Introduction:
Fish tapeworm infection, also called diphyllobothriasis, occurs when uncooked or raw fish contaminated by the parasite Diphyllobothrium latum (fish tapeworm larvae) are consumed from freshwater rivers and lakes. In addition, smoked and marinated fishes may also carry fish tapeworm cysts. It is the largest parasite to infect humans. It is mostly seen in African countries, North and South America, Asian countries like Japan, Eastern Europe, Russia, and Scandinavia. It is also seen in developing countries where sanitation and drinking water is an issue where water is contaminated by human and animal wastes and could contain fish worms. In Japan, the popularity of sushi and sashimi, which contains raw slices of fish meat, may also contribute to causing this infection.
What Is Diphyllobothrium Latum (Fish Tapeworm)?
Diphyllobothrium latum or fish tapeworm is a type of parasite (an organism that lives on or in the host and feeds on it); it is from the family of pseudophyllidean cestodes, which are tapeworms with multiple segments and has two bothria (sucking grooves) which are transmitted via aquatic species. It is one of the largest invasive parasites to infect animals and humans. Humans are infected by these when they eat uncooked or raw fish infected by this parasite. After consuming the infected fish, it may take three to six weeks to mature, and these parasites may grow up to 30 feet long in human intestines and live up to 30 years, and the person may have multiple tapeworms.
How Is the Lifecycle of Fish Tapeworm?
The eggs of the fish tapeworm hatch into embryos called coracidia after two weeks in freshwater. These embryos are ingested by the first intermediate host (usually calanoid copepods species) and develop into the first larval stage within two to three weeks. When fish eat these intermediate hosts, the larvae migrate into the muscle fibers of fish and develop into the second larval stage. These infected raw fish, when consumed, the larva gets attached to the walls of the intestine and grows. They take three to six weeks to become adult worms. This adult worm attaches to the segments of the intestine, produces up to a million eggs, and absorbs nutrients from the food that passes through the intestine. The stool may contain eggs or parts of the worm.
What Is the Morphology of Fish Tapeworms?
The parasite is white or ivory colored, the adult worm can measure up to ten meters, and it has 3000 to 4000 segments. The head of the tapeworm is elongated and has two slit-like suckers with grooves called bothria. Mature worms are broader than long. The genital pore is centrally positioned.
What Are the Symptoms of Fish Tapeworm Infection?
Most people are asymptomatic. People notice only when they discover eggs or fragments of worms in their stools. If symptoms occur, they may have;
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Gastrointestinal manifestations like diarrhea, constipation, abdominal pain or stomach cramps, or discomfort.
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Dermatological manifestations like glossitis (swollen and inflamed tongue) and pallor.
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Subacute appendicitis (inflammation of the appendix with sudden pain).
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Migrating segments can cause cholecystitis (gallbladder inflammation) and cholangitis (bile duct inflammation).
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Other manifestations include lack of appetite (chronic hunger), fatigue, and weight loss.
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Vitamin B12 deficiency, in case of prolonged infection for more than three to four years, can cause this deficiency; vitamin B12 deficiency occurs mainly when the adult worm is located in the ileum and is less risky in the duodenum, where the parasite uses up heavy amounts of vitamin B12, or by dissociation of vitamin B12 intrinsic factor in the gut, both making it unavailable to the host. Further, it can be worse if the person has folate deficiency which can cause injury to the nervous system. Long-standing deficiency of vitamin B12 can lead to:
- CNS manifestations like paresthesia (burning or numbness feeling in the extremities), encephalopathy (brain disease that can alter brain function or its structure), polyneuropathy (damage of peripheral nerves), and headaches.
- Ocular manifestations like optic neuritis (inflammation of the optic nerve) and optic nerve atrophy (damage to the optic nerve that can cause affect vision).
- Hematological manifestations like pernicious anemia (an autoimmune disease that causes vitamin B12 deficiency), megaloblastic anemia (a blood disorder where the bone marrow produces large red blood cells), pancytopenia (a condition where platelets, red and white blood cells are lower than normal), and eosinophilia (a type of white blood cells which are lower than normal).
- Respiratory manifestations like dyspnea (difficulty in breathing).
What Are the Investigations of Fish Tapeworm Infection?
Fish tapeworm infections can be diagnosed by the following methods:
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Complete blood count and differential blood count.
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Molecular biological methods are used to differentiate subspecies of Diphyllobothrium.
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Vitamin B12 level and blood tests in case of anemia.
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Stool examination, which contains fragments of worms and eggs.
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Doctors can do additional tests such as endoscopy, abdominal magnetic resonance imaging (MRI), and colonoscopy.
What Is the Treatment of Fish Tapeworm Infection?
Treatment for fish tapeworms is usually a single dose. The drug of choice is 10 to 25 milligrams per kilogram of Praziquantel, which causes severe spasms in the worm’s muscles, and the worms are passed in stools. Niclosamide is another drug that kills the worm in contact and passes in stools. The stool culture will show negative after a week of this treatment. Rarely a patient may require a second dose. These drugs are safe in pregnancy. In case of vitamin B12 deficient anemia, vitamin B12 injections and supplements are given.
What Are the Complications of Fish Tapeworm Infection?
The common complications of fish tapeworm infections include the following:
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Blockage of the intestine, which is rare.
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Megaloblastic anemia/ pernicious anemia.
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Long term presence of this worm can cause injury to the nervous system.
How to Prevent Fish Tapeworm Infection?
Prevention of fish tapeworms includes;
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People should avoid the consumption of raw fish.
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Adequate freezing (−20 °C for seven days), flash freezing (-35 ° C for 15 hours), and cooking (55 °C for 5 minutes) can kill the parasite.
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People should maintain hygiene to avoid contamination of raw fish with other food items.
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Caution should be taken while traveling to other countries where people consume more raw fish.
Conclusion:
Fish tapeworms are the largest parasite that can enter the human digestive system. However, they do not cause serious symptoms. A person may feel emotionally low because of the excretion of segments of worms in stool. Nevertheless, a person should always be aware of this infection, take care while traveling and having food outside, and avoid eating raw fish unless it is frozen and cooked properly. In restaurants, chefs should follow guidelines while cooking raw fish to avoid these infections.
