Human Parasites in Raw Fish
Can People Catch Parasitic Diseases From Raw Seafood?
Aug 12, 2009 Rosemary Drisdelle
Millions of people host parasites that they have acquired from eating raw fish. Rare in some countries, very common in others, some of these infections are dangerous
Most parasitic infections acquired from raw fish involve helminths (worms): cestodes (tapeworms), trematodes (flukes), or nematodes (round worms).Typically, these parasites live in several different hosts as they complete their life cycle, often passing from one host to the next by being eaten. Parasites like Diphyllobothrium latum (fresh water fish tapeworm) and Clonorchis sinensis (Chinese liver fluke) normally live in humans during their adult life. Other parasites, usually infesting a different animal, infect humans accidentally.
While parasites found in raw fish are typically associated with particular areas of the world, they can occur virtually anywhere today because of global travel, international trade in fish and seafood, and the spread of parasite species to new places. Most at risk, of course, are people who eat raw, lightly smoked, lightly salted, dried or pickled fish. Snails and crustaceans such as crabs and crayfish can also carry parasites.
Diphyllobothrium latum – Fresh Water Fish Tapeworm and Raw Fish
Diphyllobothrium latum is a large intestinal tapeworm of humans, found worldwide, but most common in North America, Northern Europe, and Japan. People acquire the cestode from eating raw fish, usually pike, perch, and burbot. In North America, salmon eaten with sushi, or lightly smoked, is probably the most common source, and the incidence is thought to be rising.
Though the infective plerocercoid larvae are often large enough to see in raw fish, the exercise of searching for them is likely to spoil a meal. Infection is usually without symptoms other than passing lengths of tapeworm into the toilet. Occasionally the host suffers a serious vitamin B12 deficiency with anemia.
Clonorchis sinensis – Chinese Liver Fluke and Raw Fish
Clonorchis sinensis, Chinese liver fluke, is common in the Far East. Outbreaks have occurred elsewhere after people ate imported fish.The worm is usually acquired by eating raw fresh water fish in sushi and other dishes. In addition, a number of similar flukes inhabit lake fish in other parts of the world, including North America and northern Europe, that can cause disease in humans.
At a fraction of a millimeter, the infective syage of C. sinensis in raw fish is too small to see. Though many infected people have only a few worms and no symptoms, C. sinensis can cause varying degrees of abdominal pain, diarrhea, and liver damage. The parasite is associated with liver cancer.
Paragonimus westermanii – Oriental Lung Fluke and Raw Seafood
Those who eat raw or undercooked crab or crayfish may acquire Paragonimus westermanii or a related lung fluke. Asia, Africa, Central and South America, and some Pacific islands are areas of highest risk. Adult Paragonimus spp. worms live in lung tissue.
Like C. sinensis metacercariae, the infective stage of Paragonimus spp. found in crabs and crayfish is tiny, so victims won’t have any idea the food isn’t safe. The infection may cause no symptoms, or the infected person suffers chest pains, various respiratory symptoms, and lung damage. Not uncommonly, worms travel to other parts of the body including the brain and heart where they sometimes cause fatal illness.
Anisakis simplex – Herring Worm and Raw Fish
Anisakis simplex, herring worm or whale worm, is one of a group of nematodes that are parasites of ocean mammals. Others include Pseudoterranova decipiens, cod worm or seal worm; Contracaecum spp.; and Phocascaris spp. Various salt water fish harbour larvae of these parasites and humans are infected when they eat raw, pickled, dried, smoked or salted marine fish.
Anisakid larvae in fish are typically flat and coiled, or coiled in a corkscrew shape. They are easily visible in thin slices of fish, or when thicker portions are held up to bright light. When swallowed alive, the larva attempt to continue development by boring into the wall of the stomach or small intestine. Symptoms include severe abdominal pain, vomiting, and diarrhea that can linger for long periods if the worm isn’t diagnosed and removed. Freezing at -20°C and thorough cooking kills the larvae.
Capillaria philippinensis and Raw Fish
First discovered in the Philippines, Capillaria philippinensis has infected people in Southeast Asia, Japan, the Middle East and Africa. It is thought to be a parasite of birds acquired accidentally by people when they eat small raw fish (Since the parasite is located in the fish’s entrails, a larger raw fish that has been cleaned is much less risky.)
Capillaria philippinensis infection is rare but serious. The worms can multiply to large numbers in the intestine. Symptoms include diarrhea, abdominal pain, severe intestinal damage, weight loss, dehydration, weakness, and when left untreated, death.
Gnathostoma spinigerum and Raw Fish
Gnathostoma spinigerum is commonly found in dogs and cats, and infects people accidentally when they eat raw fish, or the raw flesh of other animals. It is found in Southeast Asia and Japan, although reports of other Gnathostoma spp. come from all over the world.
In the human, an ingested larva doesn’t mature, but moves through the intestinal wall causing abdominal pain and vomiting. Thereafter it creeps about through the tissues causing swellings and inflammation that come and go and move from one place to another. The larva may exit through the skin or cause abscesses. If the larva enters the brain, the consequences can be serious.
Protozoa
Although raw fish does not typically contain parasitic protozoa, fish that is caught in contaminated water and eaten without thorough cooking can pass disease-causing protozoa, bacteria, and viruses to people.
Sources
Foundations of Parasitology 6th ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
Medical Parasitology: Cestodes of the Intestinal Tract. Drisdelle, Rosemary. Canadian Society for Medical Laboratory Science; Course no. 9887, Nov 2007.
Medical Parasitology: Trematodes of the Intestinal and Urogenital Tracts. Drisdelle, Rosemary. Canadian Society for Medical Laboratory Science; Course no. 9889, Nov 2007.
Parasites of the Colder Climates. Akuffo, Hannah, Ewert Linder, Inger Ljungström, and Mats Wahlgren, eds. London: Taylor and Francis, 2003.