A few fishy tales about worms

If you have a dog at home you’ll know full well that Buddy or Bella must be wormed regularly - for their sake as well as your own, so you don’t become infected too.

What you may not know is that there are parasitic infestations that can be contracted by humans through the food we eat when the source, like raw or undercooked meats, is contaminated1. Also, three of the more well-known ones are transmitted through raw, undercooked or pickled seafood, and tourists who are travelling on standard itineraries, taking in cities and large towns in developing countries are potentially at risk. On rare occasions these infections occur in developed nations too.

If you’re a fan of sushi, sashimi, ceviche, gravlax or marinated anchovies, this could be of interest to you …

In one high risk country, Japan, authorities have issued a health notice2 in response to a recent rise in the number of human cases of marine roundworm infections caused by the larvae of the Anisakis nematode (worm). Anisakiasis, as the infection is known, is most commonly contracted when people eat contaminated raw seafood (fish, eels, octopus and squid) in sushi and sashimi, but it can also be a risk if the dish you are scoffing contains infected fish that is cured with salt or vinegar (pickled or smoked herring). (The recommendation is for restaurants serving raw seafood to freeze it for an extended period of time before serving to ensure any larvae have been killed.)

Other countries with high rates of consumption of raw, smoked or cured seafood also have a higher incidence of infections and these include the Netherlands, Scandinavia, Spain and the west coast of South America3.

The life cycle of Anisakis worms starts as eggs released into the ocean by infected marine mammals. The eggs develop into larvae, which then become part of the food chain: eaten by crustaceans which are then eaten by fish or octopus which are then eaten by humans. A person consuming the larvae-containing raw seafood may or may not notice an unusual, tingling sensation - the worm passing through their mouth - and it can then be removed or coughed/vomited out. If the worm is swallowed, it can move to the stomach or intestines and become embedded, shielded from gastric acid by a protective coating. Ultimately the larva will die, but before that occurs, it can cause inflammation, peritonitis or obstruction.

The US Centers for Disease Control & Prevention (CDC) webpage on Anisakiasis4 lists the signs and symptoms of infection as: ‘abdominal pain, nausea, vomiting, abdominal distention, diarrhoea, blood and mucus in stool, and mild fever. Allergic reactions with rash and itching, and infrequently, anaphylaxis, can also occur.’

The very good news is that Anisakis infection can be treated by using an anthelmintic medication, or the worm can be removed by endoscope. If it has become embedded or moved outside the intestines, surgery may be required.

The other two more common helminth infections that are transmitted through eating raw or undercooked seafood, Clonorchiasis and Diphyllobothriasis, have similar life cycles to the Anisakis nematode with the exception that humans and terrestrial mammals are the infective hosts.

The fish or broad tapeworm that causes diphyllobothriasis is much more widespread, being found in Europe, North America, and Asia, as well as Chile and Uruguay in South America. Furthermore, exportation of fish from endemic countries can lead to human cases in non-endemic regions. Just over 10 years ago, and after extensive testing, a few cases were diagnosed in sushi-eaters in Brazil5. Brazil doesn’t have the climate to support fish farming and tracing of the product showed it was sourced from freshwater lakes in southern Chile.

As with aniskaniasis, the larvae are consumed when contained within the raw seafood, but Diphyllobothrium larvae move to the small intestine of the host and attach to the lining. There they mature into adult tapeworms - the largest tapeworm to affect humans – and grow up to 10 metres in length. They are also prolific egg producers – up to 1 million per day, per worm. Less than one-quarter of infected people will experience symptoms: abdominal pain or discomfort and diarrhoea are common, pernicious anaemia from Vitamin B12 malabsorption, inflammation of the gall bladder and intestinal obstruction are also possible outcomes. Treatment of uncomplicated diphyllobothriasis also involves administration of anthelmintic medication.

In the endemic countries of Korea, China, Taiwan, and Vietnam, Chinese or Oriental liver fluke (Clonorchis sinensis) infection or clonorchiasis is also known to occur through eating salted, pickled, or smoked freshwater fish containing the immature parasitic flatworm or metacercariae. The larvae mature inside the human small intestine after ingestion and move to the bile ducts to mature, producing acute phase symptoms of abdominal pain, nausea and diarrhoea. Long-term infections can lead to inflammation of the gall bladder, gall stones, pancreatitis, and cancer of the bile ducts. As with Anisakiasis, treatment is through anthelmintic medication or surgery.

While these infections are rare, they are noteworthy reasons behind the travel medicine mantra on food selection – ‘Peel it, boil it, cook it, or forget it!

1. https://www.cdc.gov/parasites/food.html
2. http://outbreaknewstoday.com/japan-sushi-rise-anisakis-90077/
3. https://web.stanford.edu/group/parasites/ParaSites2010/Lucia_Constantine/parasiteproject/Anisakiasis.htm
4. https://www.cdc.gov/parasites/anisakiasis/
5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2725803/