The world's most successful bug hits dolphins
18:04 02 June 2008
NewScientist.com news service
Catherine Brahic
http://tinyurl.com/3q44wv
How does a parasite found in kitty litter get into whales and
dolphins? By hitching a ride down the sewage pipe and into anchovy
guts, say researchers.
Gloeta Massie and Michael Black of California Polytechnic State
University say this could explain why one of the most prevalent
parasites in humans is now popping up in marine mammals. They
presented their findings at the annual meeting of the American
Microbiology Society on Monday.
"Toxoplasma gondii is the most successful protozoan pathogen on the
planet," says Michael Grigg of the US National Institutes of Health.
The US Centers for Disease Control and Prevention estimate that nearly
one in four people aged 12 or older in the US have been infected with
the parasite.
Fatal consequences
The parasite often goes unnoticed but can kill fetuses if women become
infected during pregnancy. It is the third leading cause of death by
food poisoning, and can even cause encephalitis – an acute and
potentially deadly inflammation of the brain.
Cats are the primary reservoir: they catch the parasite from eating
infected mice or birds. The parasite's eggs are dispersed in the cat's
droppings and can infect food that is poorly washed or prepared.
Over the past decade it has become clear that the problem is not
limited to land animals. T. gondii has been found in marine mammals
around the world, from seals and sea-otters to walruses and dolphins.
Beluga whales have been found dead from encephalitis thought to be
caused by the parasite.
"We don't know whether the problem has always been there, but there is
not a marine mammal that we have looked at that is not exposed to some
degree," says Grigg. "How it is getting there is the million dollar
question."
Anchovy agents
Some have suggested that the eggs get into the oceans through sewage,
and lab studies have shown that they can ac***ulate in mussels and
oysters. T. gondii has also been found in one mussel that was caught
in the wild.
But for Massie and Black, that does not explain the parasite's
infection of marine mammals.
"Although the research on bivalves is very promising, it does not
fully explain the global infestation of T. gondii in marine mammals,"
says Massie. "Many marine mammals do not eat bivalves nor do they live
near areas of freshwater runoff and sewage runoff."
Based on the global distribution of the parasite, Massie and Black
think that anchovies – a major source of food to many marine mammals –
may be part of the problem. They point out the anchovies move from
estuaries, where they come into contact with sewage efflux, to the
open ocean. They also live anywhere from the top layer of the ocean
down to the "twilight" boundary, which light from the surface only
just manages to reach.
What's more, anchovies normally eat tiny particles roughly the same
size as the T. gondii eggs.
Experimental infection
Black and Massie are in the early stages of their study. To check the
credibility of their hypothesis, they put northern anchovies
(Engraulis mordax) in seawater containing parasite eggs. Later they
dissected the fish and looked in the gut tissue for T. gondii DNA.
The tests were positive, revealing that the fish do indeed filter and
ac***ulate the eggs. When the anchovies were placed in parasite-free
water for six to eight hours after infection, the eggs remained in
their tissue. This suggests the fish could trans****t the parasite from
infested estuaries to open water.
"They're doing all the right experiments," says Grigg, but he adds
that much remains to be done. For starters, eight hours is probably
not enough time for an anchovy to pick up eggs, carry them out to sea
and be eaten by a whale.
It is also im****tant to determine what parasite strain the anchovies
can carry. Grigg has identified a new strain, which he has called
"type X". Type X is responsible for killing large numbers of
Californian sea otters. "We're wondering if it is an emerging line
that is poised to sweep the world."
Grigg's research suggests that the existence of different strains in
part explains why T. gondii sometimes kills and other times goes
unnoticed.
"Once we know how it is being transmitted we can try to mitigate the
threat," says Grigg. "If we find there are hotspots of transmission we
can try to intervene there. If Black is correct and anchovies are
being infected downstream of sewage plants, clearly what is being done
to treat the waste is not adequate."
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Weblinks
Michael Black
http://www.calpoly.edu/~bio/FacultyStaff/Faculty/Black.html
Michael Brigg
http://www3.niaid.nih.gov/labs/aboutlabs/lpd/molecularparasitologyunit/
Toxoplasma infection, CDC
http://www.cdc.gov/toxoplasmosis/
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