Immune response to Giardia lamblia in a water-borne outbreak of giardiasis in Sweden.

In one of the largest outbreaks of waterborne giardiasis reported from Europe, more than 3000 persons were exposed to contaminated water and over 1400 cases of giardiasis were diagnosed by microscopy. The outbreak resulted from an overflow of sewage water into the drinking water system of a Swedish ski resort. The period of contamination was about 1 week. Sweden is a non-endemic area for Giardia lamblia infection and, for most individuals affected, this was their first contact with the parasite. Few other enteropathogens were isolated from the patients involved. Therefore, an immune response to Giardia was unlikely to be biased by other concomitant infections. Serum samples from 352 exposed persons were collected and analysed for specific IgG and IgA antibodies to G. lamblia by indirect immunofluorescence and the results were related to the microscopic examination of faeces and the occurrence of diarrhoea. As controls, sera from 428 healthy persons were analysed at the same time by identical methods. IgG or IgA antibodies, or both, were found in 68% of patients whose diagnosis was made by microscopy, and in 22% of exposed by microscopically Giardia-negative persons, but in only 10% of healthy controls. The findings show that patients reported as negative for parasites might be infected. The time between infection and blood sampling influenced the result of the antibody test. The results suggest that stool examination should be the primary means of diagnosis of G. lamblia infection and that serological analysis performed at least 3 weeks after infection could contribute to diagnosis in a non-endemic region, when giardiasis is suspected but the parasite has not been detected.