Variable virulence of isolates of Giardia lamblia in mice.

Groups of inbred mice were inoculated intraoesophageally with Giardia lamblia cysts isolated from the stools of 15 patients with various clinical conditions: symptomatic diarrhoeic, symptomatic non-diarrhoeic and asymptomatic cyst carriers. The virulence of these isolates was studied in terms of two-hour cyst excretion rate and trophozoite counts in the duodenum. The basic pattern of the cyst excretion was similar in all the groups: cysts appeared in the stools by Day 3 +/- 1, reaching a maximum by Day 11 +/- 2. There was then a gradual decline until Day 23 +/- 2, with a low output until Day 31 post-inoculation. However, statistically significantly higher numbers of cysts were excreted by animals which had received cysts isolated from symptomatic diarrhoeic patients than by mice which received cysts from symptomatic non-diarrhoeic or asymptomatic cases. Identical results were obtained with the trophozoite counts. Haemagglutinating (HA) antibodies appeared after Day 11 post-inoculation in each group regardless of different cyst isolates, and the HA titre increased until Day 31 post-inoculation. The results indicate that variable symptomatology in patients could be due to variation in the virulence of the parasite.