What is Diphyllobothriasis (Fish Tapeworm Infection)?
Diphyllobothriasis is a type of infection caused by broad or fish tapeworms. People are paying more attention to this disease because there’s been a rise in reported cases in areas where it’s not usually seen. We’re also learning more about the worm itself thanks to molecular science studies.
These tapeworms, known as broad or fish tapeworms, hold an unfortunate record for being the longest intestinal parasites that can live in humans. These creatures mature into adult tapeworms in the intestines of their hosts after a mammal or bird consumes infected fish. A person might not show any signs of this infection. Sometimes, the only clue is when the mature worm is expelled through the person’s stool. When symptoms do appear, they can include diarrhea, stomach discomfort, constipation, fatigue, headache, and even allergic reactions.
People with this infection may also have low levels of B12, while anemia is not common. In rare instances, these tapeworms can block passageways in the body, which can lead to other health problems such as inflammation of the gallbladder or pancreas, intestinal blockages, appendicitis, and gallstones.
What Causes Diphyllobothriasis (Fish Tapeworm Infection)?
Full-grown diphyllobothriids, a type of parasite, are long and segmented, ranging from 2 to 15 meters. They have thousands of parts known as proglottids. Each tape worm has male and female organs, and can grow up to 22cm per day, living for 20 years or more. The front end of the tapeworm, called the scolex, hooks onto the intestine of the host body through a special groove. Identifying these tapeworms based on their looks is challenging due to the variability within the species.
The life cycle of these tapeworms is complex and often involves freshwater or marine environments as they begin as aquatic larvae. The cycle starts when the tapeworm eggs are deposited into water. Arthropods, like crustaceans, eat the first stage larvae. Small fish then eat the infected crustaceans, and are themselves consumed by bigger fish. The bigger fish serve as the home for the tapeworms to grow into their next stage of development. Finally, when these infected fish are eaten by humans, the tapeworm continues to mature and reproduce, thus completing the cycle.
In 2017, a new classification for members of the diphyllobothriid family was proposed, using features and molecular techniques. So far, there are six species from three groups confirmed to be human parasites, the most common of which is presumed to be Dibothriocephalus latus, although misidentification is a known issue due to the difficulty in differentiating between species in this family. In fact, infections from 13 other species were identified by look alone and have not yet been verified by molecular techniques.
Finally, human infections with these parasites have been dated back to the early Neolithic period, based on discovered eggs from around 3917 to 3905 BC. It seems that humans have always been a principal host, but because the tapeworms can adapt to survive and mature in a variety of intestinal environments, they are likely to continue infecting other mammals, including humans, in the future.
Risk Factors and Frequency for Diphyllobothriasis (Fish Tapeworm Infection)
Broad tapeworms are parasites that commonly affect marine animals like seals. However, we don’t have complete knowledge on their global distribution and host interactions. Some broad tapeworms, like D latus, are found in subarctic and temperate lakes in Eurasia, while others, like D nihonkaiensis, are commonly found in humans in countries around the Pacific Ocean. Fish species like perch, pike, hake, salmon, and salmonids often get infected. Recently, D latus started infecting freshwater salmon in South America, which could have serious health and economic impacts.
In 2002, it was estimated that diphyllobothriasis, a disease caused by these tapeworms, affected 20 million people worldwide. However, it’s hard to get accurate numbers because most countries don’t require reporting of this disease. Many people don’t know about diphyllobothriasis, and its symptoms are usually mild, so it’s often not diagnosed.
Most reported cases of diphyllobothriasis come from Japan, but the disease has been found on every continent except Africa and Australia. It’s most commonly found in regions where people eat raw fish, such as certain parts of Europe, Japan, North America, and South America.
Cleaner water and sanitation have helped lower the number of D latus infections in many places. However, not everyone benefits from these improvements. For example, in certain parts of Russia, the infection rate among non-native adults dropped from 11.1% to 1.9% between 1988-89 and 2018-19, while among indigenous populations, it only dropped from 5.3% to 4.8%. As tapeworms often infect wild animals and fish, better sanitation won’t necessarily lower infection rates for all species of the parasite.
- Many experts now see diphyllobothriasis as a re-emerging disease because there have been more cases reported in developed countries that aren’t usually affected, and global factors could increase the spread of the parasite.
- Changes like more migration and travel, increased fish consumption, and interest in diverse cuisines have led to more people being exposed to raw fish dishes in places where such diseases aren’t usually found. Modern food transportation methods even allow fish stored on ice but not frozen to be transported long distances, even from countries where certain tapeworms are common.
