A potentially deadly disease affecting marine mammals, including seals and sea otters, has been passed from the North Atlantic Ocean to the northern Pacific thanks to the melting of the Arctic sea ice.
Experts have long been concerned that sea ice melting in the northern oceans, caused by global climate heating, could allow previously geographically limited diseases to be transmitted between the two oceans.
Now scientists believe they have identified how an outbreak of distemper, similar to that suffered by dogs, was passed from northern seal populations to Alaskan seals and sea otters .
“There’s long been concern that melting Arctic sea ice could allow disease to pass between the Atlantic and the Pacific,” Tracey Goldstein, an expert in marine animal diseases at the University of California, Davis, and one of the lead authors of a report, said. “Now here we are.”
Phocine distemper virus, or PDV, has long been a threat to seal populations in the northern Atlantic, along with several strains of influenza, but had not previously been identified in the Pacific. It was first recognised in 1988 following a massive epidemic in harbour and grey seals in north-western Europe with a second event of similar magnitude and extent in 2002. The 1988 outbreak killed thousands of Britain’s seals.
The virus attacks the immune system, leaving animals susceptible to pneumonia and in the most severe cases can kill a seal within 10 days of infection. The two outbreaks, which both started on the Danish island of Anholt in the Kattegat strait, killed about 23,000 harbour seals in 1988 and 30,000 seals in 2002. The virus is believed to spread through contact between infected individuals and has killed animals in the waters of Sweden, Norway, Denmark, the UK and Germany.
The new study, published in the journal Nature Communications, based its conclusions on samples taken from seals, sea lions and sea otters in Alaska between 2001 and 2016, finding that PDV had become entrenched in Alaskan waters.
“Concern was first raised that PDV could be infecting Alaska’s sea otters so we decided to look at whether it had been transmitted via overlapping populations of seals and sea lions,” said Goldstein. “When we started testing the different species the sequence of the virus was always the same strain, with incidence peaking when the sea was open.”
The research was prompted by mounting concern in recent years that the virus could reach the Pacific via a warming Arctic and infect the large Pacific harbour seal and northern elephant seal populations or the endangered Hawaiian monk seals. Genetic sampling of the disease also established that the same strain identified in the Atlantic had emerged in Alaska, with peaks of the disease correlating to periods of low sea ice.
The study tested more than 2,000 animals from a variety of species in Alaska’s waters from the northern Beaufort Sea in the Arctic to more temperate southern areas.
The report warns of the potential for other diseases to be spread as sea ice recedes. “Climate change-driven reductions in sea ice extent in the Arctic Ocean are projected to increase and open-water routes along the northern Russian coast have occurred every August and/or September since 2008.
“The health impacts of this new normal in the Arctic are unknown, but association of open-water routes through Arctic sea ice with increased PDV exposure or infection suggests that opportunities for PDV and other pathogens to cross between North Atlantic and North Pacific marine mammal populations may become more common,” it said.
One of the concerns about the disease’s path from the North Atlantic to the Pacific is that it is unclear what impact it will have on infected species. While some may have a natural resistance to the disease, others may be more vulnerable, with the deadly 1988 outbreak in Europe believed to have been caused initially by an infected harbour seal from Greenland where the disease has long been endemic and the population has built up resistance.