Diphyllobothrium latum, the fish or broad tapeworm
Diphyllobothrium latum, otherwise known as the fish or broad tapeworm, is a cestode worm that can infect humans through consumption of raw or undercooked fish (1). It is the most commonly diagnosed cause of diphyllobothriasis, a disease which affects an estimated 20 million people today (2). D. Latum is of the phylum Platyhelminthes, class Cestoda, order Pseudophyllidea, family Diphyllobothriidae, genus Diphyllobothrium (3). There are several species of Diphyllobothrium related to D. latum that cause diphyllobothriasis in humans, including D. pacificum, which is common in fish off the coast of Peru, and D. nihonkaiense, which is the most common cause of diphyllobothriasis in Japan (7). For the purposes of this paper, references to diphyllobothriasis do not necessarily imply infection with Diphyllobothrium latum. Due to the indistinguishable clinical features of infections with different species of Diphyllobothrium spp., resources often do not differentiate between species.
History and Worldwide Distribution
The fish tapeworm has a long documented history of infecting people who regularly consume fish and especially those whose customs include the consumption of raw or undercooked fish. In the 1970’s, most of the known cases of diphyllobothriasis came from Europe (5 million cases), and Asia (4 million cases) with fewer cases coming from North America and South America, and no reliable data on cases from Africa or Australia (2). Interestingly, despite the relatively small number of cases seen today in South America, some of the earliest archeological evidence of diphyllobothriasis comes from sites in South America. Evidence of Diphyllobothrium spp. has been found in 4,000-10,000 year old human remains on the western coast of South America (5). There is no clear point in time when Diphyllobothrium latum and related species were “discovered” in humans, but it is clear that diphyllobothriasis has been endemic in human populations for a very long time.
Due to the changing dietary habits in many parts of the world, autochthonous cases of diphyllobothriasis have recently been documented in previously non-endemic areas, such as Brazil (4). In this way, diphyllobothriasis represents an emerging infectious disease in certain parts of the world where cultural practices involving eating raw or undercooked fish are being introduced. Due to its potential as an emerging infectious disease, monitoring of diphyllobothriasis should be considered, even in previously non-endemic countries. A caveat to this statement is that because diphyllobothriasis usually presents with mild clinical symptoms and has little associated morbidity or mortality, countries must weigh the costs of actively monitoring this disease with the benefits of preventing its spread (2). Due to the number of unreported and undiagnosed cases, evidence on the worldwide distribution of diphyllobothriasis is inconclusive.
The lifecycle of D. latum is quite complex and requires two intermediate hosts in which the worm must mature before reproduction can occur in the definitive host (2). The lifecycle begins with the passing of unembryonated eggs into fresh water via the feces of a variety of fish-eating carnivores (including humans) infected with an adult worm (fig. 1-1). Immature eggs embryonate in the water and hatch as ciliated coracidia, which are infective to the first intermediate host of D. latum (fig. 1-2). The first intermediate host for D. latum can be a number of different copepods, which ingest the coricidia (2). The coracidia hatches in the copepod and penetrates the intestinal wall, where it matures into a procercoid larva (2), (fig. 1-3 and 1-4).
The second intermediate host is most commonly a smaller freshwater fish, though some evidence has suggested that salmonids and marine fish may also be able to serve as secondary hosts, but probably do so infrequently (2). D. latum matures into its plerocercoid stage in nearly any tissue of the secondary host, though from an epidemiological standpoint, species in which it infects the muscle tissue are most relevant, since they are most likely to infect humans (fig. 1-5). The initial secondary intermediate host may either directly infect the definitive host, or be consumed by a larger fish before infecting the definitive host (fig. 1-6).
The adult worm attaches to the small intestines of a large number of fish-eating carnivores, including humans, after ingestion of an infected secondary host that has not been properly cooked (fig. 1-7 and 1-8). Like all cestodes, D. latum lacks a digestive system, and must rely on absorbing nutrients from their hosts (7). The fish-eating carnivores act as the definitive hosts of D. latum, in which the worm produces the proglottid segments containing the reproductive organs. The incubation period in humans is typically 4-6 weeks, but can vary from as short as 2 weeks to as long as 2 years (3). One or several of the tape-like proglottid segments regularly detach from the main body of the worm and release immature eggs in fresh water to start the cycle over again, hence the name tape-worm (fig. 1-9).
