Long Live the Broad Fish Tapeworm! All About Diphyllobothrium latum

You may or may not be aware that eating certain foods puts you at risk of picking up parasites such as Diphyllobothrium latum.  Don't worry, I'm not going to try to talk you out of eating sushi or ceviche...I would never do that because I am a lover of both! However, it is good to be aware of the dangers of such foods so that you know what you are getting yourself into...especially in places other than the states where regulations are...less stringent, to put it mildly.  The bad news is that this parasite can go undetected for many years, and may get to be about 30 feet long in humans.  The good news is that once you've been diagnosed, our old friend Praziquantel can knock out in infection rather quickly.  So sit back, and enjoy learning about this fascinating fish tapeworm!



So what is this parasite you speak of?

Sushi from a restaurant in Chicago...I ate most of this!

Diphyllobothrium latum is commonly known as the "broad fish tapeworm".  As this common name implies, it is contracted from eating raw or undercooked fish and has a broad morphology, which we will mention again a little later. The worm is native to Scandinavia, western Russia, and the Baltics, but has been spread to North America through fish importation.  It has been historically more problematic in the Pacific Northwest than anywhere else on this continent.  As Americans expand their culinary pallets to include foods with raw or undercooked fish as the main ingredient, the risk for cases of parasite infections have increased.  Cases of infection have been reported from a variety of foods from many origins.  Sushi/Sashimi are among the biggest Asian threat.  Italian carpaccio di persico and French tartare maison make up some of the European threats. In Latin American cuisine, ceviche is a candidate for transmission as well. Luckily, food standards have become more restrictive in terms of fish importation and storage in this country.  It is not likely that you will contract this parasite here in the US, but if by some chance you do, remember that it's easy to get rid of as long as you can confirm the infection.

Carpaccio di Persico

Ceviche

Taxonomy
Like all tapeworms, this animals belongs to phylum platyhelminthes under class Cestoda.  It further belongs to order Pseudophyllida because the adults of this species possess two bothria (structures that help them attach to the host's intestinal walls) and dumbbell-shaped ovaries.  These worms also have both the genital pore and the uterine pore located in the center of their ventral (belly) sides.  D. latum belongs in family Diphyllobothriidae, which infect copepods, then later infect fish (usually), and wind up in mammals, where they grow to maturity and reproduce.  D. latum is the longest tapeworm species that infects humans, averaging 10 meters (30 feet) in length.  These worms may shed as many as one million eggs in a day.  The proglottids (segments in which eggs are produced...among other functions) of this worm are more broad than other tapeworm proglottids, hence the common name.

Scolex showing bothria on left. Proglottids on right.

Life Cycle
The life cycle of this tapeworm starts with an infected mammal shedding proglottids (laden with eggs) in their feces.  These proglottids get ingested by a copepod (this takes place in water) and the eggs develop into a procercoid larva. A small fish, such as a minnow, then eats the infected copepod and the parasite develops into a plerocercoid larva or "sparganum" within the fish's muscles.  Along comes a bigger fish that eats the smaller fish, and now the parasite can make a comfy home in the muscles of something like a trout or perch.  An unsuspecting human then eats the fish without properly cooking it to rid it of the parasites within, and now our human is infected.  Once inside the human, the parasites become immature adults and eventually develop into sexually mature adults.  They begin to reproduce, releasing their eggs in the human's feces and the cycle begins again.  These worms are restricted to humans, in fact many other animals have been reported to have harbored these worms.  There are papers describing this worm from dogs, cats, bears, weasels, and even pinnipeds (such as seals and sea lions)! Once established in a mammal host, this worm can live for up to 20 years, sometimes going completely unnoticed!

Diphyllobothriasis
The disease associated with being infected with D. latum has a long history with coastal cultures that have traditionally created foods using raw or undercooked fish.  Archaeological sites in South America have revealed eggs in 4,000-10,000 year old human remains. During the 1970s, there were over 5 million cases of diphyllobothriasis reported in Europe and over 4 million cases reported in Asia.  Symptoms of this disease are often asymptomatic (having no clinical signs or symptoms), in fact, 4 out of every 5 people who contract this worm show no signs of infection.  The remaining 1 in 5 infected persons experiences vomiting, diarrhea, and weight loss in addition to fatigue, abdominal pain, general discomfort, and constipation.  In prolonged cases, patients rarely suffer from severe vitamin B-12 deficiency as this parasite may absorb as much as 80% of B-12 within a person's body.  This deficiency can lead to megaloblastic anemia, which leads to demyelination (loss of the insulation around your nerves).  Demyelination leads to degeneration of the spinal cord
and may leave people with a variety of disfunctional nerve-related symptoms.

Diagnosis and Treatment

D. latum egg under a microscope.

Due to there often being a lack of clinical symptoms, many times this disease goes undiagnosed or gets diagnosed long after infection.  There are really only two ways to diagnose this disease.  The first way is to identify proglottids or eggs from a patient's stool sample.  The second way is to perform a PCR test following sonication to remove the eggs from the proglottids.  Sometimes, these worms can be found during a routine colonoscopy. Click here to see a video of this worm shot during a colonoscopy. Once diagnosed, the treatment plan is rather easy to administer.  Typically, a patient is given praziquantel.  This drug has some side effects that are similar to those that 1/5 patients experience from the infection itself.  Patients may also be treated with niclosamide, which has no side effects because it doesn't get absorbed by the gastrointestinal tract.

Prevention
One way to prevent the spread of diphyllobothriasis is to raise public awareness of water contamination and to encourage them not to defecate in sources of drinking water. One should always avoid eating any fish that looks questionable.  The best way to prevent the spread of this disease is to educate food handlers about how to properly prepare fish.  Fish should either be cooked thoroughly or brined before serving.  If a dish requires raw fish, the fish should be frozen at -10 degrees C for 2-3 days before being thawed and used.

Gefilte Fish

Moral of the Story
If you are planning to eat sushi or gefilte fish, be sure the fish have been prepared properly to kill off any little passengers that may have been hanging out in the fish muscles.  If you think you may have accidentally ingested a fish tapeworm, see a doctor so that you don't carry around your new friend for a few decades.  Also, don't be mad at D. latum, remember, he/she is just trying to survive!  And by the way, sushi is still delicious, so the risk of getting an easily treatable tapeworm infection is certainly not going to keep me from enjoying it! :P

See? We LOVE sushi!!! :p

Here's a close-up of the sushi bridge we ordered! :)