NEW YORK: The mystery of why marine mammals worldwide have been dying from a parasite spread in cat faeces may have been solved. The culprit? The humble anchovy, new research suggests.
Toxoplasmosis, the sometimes fatal disease caused by the protozoan parasite Toxoplasma gondii, is found only in cats in its infectious form, and only the fertilised eggs of the parasite – called oocysts – cause an infection.
But over the past decade the disease has also killed a vast number of marine mammals, leaving scientists puzzled over how the parasite found its way from felines to the sea.
To date, the parasite has been detected in 33 species of marine mammal from seven families, including the southern sea otter, an endangered species. In fact, an estimated 17 per cent of sea otter deaths are caused by toxoplasmosis, and infected animals have been found in such geographically far-flung locations as the Arctic Circle, Australia and California.
Some researchers have theorised that fresh water runoff from streams, stormwater drains and sewers have carried cat faeces to the ocean, and that new products like flushable kitty litter may have increased the amount of faeces in runoff. But this doesn’t explain how the parasite has spread across entire oceans.
Now new data from researchers at California Polytechnic State University in the U.S. suggest that anchovies, an important prey for many marine animals, may be the missing link. The team reported their results at the 108th General Meeting of the American Society for Microbiology in Boston, Massachusetts, last week.
“If anchovies are really serving as hosts [to T. gondii], then this could be an explanation for how oocysts are getting from the near shore environment to the open ocean,” said Gloeta Massie, a lead researcher in the study.
Infecting the food chain
According to Massie, while it’s possible marine mammals are ingesting T. gondii directly from the water, it’s more likely the parasites are living in a host organism, which is later eaten by the mammals.
Terrestrial mammals only become infected by exposure to large numbers of oocysts, and researchers believe the case will be the same for their marine counterparts. But the low concentration of oocysts in the ocean makes it unlikely that mammals are able to ingest enough to cause infection.
Furthermore, oocysts can’t travel against currents by themselves, and the movement of the parasite across oceans suggests transportation by a host.
According to the researchers, the anchovy is a good candidate for a host because the fish often migrates between coastal waters and the open ocean. Also, anchovies themselves can’t become infected by the parasite since it only becomes active in warm-blooded animals.
Massie and her team proved that anchovies are able to pick up T. gondii when they filter ocean water for food, such as plankton or algae. The next step is to determine whether the parasite remains infectious after ingestion by anchovies.
In the next few weeks, the team will perform a bioassay in which infected anchovies will be fed to mice. If the mice become infected with T. gondii, then this may indicate that the fish can also spread the parasite to marine mammals.
“This is an important study,” said Michael Moore, a Senior Research Specialist at Woods Hole Oceanographic Institute in Massachusetts in the U.S., who specialises in marine mammal forensics and was not involved in the study. “We know very little about the microbial ecology of important diseases that affect humans and animals, such as toxoplasmosis.”