A fish tapeworm infection can occur when a person eats raw or undercooked fish that’s contaminated with the parasite Diphyllobothrium latum. The parasite is more commonly known as the fish tapeworm.

This type of tapeworm grows in hosts such as small organisms in the water and large mammals that eat raw fish. It’s passed through the feces of animals. A person becomes infected after ingesting improperly prepared freshwater fish that contain tapeworm cysts.

Fish tapeworm infections rarely present noticeable symptoms. Tapeworms are most often discovered when people notice eggs or segments of the tapeworm in stool.

Symptoms could include:

A fish tapeworm infection occurs when a person eats undercooked or raw fish that’s contaminated with fish tapeworm larvae. The larvae then grow in the intestines. It takes between three to six weeks before they’re fully grown. An adult tapeworm can grow up to 30 feet long. It’s the largest parasite to affect humans.

The journal Emerging Infectious Diseases published a report that examined the spread of fish tapeworm infections in Brazil. Infections were linked to contaminated salmon farmed at aquaculture sites in Chile. The transportation of the contaminated fish from Chile brought the infection to Brazil, a country that hadn’t seen fish tapeworms before.

The report highlighted how fish farming can spread the infection from one area to another. The cases cited in the report all stemmed from people eating salmon sushi.

This type of tapeworm parasite is most common in areas where people eat raw or undercooked fish from lakes and rivers. Such areas include:

  • Russia and other parts of Eastern Europe
  • North and South America
  • some Asian countries, including Japan

It may also be common in parts of Africa where freshwater fish are eaten.

In addition, fish tapeworms are seen in developing countries because of sanitation, sewer, and drinking water issues. Water contaminated with human or animal waste could very likely contain tapeworms. Fish tapeworm infection was regularly diagnosed in Scandinavia before improved sanitation methods were introduced.

Your doctor may order a blood test to identify the presence of a parasite. However, this type of infection is most often diagnosed by examining a person’s stool for parasites, worm segments, and eggs.

Fish tapeworm infections can be treated with a single dose of medication without any lasting problems. There are two main treatments for tapeworm infections: praziquantel (Biltricide) and niclosamide (Niclocide).

  • Praziquantel. This drug is used to treat different kinds of worm infections. It causes severe spasms in the worm’s muscles so the worm can be passed through the stool.
  • Niclosamide. This drug is prescribed specifically for tapeworm infections and kills the worm on contact. The dead worm is later passed through the stool.

If left untreated, fish tapeworm infections can cause serious problems. These complications may include:

Fish tapeworm infections can be easily prevented. Use the following guidelines:

  • Cook fish at a temperature of 130°F (54.4°C) for five minutes.
  • Freeze fish below 14°F (-10.0°C).
  • Follow proper food safety handling, such as washing hands and avoid cross-contamination with raw fish and fruits and vegetables.
  • Avoid contact with any animal known to be infected with a tapeworm.
  • Exercise caution when eating and traveling in developing countries.