Background

Amoebiasis caused by infection with E. histolytica occurs almost all over the world and is highly endemic especially in the developing countries. It is one of the major causes of dysentery/diarrhoea in Kolkata, India. According to our previous study (unpublished), detection of Entamoeba histolytica showed a marked seasonality, i.e. high peak during post-monsoon and post-winter seasons. According to the reports from other tropical countries, especially in Bangladesh, which is geographically closest to Kolkata, there is a typical pattern of detection of E. histolytic w here E. histolytica usually shows its highest peaks in the wet season and gradually decreases with the arrival of dry season [1, 2]. In a study in Bangladesh, it was shown that wet environment is not the only factor that affects the detection curve of E. histolytica, but anti-Carbohydrate Recognition Domain IgA level in the gut is another determining factor for its occurrence in a closed population [3]. However, even in that case E. histolytica detection followed a particular seasonality and trend. However in Kolkata the present scenario is different from other tropical regions.

Study design and results

For the last few years, we have been engaged with the surveillance study to determine the detection rate of common enteric parasites in Kolkata. During the last two years, about 2500 diarrhoeal stool samples were screened from hospitalized patients through a systematic sampling procedure as described previously[1] in collaboration with Infectious Diseases Hospital, Kolkata. In this surveillance, every fifth patient attending the Infectious diseases hospital from different parts of Kolkata with diarrhoea in randomly selected two days a week were included in the study. Fecal samples from these patients were collected and sent to the laboratory within half an hour of collection for detection of E. histolytica by microscopic examination using iodine wet mount method; PCR and antigen capture ELISA. The whole study was carried out from the month of November 2007 to October 2009 to include at least two seasonal cycles for better understanding of the seasonality. Although one of the major diarrhoea causing pathogen in Kolkata is Vibrio cholerae[4] among all etiologies, E. histolytica (3.6%) is one of the major diarrheagenic parasites among the parasitic etiologies of diarrhoea. It caused some sporadic diarrhea throughout the study period without any particular seasonality which is uncommon for a tropical area like Kolkata. Overall a decreasing trend of E. histolytica infection was also seen (Figure 1). Reasons behind this different nature of infection might be due to physical and environmental factors which could influence E. histolytica detection rate, such as choice of common antimicrobial drug, temperature, detection procedure of E. histolytica etc. But none of these changed strikingly in last two years that could affect E. histolytica detection rate. Indiscriminate use of anti-parasitic drugs like metronidazole and tinidazole could be attributed to the decreasing trend of detection of E. histolytica in Kolkata.

Figure 1
figure 1

Month wise distribution of samples collected throughout the study period along with percentage of E. histolytica positive among them.

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Conclusion

The present study demonstrates that there is no particular seasonality of occurrence of E. histolytica infection which is not typical of a tropical area like Kolkata. A different pattern which is almost unique in comparison to other tropical countries has been observed. Contribution of environmental and host factors as well as parasite genotypes are very important for the outcome of infection [2, 5]. Although no physical or environmental factor behind this changing pattern of E. histolytica infestation in Kolkata has yet been reported, but it is certain that E. histolytica is showing a slow but obvious change in its seasonality and this might be a signal for a transition period of changing nature of infestation by this parasite in this part of the world. It is worth mentioning that impact of climate change might lead to such changes, although no such studies are available.