Tapeworms.

Dogs and cats become infected with tapeworms by ingesting intermediate hosts that contain encysted juvenile tapeworms called larvae. The dog or cat is said to be the definitive host because it shelters the sexually reproductive, egg-producing stage of the tapeworm. The intermediate hosts, which are vertebrates in the case of Taenia and Mesocestoides and insects in the case of Dipylidium and Hymenolepis, become infected by ingesting unhatched but infective tapeworm eggs discharged in the feces of the dog or cat. The relatively less common Diphyllobothrium and Spirometra tapeworms discharge eggs that are undeveloped when passed in the feces and must fall into water to undergo development to the coracidium stage. Diphyllobothrium and Spirometra may have two or three aquatic intermediate hosts in series. The first of these, a copepod, ingests the free-swimming coracidium or ciliated oncosphere that has hatched from the egg. The final intermediate host containing the larva (plerocercoid) infective for the dog or cat is an aquatic vertebrate (fish, frog, water snake). Thus, dogs and cats become infected with tapeworms by eating uncooked meat or fish or by ingesting certain insects. These intermediate hosts are infected with juvenile tapeworms called larvae, which are the infective form for the dog or cat. The intermediate hosts, in turn, become infected by ingesting tapeworm eggs discharged in the feces of the dog or cat or, in the case of Diphyllobothrium and Spirometra, by ingesting coracidia that have subsequently developed in and hatched from such eggs. By far the most common tapeworms of dogs and cats in North America are D. caninum, T. pisiformis, and T. hydatigena. Therefore, the most common sources of tapeworm infection are, respectively, fleas, wild rabbits, and the uncooked meat and offal of ruminants and swine. Whenever a dose of tapeworm remedy is administered or dispensed, the client should be informed of these potential sources of reinfection. There is considerable overlap in the spectra of activity of currently available cestocides. Mebendazole, fenbendazole, niclosamide, bunamidine, and praziquantel are all effective against Taenia spp. Bunamidine is the drug of choice against Spirometra, Diphyllobothrium, and Mesocestoides. Praziquantel is the drug of choice against Echinococcus and Dipylidium.