The abundance of a worm that can get into people who eat raw fish has dramatically increased in the last 40 years, researchers report.
The 283-fold increase since the 1970s could have implications for the health of humans and marine mammals, which can both inadvertently eat the worm.
Thousands of papers have looked at the abundance of this parasitic worm, known as Anisakis or “herring worm,” in particular places and at particular times.
But the new study in Global Change Biology marks the first time researchers have combined the results of those papers to investigate how the global abundance of these worms has changed through time.
“This study harnesses the power of many studies together to show a global picture of change over a nearly four-decade period,” says corresponding author Chelsea Wood, an assistant professor in the University of Washington School of Aquatic and Fishery Sciences.
“It’s interesting because it shows how risks to both humans and marine mammals are changing over time. That’s important to know from a public health standpoint, and for understanding what’s going on with marine mammal populations that aren’t thriving.”
Find a worm, pick it out
Despite their name, herring worms can be found in a variety of marine fish and squid species. When people eat live herring worms, the parasite can invade the intestinal wall and cause symptoms that mimic those of food poisoning, such as nausea, vomiting, and diarrhea.
In most cases, the worm dies after a few days and the symptoms disappear. This disease, called anisakiasis or anisakidosis, is rarely diagnosed because most people assume they merely suffered a bad case of food poisoning, Wood says.
After the worms hatch in the ocean, they first infect small crustaceans, such as bottom-dwelling shrimp or copepods. When small fish eat the infected crustaceans, the worms then transfer to their bodies, and this continues as larger fish eat smaller infected fish.
Humans and marine mammals become infected when they eat a fish that contains worms. The worms can’t reproduce or live for more than a few days in a human’s intestine, but they can persist and reproduce in marine mammals.
Seafood processors and sushi chefs are well-practiced at spotting the worms in fish and picking them out before they reach customers in grocery stores, seafood markets, or sushi bars, Wood says. The worms can grow to up to 2 centimeters in length, or about the size of a US nickel.
“At every stage of seafood processing and sushi preparation, people are good at finding worms and removing them from fish,” Wood says.
Some worms can make it past these screening steps. Still, Wood—who studies a range of marine parasites—says she enjoys eating sushi regularly. For sushi consumers who remain concerned about these worms, she recommends cutting each piece in half and looking for worms before eating it.
Worse for marine mammals
For the analysis, researchers searched the published literature archived online for all mentions of Anisakis worms, as well as another parasitic worm called Pseudoterranova, or “cod worm.”
They whittled down the studies based on set criteria, ultimately keeping only those studies that presented estimates of the abundance of each worm in fish at a given point in time. While Anisakis worms increased 283-fold over the study period of 1978 to 2015, Pseudoterranova worms did not change in abundance.
Although the health risks of these marine worms are fairly low for humans, scientists think they may have a big impact on marine mammals such as dolphins, whales, and seals. The worms actually reproduce in the intestines of these animals and then get into the ocean via the marine mammals’ feces.
While scientists don’t yet know the physiological impacts of these parasites on marine mammals, the parasites can live in the mammals’ bodies for years, which could have detrimental effects, Wood says.
“One of the important implications of this study is that now we know there is this massive, rising health risk to marine mammals,” Wood says. “It’s not often considered that parasites might be the reason that some marine mammal populations are failing to bounce back.
“I hope this study encourages people to look at intestinal parasites as a potential cap on the population growth of endangered and threatened marine mammals.”
Climate change’s role
The researchers aren’t sure what caused the large increase of Anisakis worms over the past several decades, but climate change, more nutrients from fertilizers and runoff, and an increase in marine mammal populations over the same period could all be potential reasons, they say.
The Marine Mammal Protection Act has protected marine mammals since 1972, which has allowed many populations of seals, sea lions, whales, and dolphins to grow. Because the worms reproduce inside marine mammals—and their rise occurred over the same time period as the mammals’ increase—this is the most plausible hypothesis, Wood says.
“It’s possible that the recovery of some marine mammal populations has allowed recovery of their Anisakis parasites.” Wood says. “So, the increase in parasitic worms actually could be a good thing, a sign that the ecosystem is doing well.
“But, ironically, if one marine mammal population increases in response to protection and its Anisakis parasites profit from that increase, it could put other, more vulnerable marine mammal populations at risk of increased infection, and that could make it even more difficult for these endangered populations to recover.”
Additional coauthors are from Bates College, Washington Sea Grant, the National Oceanic and Atmospheric Administration, and the University of Washington. The Washington Sea Grant, the National Science Foundation, the Alfred P. Sloan Foundation, and the University of Washington funded the work.
Source: University of Washington