Date Published: August 22, 2019
Publisher: Public Library of Science
Author(s): Sivan Laviad-Shitrit, Ido Izhaki, Malka Halpern, James B. Bliska.
Vibrio cholerae is the causative agent of cholera, a life-threatening diarrheal disease. Cholera causes epidemics and pandemics, but the ways this disease spreads worldwide is still unclear. This review highlights a relatively new hypothesis regarding the way V. cholerae can be globally dispersed. Copepods and chironomids are natural reservoirs of V. cholerae and are part of different fish species’ diet. Furthermore, V. cholerae inhabits marine and freshwater fish species. Waterbird species feed on fish or on small invertebrates such as copepods and chironomids. Waterbirds have also been found to carry living copepods and/or chironomids internally or externally from one waterbody to another. All of the above points to the fact that some waterbird species might be vectors of V. cholerae. Indeed, we and others have found evidence for the presence of V. cholerae non-O1 as well as O1 in waterbird cloacal swabs, feces, and intestine samples. Moreover, hand-reared cormorants that were fed on tilapia, a fish that naturally carries V. cholerae, became infected with this bacterial species, demonstrating that V. cholerae can be transferred to cormorants from their fish prey. Great cormorants as well as other waterbird species can cover distances of up to 1,000 km/day and thus may potentially transfer V. cholerae in a short time across and between continents. We hope this review will inspire further studies regarding the understanding of the waterbirds’ role in the global dissemination of V. cholerae.
Birds are ubiquitous and globally distributed. There are 10,000 known bird species, which account for over 15% of all vertebrates . Waterbirds are birds that live on or around fresh water or marine water. Some waterbirds dive from the surface or the air to catch prey in water, and others have legs adapted to feed in water. Most studies on birds’ bacterial communities have been conducted on poultry or terrestrial birds, usually by sampling feces or swab samples [2,3]. Only a few studies have been conducted on wild waterbird microbiomes [4–11]. Billions of wild waterbirds migrate between continents twice a year in a period of only a few weeks . These wild waterbirds may have a role in pathogen (e.g., bacteria, archaea, fungi, viruses, protozoa) dissemination and are extremely important in respect to public health . Here, we review current knowledge on the topic of a relatively new hypothesis that has been presented by Halpern and colleagues , positing that waterbirds might be vectors of Vibrio cholerae, and thus may distribute this species all over the globe.
Local or intercontinental migratory movements of waterbirds and fish provide a possible mechanism for the introduction of new endemic foci of disease at short or great distances from the original source of V. cholerae infection (Fig 1). Therefore, we advocate that future studies on the occurrence of cholera outbreaks, especially across remote geographical regions, should consider the possible role of waterbirds and fish in V. cholerae transmission locally or globally. Epidemiological studies should examine the connection between environmental V. cholerae strains from waterbirds and fish to cholera cases. A fuller understanding of the ecology of V. cholerae is of vital interest to help limit the times that humans come into contact with this pathogen.
Does V. cholerae colonize some waterbird species, or does the bacterium just pass through the birds’ intestine after preying on fish or zooplankton? Feeding some waterbird species with green fluorescent protein (GFP) producing V. cholerae, should serve to answer this question. This should also be followed by observing the birds’ intestines to find out whether V. cholerae is attached to the intestines’ epithelial cells. If the bacteria colonize the intestine, are they transferred horizontally or vertically to their offspring? Does cholera toxin have some functions in waterbirds? Can we determine a model waterbird species that carries V. cholerae? Can we use this model to study and understand the role of V. cholerae pathogenic genes? Can we use this waterbird species model to monitor the dissemination of epidemic V. cholerae strains from one location to another and perhaps eventually to predict and even take measures to prevent cholera outbreaks?