The world's most successful bug hits dolphins
18:04 02 June 2008 by Catherine Brahic
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.
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."
Some have suggested that the eggs get into the oceans through sewage, and lab studies have shown that they can accumulate 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.
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 accumulate 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 transport 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 important 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."