(CN) — A pair of conservationists have found that viruses are largely to blame for the deadliest mass die-offs in marine mammal species in decades — and the changing climate will only intensify the factors contributing to these outbreaks.
Claire Sanderson and Kathleen Alexander — who both work for Virginia Tech’s fish and wildlife conservation department and the Botswana-based nonprofit CARACAL, short for the Center for Conservation of African Resources, Animals, Communities, and Land Use — published their research Thursday based on a review of scientific literature on 36 mass mortality events involving infectious diseases and marine mammal populations such as whales, dolphins, seals, manatees and polar bears between 1955 and 2018.
“The most devastating consequence will be the potential extinction of these vulnerable marine mammal species,” Alexander and Sanderson said in a joint email interview. “However, ecosystems and the species within them are so intricately linked that these events can have far reaching impacts on ecosystem function. We will likely not know the true ecological consequences of these events until years have passed and perhaps not even then.”
Mass mortality events, the researchers wrote in their article for the journal Global Change Biology, are marked by a “devastating number of fatalities within the species over a brief period of time,” though the exact number varies based on a species’ population size and conservation status.
The study found that 14% of all marine mammal species suffered a mass die-off induced by infectious disease in the reviewed time period. Of these, only a quarter were caused by bacterial infections. Viruses were not only the most frequent cause of die-offs, but were also 20 times deadlier than bacterial outbreaks.
Viruses such as morbillivirus and influenza A can infect multiple hosts in their lifetime, and because they can be carried by different species, they spread very rapidly to new populations.
But that’s not the end of the story. Infections come and go — why do they sometimes spread so rapidly and become so deadly for marine mammals?
To answer this, Alexander and Sanderson looked to species’ life history traits — how many different habitats the mammals frequent, for instance, and how social the animals are — and took note of environmental variables like the time of year the outbreak took place and the temperature of the sea surface at the time of the die-off.
The scientists learned that habitat breadth was one crucial factor. Some marine mammals are semiaquatic: pinnipeds such as seals, sea lions and walruses swim underwater but also spend time on land, where they are susceptible to being infected by more species compared to the marine mammals that never leave the water.
“The viruses … were multihost pathogens, meaning that they can infect a wide range of different species,” Alexander and Sanderson said. “This increases a pathogen’s ability to move across different landscapes, persist, and infect other susceptible hosts.”
It is perhaps unsurprising, then, that pinnipeds constituted over half of the species that suffered mass die-offs.
“Mode of transmission was also important with many spreading through aerosols and respiratory fluids, which has the ability to infect a greater number of individuals, much like the flu or Covid-19 in humans,” the researchers said. “Viruses are often associated with larger outbreaks, enhanced spread, and mortality — something we are experiencing right now during this pandemic.”
Broad habitats aren’t pinnipeds’ only worry. These mammals depend on sea ice for resting, childbirth, mating and escaping predators. As global air temperatures increase, polar ice melts, shrinking the available surfaces for these animals. They congregate more densely, and diseases spread more easily from host to host.
The scientists say their most surprising discovery was how closely abnormal sea surface temperatures correlated with the mass die-offs.
Alexander and Sanderson found that at least 61% of the mass mortality events happened while a region’s sea surface temperature had deviated from its long-term average, suggesting that climate change has invited more opportunities for deadly disease outbreaks in Earth’s waters.
“We found that for every one unit increase in global yearly [sea surface temperature] anomalies, there was nearly a 12% increase in the chance of these events occurring,” Alexander and Sanderson said. “This was startling to us. Climate change impacts will likely be far reaching.”
Ocean salinity is another important indicator. Marine mammals prey on fish, crustaceans and mollusks whose ocean water is being diluted by the freshwater runoff from melting sea ice — bad news for prey and predator alike. Underfed mammals and their compromised immune systems face greater risk of succumbing to infectious disease.
“Marine mammals represent important sentinels of aquatic health by providing information essential for managing threats to these vulnerable ecosystems,” Sanderson said in a press release. “Addressing the root causes of climate change will be of critical importance as we chart a path forward in managing these species.”
Though these variables are associated with infectious disease-driven mass deaths, the scientists noted that causal inferences are difficult to make without ruling out alternative causes and other factors, perhaps preexisting ones.
“Without historical baseline data on many of these species, it is hard to discern whether pathogens have been historically present within these populations, or they have recently emerged or been introduced,” the pair wrote. “Increased and improved marine mammal surveillance, together with infectious disease surveillance, is critical for long‐term population health monitoring of marine mammals.”
Alexander and Sanderson, who say they have both been fascinated by marine mammals since they were children, hope their research shines a light on understudied aspects of marine life.
“There has been a lot of interest in marine mammal mass mortalities and concern over these events, but there had not been a systematic assessment of the role of infectious disease and climate in these events, something we both felt was critically needed,” they said.
The researchers also noted their review of the literature only examined documents and reports published in English, which may bias their results by, for example, excluding research on marine mammals conducted in Central and South America, Asia, Africa and Europe or published in non-English journals.
“We know climate change is real and it is already having devastating consequences. We are seeing this in melting ice caps, increasing sea levels and the changing distributions of wildlife and pathogen species,” Alexander and Sanderson said. “This is yet another call to action for everyone to commit to fighting climate change. Additionally, our current experiences and studies such as this highlight the critical impact infectious diseases have on our world.”