Disease is an inevitable process, foundational to the cycling and continuation of life. As humans we have worked tirelessly to stave off disease in the many clothes it wears. For other organisms, disease is a garment that is not so easily shed. There is no treatment, there is no second chance for most organisms with acute illness. We spent $7.5 trillion fighting our own diseases last year. The vast majority of this spending occurs to fight existing illness rather than preventing illness before its onset.

Of the small fraction spent on prevention, a smaller fraction was spent understanding the risks of new diseases that may arise from emerging viruses in animals. Our short-sighted spending strategy has contributed to our unpreparedness in the wake of the new COVID-19 pandemic. Acute global events like this may temporarily shift our investments to long term proactive strategies, but when the threat has passed, our short-term reactive tendencies may once again prevail. Such is the story we are writing for diseases of wildlife. Despite the known benefits of wildlife and ecosystem services in supporting human economies and human health, we invest very little in wildlife health. Meanwhile, our movements across the globe have introduced new diseases that have ravaged wildlife populations.

Chytrid fungus has spread globally, causing massive declines in amphibian populations. Image credit: Joel Sartore, Nat Geo

Exposure to disease from new lands and seas is one of the most pressing concerns for wildlife survival. The natural process of disease that once regulated populations is now decimating populations that have no defense against novel pathogens. On land, when pestilence and plague threaten produce, pork, poultry, or peace, humans and governments invest to protect our interests. In sea, these threats are not gone; they are unseen. As humans expand our reach in the seas, threats to marine life mount, led by a disease front and backed by changing climate and habitat. 2013 saw the outbreak of sea star wasting disease, which cut through large swaths of sea star populations along the west coast of the United States, reducing populations by over 90% in some areas. The disease was driven by warming sea temperatures, which exacerbated its pathogenicity.

Sea otters are familiar faces to visitors of the west coast, though this was not the case a century ago. Hunted to just 40 individuals, the southern sea otter that previously was found in abundance up to 16,000 in California, has seen recovery in recent decades. Now at over 3,000 individuals, the majority of otter deaths is attributed to infectious disease. Toxoplasmosis is a common and pervasive disease of otters, contributing to over 15% of deaths. Where does the pathogen come from? House cats. Run-off and sewage flow transport a pathogen commonly found in these pets to the sea, where otters can be infected.

The thorny-headed worm (known by scientists as an acanthocephalan) is another pathogen on the rise in recent years that is also likely contributing to more otter deaths. This parasite is found in sand crabs and transmitted to birds that feast on these crabs. Simultaneously growing otter populations and declining presence of high-quality prey like abalone has contributed to an increase in consumption of sand crabs by otters. This is like trading a steak for a bag of potato chips! The potato chips are cheap and easy to eat, but not very good for you. In the past 50 years, the thorny-headed worm has gone from infecting 1% of otters to nearly 50%. Now, the parasite contributes to one of every four otter deaths.

Solving this problem is an incredibly large task, and I am searching for answers to how and why otters become infected. This includes working with an incredible team of scientists and staff at the California Department of Fish and Wildlife without whom my work would not be possible. With them, I am able to dissect many of the otters that die along the coast of California to determine the cause of death. It is certainly not always glamorous (or clean) work, but it is important to investigate the presence of the parasite in otters.

Thorny-headed worm of the Profilicollis spp. variety. Image credit: Colleen Young and Richard Grewelle

Parasites themselves are not an appetizing subject, not especially good dinner table conversation. They, however, are fascinating and important components of natural ecosystems. Thorny-headed worms compose their own phylum and are the only animal phylum to be completely parasitic. They rarely infect humans, which means that many of us have never heard of them. I have found through my work that the species that infect otters also infect lots of bird species. Besides having the ability to live in a variety of hosts, these worms are uniquely skilled in many ways, including their ability to absorb nutrients through their outer layer. Their absorption capabilities also make them good bioaccumulators of toxins and heavy metals, a property which some scientists have used to measure pollution in aquatic ecosystems.

There is a lot we don’t know about these parasites, but we do know that at low numbers they are not particularly harmful to birds. The problem comes when high numbers of them are ingested by sea otters. Reducing disease caused by thorny-headed worms may rely on aiding the recovery of abalone populations. Future recovery of sea otter populations relies on our ability to understand and act to mitigate their biggest threats. Their presence adds value to ecosystems, and as a result, improves ecological and economic services we receive from the coastal ocean. Fighting their disease improves our well-being more than we know.

Richard Ernest Grewelle IV is a 5th year PhD student at Hopkins. He studies how disease impacts wildlife and humans. Funding for this fieldwork and research comes from the Sea Otter Foundation & Trust, Myers Trust, Stanford Graduate Fellowship, and ARCS Fellowship.