The Relative Influence of Polyparasitism, Environment, and Host Factors on Schistosome Infection Rita de Cássia Ribeiro Silva, Maurício Lima Barreto, Ana Marlúcia Oliveira Assis, Mônica Leila Portela de Santana, Isabel M. Parraga, Mitermayer Galvão Reis, and Ronald E. Blanton* School of Nutrition, Federal University of Bahia, Salvador, Bahia, Brazil; Institute of Collective Health, Federal University of Bahia, Salvador, Bahia, Brazil; Department of Nutrition, Case Western Reserve University, Cleveland, Ohio, USA; Oswaldo Cruz Foundation, Fiocruz, Salvador, Bahia, Brazil; Center for Global Health and Diseases, Case Western Reserve University, Cleveland, Ohio, USA Abstract. Where prevalence of geohelminths and schistosomes is high, co-infections with multiple parasite species are common. Previous studies have shown that the presence of geohelminths either promotes or is a marker for greater prevalence and intensity of Schistosoma mansoni infections. Some of this apparent synergy may simply represent shared conditions for exposure, such as poor sanitation, and may not suggest a direct biologic interaction. We explored this question in a study of 13,279 school children in Jequié, Bahia, Brazil, with a survey of demographic characteristics and stool examinations. Cross-sectional analysis revealed a statistically significant increase in the prevalence and intensity of S. mansoni infection with increasing numbers of geohelminth species (OR 2.5, 95% CI 1.38–3.64). Less than 20% of the strength of this association was contributed by socioeconomic status or environmental conditions. Thus, polyparasitism itself, as well as intrinsic host factors, appears to produce greater susceptibility to additional helminth infections. INTRODUCTION Among helminth infections, schistosomiasis is considered one of the most important health problems, not only for its high prevalence but also for its potential to develop into se- vere clinical forms precisely at a time of greatest human pro- ductivity. 1 More than 200 million people are infected with schistosomes according to the World Health Organization, and 500–600 million are at risk because of inadequately pro- tected water supplies and precarious sanitary conditions. 2 In most areas, schistosomiasis is not the only chronic para- sitic infection found. It is estimated that 26–68% of the indi- viduals with schistosomiasis are carriers of an additional hel- minthic infection, such as hookworms, Ascaris lumbricoides, or Trichuris trichiura. 3 These parasites can aggravate the mor- bidity due to schistosome infection, especially in young chil- dren. 4 Various authors have demonstrated a synergism between simultaneous infection with geohelminths and the burden of schistosomiasis, 5–8 but whether this effect was the result of shared environments, host factors, intrinsic parasite biology, or a combination of these factors is usually not explored. This study will address the relative contribution of these different factors by evaluating correlations between prevalence and in- tensity of Schistosoma mansoni infection and co-infection with hookworms, A. lumbricoides, and T. trichiura. We fur- ther evaluated the influence of socioeconomic status and en- vironmental conditions. MATERIALS AND METHODS Population and study area. The study was located in the city of Jequié, a district center in the southern region of the state of Bahia, Brazil. The total population of the municipal district was 120,396 in 1991. This city is situated in one of the most prosperous districts in the State, and it constitutes an economic pole of regional attraction for surrounding cities and rural areas. The Contas River flows through the district. This river is highly polluted with raw sewage from homes and businesses present along its banks. It is fed by equally pol- luted streams flowing from the rest of the city. In the course of 2 months, some 13,279 individuals 7–17 years of age were examined for intestinal helminths. This corresponds to 40% of individuals in this age range who were resident in the city. Informed consent. Informed consent was obtained for all study subjects before clinical and parasitologic studies. This study was approved by the Human Investigation Committees of University Hospitals of Cleveland and of the Oswaldo Cruz Foundation, Salvador, Bahia, Brazil. Individuals infected with intestinal helminths were treated with albendazole, and indi- viduals infected with S. mansoni were treated with oxam- niquine at WHO-recommended doses. Stool examination. The collection of stool samples was car- ried out by house-to-house distribution of lidded plastic con- tainers that were collected on the following day. The Kato– Katz quantitative stool exam, 9 carried out by laboratory staff trained by the National Health Foundation (FNS), was used to determine parasite species and intensity of infection. Iden- tification of hookworm eggs was performed 2 hours after pre- paring the slides. Quality control for the stool examinations was carried out by a technician hired independently for this job who re-examined 10% of all slides. Index of socioeconomic and environmental conditions. An indicator of the environmental and household conditions was constructed from demographic information obtained by inter- views with childcare providers. A questionnaire was admin- istered for a sample of 1,766 schoolchildren infected with a low or moderate intensity of at least 1 species of helminth. The sample was obtained in areas of the city with the highest prevalence of S. mansoni infections. Variables describing household conditions and sanitation were dichotomized as favorable (0) or unfavorable (1) and tested for statistically significant associations with the presence or absence of ane- mia in a logistic regression model. Those that remained sig- nificant (P < 0.05) were included in the index (Table 1). Thir- teen variables were found to be significant, and the sum of * Address correspondence to Ronald Blanton, Center for Global Health and Diseases, 2103 Cornell Road, Case Western Reserve Uni- versity, Cleveland, OH. E-mail: reb6@case.edu Am. J. Trop. Med. Hyg., 77(4), 2007, pp. 672–675 Copyright © 2007 by The American Society of Tropical Medicine and Hygiene 672