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      Influence of Exposure History on the Immunology and Development of Resistance to Human Schistosomiasis Mansoni

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          Previous studies suggest that humans can acquire immunity to reinfection with schistosomes, most probably due to immunologic mechanisms acquired after exposure to dying schistosome worms.

          Methodology/Principal Findings

          We followed longitudinally two cohorts of adult males occupationally exposed to Schistosoma mansoni by washing cars (120 men) or harvesting sand (53 men) in Lake Victoria. Men were treated with praziquantel each time S. mansoni infection was detected. In car washers, a significant increase in resistance to reinfection, as measured by the number of cars washed between cure and reinfection, was observed after the car washers had experienced, on average, seven cures. In the car washers who developed resistance, the level of schistosome-specific IgE increased between baseline and the time at which development of resistance was first evidenced. In the sand harvesters, a significant increase in resistance, as measured by the number of days worked in the lake between cure and reinfection, was observed after only two cures. History of exposure to S. mansoni differed between the two cohorts, with the majority of sand harvesters being lifelong residents of a village endemic for S. mansoni and the majority of car washers having little exposure to the lake before they began washing cars. Immune responses at study entry were indicative of more recent infections in car washers and more chronic infections in sand harvesters.


          Resistance to reinfection with S. mansoni can be acquired or augmented by adults after multiple rounds of reinfection and cure, but the rate at which resistance is acquired by this means depends on immunologic status and history of exposure to S. mansoni infection.

          Author Summary

          Schistosomiasis is a parasitic blood fluke infection of 200 million people worldwide. We have shown that humans can acquire immunity to reinfection after repeated exposures and cures with the drug praziquantel. The increase in resistance to reinfection was associated with an increase in schistosome-specific IgE. The ability to develop resistance and the rate at which resistance was acquired varied greatly in two cohorts of men within close geographic proximity and with similar occupational exposures to schistosomes. These differences are likely attributable to differences in history of exposure to Schistosoma mansoni infection and immunologic status at baseline, with those acquiring immunity faster having lifelong S. mansoni exposure and immunologic evidence of chronic S. mansoni infection. As many conflicting results have been reported in the literature regarding immunologic parameters associated with the development of resistance to schistosome infection, exposure history and prior immune status should be considered in the design of future immuno-epidemiologic studies.

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          Most cited references 30

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          Human IgE, IgG4 and resistance to reinfection with Schistosoma haematobium.

          A well recognized feature of the immune response to parasitic helminth infections, including schistosomiasis, is the production of large amounts of specific and nonspecific IgE1,2. Immunological pathways involving IgE can lead to damage to the developing schistosomulum and it has been suggested that responses involving IgE could have evolved as protection against helminth infections. There has been no epidemiological evidence to support this idea and the only significant IgE responses known in man are those involved in the pathogenesis of allergic disease. Here we measure serological response during reinfection with S. haematobium and demonstrate that IgE antibodies in man can be beneficial. Our results support the hypothesis that the slow build-up of IgE to high levels and the early production of IgG4 antibodies, which may block IgE pathways are responsible for delaying the development of protective immunity to S. haematobium.
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            Immunity after treatment of human schistosomiasis: association between IgE antibodies to adult worm antigens and resistance to reinfection.

            Previous studies in school children have demonstrated the slow development with age of resistance to reinfection after chemotherapy of Schistosoma mansoni infections, and have indicated that inappropriate ("blocking") antibody responses prevent the expression of immunity in young children. The present study was designed to investigate further the nature of the protective responses, by serological studies on a group of 151 S. mansoni-infected individuals resident in an endemic area in Machakos District, Kenya. Antibody levels against various antigens in blood samples before treatment were related to intensity of previous infections; antibodies in blood samples taken 6 months after treatment were related to cumulative reinfection rates over the following 30 months. IgE against an adult-worm antigen preparation correlated positively with age and negatively with reinfection. In contrast, IgE antibodies against other life-cycle stages showed either no relationship or the reverse correlation. Furthermore, antibodies of other isotypes against adult-worm antigens showed no correlations with reinfection. The correlation with IgE could be demonstrated for different preparations of adult worms, including a periodate-treated preparation presumptively depleted of carbohydrate epitopes. For both the intact and the periodate-treated preparations, multiple regression analysis of the results for children less than or equal to 16 years old demonstrated an IgE effect after allowing for age, although this effect was not observed in a previously studied group of school children. Western blot analysis of the adult-worm preparation revealed a limited set of antigens recognized by IgE, among which an antigen of 22 kDa was prominent. The qualitative presence of IgE against this antigen could also be shown to be related to a lack of subsequent reinfection.
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              Schistosoma mansoni in infants (aged < 3 years) along the Ugandan shoreline of Lake Victoria.

              In two complementary epidemiological surveys of villages on the Ugandan shoreline of Lake Victoria, the putative occurrence of intestinal schistosomiasis in the local infants (children aged < 3 years) was investigated. When, during the first survey, 136 mother-and-infant pairs from a total of 12 villages were studied, only 7% of the infants but 45% of the mothers were found to be egg-patent for Schistosoma mansoni infection. The use of dipstick tests for urine-circulating cathodic antigen indicated, however, a much higher prevalence, of approximately 40%, among the infants. In the second survey, urine samples and multiple, not single, stool samples were collected from another 19 mother-and-infant pairs in two of the 12 study villages (Bugoto and Bwondha), and a standardized questionnaire was implemented. The prevalence of egg-patent infection was then found to be markedly higher in the study infants from Bugoto (86%) than in those from Bwondha (25%). A greater level of mother-and-infant water contact, a higher abundance of (infected) Biomphalaria choanomphala, and an unusual lakeshore topology may explain why S. mansoni infection was so much more common in the Bugoto subjects than in the Bwondha. All but one of the infants studied in the second survey were found to be anaemic (with <110 g haemoglobin/litre). Taken together, these children were less likely to be found infected with hookworm (16%), Hymenolepis nana (11%) or Trichuris trichiura (5%) than with S. mansoni (47%). Infection with the parasites causing intestinal schistosomiasis can be common among the infants living in these lakeshore villages. Although the immediate and later-life clinical impacts of such infection have yet to be elucidated, such infants would probably benefit from regular de-worming. Mothers should be strongly encouraged to visit the nearest health-services clinic, with their infants, for any necessary anthelmintic treatment.

                Author and article information

                Role: Editor
                PLoS Negl Trop Dis
                PLoS Neglected Tropical Diseases
                Public Library of Science (San Francisco, USA )
                March 2010
                23 March 2010
                : 4
                : 3
                [1 ]Center for Tropical and Emerging Global Diseases and Department of Microbiology, University of Georgia, Athens, Georgia, United States of America
                [2 ]Centre for Global Health Research, Kenya Medical Research Institute, Kisumu, Kenya
                [3 ]Department of Pathology, University of Cambridge, Cambridge, United Kingdom
                [4 ]Division of Parasitic Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America
                George Washington University, United States of America
                Author notes

                Conceived and designed the experiments: DMSK WES DGC. Performed the experiments: PNMM EMOM BA. Analyzed the data: CLB. Contributed reagents/materials/analysis tools: CMF DWD WES. Wrote the paper: CLB. Organized and implemented longitudinal field work: PNMM EMOM BA DMSK.

                This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.
                Page count
                Pages: 11
                Research Article
                Immunology/Immunity to Infections
                Infectious Diseases/Helminth Infections

                Infectious disease & Microbiology


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