiasis in ancient Nubia

Schistosomiasis has been deemed "the most important water-based disease from a global public-health perspective" in modern populations. To better understand the burden of schistosomiasis in ancient populations, we conducted immunologic examinations of desiccated tissue samples from two ancient Nubian populations, Wadi Halfa (N = 46) and Kulubnarti (N = 191). Saqia irrigated agriculture increases the available habitat for the aquatic vector snails and the risk of exposure. On the basis of evidence regarding the impact of saqia irrigation on schistosomiasis prevalence and transmission in modern populations, we predicted that the prevalence of Schistosoma mansoni infection would be higher in Wadi Halfa (saqia irrigation) than Kulubnarti (annual flooding). We also predicted that peak infection prevalence would occur at an earlier age within the Wadi Halfa population than the Kulubnarti population and that in both populations the prevalence of schistosomiasis would be higher in males than females due to differential water contact. The prevalence of S. mansoni was greater in the Wadi Halfa population (26.1%) than at Kulubnarti (9.4%) (P = 0.002). However, peak prevalence of infection did not occur in a younger age category within the Wadi Halfa population; prevalence of infection peaked at 66.7% in the mature adult age group (46+ years) in the Wadi Halfa population and at 16% in the later child age group (6-10 years) in the Kulubnarti population. There were no statistically significant differences in prevalence between males and females of either population. The impact of human alteration of the environment on the transmission of schistosomiasis is clearly shown in these populations.     

Interspecific antagonism and virulence in hosts exposed to two parasite species

Co-infection of host organisms by multiple parasite species has evolutionary consequences for all participants in the symbiosis. In this study, we co-exposed aquatic-snails (Biomphalaria glabrata) to two of their trematode parasites, Schistosoma mansoni and Echinostoma caproni. In co-exposed snails, E. caproni prevalence was 63% compared to only 23% for S. mansoni. Co-exposed E. caproni-infected snails exhibited reduced fecundity, higher mortality, and higher parasite reproduction (higher virulence) compared to hosts exposed to echinostomes alone. Conversely, co-exposed S. mansoni-infected snails released fewer parasites and produced greater numbers of eggs compared to hosts exposed to S. mansoni alone. These results suggest that co-exposure not only influences the establishment (presence or absence) of particular parasite species, but also impacts host life history, parasite reproduction, and the virulence of the interaction.