Prepatency of giardiasis.

Recently acquired giardiasis was suspected in 199 patients because of typical symptoms following a trip to a known endemic area. In 130 cases giardiasis was confirmed by stool examination, although most of the samples obtained less than 3 wk after probable infection were negative for Giardia lamblia. Prepatency was documented in 35 patients who, after one or more negative stool examinations, started to excrete Giardia. The median prepatent period was 14 days, and in most cases the prepatent period was less than 3 wk. The median incubation time of giardiasis was 8 days, and in two-thirds of the cases the symptoms had continued for over a week before the parasite became detectable in faeces. A practical conclusion was that, in suspected cases of giardiasis with negative stool findings during the first 3 wk after possible exposure to Giardia, examination of repeated faecal samples is still effective in confirming the diagnosis.