Manatee, Sea Lion Deaths May Be Health Warnings for Humans

Scott Norris in St. Louis, Missouri
for National Geographic News
February 28, 2006
You may think you have little in common with a sea lion, an otter, or a manatee.

But if you happen to eat seafood, own a cat, or live near the coast, the health of these ocean animals might be relevant to your own.

Marine mammals, scientists say, are often indicators of environmental problems such as disease-causing life-forms and biologically produced toxins.

Earlier this month experts presented a new picture of contaminants in the marine environment that shows some surprising health connections between land and sea.

In Florida (map), for example, scientists are finding that powerful poisons known as brevetoxins—produced by the algae responsible for so-called red tides—are more durable and spread in more ways than previously suspected.

Recent die-offs of bottlenose dolphins and endangered Florida manatees led researchers to the new discoveries, said Gregory Bossart, director of marine mammal research at the Harbor Branch Oceanographic Institution in Fort Pierce, Florida.

Manatees and Red Tides

This month Florida wildlife officials announced record manatee deaths for January—one-third more than were reported for the same month last year.

The cause of half the deaths is still unknown, but officials suspect red tide might have played a role.

Red tide algal blooms—sudden, massive growths of algae—are known to cause massive fish die-offs, and often prompt authorities to shut down beaches and shellfish beds.

These blooms are natural occurrences but have been increasing in frequency in many parts of the world. Experts suspect water pollution and rising ocean temperatures are to blame.

Bossart's group has shown that toxins produced during a red tide become airborne, forming a chemical cloud over the affected waters.

By breathing in the toxins, Bossart said, "manatees were literally gassed to death. They died of acute toxic shock."

These poisonous clouds may also drift inland and affect humans, Bossart said during the annual meeting of the American Association for the Advancement of Science, held earlier this month in St. Louis, Missouri.

In coastal areas of South Florida, researchers have noted a sharp rise in emergency room admissions for lung conditions, such as bronchitis and asthma, during red tide events.

Scientists studying red tides previously believed that after an algal bloom, high concentrations of brevetoxins didn't last long in the marine environment.

They also thought that exposed fish would die and not transfer the chemicals to fish-eating animals higher up the food chain.

But, Bossart said, in some recent die-offs, animals didn't have the blisters and lung hemorrhaging characteristic of brevetoxin inhalation.

He and his coworkers discovered that manatees feeding on sea grass and dolphins feeding on menhaden—a plankton-eating fish—received lethal doses of brevetoxin from their diet.

These poisonings occurred weeks after the most recent red tide.

"Now we know poisoning can occur in the absence of the algae," Bossart said. "Just issuing warnings and closing shellfish beds when a red tide is occurring is not enough."

The potential for long-term dietary and airborne exposure to brevetoxins should be a cause for concern, Bossart says.

"Most worrying to me are the effects of chronic exposure. The west coast of Florida is consistently exposed to red tides."

Bossart said that ongoing work has shown a link between brevetoxin exposure and weakened immune systems in laboratory rats.

"We also found that manatees are immune suppressed," he said.

The findings raise concerns that marine mammals—and humans—exposed to the toxin could be at higher risk for other kinds of infections.

Hold the Anchovies

Algal toxins are also a problem for California sea lions. More than a thousand of the marine mammals have died over the past five years from domoic acid poisoning.

Like brevetoxins, domoic acid is a compound produced by algal blooms that is toxic to the nerves or nervous tissue.

But sea lions with neurological damage have been found stranded on beaches even when no toxin-producing blooms are evident.

Frances Gulland, of the Marine Mammal Center in Sausalito, California, said sea lions obtain lethal doses of the toxin by feeding on algae-eating fish, such as northern anchovy.

"Low levels of domoic acid can have severe long-term effects on sea lions, beyond acute mortality," Gulland said. These effects include brain damage and reproductive failure.

Gulland said pregnant sea lions exposed to the poison either experienced spontaneous abortions or gave birth prematurely (photo: sea lion pups).

Researchers found domoic acid in these fetuses, showing that the poison can pass from mother to pups. The findings have serious implications about the effects of a seafood diet on human reproductive health.

"Sea lions share our coastal environment and eat many of the same fish as us, only more of them," Gulland said. "They may be giving us an early warning."

Cat Quandary

Patricia Conrad, a veterinarian at the University of California, Davis, also spoke at the St. Louis meeting.

She reported on studies seeking to explain the stagnant growth rate of southern sea otter populations from California to Washington State.

Conrad and other researchers were surprised to discover high mortality rates of young and adult otters due to disease. A leading contributor turned out to be the widespread parasite Toxoplasma gondii.

The parasite is known to reproduce only in cats. The domestic animals then release the parasites' infectious oocysts—egglike structures that transfer parasites to new hosts—into the environment through their feces.

A single cat can shed millions of the tiny, long-lived oocysts.

While many mammals carry the parasite, Conrad said, sea otters do not feed on other mammals and so cannot become infected through their food.

"The way sea otters get infected is from the oocysts shed in cat feces," Conrad concluded.

Her team's theory is that oocysts released outdoors by domestic and feral cats slowly make their way into streams and eventually into the ocean.

Their latest data show that 38 percent of the live sea otter population and over half of the dead otters analyzed have been exposed to the parasite.

Infected otters may die of severe brain disease and may also be at higher risk of shark attack, since the illness would make them easier prey.

Although not dangerous to humans under most circumstances, the parasite can cause severe disease in people with suppressed immune systems and can interfere with fetal development.

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