You’re complaining of having nightmares about your teeth falling out? I dream of intestinal colonization with a 30 foot tapeworm. Everyone’s got their own hang-ups and quite frankly the largest parasite of man, the freak of nature Diphyllobothrium latum, unnerves me. What’s not to dislike? The longest lifespan of any human parasite and the jaw-dropping lengths it can reach are just a few of its charms.
An unfragmented fish tapeworm excreted after injection of amidotrizoic acid. Arrow identifies the scolex. Inset shows magnification of the proglottids. Click for source.
The fish tapeworm has long been an intestinal bedfellow of mankind. The earliest evidence of infection has been found in archaic coprolites along the Pacific coast of Chile and Peru dating from 4110 to 1950 BC (1). Tapeworm eggs have also been found at Neolithic lakeside settlements dating from 3900 BC in Germany and Switzerland and represent the earliest occurrence of the tapeworm in the Old World; diphyllobothriasis, as the infection is called, may have been one of the leading parasitic infections of Neolithic people during that period (2). It still remains a modern scourge, with an estimated 20 million people worldwide infected (3).
Scandinavians brought the creature to the midwestern United States as they left their homeland and moved to the lake regions of Minnesota and Wisconsin in the 1800s (4). As the sewage of sturdy Norwegians and what-have-yous contaminated the lakes of the Midwest, tiny copepods ingested the eggs. These little crustaceans would be consumed by predatory freshwater fish such as pike, carp, salmon, perch, turbot and pickerel and the tapeworm larva would embed itself in their muscle or viscera. The larval form of the tapeworm, the plerocercoid, makes its final home in the small intestine of humans or bears when the fish is eaten in a raw or undercooked state. The tapeworm can live quite comfortably there, in relative parasitic harmony with its host for several decades.
D. latum has one the most magnificent anatomies of the helminthic family. It has two suction cup-like grooves located at its head, known as a scolex, that allow it to grip the small intestine. The scolex looks like some sort of bizarre, antique tool you’d stumble upon in your grandfather’s shed. Or a sex toy. Anyway! The body is composed of thousands of rectangular segments known as proglottids; the worm can be made of up to thousands of these guys stretching through the intestine for several feet. It’s a rapidly growing beast too - tapeworms can grow 22 cm a day or 1 cm per hour (5). The longest specimen every found was recorded at an alarming 25 m or 82 ft (6).
The scolex head of the fish tapeworm. Note the bothria "grooves" that it uses to attach to the intestinal wall. Click for source
Each proglottid contains all of the necessary machinery to reproduce - testes, ovaries and uterus are neatly compacted into a globular, rosette shape that churns out eggs. Most interestingly, egg production seem to be more common in the late summer and fall (7). The tapeworm continuously grows new proglottids downwards from the head and neck, shifting the older segments further down the intestines where they may be eventually lost as the tapeworm ages. If you’re infected and in great luck, you may see wildly thrashing proglottids (either singly or in a chain) upon defecation. A proglottid segment has been evocatively described as “tagliatelle” like (8). Splendid!
We can thank gefilte fish for the alarming sight of a wriggling proglottid in the toilet bowl of a Jewish housewife in Brooklyn. Gefilte fish is quite the culinary amuse bouche - a ground-up concoction of freshwater fish, eggs, matzo meal and your requisite salt, which is then smushed into a ball and promptly boiled. These balls may then be stuffed into a whole fish and served. Determining whether your gefilte fish is properly cooked depends on instinct and you and your family members’ standards of taste and “doneness” - in other words, frequent taste testing which may result in self-inoculation with the larval plerocercoid. Handling of the raw fish can also deliver the infective larva to the intestinal system. Tapeworm infection is most likely to occur in the Jewish population during Passover when gefilte fish is much in demand (9).
A stained proglottid segment from the fish tapeworm showing the uterus. Image: CDC. Click for source.
Scandinavians have a similarly constructed dish, fish balls or “fiskeboller”, though it is typically served with a cream sauce instead of inhabiting the interior of another fish. The high prevalence of diphyllobothriasis due to the culinary adventures in these two communities have earned the tapeworm the name “Jewish/Scandinavian housewife’s disease”.
This parasite is quite tolerant of other religions and eating habits - all cultures that consume freshwater fish are susceptible to infection. The Japanese may be infected by D. nihonkaiense* by eating uncooked or raw sushi or sashimi made of anodromous fish such as salmon that is used as an intermediate host by the worm. The global epidemiology of infection with this species has truly evolved as transportation and refrigeration systems have improved thus enabling the transport of fresh fish from rural, isolated regions to urban locales (10).
D. latum has also reared its narrow scolex head in Brazil thanks to a burgeoning aquaculture industry that rears salmon in southern Chile (11). Consuming dishes of contaminated raw or smoked salmon are typically responsible for tapeworm transmission, though Japanese cuisine has also made its mark in South America where raw sushi has become quite popular. Diphyllobothriasis was also quite common in the Canadian Inuit population but has declined significantly over the past few decades due to cultural changes in eating behaviors (11).
