How might El Niño serve to facilitate such long trans-oceanic transmission? El Niño manifests as a zonal displacement of warm water from the western to eastern side of the Pacific Ocean, which has been shown to transport foreign zooplankton populations into Peru and Chile12. As the survival and spread of vibrios in the marine environment under adverse conditions has been linked to the ability of these bacteria to attach to plankton13,14 — which form a major component of biomass in the open ocean — it is possible that zooplankton provide a source of nutrients and protection, enabling vibrios to potentially ‘piggyback’. As El Niño waters can translocate larger organisms, notably exotic Asiatic marine mammals, fish, crustacean and snake species into the Americas, microbial transmission seems also likely. Further studies are clearly needed to elucidate the relationship between El Niño and long-distance transmission.

There is almost unanimous consensus that the 2015–2016 El Niño may be the most powerful since records started in the early 1950s (Fig. 1). An event of this magnitude may offer a valuable opportunity to study the potential introduction of another wave of pathogenic Vibrio populations into South America. The use of whole genome sequencing has been instrumental in using human clinical samples to track the introduction and human spread of V. cholerae epidemics15, but it remains difficult to use only these samples to assess human versus environmental transmission. For example, sequencing of the recent cholera outbreak in Haiti suggested that the infection originated from Asia and may have been transmitted through humans, rather than through purely environmental means. To unambiguously show the connection between El Niño and waterborne disease emergence into Latin America, extensive sampling and sequencing of environmental samples (for example, in both South American and Asian marine environments and in the open ocean as have been done in the recent TARA study of microbial diversity16), with a particular focus on the analysis of zooplankton is required. Furthermore, the selection of V. parahaemolyticus as a model organism for assessing the potential role of El Niño in the oceanic dispersion of pathogenic vibrios could provide some advantages over other species. One of the main difficulties for an effective tracking of pathogenic V. cholerae is the explosive nature of the outbreaks, and the rapid spread of infections due to the contribution of human transmission through contaminated drinking water. However, V. parahaemolyticus would offer a more pertinent model for tracing the dispersion of strains across the ocean: V. parahaemolyticus is a strictly halophilic organism with a habitat limited to the marine environment and infections are exclusively linked to environmental sources ruling out the confounding role of human transmission inherent with cholera. Finally, infections caused by V. parahaemolyticus are endemic in Peru, which provides a stable source of strains to identify the introduction of novel genetic variants in the region. The ability to identify pathogenic strains in intermediate sources, such as zooplankton as a potential vehicle for dissemination or shellfish (which represent a direct route into the human population), at both sides of the El Niño track coupled to phylogenetic reconstructions9 would provide an even more persuasive strand of evidence that this link exists and drives waterborne disease transmission.

In conclusion, the positive identification of Asiatic strains of vibrios in South America suggest that regional oceanography conditions provide both a periodic and unique source of new pathogens into Latin America with serious implications for the spread of waterborne infections. While more work will be necessary to validate El Niño's role in Vibrio transmission, it is interesting to speculate that it may also play roles in other diseases, such as waterborne disease of Crustacea. And, in addition to physical translocation of disease agents, El Niño could mediate other climactic effects on temperature and chemical distribution that intersect with other disease parameters, such as vector and host redistribution, physiochemical stress or availability of nutrients. For example, there is some evidence that increased sea surface temperatures may reduce salinity and aid in Vibrio fitness in the Bay of Bengal9. These avenues of investigation will be particularly interesting to study in the general context of climate change, as environmental effects are becoming major contributing factors in the emergence and spread of infectious disease.