Identification of chironomid species as natural reservoirs of toxigenic Vibrio cholerae strains with pandemic potential.

Sivan Laviad-Shitrit,Malka Halpern,Ido Izhaki,Leena Thorat,Rotem Sela,Bimalendu B Nath,Yehonatan Sharaby,Husain Poonawala

doi:10.1371/journal.pntd.0008959

Abstract

Vibrio cholerae causes the fatal cholera diarrhea. Chironomids (Diptera; Chironomidae) are abundant in freshwater aquatic habitats and estuaries and are natural reservoirs of V. cholerae. Until now, only the non-O1/O139 serogroups of V. cholerae were identified in chironomids. Here, we explored whether chironomids are natural reservoirs of V. cholerae O1/O139 serogroups, which are associated with cholera endemics and pandemics. All four life stages of chironomids were sampled from two rivers, and a laboratory culture in Pune, India, and from a pond in Israel. In total, we analyzed 223 chironomid samples. The presence of V. cholerae O1/O139 serogroups was verified using molecular tools. Nine chironomid species were identified; of them, Chironomus circumdatus was the most abundant. The presence of V. cholerae serogroup O1 and the cholera toxin genes were detected in samples from all chironomid species. However, serogroup O139 was detected in only two chironomid species. Besides PCR to detect specific genes, a metagenomic analysis that was performed in three selected C. ramosus larvae, identified a list of virulence genes associated with V. cholerae. The findings provide evidence that chironomids are natural reservoirs of toxigenic V. cholerae O1/O139. Chironomid populations and V. cholerae show biannual peak patterns. A similar pattern is found for cholera epidemics in the Bengal Delta region. Thus, we hypothesize that monitoring chironomids in endemic areas of the disease may provide a novel tool for predicting and preventing cholera epidemics. Moreover, serogroup O139 was detected only in two chironomid species that have a restricted distribution in the Indian subcontinent, possibly explaining why the distribution of the O139 serogroup is limited.

Highlights

Vibrio cholerae, is the causative agent of the diarrheal cholera disease which infects millions of people and causes 21,000 to 143,000 deaths, annually [1]
We explored whether chironomids are natural reservoirs of V. cholerae O1/ O139 serogroups, which are associated with cholera endemics and pandemics
There are more than 200 V. cholerae serogroups; only serogroups O1 and O139 have been associated with cholera epidemics and pandemics [2]

Summary

Introduction

Vibrio cholerae, is the causative agent of the diarrheal cholera disease which infects millions of people and causes 21,000 to 143,000 deaths, annually [1]. Human infections occur mainly via the handling or consumption of contaminated food and water. Toxigenic strains pass through the human gastric acid barrier and colonize the epithelial cells of the small intestine. The bacterium produces the cholera toxin which causes acute dehydration that if untreated–in many cases leads to death [2]. Chironomids (Diptera; Chironomidae), known as non-biting midges, are globally distributed and one of the most abundant insects in freshwater habitats. Chironomids undergo a complete metamorphosis comprising the aquatic egg, larval, and pupal stages, while the terrestrial adults emerge into the air. Females lay egg masses on the water’s edge. Each egg mass contains hundreds of eggs surrounded by a gelatinous matrix which is composed of glycoprotein and chitin [4,5,6]

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