Longitudinal study of intestinal Entamoeba histolytica infections in asymptomatic adult carriers

Abstract

To gain insight into the dynamics of intestinal Entamoeba histolytica infection, a longitudinal study was performed over an observation period of 15 months with a group of 383 randomly selected adult individuals (mean age, 38.5 years) living in an area of amebiasis endemicity in central Vietnam. Ameba infection was diagnosed by using species-specific PCR and DNA extracted directly from fecal samples. The results indicated an E. histolytica prevalence of 11.2% and an annual new infection rate of 4.1% in the study population. Follow-up of the 43 individuals who were E. histolytica positive at enrollment suggested a regular exponential decline in infection of about 3% per month and a mean half-life of infection of more than 15 months. However, the reinfection rate for this group of participants was 2.7 times higher than that predicted for the study population as a whole. Both the reappearance of the parasite after successful treatment of E. histolytica infection and changes in "genetic fingerprints" of parasites during the course of infection revealed an annual new infection rate of about 11.5%. Thus, the mean half-life of E. histolytica infection was calculated to be 12.9 months (95% confidence interval, 10.2 to 15.6 months). Notably, none of the participants developed symptoms compatible with invasive intestinal amebiasis, and only one of the subjects developed an amebic liver abscess during the observation period.

FIG. 1.

Time course of intestinal E. histolytica infection in asymptomatic carriers and half-life of infection. A total of 46 E. histolytica carriers (100%) were identified by fecal PCR in April 2000 (time point, 0 months) and were monitored over a 15-month observation period. Curve a represents the relative number of individuals infected with the parasite at various time points according to the PCR results obtained during follow-up in August 2000 (4 months), April 2001 (12 months), and July 2001 (15 months). Curve b is a modification of curve a, which has been corrected for reinfection; in addition, the 95% confidence intervals have been included. The annual reinfection rate was determined by two independent approaches: (i) analysis of changes in DNA fingerprints of E. histolytica infections during follow-up and (ii) analysis of new infections in previous E. histolytica carriers 13 months after successful antiamebic therapy. Time points at which 50% of individuals have lost their infections are indicated by dotted lines.

FIG. 2.

DNA fingerprints of a representative number of E. histolytica organisms during follow-up. Gel photographs of PCR-amplified fecal DNA for locus NK and locus 5-6 of E. histolytica from 10 individuals infected with the parasite in April 2000 and April 2001 are shown. Note that the band patterns for different individuals are highly divergent. However, only for one individual (493-4) did the parasite DNA pattern change between April 2000 and April 2001.