Risk factors for giardiasis: a case-control study in Avon and Somerset.

Giardia lamblia is a common and increasing cause of gastrointestinal illness in the UK. We report a case-control study that examined risk factors for giardiasis. Patients with giardiasis were identified from reports to the Consultants in Communicable Disease in Avon and Somerset, and age-sex matched controls were obtained from their general practitioners' lists. Details of travel history, water consumption and recreational water use were collected by postal questionnaire. Over the period July 1992 to May 1993, 74 cases and 108 matched controls were obtained. The data were analysed using conditional logistic regression. Swimming appeared to be an independent risk factor for giardiasis (odds ratio 2.4, 95% CI 1.0 to 6.1, P = 0.050). Travel (P = 0.001), particularly to developing countries, and type of travel (P = 0.004)--that is, camping, caravanning or staying in holiday chalets--were also observed to be significant risk factors. Other recreational water use and drinking potentially contaminated water were found to be not statistically significant after adjustment for other factors.