Current Advice on Parasite Control:
Intestinal Parasites - Diphyllobothriidean Tapeworm
*Diphyllobothriidean cestodes are sometimes referred to as “primitive” tapeworms
Overview of Life Cycle
Diphyllobothriidean cestodes have indirect life cycles that require two intermediate hosts before becoming infectious to the definitive host.
- Adult tapeworms discharge operculated eggs from a midventral genital pore; these eggs are then passed in the feces. When the egg contacts water, a ciliated embryo hatches and infects the first intermediate host, a copepod that eats it.
- In the copepod the first larval stage (procercoid) develops in 2-3 weeks.
- When the copepod is consumed by a second intermediate host, the next larval form develops (plerocercoid also called sparganum). Dogs and cats are infected when they ingest these larval forms in the second intermediate host.
Transport (paratenic) hosts may also be important in the life cycle of Diphyllobothriidean tapeworms.
- With D. latum, if a small infected fish is eaten by a larger fish, the tapeworm larva will transfer to the muscles of the larger fish and remain infective for the fish eating mammal.
- With Spirometra, if an infected second intermediate host is eaten by another second intermediate host, the plerocercoid larvae transfers to the tissues of the new host and is capable of infecting the dog or cat that ingests these infected animals.
- Diphyllobothriidean cestodes have indirect life cycles that require two intermediate hosts before becoming infectious to the definitive host.
- The egg of a diphyllobothriidean tapeworm shows the characteristic operculum (arrow).
- The ciliated first stage larva, termed a coracidium, emerges from the egg.
- The copepod is the first intermediate host..
- The larval form that is infective for vertebrates, sometimes referred to as a sparganum, develops in the second intermediate host after it ingests the copepod.
- The adult diphyllobothriidean cestode (see image) is found in the small intestine of dogs or cats and other fish-eating mammals including people.
- The scolex of adult diphyllobothriidean tapeworms does not have hooks or suckers like those found in “true” (Cyclophyllidean) tapeworms. The worms attach to the gut with dorsal and ventral longitudinal groves that are on the scolex. These slits are called bothria.
- Diphyllobothriidean eggs may be confused with trematodes eggs and eggs of D. latum and Spirometra species are hard to distinguish from each other because of similar size range (D. latum 67-71 x 40-50 um, Spirometra spp 65-70 x 35-37 um). However, Spirometra eggs are more likely to float in sugar or zinc solutions than D. latum, and Spirometra eggs are more narrow at the anterior end that D. latum eggs.
- Unlike “true” tapeworms, which are widely regarded as almost nonpathogenic to the dog or cat definitive host, infection with diphyllobothriidean cestodes 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.
- Infection of dogs and cats with diphyllobothriidean tapeworms 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.
Host Associations and Transmission Between Hosts
- Both dogs and cats are susceptible to infection with diphyllobothriidean tapeworms following ingestion of infected prey, but infection is not passed directly between dogs and cats.
- Diphyllobothrium infection of dogs and cats develops following ingestion of an infected fresh-water fish that ingested the copepod first intermediate host.
- Dogs and cats become infected with Spirometra when they ingest an infected vertebrate. Spirometra larvae have a broad host range; infective spargana may develop in amphibians, reptiles, birds, or mammals that ingest an infected copepod first intermediate host or a transport vertebrate host.
- Domestic and wild carnivores like the dog, cat, and raccoon are definitive hosts; the adult tapeworm lives in the small intestine and passes eggs in feces. The bobcat (Lynx rufus is thought to be the natural definitive host.
Prepatent Period and Environmental Factors
- Dogs and cats may begin shedding eggs of Diphyllobothrium latum 5 – 6 weeks after infection and Spirometra eggs as soon as 10 days following infection. Infections will occur only when dogs and cats ingest larvae in prey species or in undercooked animal tissue in an area where infection is cycling in nature.
Site of Infection and Pathogenesis
- Tapeworms are found in the small intestine of dogs and cats. Although not all infections with diphyllobothriidean cestodes in dogs and cats result in overt clinical disease, vomiting, diarrhea, and weight loss that resolve following treatment have been reported.
- Diagnosis of infection with diphyllobothriidean tapeworms is made by recognizing the characteristic operculated eggs on fecal flotation or sedimentation.
- Chains of segments may also be identified in vomit or feces of an infected dog or cat. The proglottids are wider than long with a medial genital pore. These senile segments have already shed their eggs when seen in feces or vomitous.
- No products have been approved for treatment of diphyllobothriidean tapeworm infections in dogs and cats.
- Praziquantel has been used successfully to treat diphyllobothriidean 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.
- Treatment of diphyllobothriidean tapeworms in dogs and cats must be combined with prevention of ingestion of prey species (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
- Dogs and cats infected with diphyllobothriidean cestodes do not create an immediate zoonotic risk because the stage (coracidium) that hatches from the eggs shed in pet feces are 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.
- Spirometra spp. are also zoonotic; people who inadvertently ingest Spirometra-infected copepods in water or pleurocercoid larvae (spargana) in the tissue of an infected second intermediate or paratenic host can develop a larval infection in their tissues. The plerocercoid larvae can also migrate from infected animal tissue into wounds or across a mucous membrane. In both cases, the human becomes a paratenic host with Spirometra larvae in the tissues. Zoonotic infections with Spirometra spp. in North America usually present as a flocculent subcutaneous mass; larvae may also develop in ocular tissue and in the central nervous system.
- Little S. and Ambrose D. 2000. Spirometra infections in cats and dogs. Compendium on Continuing Education for the Practicing Veterinarian 22, 299–305, 2000.
- 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.
- Zajac, A. M. and G. A. Conboy. 2012. Veterinary Clinical Parasitology, 8th ed. Wiley-Blackwell, Ames, Iowa 368p
- Bowman DD. 2009. Helminths, in Georgis’ Parasitology for Veterinarians, ed 9. WB Saunders Co, pp131-137.