A study led by the University of Washington finds dramatic increases in the abundance of a worm that can be transmitted to humans who eat raw or under-cooked seafood. Its 283-fold increase in abundance since the 1970s could have implications for the health of humans and marine mammals, which both can 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 this is the first study to combine 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," said corresponding author Dr Chelsea Wood, an assistant professor in the UW 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."
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 diarrhoea. 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 explained.
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 explained. The worms can be up to 2 centimetres in length, or about the size of a US 5c nickel.
"At every stage of seafood processing and sushi preparation, people are good at finding worms and removing them from fish," Wood said.
Some worms can make it past these screening steps. Still, Wood – who studies a range of marine parasites – said 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.
For the analysis, the study's authors 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 be having a big impact on marine mammals such as dolphins, whales and seals. The worms actually reproduce in the intestines of these animals and are released into the ocean via the marine mammals' faeces. 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 said.
"One of the important implications of this study is that now we know there is this massive, rising health risk to marine mammals," Wood said. "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."
The authors 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 said.
Marine mammals have been protected under the Marine Mammal Protection Act 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 said.
"It's possible that the recovery of some marine mammal populations has allowed recovery of their Anisakis parasites." Wood said. "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."
Abstract
The Anthropocene has brought substantial change to ocean ecosystems, but whether this age will bring more or less marine disease is unknown. In recent years, the accelerating tempo of epizootic and zoonotic disease events has made it seem as if disease is on the rise. Is this apparent increase in disease due to increased observation and sampling effort, or to an actual rise in the abundance of parasites and pathogens? We examined the literature to track long‐term change in the abundance of two parasitic nematode genera with zoonotic potential: Anisakis spp. and Pseudoterranova spp. These anisakid nematodes cause the disease anisakidosis and are transmitted to humans in undercooked and raw marine seafood. A total of 123 papers published between 1967 and 2017 met our criteria for inclusion, from which we extracted 755 host–parasite–location–year combinations. Of these, 69.7% concerned Anisakis spp. and 30.3% focused on Pseudoterranova spp. Meta‐regression revealed an increase in Anisakis spp. abundance (average number of worms/fish) over a 53 year period from 1962 to 2015 and no significant change in Pseudoterranova spp. abundance over a 37 year period from 1978 to 2015. Standardizing changes to the period of 1978–2015, so that results are comparable between genera, we detected a significant 283‐fold increase in Anisakis spp. abundance and no change in the abundance of Pseudoterranova spp. This increase in Anisakis spp. abundance may have implications for human health, marine mammal health, and fisheries profitability.
Authors
Evan A Fiorenza, Catrin A Wendt, Katie A Dobkowski, Teri L King, Marguerite Pappaionou, Peter Rabinowitz, Jameal F Samhouri, Chelsea L Wood
[link url="https://www.sciencedaily.com/releases/2020/03/200319090232.htm"]University of Washington material[/link]
[link url="https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.15048"]Global Change Biology abstract[/link]