The death of a beluga whale mother at the Shedd Aquarium last month apparently was caused by bacteria-contaminated fish fed to her by keepers and sold by wholesalers who supply the nation’s restaurants and supermarkets, according to test results released Wednesday.
The 14-year-old whale, Immiayuk, was killed by an invisible bacteria found on the skin of fish that can cause a lethal blood-borne infection in whales and dolphins, Shedd officials said.
Five years ago, the same disease, erysipelas, killed Quitz, a young Pacific whitesided dolphin at the Shedd. The cause of Immiayuk’s death on Dec. 26 was confirmed through detailed, microscopic analysis done on tissue taken from her carcass.
The Shedd buys fish for its marine mammals from about 50 brokers on both coasts. Government inspected, it is the highest grade of fish available, rated as “human consumption quality.”
“The vendors I deal with all sell their fish to restaurants all over the country,” said Ken Ramirez, the Shedd’s chief marine mammal trainer. “We give our animals only the very highest-quality food.”
Cooking the fish kills the bacteria. Whales and dolphins, of course, eat the fish raw, including the scales and outer mucous membrane, where the bacteria resides.
Humans eating raw fish, such as sushi, presumably don’t eat the outer mucous membrane and scales and so are unaffected by the bacteria, except for a slight chance of minor skin irritations.
Erysipelas is one of the known causes of death for whales and dolphins in the wild and has been a cause of death for whales and dolphins in many zoo and aquarium collections in the past as well, said Jeff Boehm, Shedd vice president and chief of veterinary services.
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“The course of this disease is so swift, and the symptoms typically are not visible until the animal is near death, making it difficult to detect in time to treat effectively,” Boehm said.
The bacteria lives in the exterior mucous coating of healthy wild fish. All wild and captive dolphins are thought to be exposed to the bacteria from time to time, but it usually passes harmlessly through their digestive systems.
Why it occasionally kills, as it did in the case of Quitz and Immiayuk, who left behind a nursing five-month-old female calf, is an answer that has eluded Shedd officials and researchers all over the country.
The bacteria appears in and can be harmful to many kinds of land animals too. Veterinary medicine long has provided farmers with inoculations to protect turkeys and pigs from the disease. A version of the vaccine used on swine was tried several years ago on captive dolphins, but Ramirez said side effects from the drug were so severe that its use was discontinued.
It is impossible to obtain fish without bacteria, Boehm said.
Since Quitz died from erysipelas in 1995, Shedd keepers periodically take samples from the frozen fish it stocks to grow cultures in search of the bacteria.
“We have never been able to isolate this bacteria,” Boehm said. “It just seems to appear in wild fish from time to time.”
Most often the fish supplied by wholesalers is frozen, but freezing does not kill the bacteria. It is thawed but raw when fed to the whales and dolphins. Boehm said researchers are working to discover how often the fish might be carrying the erysipelas bacteria, and why the bacteria can be so deadly to one animal while others around it appear unaffected.
In any infectious disease, in humans or animals, Boehm said, a combination of factors usually is present before it can cause harm. Those include exposure to the bacteria, the disease getting access to vital organs and its being able to get past the host’s immune defense systems.
The most logical way it could breach the immune system would be when the animal is weakened by injury, other illnesses or age, he said. That did not seem to be the case with Immiayuk, who checked out as perfectly healthy in an exam a few weeks before her death.
“Her immune system was robust,” Boehm said. “She even had evidence of antibodies for erysipelas a few years ago. So now we have to wonder if this is a different strain of the bacteria from the one we have seen here before. If so, we want to know how often and how much of the bacteria shows up.”
Because it is so difficult to detect, he said nobody knows how the bacteria presents itself: “Do you find it in great amounts on a single, isolated fish out of the thousands that come in a shipment? Is it found in moderate amounts on a few fish in the batch? Is it present in small amounts on all the fish of a given batch? We don’t know.”
The Shedd feeds its marine mammal collection 650 pounds of fish a day. Every day, Ramirez said, about 20 people examine each fish individually, looking for signs of disease and deterioration. Any fish with missing scales, a missing eye or any other telltale sign of trouble is thrown away.
Shedd officials have discussed the possibility of using a process of irradiating the fish to kill the bacteria, but that process would sterilize the food so completely, it might be even more dangerous than the erysipelas bacteria, Boehm said.
“There are all sorts of bacteria that are part of normal physiological system for which the animal has a natural immunity,” he said. “If you killed (all the bacteria), then any bacteria that later slipped back into their system could be a killer. We don’t want animals that are naive of all potential pathogens.”
Boehm said researchers at the Shedd and elsewhere are trying to find out more about how the erysipelas bacteria spreads in nature and how it conquers the immune systems of some animals and not others. From that knowledge, they hope to develop a vaccine that would protect captive and wild whales, porpoises and dolphins.
Since Immiayuk’s death, keepers have been training her calf Kayavak to eat whole fish. Still nursing when her mother died, she has been weaned from her milk diet and gained 10 pounds on her fish diet.
“It’s an around-the-clock process to feed her,” Ramirez said. “If she were nursing, she’d be feeding from her mom every hour or so, so we have to accommodate her digestion by feeding her throughout the day and night, about once every three hours.”
For the time being, the calf remains in the Shedd’s off-viewing medical pool. For a few days after Immiayuk’s death, an older adolescent female beluga, Naya, swam with her in the pool, but Ramirez said Naya showed signs of being bored with constant baby-sitting.
Kayavak’s current companions are keepers who remain with her 24 hours a day.