An investigation with the objective of studying seroprevalence of infection by the bacterium Helicobacter pylori was carried out with 100 children, between 1 and 12 years of age at the João de Barros Barreto University Hospital in Belém, Brazil, and with their respective mothers. Possible risk factors related to infection were analysed and possible associations of infection between mothers and their children, including the CagA strains. Blood and saliva samples were collected from the participants and stool samples from the children. Anti-H. pylori serology was done using the indirect haemoagglutination method and anti-CagA was done by Elisa. The ABH and Lewis phenotypes in blood were determined with the direct haemoagglutination test and in saliva by Elisa dot-blot. Antigens of the bacteria were studied in 79 stool samples from the children by Elisa capture. Personal and family information was obtained using a standard questionnaire. Seroprevalence among children was 50.0% and 86.0% among mothers. Seroprevalence among children increased with age (p < 0.05) and the habit of attending schools or creches (p < 0.05). The diagnostic methods Elisa capture and indirect haemoagglutination showed similar performance in children, with greater discordance observed in the 1 to 4 year age group (p < 0.05). Infected mothers represented a risk factor for infection in their children (p < 0.05), above all mothers with CagA strains (p < 0.05). The fact of coming from municipalities with 100 thousand or more inhabitants (p < 0.05), piped water (p < 0.05), absence of sanitary installations (p < 0.05) and sanitation in homes (p < 0.05) represented a risk for family infection. Transmission of H. pylori was facilitated by precarious hygiene and sanitation conditions, urban conglomerations and by human contact between children and their mothers, through faecal-oral, oral-oral and/or gastro-oral routes.
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