A three-year surveillance study from March 2017 to February 2020 in Kolkata has found an amoeba pathogen that previously did not cause any amoebiasis (a form of diarrhoea) in humans has now become pathogenic. Surprisingly, a team of researchers from the Kolkata-based National Institute of Cholera and Enteric Diseases (ICMR-NICED) found that not only had the amoeba pathogen — Entamoeba moshkovskii — turned pathogenic, it was the leading cause of amoebic infections in humans; more than half of the amoebic infections were caused by this pathogen. The researchers studied the stool samples of diarrheal patients admitted in two hospitals in Kolkata.
Of particular concern is the fact that infections caused by E. histolytica, which used to be the predominant amoeba pathogen that caused amoebiasis, were decreasing and the newly pathogenic E. moshkovskii was taking its place. The researchers identified a few mutations that signify an essential role of the new pathogenic parasite in adapting to the gut environment of humans or in acquiring other enteric pathogens.
The results of the study were published recently in the journal PLOS Neglected Tropical Diseases.
Diarrhoea can be caused by bacteria, viruses and amoeba pathogens. In the study, the team of researchers led by Dr. Sandipan Ganguly of NICED found nearly 5% of patients with diarrhoea were caused by different Entamoeba species and over 3% of patients were infected with E. moshkovskii. While there were no statistically significant differences between infections in males and females, E. moshkovskii infections were most predominant in children aged 5-12 years.
While infections caused by E. histolytica usually peaked during the wet season and gradually decreased with the arrival of the dry season, the seasonal pattern of E. moshkovskii infection in Kolkata was quite unique — there were two infection peaks coinciding with summer and post-fall season. During the over two-decade-long active surveillance study for the detection of common enteric parasites in and around Kolkata, the researchers found infections coming up during non-seasonal periods for E. histolytica. They also observed a significant proportion of cysts/trophozoites of amoeba in stool samples that have a similar morphological feature to E. histolytica throughout the year. When they carried out PCR-based molecular identification to identify the similar looking amoeba trophozoites, they found that the morphologically indistinguishable amoeba from E. histolytica was indeed the related species E. moshkovskii.
According to the authors, another notable feature was that infection with the new pathogenic amoeba alone was statistically associated with diarrhoeal occurrence. “The diarrheal incidents associated with E. moshkovskii were not commonly coinfected in Kolkata. These results indicate that E. moshkovskii may not simply be a commensal of the human gut; instead, it acts as a “potential” pathogen causing diarrhoea and other gastrointestinal disorders in the study area,” they write.
In most cases, amoebiasis is routinely diagnosed by light microscopy. However, light microscopy has limited sensitivity and specificity, and as a result, it becomes difficult to differentiate between the cyst and trophozoites of the pathogenic E. histolytica and E. moshkovskii. While trophozoites of E. histolytica are generally found in large numbers in stool samples, that is not the case with E. moshkovskii. So in order to identify the similar looking trophozoites and identify the pathogenic amoeba that was causing diarrhoea during off-seasons, the researchers turned to PCR-based molecular identification. This led to the identification of E. moshkovskii in over 50% of diarrhoea cases caused by amoebic parasites.
The team has so far not carried out drug-susceptibility tests for E. moshkovskii.
