life is getting sicker
16:55 19 April 04
NewScientist.com news service
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
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
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.
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.
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
· 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.
PLoS Biology (vol 2, p 542)