Dipylidium caninum - The Dog Tapeworm
One of the more common parasites of domestic dogs is the tapeworm Dipylidium caninum (DIE-pill-ID-ee-um kay-NINE-um). This parasite grows to around 10-15cm, with individual segments resembling cucumber seeds in size and shape. The intermediate host for dog tapeworms are the dog and cat fleas Ctenocephalides canis (TEN-oh-KEFF-ah-LIE-dees KAY-niss) and Ctenocephalides felis (FEEL-iss). The eggs passed out in the dog's faeces are eaten by the larval stage of the flea, and the immature tapeworm stays with the flea through its metamorphosis to the blood-sucking adult. When the flea bites, the dog may bite at the flea and swallow the larval tapeworm. In heavily infested dogs, their mouths may harbour many of the parasites in the bodies of fleas removed during grooming.
The segments of Dipylidium are capable of limited movement, and if this happens in the dog's rectum or anus, it causes intense itching. Afflicted dogs may be observed to "scoot" or drag their backsides along the ground to relive the itching. This, of course helps to crush the segments and release the eggs into the environment.
If humans swallow infected fleas, they too may become infected. This is easier than it sounds. Allowing the dog to lick your mouth may transfer the larval tapeworms to your mouth, while crushing the fleas between your fingernails spreads the parasites across your hands. Children are particularly prone to infection with Dipylidium. Like other flatworms, this tapeworm is not affected by routine dog worming treatments (ie. those directed against the intestinal nematodes like roundworm, whipworm or hookworm). Instead, dogs should be routinely wormed with a broad spectrum anthelmintic - one which has been shown to be effective against tapeworms.