A water-borne outbreak of giardiasis in Sweden.

In the middle of October 1982 a large number of people living at Mjövik, a small community in the south-east of Sweden, fell ill after drinking municipal water contaminated with waste water. The epidemic curve showed one single outbreak without secondary cases. No pathogenic agents could be isolated from the water or sick persons. From some persons who fell ill again or did not recover new samples were taken and Giardia lamblia was then isolated from persons with remaining symptoms and previously affected persons. No parasite was found in persons without gastroenteritis. The outbreak at Mjövik was caused by at least 2 agents, 1 unidentified, with a short incubation time, contracted by 557 persons, and Giardia lamblia, with a longer incubation time, contracted by at least 56 persons.