'Sushi parasites' have increased 283-fold in past 40 years

Before you eat sushi, nigiri, or any other raw fish, you might want to quickly check for worms.

A new study led by the University of Washington finds that a worm that can infect people who eat raw or undercooked fish is becoming much more common. The worm's frequency has grown 283 times since the 1970s. This could be bad for the health of people and sea mammals, since both can eat it accidentally.

Many studies have looked at how common this parasitic worm, called Anisakis or "herring worm," is in certain places and at certain times. But this is the first study that looks at how the number of these worms around the world has changed over time by combining the data of those works. The results were written up in the journal Global Change Biology on March 19.

"Many studies were used together to show a global picture of change over almost 40 years," said Chelsea Wood, an assistant professor in the UW School of Aquatic and Fishery Sciences and the study's corresponding author. "I find it interesting that it shows how risks to people and marine mammals change over time." If you care about public health or want to know what's going on with sea mammal species that aren't doing well, you should know that.

Even though they are called herring worms, they can be found in many types of sea fish and squid. When people eat live herring worms, the parasite can get into the intestines and make people sick in ways that look like food poisoning, like throwing up, sickness, and diarrhea. Most of the time, the symptoms go away after a few days when the worm dies. Anisakiasis or anisakidosis is the name of this disease. Wood explained that it is rarely identified because people think they just had a bad case of food poisoning.

It's the bottom-dwelling shrimp or copepods that get infected first when the worms hatch in the ocean. When small fish eat infected crabs, the worms move to their bodies. This process keeps going as bigger fish eat smaller infected fish.

When people or sea mammals eat fish that has worms in it, they get sick. In humans, the worms can only live for a few days and reproduce, but in marine animals, they can stay and breed.

Chefs who work with seafood and sushi are very good at finding worms in fish and removing them before they reach customers in grocery stores, seafood markets, and sushi bars, Wood said. The worms can grow to be as long as 2 centimeters, which is about 5 cents.

"At every stage of seafood processing and sushi preparation, people are good at finding worms and removing them from fish," said Wood.

There are some worms that can get through these steps. Wood, who studies many kinds of marine bugs, said she still likes eating sushi often. People who still want to eat sushi but are worried about these worms should cut each piece in half and check each half for worms before eating it.

For the research, the study's authors looked through all of the online literature that talked about Anisakis worms and another type of parasite worm known as Pseudoterranova, or "cod worm." Based on certain criteria, they cut down the studies until they were left with only those that gave estimates of how common each worm was in fish at a certain point in time. From 1978 to 2015, the number of Anisakis worms grew 283 times, but the number of Pseudoterranova worms did not change.

Scientists think that these marine worms may be having a big effect on marine species like dolphins, whales, and seals, even though they don't pose much of a health risk to people. The worms actually lay their eggs in the guts of these animals and then get into the ocean through their waste. Scientists don't yet know how these parasites affect sea mammals' bodies, but Wood said that the parasites can live in the mammals' bodies for years, which could be bad.

"One of the important implications of this study is that now we know there is this massive, rising health risk to marine mammals," said Wood. No one often thinks that bugs could be the reason why some groups of sea mammals aren't recovering. I hope that this study makes people more interested in intestinal parasites as a way to limit the population growth of marine beasts that are rare or dangerous.

The authors aren't sure what caused the big rise in Anisakis worms over the last few decades, but they said that climate change, more nutrients from pesticides and waste, and more marine mammals over the same time period could all be factors.

The Marine Mammal Protection Act has been in place since 1972 to protect marine animals. This has helped many seal, sea lion, whale, and dolphin species grow. This is the most likely explanation, according to Wood, since the worms reproduce inside sea mammals and their numbers grew at the same time as the animals' numbers grew.

"It's possible that the recovery of some marine mammal populations has allowed recovery of their Anisakis parasites." Wood told them. "So, the rise in parasitic worms might be a good thing; it could mean that the ecosystem is doing well." In a strange twist, if the number of one marine mammal population grows because of protection and its Anisakis parasites benefit from that growth, it could make it easier for other, less protected marine mammal populations to get infected, which could make it even harder for those populations to recover.