Figure. 1 Lifecycle of Diphyllobothrium latum. See above text for description.
Morphology of the Adult Worm
The adult worm is comprised of three fairly distinct morphological segments; the scolex, the neck and the lower body. The scolex is the head portion of the worm, and is equipped with a slit-like groove (the bothrium) for attachment to the intestine (bright, opaque bulb in Fig. 2-C, 2-D and 2-E). The scolex attaches to the neck, or proliferative region (the translucent region in Fig. 2-A, 2-C, 2-D). From the neck grows many proglottid segments which contain the reproductive organs of the worm (several proglottids shown in Fig. 2-B). Mature proglottids are typically wider than they are long, which is why D. latum is known as the broad tapeworm. Adult worms grow as quickly as 1 cm/hr and can reach extremely long lengths in the intestine of the definitive host. Adults can reach 25 m in length, but more often grow to between 2-15 m (1, 2).
Figure 2. Diphyllobothrium latum in human intestine. See above text for description.
Clinical Presentation in Humans
Diphyllobothriasis can present with diarrhea, abdominal pain, vomiting, weight loss, fatigue, constipation and discomfort (1). Approximately four out of five cases are asymptomatic and may go many years without being detected (2). The most potentially serious symptom can occur if the adult worm attaches to the proximal area of the jejunum. In a small number of cases, this leads to severe vitamin B12 deficiency due to the parasite absorbing 80% or more of the host’s B12 intake, and a megaloblastic anemia indistinguishable from pernicious anemia (7). If the parasite load is large, patients can also become constipated or experience bowel obstruction (1).
The only known cases of D. latum in humans have been acquired directly from the secondary host through ingestion (1). There are many reservoirs for the adult parasite, as it is very non-specific in its ability to infect definite hosts. Reservoirs include humans, dogs, bears, and many other carnivorous mammals as well as fish-eating birds (3, 2).
There are no vectors for this disease (3).
Diagnosis and Treatment
Diagnosis is usually made by identifying proglottid segments, or characteristic eggs in the feces (3). These simple diagnostic techniques are able to identify the nature of the infection to the genus level in most cases, which is usually sufficient in a clinical setting (2). However, when the species needs to be determined (in epidemiological studies, for example), restriction fragment length polymorphisms can be effectively used. PCR can be performed on samples of purified eggs, or native fecal samples following sonication of the eggs to release their contents (2).
Figure 3. Egg with characteristic knob (at bottom of picture) opposite the operculum. Each major unit on the scale at left represents 10 μm.
Upon diagnosis, treatment is quite simple and effective. The standard treatment for diphyllobothriasis, as well as many other tapeworm infections is a single dose of Praziquantel, 5-10 mg/kg PO once for both adults and children. An alternative treatment is Niclosamide, 2 g PO once for adults or 50 mg/kg PO once (8). Praziquantel has few side effects, many of which are similar to the symptoms of diphyllobothriasis. They include malaise, headache, dizziness, abdominal discomfort, nausea, rise in temperature and occasionally allergic skin reactions (2). The side effects of Niclosamide are very rare, due to the fact that it is not absorbed in the gastrointestinal tract (2). Another interesting potential diagnostic tool and treatment is the contrast medium, Gastrografin, introduced into the duodenum, which allows both visualization of the parasite, and has also been shown to cause detachment and passing of the whole worm (9). This treatment provides benefits over traditional treatment of diphyllobothriasis in that there are no side effects associated with Gastrografin and results in the passing of a whole worm which is useful for identification. However the therapy is more expensive, insertion of the duodenal tube causes considerable pain, and fluoroscopic images are needed to confirm the infection in the intestine (2).
