Diphyllobothrium spp.
Species
Diphyllobothrium latum
Diphyllobothrium dendriticum
Other Diphyllobothrium spp.
The most common of the Diphyllobothrium spp. in North America is D. latum, commonly referred to as the “broad fish tapeworm.”
Overview of Life Cycle
- Diphyllobothrium spp. have indirect life cycles that require two intermediate hosts before becoming infectious to the definitive host.
- Adult tapeworms release operculate eggs from a midventral genital pore; these eggs are then passed in the feces. When the egg contacts water, a ciliated embryo (coracidium) hatches and is ingested by the first intermediate host, a copepod.
- In the copepod the first larval stage (procercoid) develops in 2-3 weeks.
- When the copepod is consumed by a second intermediate host, any of a number of species of fish, the next larval form develops (plerocercoid).
- Other fish species can serve as transport (paratenic) hosts and also may be important in the life cycle of Diphyllobothrium spp.
- Dogs and cats are infected when they ingest the plerocercoid in the fish intermediate or paratenic host.
- Fish hosts of D. latum include perch, pike, burbot, sauger, and walleye
- Fish hosts of D. dendriticum include salmonids, three-spine sticklebacks, and osmerids.
- Adult D. latum in the small intestine of the definitive host can grow to a length of 3 to 25 m in length, while D. dendriticum can be more than 2 m in length
Stages
- The egg of a Diphyllobothrium spp. shows the characteristic operculum (arrow).
The ciliated first stage larva, termed a coracidium, emerges from the egg in the presence of water and is ingested by a copepod first intermediate host where it develops into a procercoid.
A second intermediate host ingests the copepod and a plerocercoid develops. This plerocercoid stage is infective to larger fish paratenic hosts and to any definitive hosts that eat raw fish.
Adult Diphyllobothrium spp. are found in the small intestine of dogs, cats, and other fish-eating mammals including people.
The scolex of adult Diphyllobothrium spp. does not have hooks or suckers like those found in Cyclophyllidean tapeworms. Rather, the cestodes attach to the small intestinal mucosa with dorsal and ventral longitudinal groves on the scolex called bothria.
Because of the presence of an operculum, diphyhillobothridean eggs may be confused with trematode eggs. Eggs of Diphyllobothrium spp. and Spirometra species may be difficult to distinguish from one another because of similar size range (Diphyllobothrium spp. 58-76 x 40-51 µm, Spirometra spp.  55-76 x 30-43 µm). However, Spirometra eggs are more likely to float in sugar or zinc solutions than Diphyllobothrium spp., and Spirometra eggs are narrower at the anterior end than Diphyllobothrium spp. eggs.
Diphyllobothrium egg 400X Flotation
Diphyllobothriidean Adult
Disease
- Unlike cyclophyllidean tapeworms, adults of which are widely regarded as almost nonpathogenic to the dog or cat definitive host, infection with Diphyllobothrium spp. has been associated with gastrointestinal disease in dogs and cats. Clinical signs reported include diarrhea, weight loss, and vomiting which usually resolve following appropriate anthelmintic therapy.
Prevalence
Infection of dogs and cats with Diphyllobothrium spp. is not as common as infection with cyclophyllidean cestodes, and studies reporting prevalence estimates have not been published. Nevertheless, these tapeworms may be frequently seen and thus regionally important in some areas of the United States.
D. latum is most commonly seen in the Great Lakes region of the upper Midwestern United States and in Canada.
D. dendriticum is less common but has been reported in Alaska, Maine, Michigan, Minnesota, Montana, Nevada, Oregon, Wyoming, British Columbia, Manitoba, Newfoundland-Labrador, Northwest Territories, Ontario, and Quebec.
Host Associations and Transmission Between Hosts
Both dogs and cats are susceptible to infection with Diphyllobothrium spp. following ingestion of infected fish but infection is not passed directly between dogs and cats.
Diphyllobothrium spp. infection of dogs and cats develops following ingestion of an infected fish that ingested the copepod first intermediate host.
Prepatent Period and Environmental Factors
- Dogs and cats may begin shedding eggs of Diphyllobothrium latum about 3-4 weeks after infection. Infections will occur only when dogs and cats ingest larvae in fish in an area where infection is cycling in nature.
Site of Infection and Pathogenesis
- Adult Diphyllobothrium spp. are found in the small intestine of dogs and cats. Although not all infections with Diphyllobothrium spp. in dogs and cats resulting in overt clinical disease, vomiting, diarrhea, and weight loss that resolve following treatment have been reported.
Diagnosis
Diagnosis of infection with Diphyllobothrium spp. is made by recognizing the characteristic operculum, light brown, undifferentiated eggs (58-76µm by 40-51µm) on sedimentation. Eggs may be detected by flotation but sedimentation is a more reliable technique.
Eggs of D. latum and D. dendriticum cannot be differentiated morphologically.
