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Climate Change

Arctic Viruses Infecting sub-Arctic Seals: How Climate Change is Creating Marine Epidemics

With all the preparations being made to help human populations get through Covid-19, many are wondering about the effect the coronavirus or similar viruses may have on our animal counterparts. Some panicky pet owners are even rushing to get their dogs special Covid masks.

While it’s highly unlikely your dog will catch Covid-19, SARS-type viruses do infect more animals than just humans, and there are plenty of other viruses that preferentially infect everything from birds, to whales, to plants.

Sea ice is melting at a dramatic rate across the Arctic, leading to large changes in the biological communities that depend on sea ice. (NOAA)

In the Arctic, where melting sea-ice is opening up new territory for all lifeforms, scientists are seeing marine viruses act in new and concerning ways.

Phocine distemper virus (PDV), a pathogen that causes respiratory and neurological issues in seals and is related to Canine distemper virus, has caused mass die-offs in European harbour seals in the North Atlantic since at least the 1980s. Also present in Arctic seal populations, including Harp and Grey seals, PDV seems to exist in a few different strains around the Atlantic but is markedly more deadly for the southerly Harbour Seals.

It’s not uncommon for the occasional viral epidemic to kill off a large number of animals. Related species (or even populations of a single species) that overlap in range may routinely exchange viral infections. As long as enough infected animals survive, they’re likely to pass on whatever adaptive trait got them through the infection to the next generation – running a never-ending arms race with the virus’s own evolution. When a virus is novel to the population, though, mass mortality events like those seen in the Atlantic Harbour seals can occur.

Potential routes for movement of seals infected with PDV through the Arctic Ocean opened by reductions in sea ice extent. Routes along northern Russia (green) and northern Canada (orange) are shown with August 2002 sea ice extent. (From VanWormer et. al. 2019)

PDV wasn’t found in the North Pacific until very recently. Scientists are now wondering if melting sea-ice is to blame for a rise in cases seen in Northern Sea Otters, found along the South-Western coast of Alaska.

Blood and Snot for Science

A collaborative study by American and UK researchers published in Nature last year compared known ranges for seal species, sea-ice records, and confirmed PDV cases to see what role the distemper virus may be playing in Pacific mortality events.

Bearded seals, Northern fur seals, and Stellar sea lions are all tagged and tracked by the Alaska Fisheries Science Centre’s Marine Mammal Laboratory, so the researchers were able to create accurate estimates of when and where the different species have overlapped through the years. This is important, because we know Northern Sea Otters didn’t get PDV from infected Harp or Grey Seals – they’re just too far apart from each other. Bearded seals, however, do share territory with other Arctic seals and are much more likely to have come in contact with infected species from the Atlantic.

From 2001-2016, the researchers routinely sampled the blood and swabbed the noses of the different seal species found from Alaska to Russia. The blood was used to search for PDV antibodies, something currently- or previously-infected seals would be producing to help their bodies fight the virus. The nose swab was used to test for the virus itself, by amplifying and sequencing the virus’ genetic material.

Signs of current or very recent PDV infection were found in all the monitored Phocid species, indicating that the 2004 Sea otter cases were not an isolated phenomenon. Crucially, they also found a significant relationship between the percentage of animals showing PVD infection and the state of the sea-ice. In years the sea-ice opened up enough to allow for trans-ocean travel across the Arctic, as much as 50+% of the approximately 2,700 Phocids tested positive for PDV.

A Ringed Seal (Pusa hispida), an Arctic and subArctic seal species. (NOAA Seal Survey)

So PDV has entered the Pacific. Scientists aren’t sure yet what the presence of the virus means for the health of native seals and their relatives in the Pacific, though they suspect it may have played a role in a 2004-2006 die-off event in Sea otters as well as in sporadic Stellar sea lion deaths. It’s possible that, as in the Arctic, PDV is able to exist within some species or populations without causing mass mortality, but the potential for outbreaks in sensitive southern species is worrying.

Open ocean is quickly becoming the new normal in the Arctic. New and extreme disease events may also become more common as we more forward into this warmer age and will carry important implications for the fate of marine species. In seals, which are key components of coastal ecosystems at all latitudes, the effects of die-off events will be felt throughout the marine food chain. We need to make sure our marine economies are ready to adapt.

VanWormer, E., Mazet, J.A.K., Hall, A. et al. Viral emergence in marine mammals in the North Pacific may be linked to Arctic sea ice reduction. Sci Rep 9, 15569 (2019). https://doi.org/10.1038/s41598-019-51699-4

Estimated distances animals can travel during the PDV latent and infectious period (1 week, 2 weeks, and 4 weeks) illustrating the areas where viral transmission could occur, based on median travel speeds calculated for satellite-tagged bearded seals (green circles), spotted seals (orange), Steller sea lions (blue), and northern fur seals (purple). (b) Recorded tracks of a PDV seropositive bearded seal followed in July 2009 and a seropositive northern fur seal followed in November 2010 shown with sympatric PCR positive spotted seals, ribbon seals, and northern fur seals sampled 2009–2010. Sea ice is shown at its minimum extent in September (panel a) and retreating the following July after reaching a maximum winter extent (panel b). (From VanWormer et. al. 2019)

(a) Yearly seroprevalence for antibodies to PDV in Steller sea lion pups, juveniles, and subadults (black dots) with seroprevalence of canine distemper virus antibodies (blue dots) measured in a subset of Steller sea lions (n = 80); (b) PDV seroprevalence (black dots) and viral infection prevalence (PDV nucleic acid detected from nasal swabs; green squares) for all species combined (ice-associated seals, Steller sea lions, northern fur seals, and northern sea otters) from 2001–2016. Error bars represent 95% exact confidence intervals (CI). A 95% CI was not included for viral infection prevalence in 2008 as only one animal was tested. Presence of an open water route through Arctic sea ice along the northern Russian coast following a year of closed sea ice (grey bars) was significantly associated with animals testing seropositive or PCR positive for PDV. The strain of PDV responsible for an outbreak in harbour seals in the North Atlantic Ocean during 2002 (red star) was detected in PCR positive animals in the North Pacific Ocean throughout the study period. (From VanWormer et. al., 2019)

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