Parasitic worms that infect humans are on the increase in ocean fish

A University of Washington researcher has found that infestations of two parasitic worms in fish have increased substantially in recent decades.

The marine-parasite researcher, Chelsea Wood, studied changes in parasitic nematode worms—specifically, the genera Pseudoterranova and Anisakis—that infect shellfish and fish as well as marine mammals such as seals and whales.

Both of the parasitec can cause illness in humans who eat undercooked or raw fish and squid.

According to a December 21 article in the online journal Hakai Magazine, Wood and fellow researchers found a one-and-a-half-times increase in Pseudoterranova numbers in fish globally between 1978 and 2015.

But the average number of Anisakis per fish grew a whopping 90 times between 1962 and 2015.

The researchers examined preserved fish specimens from a local museum collection. “The parasites are preserved along with the specimen and are still detectable,” Wood says in the Hakai article. “You can see what the parasites in a fish from 1888 were like.”

Another parasitic worm, Clavinema mariae, which doesn't cause illness in humans, increased its numbers eight times in local English sole since the 1930s.

Wood says that normal public-health precautions for consuming raw fish should protect against people becoming infested.

Meanwhile, an August 24, 2018, scientific paper published in the journal Emerging Infectious Diseases by a team of Japanese researchers concluded that human infection by a tapeworm found in raw North Pacific salmon, Dyphyllobothrium nihonkaiense, "is becoming a global threat due to the increasing consumption of raw salmon worldwide".

D. nihonkaiense can grow to lengths of up to 10 metres in human digestive tracts and lay millions of eggs, which are excreted by the hosts in feces. In humans, effects of D. nihonkaiense infection are often asymptomatic or produce abdominal pain and diarrhea.