In contrast, adult pseudophyllidean cestodes, such as Spirometra species or Diphyllobothrium latum, discharge individual operculated eggs through a median genital pore. These eggs hatch upon contact with water and develop in a copepod first intermediate host and a vertebrate second intermediate host before being ingested by a cat or dog definitive host and developing into an adult tapeworm. Dogs and cats may begin shedding pseudophyllidean tapeworm eggs as soon as 10 days after infection. Infections will only occur when dogs and cats ingest larvae in prey species or in undercooked animal tissue in an area where infection is cycling in nature.1
Adult tapeworms reside in the small intestines of dogs and cats. Motile proglottids may be seen in the perianal region as they exit the animal, in the pet's environment (e.g. on bedding), or in the fecal material itself.
Although the common cestodes of dogs and cats, such as Dipylidium caninum and Taenia species, usually do not cause significant disease in pets,1 tapeworm infections are aesthetically unpleasant. The presence of proglottids on pets or in the home may cause distress to clients and can fracture the human-animal bond. In addition, infection of pets with these parasites poses a zoonotic health risk in the home. In contrast, the less commonly seen Mesocestoides species may occasionally induce peritoneal cestodiasis, and both Spirometra and Diphyllobothrium species have been associated with gastrointestinal disease characterized by vomiting, diarrhea, and weight loss in infected cats and dogs.2 For all of these reasons, control and treatment of tapeworms in pets is warranted.
PREVALENCE AND GEOGRAPHIC DISTRIBUTION
The reported prevalence of tapeworms in the Americas in published studies varies from 4% to 60% in dogs and 1.8% to 52.7% in cats.3-5 A number of factors influence the likelihood that a particular dog or cat will become infected with tapeworms, including the geographic region where the animal lives and the opportunity it has to ingest an infected intermediate host. Prevalence data generated by fecal flotation alone almost certainly underestimate the frequency of infection with Dipylidium caninum and Taenia species. Because proglottids, and thus eggs, are focally distributed in fecal material, a given fecal sample may not contain tapeworm proglottids or eggs even in the presence of an infection.3
Dipylidium caninum and Taenia species are found throughout North America. Mesocestoides species is less common than either D. caninum or Taenia species but does occasionally appear in foci throughout the United States. At present, Echinococcus species are thought to be largely limited to areas of Alaska and of the north-central, midwestern, and southwestern United States as well as areas of Canada.6
Spirometra and Diphyllobothrium species infections in dogs and cats are not as common as infection with cyclophyllidean cestodes, and studies reporting prevalence estimates have not been published. However, these tapeworms are known to be common in pets in some areas of the United States.
STRATEGIES TO CONTROL TAPEWORMS IN PETS
In North America, Echinococcus species infections in people are rare6 ; isolated reports of zoonotic infection with larval Taenia species of dogs and cats also exist.9 Although the overall risk of human infection with these parasites in North America is extremely low, dogs and cats infected with these tapeworms do create a potential zoonotic risk. Taenia and Echinococcus species eggs shed in the feces of an infected dog or cat are immediately infectious to the intermediate host.1 People who consume these eggs may develop cestode cysts requiring drainage, surgical removal, or extended chemotherapy. In the case of Echinococcus multilocularis, surgery is unlikely to be successful, and long-term anthelmintic therapy may be required.6
Dipylidium caninum infections in children who have ingested infected fleas are occasionally reported.10 The disease induced in children is generally mild, confined to the intestinal tract, and readily treated but can still be distressing to the families. People are a normal definitive host of Diphyllobothrium latum and may become infected with this tapeworm after ingesting larvae in raw fish.1 Spirometra species are also zoonotic; people who inadvertently ingest Spirometra species-infected copepods in water or spargana in the tissue of an infected second intermediate host can develop the larval form. In North America, people with Spirometra species infections usually present with flocculent subcutaneous masses; larvae have also been reported to develop in ocular tissue and in the central nervous system.2
Editors' note: Dr. Little has received research funding from Bayer Animal Health.
Susan E. Little, DVM, PhD, DEVPC
Department of Veterinary Pathobiology
Center for Veterinary Health Sciences
Oklahoma State University
Stillwater, OK 74078
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