Epidemiology and Public Health Strategies
People at high risk for infection have traditionally been those who regularly consume raw fish, including fishermen who eat the raw liver or roe of their catches and women preparing and tasting foods that contain raw fish (2). Many regional cuisines include raw or undercooked food, including sushi and sashimi in Japanese cuisine, carpaccio di persico in Italian, tartare maison in French-speaking populations, gefilte fish in Jewish populations, ceviche in Latin American cuisine (2). With emigration and globalization, the practice of eating raw fish in these and other dishes has brought diphyllobothriasis to new parts of the world and created new endemic foci of disease (2).
The number of cases of human diphyllobothriasis has increased in recent years, likely due to increased consumption of raw fish and importation of fish from areas of the world where D. latum is endemic (2). According to a recent study of D. latum in Western Europe, some lakes in Italy, Switzerland and France had infection levels of secondary hosts as high as 33.3% (6). The same study found the prevalence of diphyllobothriasis to be decreasing in the Baltic and Scandinavian countries, which traditionally have had some of the highest rates in the world. Epidemiological studies such as these are necessary for tracking the distribution of different Diphyllobothrium species and monitoring epidemics worldwide. However, it is probably the case that many countries lack the resources or impetus for epidemiological studies looking at D. latum in the environment and in human populations.
Factors contributing to the persistence of the disease and the recent increase in infections worldwide are many-fold, and include: the number of animals that can serve as reservoirs; improper sanitation practices which introduce eggs into bodies of fresh water; and emigration of peoples whose cultural practices include consumption of raw fish (2). These factors highlight some of the potential avenues for public health interventions in decreasing the prevalence of diphyllobothriasis. The most viable interventions include: prevention of water contamination both by raising public awareness of the dangers of defecating in recreational bodies of water and by implementation of basic sanitation measures; screening and successful treatment of people infected with the parasite; and prevention of infection of humans via consumption of raw, infected fish (2). The last of these can most easily be changed via education about proper preparation of fish. Fish that is thoroughly cooked, brined, or frozen at -10˚C for 24-48 hours can be consumed without risk of D. latum infection. Although it is not morally justified to banish the cultural practices that contribute to epidemics of diphyllobothriasis, public awareness and regulation of commercial distributors is likely to go a long way, especially in endemic areas.
For questions concerning information on this website, please refer to the resources below, or contact Jack Hunt at firstname.lastname@example.org. This website was created as part of Human Biology 153: Parasites and Pestilence, Stanford University 2009.
Useful Web Links
The CDC’s website on Diphyllobothriasis including information on causal agent, life cycle, geographic distribution, clinical features, laboratory diagnosis and treatment.
Wikipedia article on the genus Diphyllobothrium
Gideon Online, great resource for information on clinical and epidemiological features of an exhaustive list of infectious diseases.
Ko, S.B. “Observation of deworming process in intestinal Diphyllobothrium latum parasitism by Gastrografin injection into jejunum through double-balloon enteroscope.” (2008) from Letter to the Editor; American Journal of Gastroenterology, 103; 2149-2150. Figure 1.
Scholz, T., et al. “Update on the human broad tapeworm (genus Diphyllobothrium), including clinical relevance.” (2009) Clinical Microbiology Reviews, 22 (1), 146-160. Figure 1 (C).
2. Scholz, T., et al. “Update on the human broad tapeworm (genus Diphyllobothrium), including clinical relevance.” (2009) Clinical Microbiology Reviews, 22 (1), 146-160.
4. Llaguno, Mauricio M., et al. “Diphyllobothrium latum infection in a non-endemic country: case report.” (2008) Revista da Sociedade Brasileira de Medicina Tropical, 41 (3), 301-303
5. Reinhard, K. J. “Parasitology as an interpretive tool in archaeology.” (1992) American Antiquity, 57(2), 231-245.
6. Dupouy-Camet, J. and Peduzzi, R. Current situation of diphyllobothriasis in Europe. (2004) Eurosurveillance; 9(5).
7. John, David T., and William A. Petri. Markell and Voge’s Medical Parasitology. Elsevier; St. Louis (2006).
9. Ko, S.B. “Observation of deworming process in intestinal Diphyllobothrium latum parasitism by Gastrografin injection into jejunum through double-balloon enteroscope.” (2008) from Letter to the Editor; American Journal of Gastroenterology, 103; 2149-2150.