Virus related to measles could push Puget Sound orcas to extinction, study says

Researchers studying the killer whales that frequent Puget Sound are growing increasingly concerned that a dangerous virus or other disease-causing organism could spread through the population and hasten extinction of these critically endangered southern resident orcas.

Researchers led by veterinarian Pete Schroeder capture the breath of orcas in a search for pathogenic organisms from 2007 to 2009. // Photo courtesy of Pete Schroeder
Researchers led by veterinarian Pete Schroeder capture the breath of orcas in a search for pathogenic organisms from 2007 to 2009. // Photo courtesy of Pete Schroeder

Without dramatic changes to their environment, extinction is already considered the likely future for the southern residents, as they continue to face shortages of food, high levels of chemical contamination and stress from the noise around them. Their numbers have declined from 98 animals in 1995 to 72 today.

New research suggests that extinction could come sooner if the whales were to become infected with a novel pathogen, such as cetacean morbillivirus, which has killed thousands of Atlantic bottlenose dolphins on the U.S. East Coast but has not been seen in the Pacific Northwest.

“Given its fragile state, it is unlikely that this population would recover from the sudden increase in mortality that would result from a majority of the population becoming infected with CeMV,” states the new report.

Michael Weiss, a researcher with the Center for Whale Research and lead investigator on the study, said the prospect of CeMV in the southern residents can be compared in some ways to the recent outbreak in humans from the novel coronavirus: Just as humans lack immunity to the new coronavirus, the orcas have no history of exposure to CeMV, thus they are vulnerable to the worst effects of the organism.

“When I think of risks to the southern residents, I think the main risk is not getting enough food to remain nutritionally healthy,” said Weiss, a doctoral candidate at the University of Exeter in England. “But an outbreak of cetacean morbillivirus would be a nuclear meltdown. It has a low probability of happening, but the results would be absolutely catastrophic.”

Southern residents eat fish, primarily Chinook salmon — another species at risk of extinction. The lack of food, chemical contamination and stress can be thought of as contributing to the disease process among the whales. A high rate of inbreeding in this population also can affect their immunity.

“The misconception is that these animals are starving to death,” said Joe Gaydos, a veterinarian with the SeaDoc Society. “You need to look at the bigger picture of what these animals are dying from” — and the causes are varied.

In the realm of infectious disease, an orca may not be able to fight off an infection if it is already weak from lack of food, Gaydos said. If the infection persists, the animal could have a hard time catching fish to eat; it could become disinterested in food; or it might even be unable to adequately process the food that it does eat.

Toxic chemicals, particularly the polychlorinated biphenyls found in killer whales, can reduce an animal’s immunity, as can stress from noise or other causes. A whale in a weakened condition is more likely to succumb to any number of problems, including disease, congenital problems or trauma, such as being struck by a boat.

Strong social bonds among the southern resident killer whales can increase the opportunity for disease transmission, according to the new study published in the journal Biological Conservation. While CeMV is not the only organism that could threaten the population, Weiss said, the disease is frequently discussed as a significant threat.

Cetacean morbillivirus exists within a family of viruses that cause human measles, canine distemper and related diseases among cats and ruminants, such as goats, sheep and camels. The disease caused by the virus is highly contagious and can be spread through airborne droplets from the breath of an infected animal. Based on studies of other populations, CeMV is likely to kill 70-80 percent of the orcas that become infected, the new report says.

The study examined how infection could spread from one whale to another. In the wild, several orcas often surface together, exhaling plumes of mist that can be inhaled by a nearby whale. In the study, the frequency of close contact actually observed by whale researchers was factored into a new model, which can be used to predict how any infection could spread through the population from a single individual.

The initial infection could come from another species. Southern residents have been known to interact with harbor porpoises, humpback whales and Pacific white-sided dolphins — all thought to be susceptible to CeMV, making them potential sources for an outbreak.

Killer whales travel in family groups, led by an elder female and her descendants. Groups of these so-called matrilines make up socially related pods — specifically J, K and L pods among the southern residents. The new study showed that this modular organization could help reduce the spread of infection — but only minimally compared to groups of animals that interact in a more uniform pattern.

Using a variety of assumptions, the new model showed that occasionally the disease would fail to spread much beyond the initially infected individual, but in most cases about 90 percent of the orcas would come down with the disease and about 70 percent of those would die.

A vaccine to protect against cetacean morbillivirus has been tested, but not deployed, in bottlenose dolphins. If a vaccine were to be developed for the killer whales — and there are many challenges — one would need to vaccinate at least 42 of the 72 southern residents to substantially reduce the risk of a major outbreak, according to the analysis.

“The logistical challenges of vaccinating and monitoring individuals at sea and the potential stress these activities may cause the animals likely make the prospect of wide-scale vaccinations impractical, as well as potentially unethical,” the report concludes.

Other stories about disease in Puget Sound species include the three-part series “The Orca Docs: Can medical interventions help?” See also the entire section on disease in the Encyclopedia of Puget Sound.

About the Author: 
Christopher Dunagan is a senior writer at the Puget Sound Institute.