A scientific expedition using a Continuous Plankton Recorder from the Marine Biological Association has helped investigate the role of tropical lakes as reservoirs of the cholera-causing bacteria, Vibrio cholerae.
Cholera is an extremely virulent disease with an estimated 1.3-4 million cases, and 21,000-143,000 deaths worldwide. Little research has been carried out in cholera hotspots, such as tropical lake ecosystems, due to the sheer size and remoteness of such locations, and the difficulties associated with identifying virulent strains of the bacteria using traditional molecular analysis methods.
Lake Tanganyika, in Central Africa, is the world’s second largest freshwater lake, and a recognised cholera hotspot. Funded by the National Geographic Society (USA) and coordinated by Prof Luigi Vezzulli from the University of Genoa, an expedition team set out to investigate the role tropical lakes play as reservoirs of V. cholerae in this area.
Led by Dr. Pierre-Denis Plisnier (University of Liege), Lance Gregory (Marine Biological Association) and Dr. Ishmael Kimirei (Tanzania Fisheries Research Institute) the expedition team travelled to Tanzania to collect plankton samples from Lake Tanganyika using a Continuous Plankton Recorder (CPR), a high-speed plankton sampler. V. cholerae is associated with small planktonic copepods, known as copepods, which constitute its main reservoir. CPRs are traditionally deployed in marine environments by commercial ships, and several modifications to the expedition’s vessel were required before sampling could commence.
The team encountered many technical and practical challenges during their expedition, with transporting all the required equipment to a highly remote area, a lack of common language on-board and serious storms while carrying out their 322 nautical mile (596km) transect of the lake. Despite all this, the ship’s crew and expedition team performed admirably, achieving a 100% success rate collecting CPR plankton samples.
Lance Gregory, Operations Manager for the CPR Survey said “It was real privilege to co-lead this international expedition to a remote region of our world. Surrounded by so much outstanding natural beauty, the multinational team met and overcame some unique challenges, and with perseverance and team work, we achieved our scientific aims and friendships were made”.
Following the completion of a successful research expedition, the CPR samples were analysed using high-tech molecular analysis methods, known as last generation molecular techniques, for the detection and genetic analysis of V. cholerae. These sophisticated analysis methods revealed V. cholerae was present in most of the samples analysed over a very large area of the lake. However, thorough genetic analysis found the microorganisms identified were not capable of causing large-scale cholera outbreaks. These results are in contrast to results from coastal areas where cholera is endemic. It may be therefore, toxic strains of V. cholerae only establish in confined local settings, e.g. very near to the shore linked with human pollution, coastal upwelling or episodic plankton blooms.
This study, published in the scientific journal ISME Communications, represents to date the largest investigation ever conducted for the research of Vibrio cholerae in large tropical lakes. It provides an important proof of concept on the use of the CPR technology in cholera studies which could be employed in other areas of the world, where the establishment of environmental reservoirs of epidemic V. cholerae strains are suspected to play a role in the origin of cholera outbreaks.
Prof Luigi Vezzulli from University of Genoa, Italy, who led the study, said “I was really enthusiastic about the success of this study considering the considerable organizational and technical difficulties we had to face. This work opens a new perspective on the use of CPR coupled with modern genetic methods to investigate microbial waterborne pathogens and their associated human diseases”.
To read the full research paper, see:Vezzulli, L., Oliveri, C., Borello, A. et al. Aquatic reservoir of Vibrio cholerae in an African Great Lake assessed by large scale plankton sampling and ultrasensitive molecular methods. ISME COMMUN. 1, 20 (2021). https://doi.org/10.1038/s43705-021-00023-1