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Common Intestinal Parasites of Dogs and Cats

Ron Hines DVM PhD 2/10/03 -

Intestinal worms are the most common disease problem in dogs and cats. Left untreated in young animals, they cause stunting, lack-luster hair coats, thinness, listlessness and diarrhea. In adult animals the signs are often more subtle.

Dogs and cats often carry hookworms (Ancylostoma species). These parasites are barely visible to the naked eye. Young puppies and occasional kittens are often born with these parasites since they can migrate from encysted stages in the mother. Others ingest the eggs of hookworms when the pets are kept in unsanitary conditions or on soil. Hookworm eggs are quite fragile, so bright sunshine and dryness kills them rapidly. When a living egg is eaten, it hatches in the petís stomach and quickly burrows through the stomach lining and into the pets tissues. It then migrates through the body Ė often the liver Ė as it matures. Eventually, the worm is coughed up and re-swallowed. This time, the worm makes its home in the petís small intestine where it chews and damages the lining as it sucks blood. Pets often become anemic due to this blood loss. Their stools are often abnormally dark and loose as well. Heavy hookworm infections in young or debilitated pets can be fatal. Hookworm disease is easily eliminated by administering oral pyrantel pamoate and prevented by giving monthly heartworm preventative such as Heartgard, which contains the same drug. These parasites can also infect human beings where they can cause a disease called visceral larval migrans or severe eye problems. Hookworms are diagnosed by microscopic examination of the petís stool for characteristic parasite eggs.

Dogs and cats often carry roundworms (Toxocara & Toxascaris species). These are larger worms, which look like spaghetti when passed in the stool. Again, puppies and kittens can be born with these parasites. When present in large numbers, the puppies and kittens often have a potbelly and dry, scaly hair coat. Unsanitary conditions increase the likelihood of these parasites and they are often found in conjunction with hookworms. Roundworms have no mouth so the do less damage to the lining of the intestine. They do their damage as they migrate through the body and by producing colic as the thrash about in the intestine. Occasionally they will obstruct the flow of bile from the liver. Unlike hookworm eggs, roundworm eggs are very resistant to drying, sunshine or disinfectants. They can last for years in soil and still be infective. They can also transfer to young children and cause eye inflammations in children and adults. The same pharmaceuticals, which cure and prevent hookworm disease cure and prevent roundworms.

Another common worm (nematode) of dogs are whipworms (Trichuris species). Although not nearly as common as hookworms or roundworms, these worms are most common in adult animals Ė especially those housed in groups or kennels. These worms have a direct life cycle and do not leave the intestine. Eggs are passed in the dogís stool and after 2-4 weeks in a warm, moist environment, they become infective to another dog. The eggs hatch and the worms mature in the cecal area of the intestine where they can cause chronic bowel inflammation. Many dogs show no signs. Others have soft or mucoid stools and fail to gain weight. It is often difficult to diagnose infection because eggs are sparsely shed from time to time. Dogs with symptoms of whipworms are best placed on six months of a heartworm preventative containing milbemycin oxime (Interceptor) which also destroys these parasites. A second drug, fenbendazole is also effective in eliminating this parasite. Dog whipworms are not infectious to humans.