Viruses belonging to the Morbillivirus genus (in the Paramyxoviridae family of enveloped negative-sense RNA viruses) have been recognized as significant causes of disease in cetaceans and pinnipeds for several decades. Morbillivirus species are also responsible for measles in humans, distemper in dogs and Rinderpest in cattle. In all cases, infection usually leads to either rapid death or recovery with lifelong immunity. Occasional epizootics caused by phocine distemper virus (PDV, Phocine morbillivirus) have been particularly severe in populations of Atlantic harbour seals and grey seals in northern Europe. These are thought to have been caused by cross-infection introduced from Arctic pinniped species, which act as a reservoir (Jo et al., 2018). Here, PDV spreads rapidly among large populations of young animals in in close contact during the breeding season. The rapid spread can be explained by the decline in “herd immunity”, comparable to periodic measles epidemics in humans which can occur when there are low levels of vaccination.
Changing patterns of ice cover affect the distribution, breeding and ecology of marine mammals, leading VanWormer et al. (2019) to test the hypothesis that reduction in sea ice cover will lead to greater contact between different species of Arctic and sub-Arctic mammals and the consequent introduction of PDV into the North Pacific. Their long-term study from 2001-2016 combined satellite telemetry of the movement of ice-associated mammals with blood and tissue sampling of nearly 2700 animals to test for viral infection using immunological tests for the presence of antibodies to the virus and molecular detection of the virus by Q-PCR. VanWormer et al. confirmed that exposure and infection by PDV is spreading to seals, sea lions and sea otters in the northern Pacific, with peaks linked to reductions in the extent and thickness of sea ice due to global warming. However, although sporadic mortalities have been observed in sea otters in Alaska, there do not appear to have been large mortalities in North Pacific seals and sea lions. This suggests that the virus may be able to persist in these species without causing widespread disease and it is not yet clear whether the spread of PDV to North Pacific mammals will lead to high mortalities like those seen in the North Atlantic. Ice-free open water routes through the Arctic are now occurring every summer, and Van Wormer and colleagues highlight the potential health impacts of this “new normal” on long-distance transmission of pathogens across the Arctic region and the need for ongoing monitoring.