August 19, 2020 — The ongoing coronavirus pandemic is a reminder of the devastation disease outbreaks can cause. But such disasters do not only affect humans. New research led by Claire Sanderson, a wildlife epidemiologist and immunologist at Virginia Polytechnic Institute and State University, shows that disease outbreaks among marine mammals have quietly been on the rise. Between 1955 and 2018, a sixth of marine mammal species have suffered a mass die-off caused by an infectious disease.
Reports of disease-induced mass die-offs in marine mammals have been increasing since at least 1996. This could be due in part to increased surveillance. However, it’s also likely that scientists are still underestimating the true numbers of outbreaks in these populations. Marine mammals travel great distances in remote parts of the oceans, and often the only indication that something has gone wrong is when carcasses start washing up on shore.
Disease dynamics in marine systems are relatively unexplored compared to those on land. To address this, Sanderson combed through decades of published work documenting the occurrence of disease-driven mass deaths. The majority of outbreaks, she found, were caused by viruses such as influenza A and strains of Morbillivirus—viruses that cause pandemic flus and measles in humans, respectively.
Bacteria are the next most common causes of mass die-offs, but these die-offs tend to be less severe. On average, a viral outbreak causes roughly 7,000 marine mammal deaths, while a bacteria-induced mass mortality event causes 350 deaths. Compared with death tolls from the largest outbreaks in human populations these may appear small, but for already threatened animals such as Mediterranean sperm whales and pilot whales, even the loss of a few animals endangers the population’s long-term survival.