Diphyllobothrium latum (fish or broad tapeworm) Epidemiology
Fish tapeworm infection is distributed worldwide, in the subarctic and temperate regions; it is associated with eating of raw or improperly cooked fresh water fish.
Morphology
This is the longest tapeworm found in man, ranging from 3-10 meters with more than 3000 proglottids. The scolex resembles two almond-shaped leaves and the proglottids are broader than they are long, a morphology reflected in the organism's name. Eggs are 30 x 50 micrometers in size and contain an embryo with 3 pairs of hooklets (figure 4).
Life cycle
Man and other animals are infected by eating uncooked fish that contains plerocercoid larvae (15 x 2 mm) which attach to the small intestinal wall and mature into adult worms in 3 to 5 weeks. Eggs discharged from gravid proglottids in the small intestine are passed in the feces. The egg hatches in fresh water to produce a ciliated coracidium which needs to be ingested by a water flea (Cyclops) where it develops into a procercoid larva. When infected Cyclops are ingested by the freshwater fish, the procercoid larva penetrates the intestinal wall and develops into a plerocercoid larva, infectious to man (figure 3).
Symptoms
Clinical symptoms may be mild, depending on the number of worms. They include abdominal discomfort, loss of weight, loss of appetite and some malnutrition. Anemia and neurological problems associated with vitamin B12 deficiency are seen in heavily infected individuals.
Diagnosis
Diagnosis is based on finding many typical eggs and empty proglottids in feces (Figure 3). A history of raw fish consumption and residence in an endemic locality is helpful.
Treatment and control
Praziquantel is the drug of choice. Freezing for 24 hours, thorough cooking or pickling of fish kills the larvae. Fish reservoirs should be kept free of raw sewage.
Figure 3
Immature eggs are passed in feces
. Under appropriate conditions, the eggs mature (approximately 18 to 20 days)
and yield oncospheres which develop into a coracidia
. After ingestion by a suitable freshwater crustacean (the copepod first intermediate host) the coracidia develop into procercoid larvae
. Following ingestion of the copepod by a suitable second intermediate host, typically minnows and other small freshwater fish, the procercoid larvae are released from the crustacean and migrate into the fish flesh where they develop into a plerocercoid larvae (sparganum)
. The plerocercoid larvae are the infective stage for humans. Because humans do not generally eat undercooked minnows and similar small freshwater fish, these do not represent an important source of infection. Nevertheless, these small second intermediate hosts can be eaten by larger predator species, e.g., trout, perch, walleyed pike
. In this case, the sparganum can migrate to the musculature of the larger predator fish and humans can acquire the disease by eating these later intermediate infected host fish raw or undercooked
. After ingestion of the infected fish, the plerocercoid develop into immature adults and then into mature adult tapeworms which will reside in the small intestine. The adults of D. latum attach to the intestinal mucosa by means of the two bilateral groves (bothria) of their scolex
. The adults can reach more than 10 m in length, with more than 3,000 proglottids. Immature eggs are discharged from the proglottids (up to 1,000,000 eggs per day per worm)
and are passed in the feces
. Eggs appear in the feces 5 to 6 weeks after infection. In addition to humans, many other mammals can also serve as definitive hosts for D. latum. CDC DPDx Parasite Image Library
Figure 4A
Eggs of Diphyllobothrium latum. These eggs are oval or ellipsoidal, with at one end an operculum (arrows) that can be inconspicuous (right). At the opposite (abopercular) end is a small knob that can be barely discernible (left). The eggs are passed in the stool unembryonated. Size range: 58 to 76 µm by 40 to 51 µm. CDC. Image A contributed by Georgia Division of Public Health
Figure 4B
Gravid proglottids of Diphyllobothrium latum. CDC/Dr. Mae Melvin
Figure 4C
Proglottids of Diphyllobothrium latum. The species characteristics are: the proglottid is broader than it is long; size 2 to 4 mm long by 10 to 12 mm wide; uterus coiled in rosette appearance; genital pore at the center of the proglottid. CDC
Figure 4E
Diphyllobothrium latum scolex and gravid proglottids © Dr Peter Darben, Queensland University of Technology clinical parasitology collection. Used with permission
Figure 4D
Proglottids of Diphyllobothrium latum. These proglottids tend to be passed in strands of variable length in the stool. The proglottids tend to be broader than long. CDC. Image contributed by Georgia Division of Public Health.
Figure 4F
Diphyllobothrium latum egg © Dr Peter Darben, Queensland University of Technology clinical parasitology collection. Used with permission