- Growing young salmon in lakes and the possible contamination and escape of infected fish from ocean pens have raised concerns about fish farming. These practices could be why D latus has been found in places like Brazil, Taiwan, and South India.
These parasites can infect anyone, regardless of age, gender, or ethnicity. However, certain groups are at a higher risk, such as:
- People who regularly eat raw fish, like in Scandinavia, Peru, and Japan.
- People who prepare dishes that involve tasting raw fish, like gefilte fish.
- Fishermen, who often consume fresh catches.
Additionally, high infection rates are often found in populations with poor access to clean water and sanitation.
Signs and Symptoms of Diphyllobothriasis (Fish Tapeworm Infection)
Being infected with the human diphyllobothriid, a type of tapeworm, often causes no symptoms. In fact, it’s usually only discovered during a colonoscopy or when segments of the worm are passed naturally. This symptom, while harmless, can have a marked psychological effect on the person.
About one fourth of people infected with this tapeworm will experience mild symptoms. These might include:
- Stomach pain or bloating,
- Indigestion
- Vomiting or diarrhea
- Constipation
- Fatigue
- Dizziness
- Muscle pain
- Headaches.
It’s rare but possible for these individuals to also experience weight loss, skin problems, or an intestinal blockage.
If the tapeworm doesn’t cause any complications and the patient isn’t seeing segments of the worm, the physical exam is often normal. However, some people may show signs of an allergy to the tapeworm, and very rarely, severe blockage or vitamin B12 deficiency may cause symptoms like paler skin, yellow skin, difficulty breathing, nervous system issues that could include headaches, vision changes or unsteady movements.
A health provider would diagnose a tapeworm infection based on the patient’s symptoms along with important details from their personal and medical history, like their job, hobbies, eating habits, and travel experiences.
Testing for Diphyllobothriasis (Fish Tapeworm Infection)
To diagnose a particular type of tapeworm infection called Diphyllobothriidae, doctors typically look for specific eggs or pieces of the worm in the patient’s stool. Since these eggs are often plentiful, no special efforts need to be made to concentrate the sample. However, there may be times when no eggs appear in the stool for several months, especially if a large piece of the tapeworm has recently been passed. In such cases, repeated testing may be necessary.
Blood tests, particularly complete blood count (CBC) and vitamin B12 tests, can be helpful to identify any related health issues this tapeworm can cause. This includes conditions like anemia caused by large red blood cells (megaloblastic anemia), vitamin B12 deficiency, low levels of all blood cells (pancytopenia), higher than normal presence of a specific type of white blood cell (eosinophilia), and anemia due to vitamin B12 absorption issues (pernicious anemia).
Although it’s technically possible to identify the exact species of the tapeworm causing the infection, it’s not usually necessary. The treatment for all tapeworm species is the same, and devising an accurate identification often needs molecular-level techniques, which are not needed for treatment purposes. However, knowing the species can be helpful in research or when the tapeworm’s characteristics seen through the microscope don’t match the type typically seen in the local area.
Further identification of the species and its spread is done through a complex process involving reaction to amplify DNA and detecting specific changes in the DNA structure. For this process to work, the clinical samples must be either fresh, frozen, or preserved in a special solution.
While there are numerous methods to visualize the tapeworm within the body, these are often not necessary. Some doctors recommend an abdominal ultrasound as it’s non-invasive and widely available, and it might show the worm within the intestines. However, other techniques like endoscopy, abdominal MRI, or colonoscopy are not typically used since they are costly and invasive.
Treatment Options for Diphyllobothriasis (Fish Tapeworm Infection)
For uncomplicated diphyllobothriasis, a type of tapeworm infection, outpatient treatment is a safe and effective option. The first-choice treatment is a medicine called praziquantel. This drug is absorbed but not broken down by the parasite, and it interrupts the parasite’s calcium metabolism, which leads to paralysis. The Centers for Disease Control and Prevention (CDC) advises one dose of 5 to 10 mg/kg for both children and adults.
However, there is less data on the safety of praziquantel during pregnancy and in children under the age of 4, as there haven’t been proper studies conducted. But, it has been used safely in programs targeting mass infections without any observed harmful effects on pregnancy outcomes or young children. In pregnant women or young children, the potential risk of untreated infection needs to be weighed against the possible harms of the drug.