Chains of segments may also be identified in vomit or feces of an infected dog or cat.  The proglottids are wider than long and have a distinct medial genital pore. These segments are considered senile and have already shed their eggs when seen in feces or vomitus.
Treatment
No products have been approved for treatment of Diphyllobothrium spp. infections in dogs and cats.
Praziquantel has been used successfully to treat pseudophyllidean tapeworms in dogs and cats; however, a higher-than-labeled dose (25 mg/kg orally) and extended duration of treatment (2 consecutive days) may be required to eliminate the infection.
A single dose of praziquantel (35 mg/kg orally) has also been recommended for treating infection in cats.
Treatment of Diphyllobothrium spp. in dogs and cats must be combined with prevention of ingestion of raw fish (see Control and Prevention) or reinfection is likely to occur.
Control and Prevention
- Preventing predation and scavenging activity by keeping cats indoors and dogs confined to a leash or in a fenced yard will limit the opportunity for dogs and cats to acquire infection with these cestodes.
Dogs and cats should not be fed raw or undercooked fish or other vertebrate tissue.
Public Health Considerations
- Diphyllobothrium spp. infection is the cause of diphyllobothriasis in humans, the most important fish-borne zoonoses caused by a cestode parasite, with an estimated 20 million people infected worldwide.
Dogs and cats infected with Diphyllobothrium spp. do not create an immediate zoonotic risk because the stage (coracidium) that hatches from the eggs shed in pet feces is infectious only to the copepod first intermediate host.
People are a normal definitive host of D. latum and may become infected with this tapeworm upon ingestion of larvae in raw fish. Clinical signs in people include abdominal pain, vomiting, diarrhea, weight loss, and fatigue.
Pernicious anemia due to absorption of vitamin B-12 by Diphyllobothrium spp. has been described in human infections.
Selected References
- Conboy G. Cestodes of dogs and cats in North America. Vet Clin North Am Small Anim Pract 39:1075-1090, 2009.
Kuchta R., Brabec J., Kubackova P., and Scholz T. 2013. Tapeworm Diphyllobothrium dendriticum (Cestoda)- neglected or emerging human parasite? PLOS Neglected Tropical Diseases. 7(12).
Kuchta R., Scholz T., Brabec J., and Bray R. A.2008. Suppression of the tapeworm order Pseudophyllidea (Platyhelminthes: Eucestoda) and the proposal of two new orders, Bothriocephalidea and Diphyllobothriidea. International Journal for Parasitology. 38(1):49-55.
Species
Diphyllobothrium latum
Diphyllobothrium dendriticum
Other Diphyllobothrium spp.
The most common of the Diphyllobothrium spp. in North America is D. latum, commonly referred to as the “broad fish tapeworm.”
Overview of Life Cycle
- Diphyllobothrium spp. have indirect life cycles that require two intermediate hosts before becoming infectious to the definitive host.
- Adult tapeworms release operculate eggs from a midventral genital pore; these eggs are then passed in the feces. When the egg contacts water, a ciliated embryo (coracidium) hatches and is ingested by the first intermediate host, a copepod.
- In the copepod the first larval stage (procercoid) develops in 2-3 weeks.
- When the copepod is consumed by a second intermediate host, any of a number of species of fish, the next larval form develops (plerocercoid).
- Other fish species can serve as transport (paratenic) hosts and also may be important in the life cycle of Diphyllobothrium spp.
- Dogs and cats are infected when they ingest the plerocercoid in the fish intermediate or paratenic host.
- Fish hosts of D. latum include perch, pike, burbot, sauger, and walleye
- Fish hosts of D. dendriticum include salmonids, three-spine sticklebacks, and osmerids.
- Adult D. latum in the small intestine of the definitive host can grow to a length of 3 to 25 m in length, while D. dendriticum can be more than 2 m in length
Stages
- The egg of a Diphyllobothrium spp. shows the characteristic operculum (arrow).
The ciliated first stage larva, termed a coracidium, emerges from the egg in the presence of water and is ingested by a copepod first intermediate host where it develops into a procercoid.
A second intermediate host ingests the copepod and a plerocercoid develops. This plerocercoid stage is infective to larger fish paratenic hosts and to any definitive hosts that eat raw fish.
Adult Diphyllobothrium spp. are found in the small intestine of dogs, cats, and other fish-eating mammals including people.
The scolex of adult Diphyllobothrium spp. does not have hooks or suckers like those found in Cyclophyllidean tapeworms. Rather, the cestodes attach to the small intestinal mucosa with dorsal and ventral longitudinal groves on the scolex called bothria.
Because of the presence of an operculum, diphyhillobothridean eggs may be confused with trematode eggs. Eggs of Diphyllobothrium spp. and Spirometra species may be difficult to distinguish from one another because of similar size range (Diphyllobothrium spp. 58-76 x 40-51 µm, Spirometra spp.  55-76 x 30-43 µm). However, Spirometra eggs are more likely to float in sugar or zinc solutions than Diphyllobothrium spp., and Spirometra eggs are narrower at the anterior end than Diphyllobothrium spp. eggs.
