Tapeworm (Pseudophyllidean Cestodes*)

Species*

Diphyllobothrium latum
Spirometra spp.

*Pseudophyllidean cestodes are sometimes referred to as “primitive” tapeworms

Pseudophyllidean cestodes have indirect life cycles that require two intermediate hosts before becoming infectious to the definitive host.
- Adult pseduophyllidean tapeworms discharge operculated eggs from a 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.
- The copepod is then consumed by a second intermediate host, and the larval form develops. Dogs and cats are infected when they ingest these larval forms in the second intermediate host.

The egg of a pseduophyllidean cestode shows the characteristic operculum (arrow).
The ciliated form, termed a coracidium, emerges from the egg.
The copepod is the first intermediate host of a pseudophyllidean cestode.
The larval form, sometimes referred to as a sparganum, develops in the second intermediate host.
The adult pseudophyllidean cestode (see image) is found in the small intestine of a dog or cat.

Unlike “true” tapeworms, which are widely regarded as nonpathogenic to the dog or cat definitive host, infection with pseudophyllidean cestodes has been associated with gastrointestinal disease in dogs and cats. Clinical signs reported include diarrhea, weight loss, and vomiting and usually resolve following appropriate anthelmintic therapy.

Infection of dogs and cats with pseudophyllidean 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.

Both dogs and cats are susceptible to infection with pseudophyllidean 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.

Dogs and cats may begin shedding eggs of pseudophyllidean tapeworms 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.

Tapeworms are found in the small intestine of dogs and cats. Although not all infections with pseudophyllidean 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 pseudophyllidean tapeworms is made by recognizing the characteristic operculated eggs on fecal flotation or by identifying the long chains of segments of the adult worm, which may be found in vomit or feces of an infected dog or cat. Adult tapeworms can be identified grossly by the presence of a distinct medial genital pore.

No products have been approved for treatment of pseudophyllidean tapeworm infections in dogs and cats.
Praziquantel has been used successfully to treat pseduophyllidean 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 pseudophyllidean 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.

Prevention of 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 pseudophyllidean cestodes.
Dogs and cats should not be fed raw or undercooked fish or other vertebrate tissue.

Dogs and cats infected with pseudophyllidean cestodes do not create an immediate zoonotic risk because the coracidia that hatch 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 spargana in the tissue of an infected second intermediate host can develop the larval infection. Zoonotic infections with Spirometra spp. in North America usually present as a flocculent subcutaneous mass; larvae have also been reported to develop in ocular tissue and in the central nervous system.