Population Biology of Gambierdiscus toxicus in the U.S. Virgin Islands: Towards an Integrated Study of Ciguatera Fish Poisoning

Donald M. Anderson and Deana Erdner, Biology Department, WHOI

Ciguatera is a complex clinical syndrome caused by the consumption of fish contaminated with toxins that originate from the benthic dinoflagellate Gambierdiscus toxicus in tropical regions. It is the most common form of illness associated with harmful algal blooms (HABs), and the one with by far the greatest public health and economic impact. It has an international distribution and disproportionately affects persons in the lowest socioeconomic groups who are most dependent on reef fish for food. CDC data show it to be the most common foodborne illness related to finfish consumption in the United States, with international data suggesting that it is the most common of all marine food poisonings. Despite the frequency with which it occurs and the vast number of victims globally, the restriction of the disease to tropical islands has resulted in minimal scientific attention (exacerbated by local concerns that any mention of the disease will have a negative impact on the tourist trade). However, because of its presence, major reef fisheries around the globe are not available for commercialization, and many island societies are unable to exploit their local fish as sources of much-needed protein.

The incidence, prevalence, and worldwide distribution of ciguatera appear to be increasing, paralleling a worldwide increase in toxic or harmful algae. The increase is thought to be linked with degradation of reef environments due to both anthropogenic and naturally occurring environmental changes, such as global warming, tourism, eutrophication, sewage and freshwater runoff, sedimentation due to erosion or dredging, and ship groundings. Some have argued that ciguatera may be one of our most sensitive indicators of environmental disturbance in tropical marine ecosystems, though at present it is not possible to determine whether environmental degradation and change in tropical regions are affecting its incidence or severity. Given the rate at which we are modifying tropical coastal ecosystems, there is an urgent need to increase our understanding of this poorly studied poisoning syndrome and the algal species that cause it, to allow us to predict and minimize the risk of illness.

Here we propose to survey the distribution, abundance, and population genetic structure of Gambierdiscus toxicus around the island of St. Thomas in the U.S. Virgin Islands. St. Thomas provides a model study site, as the island has a well-documented and significant annual incidence of ciguatera as well as a history of epidemiologic studies of the poisoning on the island. Two surveys will be conducted in the April-May and August-October time periods, which coincide with the seasons of highest rainfall and also highest Gambierdiscus abundance in the Virgin Islands. Dinoflagellate samples will be collected at six study sites and one control site around the island, along with a suite of environmental measurements. The samples will be used to determine G. toxicus abundance and population composition, using ribosomal RNA genes as markers. The resulting data would contribute to our understanding of a global public health issue in tropical areas, by providing a description of the distribution and genetic structure of the causative dinoflagellate in the U.S. Virgin Islands, an area with a significant annual incidence of ciguatera. This type of survey has not been conducted for decades, and new data is sorely needed given the occurrence and rate of habitat degradation in tropical areas in the intervening time period. The information gained from this project would also contribute to a larger effort by the PIs and a  multidisciplinary team of epidemiologists and toxicologists to establish a long-term study on the causative and temporal links between toxic dinoflagellates, the environment, and the clinical manifestations of ciguatera.