A Novel Bacterial Pathogen of Biomphalaria glabrata: A Potential Weapon for Schistosomiasis Control?

Abstract

Background

Schistosomiasis is the second-most widespread tropical parasitic disease after malaria. Various research strategies and treatment programs for achieving the objective of eradicating schistosomiasis within a decade have been recommended and supported by the World Health Organization. One of these approaches is based on the control of snail vectors in endemic areas. Previous field studies have shown that competitor or predator introduction can reduce snail numbers, but no systematic investigation has ever been conducted to identify snail microbial pathogens and evaluate their molluscicidal effects.

Methodology/Principal findings

In populations of Biomphalaria glabrata snails experiencing high mortalities, white nodules were visible on snail bodies. Infectious agents were isolated from such nodules. Only one type of bacteria, identified as a new species of Paenibacillus named Candidatus Paenibacillus glabratella, was found, and was shown to be closely related to P. alvei through 16S and Rpob DNA analysis. Histopathological examination showed extensive bacterial infiltration leading to overall tissue disorganization. Exposure of healthy snails to Paenibacillus-infected snails caused massive mortality. Moreover, eggs laid by infected snails were also infected, decreasing hatching but without apparent effects on spawning. Embryonic lethality was correlated with the presence of pathogenic bacteria in eggs.

Conclusions/Significance

This is the first account of a novel Paenibacillus strain, Ca. Paenibacillus glabratella, as a snail microbial pathogen. Since this strain affects both adult and embryonic stages and causes significant mortality, it may hold promise as a biocontrol agent to limit schistosomiasis transmission in the field.

Author Summary

The present paper reports the isolation and the characterization of a new microbial pathogen of the freshwater snail, Biomphalaria glabrata. Genetic analyses revealed that the species has not been previously described and could be classified into the Paenibacillus genus. These bacteria invade most snail tissues and proliferate, causing massive lethality. Moreover, the bacterial infection can be transmitted both vertically and horizontally to other snails, causing their death in 30 days. This discovery is potentially important because B. glabrata, as an intermediate host, plays an important role in transmitting schistosomiasis, the second-most widespread human parasitic disease. The World Health Organization’s objective of schistosomiasis eradication in a decade encourages the development of multiple approaches for countering the disease, one of which is vector population control. This new bacterial strain clearly could be a potential agent for such a strategy.