Bubbe's geftile fish. What could go wrong? Click for source.
Despite its preposterous length, tapeworm infection itself is rather benign. At the most, a person may initially suffer abdominal discomforts such as diarrhea, cramping, flatulence, vomiting, nausea, and weakness. Chronic infections may result in weight loss, fatigue and pernicious anemia. This type of anemia occurs when there are exceptionally low levels of folic acid, otherwise known as vitamin B12, a macromolecule necessary for the production of red blood cells by bone marrow. The tapeworm actively competes for the vitamin, nicking it from its human host. Scandinavians and the Finnish in particular have a genetic predisposition to B12 deficiency and are more susceptible to suffering from pernicious anemia.
For those who have no qualms about body infestation, flailing worms and colonoscopies, I highly recommend watching the Youtube video below of an accidental discovery. [embed]http://www.youtube.com/watch?v=qQhaO1pWztw[/embed]
Treatment is, shockingly, quite simple - administering a single dose of the anthelmintic praziquantel should expel the worm. It is important that the entire worm is removed from the intestine; if the scolex and head remain, the tapeworm will continue to quite happily grow proglottids.
Thankfully, incidence of tapeworm infection in the United States has declined due in part to the increased inspection of fish imports, improved sewage management as well as the modern convenience of cheap, cooking thermometers (7). Most sushi served in the United States is used with previously frozen fish which kills the plerocercoid. Diphyllobothriasis is still common in Europe, especially in the Baltic and Scandinavian countries (11).
For those of you who will be joining me in dreaming of tapeworm tangos in your belly, just remember that sushi, sashimi, carpaccio or ceviche that is prepared with never-frozen freshwater fish should be eaten with caution. Or gusto depending on how you live your life - who am I to say? Bon appetite!
Parasitologists are still disentangling the morphological and ecological variations between Diphyllobothrium species - much of the genus’ phylogeny needs rigorous molecular and genetic characterization. It could very well be that there is only one species of tapeworm or gazillions of them. A super cool hypothesis about the morphological differences seen in different tapeworms found throughout the world is that human hosts alter the tapeworm’s morphology. For a review, click here.
For more in-depth information on the life cycle of D. latum, the indispensable CDC is always here to help you.
A charming infestation story a few years back from the NYT. Also the great Robert Desowitz goes into a nice story about a Jewish grandmother infected with a new Scandinavian friend. Get the book here.
Looking to buy antiquated tapeworm cures and other parasitological ephemera for beloved relatives? I know just the thing!
References (1) KJ Reinhard (1992) Parasitology as an Interpretive Tool in Archaeology. American Antiquity. 2(57): 231-45 (2) M Le Bailly, U Leuzinger, H Schlichtherele & F Bouchet. (2005) Diphyllobothrium: Neolithic Parasite? J Parasitol. 91(4): 957-9 (3) T Scholz, HH Garcia, R Kuchta, B Wicht. (2009) Update on the Human Broad Tapeworm (Genus Diphyllobothrium), Including Clinical Relevance. Clin Microbiol Rev. 22(1): 146-60 (4) Weintraub, S. The Parasite Menance. Woodland Publishing, 2000 (5) F Kuhlow (1955) Studies on the development of Diphyllobothrium latum. Z. Tropenmed. Parasitol.6 :213–225 (6) von Bonsdorff, B. 1977. Diphyllobothriasis in Man. Academic Press, New York, NY. (7) EJ Jenkins, JM Schurer, KM Gesy. (2011) Old problems on a new playing field: Helminth zoonoses transmitted among dogs, wildlife, and people in a changing northern climate. Vet Parasitol. Epub ahead of print (8) Y Jackson, R Pastore, P Sudre, L Loutan, F Chappuis. (2007) Diphyllobothrium latum Outbreak from Marinated Raw Perch, Lake Geneva, Switzerland. Emerg Infect Dis. 13(12): 1957-8 (9) D Gibbs (1984) Gut feelings in the old. Brit Med J. 289: 1065 (10) N Arizono, M Yamada, F Nakamura-Uchiyama, K Ohnishi. (2009) Diphyllobothriasis Associated with eating raw pacific salmon. Emerg Infect Dis. 15(6): 866-70 (11) FC Cabello (2007) Salmon Aquaculture and Transmission of the Fish Tapeworm. Emerg Infect Dis. 13(1): 169-71
Scholz, T., Garcia, H., Kuchta, R., & Wicht, B. (2009). Update on the Human Broad Tapeworm (Genus Diphyllobothrium), Including Clinical Relevance Clinical Microbiology Reviews, 22 (1), 146-160 DOI: 10.1128/CMR.00033-08
Cabello, F. (2007). Salmon Aquaculture and Transmission of the Fish Tapeworm Emerging Infectious Diseases, 13 (1), 169-171 DOI: 10.3201/eid1301.060875