It’s important to note that praziquantel is broken down by a bodily process known as CYP3A4 metabolism, and this could potentially interact with other medicines a patient is taking. Some side effects of praziquantel include fatigue, headaches, dizziness, stomach pain, a fever, and possibly a skin rash referred to as urticaria.
Interestingly, a one-time dose of another medicine, niclosamide, can be used as an alternative treatment in many countries, though not in the U.S. It works by interfering with the tapeworm’s energy production. Adults typically receive a 2-gram dose while children over six take a 1-gram dose.
After treatment, it’s recommended to check the patient’s stool a month later to confirm the tapeworm has been eradicated. You may notice that the recommended dose of praziquantel is lower than past suggestions, as a higher dose may be more effective in the event of treatment failure.
If the tapeworm infection has led to a deficiency of vitamin B12, this typically improves once the tapeworm is treated. However, for patients with severe deficiency symptoms that affect the blood or nerves, supplemental vitamin B12 may be required.
What else can Diphyllobothriasis (Fish Tapeworm Infection) be?
Diphyllobothriasis is tricky to diagnose because it can has multiple symptoms. The most common ones are mild stomach discomfort and diarrhea. To identify the cause of these symptoms, doctors often ask for a stool sample to test for parasites. This is especially the case if the patient may have been exposed to parasitic infections through travel or eating raw or undercooked meat.
Doctors can then determine whether the parasite is a type of diphyllobothriasis or something different, like a Taenia species, by studying the sample under a microscope.
If the patient is suffering from a type of anemia (a lower than normal number of red blood cells) known as a “macrocytic anemia,” then doctors need to consider several other possible causes. This could include a condition that interferes with the body’s ability to produce red blood cells, known as pernicious anemia, or things like exposure to harmful substances that might damage the bone marrow where red blood cells are produced.
It could also be caused by something in the patient’s diet – either they’re on a restrictive diet, or they’re just not getting enough of the right nutrients. It could also be caused by certain medications, or by conditions that interfere with the body’s ability to absorb nutrients from food, such as malabsorption or short gut syndrome.
In very rare cases, patients may also experience severe stomach pain or discomfort, which could indicate a number of more serious conditions. These could include inflammation of the gallbladder (cholecystitis), appendicitis, or even a blocked intestine. In these situations, the diagnosis might only become clear when doctors take a closer look, such as with an ultrasound scan or a surgical procedure called a colonoscopy.
What to expect with Diphyllobothriasis (Fish Tapeworm Infection)
The outlook for people who get diphyllobothriasis, a type of parasitic infection, is usually very good. This is because the disease tends to be mild and the treatment works effectively. However, the situation can get complicated if there are too many worms, other health conditions, or if there is a blockage in the body’s passageways. More details about this are provided in the complications section.
Possible Complications When Diagnosed with Diphyllobothriasis (Fish Tapeworm Infection)
Diphyllobothriasis often leads to certain complications, which are usually related to the worm’s inappropriate attachment or when a large number of worms are present. These complications include blockages inside the intestines, symptoms similar to appendicitis, inflammation of the gallbladder, and inflammation of the bile ducts. Some of these problems could necessitate surgery.
Commonly, if you have a deficiency in Vitamin B12, you might experience symptoms like tiredness and paleness due to a specific type of anemia known as megaloblastic anemia. If this deficiency becomes severe and persists for a long time, it can even lead to heart failure and neurological symptoms like shortness of breath, headaches, tingling sensations, changes in vision, and lack of coordination.
People with conditions that already entail issues with nutrient absorption (such as HIV) or those whose food intake is less than adequate, could face more serious consequences due to malabsorption from diphyllobothriasis.
Preventing Diphyllobothriasis (Fish Tapeworm Infection)
To avoid the risk of infection with Diphyllobothriidae, an easy rule to remember is always to eat well-cooked or deep-frozen fish. You should avoid fish that is undercooked, raw, pickled, smoked, or dried unless it’s been frozen deep. Also, try not to taste fish mixtures like gefilte fish before they are completely cooked.
The Centers for Disease Control and Prevention (CDC) advise cooking fish to a minimum internal temperature of 145 °F (63 °C) or deep-freezing it at -4 °F (-20 °C) for at least seven days. If you’re interested in using salt to kill the larvae of this parasite, a solution of 12% NaCl has been found to be effective.
For more details about commercial freezing of fish and how it’s done to ensure it’s safe to eat, look for the section titled ‘Enhancing Healthcare Team Outcomes’.
