Sashimi, nigiri and other forms of raw fish have just gotten a whole lot fishier. According to research led by the University of Washington (UW), the abundance of a parasitic worm, known as Anisakis or “herring worm”, has dramatically increased since the late-1970s. The worms, which thrive in fish and marine animals, can also be transmitted to humans who eat raw or undercooked seafood.
"This study harnesses the power of many studies together to show a global picture of change over a nearly four-decade period," corresponding author Chelsea Wood, an assistant professor in the UW School of Aquatic and Fishery Sciences, said in a statement. "It's interesting because it shows how risks to both humans and marine mammals are changing over time."
From a collection of over 100 papers, Wood and her colleagues analyzed the data to generate an average number of worms per fish from different locations and in different years. The results, which were published in Global Change Biology, show a significant 283-fold increase in Anisakis worms between 1978 and 2015. This information is important to know from a public health standpoint and for understanding the populations of marine mammals that host these worms, Wood explained.
For humans that ingest Anisakis worms, symptoms can include nausea, vomiting, and diarrhea – similar to food poisoning. In most cases, these symptoms disappear quickly, as the worms die out within a few days. Thankfully, seafood processors and sushi chefs are well-practiced at spotting the 2-centimeter-long worms and can extract them from fish before any consumer tucks into their meal. Even after studying a range of marine parasites, Wood still eats sushi regularly, but for those concerned she recommends cutting each piece in half and looking for worms before eating it.
Under the sea, the worms follow a different story. Beginning their lifecycle in the intestinal tracks of marine mammals, such as whales and dolphins, they are excreted out in their feces before hatching and infecting small crustaceans, such as bottom-dwelling shrimp or copepods. As small fish eat these crustaceans, and then larger fish eat the smaller fish, the worms are transferred to their bodies.
Unlike in humans, where health risks of these worms are fairly low, they can persist and reproduce in the aquatic animals, before being released via feces, starting the whole cycle again. Whilst the physiological impacts are not yet known by scientists, the fact that these worms can survive in these mammals’ bodies for years could indicate some side-effects.
"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."
Although the authors aren’t sure what caused the huge increase of Anisakis worms of the last few decades, climate change and more nutrients from fertilizers and run-off, could all be potential reasons. However, the complex relationship between marine mammals and the worms seems like the most plausible hypothesis, according to the researchers.
"It's possible that the recovery of some marine mammal populations [under the Marine Mammal Protection Act, since 1972] has allowed recovery of their Anisakis parasites," Wood said. “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."
Further questions about the meaning of the Anisakis population increase were also acknowledged by the researchers. However, as the literature was heavily weighted to the past 50 years, the authors couldn’t obtain a baseline figure for their abundance before ocean ecosystems were impacted by fishing, pollution, and climate change. Therefore, questions such as whether the worms’ increasing population is a result of a rise in infection or a recovery of Anisakis to a pre-impact figure remain unknown.