Anisakis Worms
© GONZALO JARA/SHUTTERSTOCK
Anisakis worms in blue whiting fish.
The presence of parasitic worms in raw or undercooked seafood, popular in dishes such as sushi, sashimi, poke and carpaccio, has increased 283-fold since the 1970s, according to a study published in the journal Global Change Biology.

The parasite, called Anisakis or "herring worm" is found in several species of fish and squid. When people inadvertently eat the worm, it can attach to the intestinal wall and cause symptoms akin to food poisoning, such as nausea, vomiting and diarrhoea.

This disease, known as anisakiasis or anisakidosis, often goes under the radar because people understandably mistake it for a bad case of food poisoning, says senior author Chelsea Wood from the University of Washington, US.

Usually, the worm dies after a few days in humans and the symptoms disappear, although some cases can cause more severe reactions, including acute stomach pain, and persevere for months.

In marine animals, the parasite reproduces and is expelled in their faeces, from where it can infect crustaceans, such as bottom-dwelling shrimp or copepods, which are then eaten by small fish and from there on up the food chain.

Recent upsurges in the disease from several regions including Japan, the US and Europe has led to it being labelled an emerging zoonosis; whether this due to better detection, increased raw fish consumption or more parasites is unclear.

To investigate, Wood and colleagues collated data from thousands of papers that have investigated the proliferation of this worm over time.

They searched published literature on the global abundance of Anisakis and another parasite called Pseudoterranova, or "cod worm", which have different hosts, habitats and reproductive requirements, and ran a meta-analysis.

Results showed that while Anisakis numbers increased over the studied period, Pseudoterranova abundance remained the same.

While the health risks to humans are relatively low, scientists are concerned about the impact the worms could be having on marine mammals such as seals, dolphins and whales, in whose bodies they can thrive for years.

"One of the important implications of this study is that we know there is this massive, rising health risk to marine animals," says Wood. "It's not often considered that parasites might be the reason that some marine mammal populations are failing to bounce back."

She hopes their study will prompt intestinal parasites to be considered as a contributing factor to endangered or threatened marine animal populations.

Although the researchers aren't sure why Anisakis worm populations have increased, they suggest climate change, increased nutrients from fertiliser and runoff and increased marine populations - due to their protection - could all play a role.

But they note that "this is the story of only two parasite species among millions that are extant," proposing further broad-scale exploration of other marine parasites.

"Only then will we have the data to indicate whether contemporary oceans are facing a 'rising tide' of marine disease," they write.

As far as humans go, seafood processors and sushi chefs are experienced at spotting the worms, which can be up to two centimetres long, and removing them from fish before they reach people's dinner plates.

But some worms elude these screening processes. Wood herself enjoys a regular meal of sushi and recommends that consumers who are concerned can cut each piece in half to search for the little parasites before indulging.