Marine pandemics fly under the radar

While infectious disease outbreaks are generally understudied in marine mammals compared to their terrestrial counterparts, a new study points to certain environmental factors as contributors to mass mortality events. As climate change amplifies these factors, it becomes crucial to monitor our oceans more closely. As the COVID-19 pandemic spreads across the globe, the reality of the scale of infectious diseases on Earth is becoming clear to everyone. But it is not enough to look only on land; we must also anticipate these kinds of events underwater. Infectious diseases are responsible for deadly pandemic waves that can ravage invertebrates, birds, mammals, and fish alike. However, such events are generally understudied in aquatic mammals compared to their terrestrial counterparts. In an effort to address these gaps, researchers from CARACAL (Center for African Resources: Animals, Communities, and Land Use) sought to assess the factors contributing to the occurrence of mass infectious events in marine mammals. As a result, mass mortality events of infectious origin occurred in 14% of marine mammal species between 1955 and 2018. Viruses are the primary cause. They account for 72% of these events and have caused 20 times more deaths than bacteria. Outbreaks of morbilliviruses and influenza A, in particular, are of concern due to their life cycles, which allow them to cross the species barrier and infect multiple types of hosts. However, it appears that the social behavior and trophic level (position in the food chain) of marine mammals do not seem to be aggravating factors in mass infectious events. In contrast, habitat size and fluctuations are. More than half of the species that have experienced mass mortality events within a short period of time are pinnipeds—that is, members of the family of seals, sea lions, and walruses.

In 2000, for example, more than 10,000 endangered Caspian seals died in less than four months from an infection with the canine distemper virus.

These semi-aquatic animals, which bridge the land and sea, appear to be more vulnerable to pathogens from both environments. Furthermore, environmental variables—such as seasonal and temperature anomalies—are also associated with disease outbreaks among marine mammals. 61% of mass mortality events among marine mammals occurred during periods when regional sea surface temperature anomalies were reported. Thus, the incidence rate of a mass infectious event increases by nearly 12% for each temperature anomaly. The chemical balance of the oceans is also changing, and melting ice and changes in ocean salinity have implications for the spread of disease. As climate change intensifies and temperatures warm, complex chains of events will be disrupted, thereby destabilizing the delicate balances of marine ecosystems. This is all the more true given that 37% of marine mammals that have experienced an infectious mass mortality event are endangered or vulnerable. Marine mammals serve as sentinels of aquatic health. They provide essential information for managing the threats to these vulnerable ecosystems. This study highlights the critical threat that infectious diseases can pose to marine species and the potential significance of climate change as a major driver of this process.