Marine disease on the rise?

Humans can affect marine life in unexpected ways, as when large numbers of seals succumbed to canine distemper virus in 2000, presumably contracted from domestic dogs. Such human incursions cause even more damage by exacerbating the effects of naturally occurring parasitic and pathogenic diseases. While all indicators point to a real increase in disease in marine organisms, scientists have no baseline data to measure these increases against and so cannot directly test whether marine diseases are genuinely increasing. Now Jessica Ward and Kevin Lafferty report a new method in PLoS Biology that uses the recorded incidence of disease in the scientific literature to identify disease trends in major groups of marine organisms. Their analysis not only confirms fears but also throws up some unexpected results.

Ward and Lafferty conducted an online search of 5,900 journals published from 1970 to 2001 to measure the proportion of reports of disease in nine groups or marine organisms: turtles, corals, mammals, urchins, mollusks, seagrasses, decapods (crustaceans), sharks/rays, and fishes. Their approach also takes into account potential confounding factors, such as the effect of a particularly prolific author or a single disease event reported multiple times. They found a clear increase in disease in all groups except seagrasses, decapods, and sharks/rays. And they found that disease reports actually decreased for fishes. (One explanation for this decrease could be that drastic reductions in population density caused by overfishing present fewer opportunities for transmitting infection.)

These results confirm scientists’ perceptions about the rising distress of threatened populations and thus reflects a real underlying pattern in nature. That disease did not increase in all taxonomic groups suggests that increases in disease are not simply the result of increased study and that certain stressors, such as global climate change, will impact disease in complex ways. Ward and Lafferty have created a powerful tool to help evaluate trends in disease in the absence of baseline data. It is only by understanding the dynamics of disease outbreaks that scientists can help develop sound methods to contain them.