Diphyllobothrium egg 400X Flotation
Diphyllobothriidean Adult
Disease
- Unlike cyclophyllidean tapeworms, adults of which are widely regarded as almost nonpathogenic to the dog or cat definitive host, infection with Diphyllobothrium spp. has been associated with gastrointestinal disease in dogs and cats. Clinical signs reported include diarrhea, weight loss, and vomiting which usually resolve following appropriate anthelmintic therapy.
Prevalence
Infection of dogs and cats with Diphyllobothrium spp. is not as common as infection with cyclophyllidean cestodes, and studies reporting prevalence estimates have not been published. Nevertheless, these tapeworms may be frequently seen and thus regionally important in some areas of the United States.
D. latum is most commonly seen in the Great Lakes region of the upper Midwestern United States and in Canada.
D. dendriticum is less common but has been reported in Alaska, Maine, Michigan, Minnesota, Montana, Nevada, Oregon, Wyoming, British Columbia, Manitoba, Newfoundland-Labrador, Northwest Territories, Ontario, and Quebec.
Host Associations and Transmission Between Hosts
Both dogs and cats are susceptible to infection with Diphyllobothrium spp. following ingestion of infected fish but infection is not passed directly between dogs and cats.
Diphyllobothrium spp. infection of dogs and cats develops following ingestion of an infected fish that ingested the copepod first intermediate host.
Prepatent Period and Environmental Factors
- Dogs and cats may begin shedding eggs of Diphyllobothrium latum about 3-4 weeks after infection. Infections will occur only when dogs and cats ingest larvae in fish in an area where infection is cycling in nature.
Site of Infection and Pathogenesis
- Adult Diphyllobothrium spp. are found in the small intestine of dogs and cats. Although not all infections with Diphyllobothrium spp. in dogs and cats resulting in overt clinical disease, vomiting, diarrhea, and weight loss that resolve following treatment have been reported.
Diagnosis
Diagnosis of infection with Diphyllobothrium spp. is made by recognizing the characteristic operculum, light brown, undifferentiated eggs (58-76µm by 40-51µm) on sedimentation. Eggs may be detected by flotation but sedimentation is a more reliable technique.
Eggs of D. latum and D. dendriticum cannot be differentiated morphologically.
Chains of segments may also be identified in vomit or feces of an infected dog or cat.  The proglottids are wider than long and have a distinct medial genital pore. These segments are considered senile and have already shed their eggs when seen in feces or vomitus.
Treatment
No products have been approved for treatment of Diphyllobothrium spp. infections in dogs and cats.
Praziquantel has been used successfully to treat pseudophyllidean tapeworms in dogs and cats; however, a higher-than-labeled dose (25 mg/kg orally) and extended duration of treatment (2 consecutive days) may be required to eliminate the infection.
A single dose of praziquantel (35 mg/kg orally) has also been recommended for treating infection in cats.
Treatment of Diphyllobothrium spp. in dogs and cats must be combined with prevention of ingestion of raw fish (see Control and Prevention) or reinfection is likely to occur.
Control and Prevention
- Preventing predation and scavenging activity by keeping cats indoors and dogs confined to a leash or in a fenced yard will limit the opportunity for dogs and cats to acquire infection with these cestodes.
Dogs and cats should not be fed raw or undercooked fish or other vertebrate tissue.
Public Health Considerations
- Diphyllobothrium spp. infection is the cause of diphyllobothriasis in humans, the most important fish-borne zoonoses caused by a cestode parasite, with an estimated 20 million people infected worldwide.
Dogs and cats infected with Diphyllobothrium spp. do not create an immediate zoonotic risk because the stage (coracidium) that hatches from the eggs shed in pet feces is infectious only to the copepod first intermediate host.
People are a normal definitive host of D. latum and may become infected with this tapeworm upon ingestion of larvae in raw fish. Clinical signs in people include abdominal pain, vomiting, diarrhea, weight loss, and fatigue.
Pernicious anemia due to absorption of vitamin B-12 by Diphyllobothrium spp. has been described in human infections.
Selected References
- Conboy G. Cestodes of dogs and cats in North America. Vet Clin North Am Small Anim Pract 39:1075-1090, 2009.
Kuchta R., Brabec J., Kubackova P., and Scholz T. 2013. Tapeworm Diphyllobothrium dendriticum (Cestoda)- neglected or emerging human parasite? PLOS Neglected Tropical Diseases. 7(12).
Kuchta R., Scholz T., Brabec J., and Bray R. A.2008. Suppression of the tapeworm order Pseudophyllidea (Platyhelminthes: Eucestoda) and the proposal of two new orders, Bothriocephalidea and Diphyllobothriidea. International Journal for Parasitology. 38(1):49-55.