But is that due to better detection or a real increase? And what implications does it have for humans and marine mammals?
The herring worm, Anisakis, is found naturally in a variety of marine fish and squid species. They are less than an inch long, but that is obviously large in a fish, so seafood processors and sushi chefs are well-practiced at spotting them and picking them out before they reach customers. Some will still make it through and 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 often undiagnosed because most people assume they merely suffered a bad case of food poisoning.
While they can't reproduce or live for more than a few days in a human's intestine, in the ocean, the scenario is much different. After hatching, 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.
Why the dramatic increase in parasites? It's ironically the result of better marine animal protection
The study's authors searched the published literature archived 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.
The dramatic increase isn't impacting humans in a meaningful way, the last thing sushi chefs want is showing up in your Instagram feed with a worm the size of a nickel, but since the worms actually reproduce in the intestines of marine creatures animals and are released back into the ocean via the marine mammals' feces or live in those mammals' bodies for years, the impact is still unknown.
The reason for the increase in marine mammal populations since the 1970s could be due to record-keeping or it could be better marine animal protection.
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.
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