By Debora MacKenzie
New Scientist
April 19, 2004


For years, apparent increases in illness among marine creatures, from whales to coral, have left marine scientists with the uneasy suspicion that the seas are increasingly plagued by disease. Now, US researchers have uncovered the first good evidence that they are right.

In 1998, a dozen of the world's top experts on diseases of marine animals warned that sea creatures seemed to be getting sick more often, with more diseases.

New viruses had appeared in whales and seals, while corals were dying of fungal and algal infections. Pilchards succumbed to viruses and an aggressive parasite expanded its range to attack commercial oysters, scallops and clams. In the Caribbean, some unknown bacteria wiped out what had been the dominant sea urchin.

But there was no way to tell if the apparent increase was simply due to more scientists paying more attention to marine disease. There was no baseline, as no one had ever measured disease incidence in any of these species decades ago.

Now, Jessica Ward, at Cornell University in Ithaca, New York, has shed important new light on the problem by looking at how the number of reports of marine diseases in nine different groups of marine creatures has changed in the scientific literature since 1970.

"We wanted to find out if something was actually happening," Ward told New Scientist. "For most groups of organisms, we found that yes, there is something going on out there. Now we hope more people will try and figure out where it is coming from."

True incidence

Ward, with Kevin Lafferty, of the University of California in Santa Barbara, first tested whether changing numbers of scientific reports of rabies in US raccoons matched the true incidence of the disease, which is known independently. They matched, suggesting more scientific reports really do mean more disease.

The pair further tested the relationship by removing the most prolific laboratory from the publications they collected for each group of marine creatures -- just in case increased reporting reflected only one scientist's funding success. This did not change any apparent disease trends. Neither did taking out multiple papers on one well-reported disease event, such as the Caribbean urchin die-off.

So using scientific reports as a measure, Ward and Lafferty found that disease has increased in turtles, corals, marine mammals, urchins, and molluscs such as oysters.

Illness seems to have remained steady in the shark and shrimp families, and in seagrasses. Surprisingly, disease reports have diminished for fish.

Easy prey

There are numerous possible reasons for rising disease. One, Ward suggests, is increasing sea surface temperatures due to global warming. This can cause corals to bleach, making them easier prey for infections.

Warming has also led to the northward spread of the oyster parasite Perkinsus. And warming is thought to accelerate the growth of tumours in turtles caused by a herpes virus.

Another possible factor is that human over-fishing has destabilised marine ecosystems. For example, when the urchins in the Caribbean died, corals were overwhelmed by the algae the urchins used to eat. "Normally fish would have eaten the algae instead, but they weren't there," says Ward.

Other suggested causes include:
  • new pathogens from domestic animals, such as dog distemper virus and the parasite Toxoplasma
  • bioaccumulation of toxins weakening marine mammals' immunity
  • new species carried across oceans in ships' ballast tanks introducing new diseases
In the face of all this, the apparent health of fish is intriguing. Ward says this could be because the fish are simply fewer in number. Many pathogens die out among animals that are not packed densely enough to pass the infection on. But it is also possible, she says, that the frequency of disease is just as bad or worse -- but fewer fish mean fewer observations, and fewer reports.

Journal reference: PLoS Biology (vol 2, p 542)

Informant: NHNE


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