Roundworms in Dogs

Ascarids are the most frequent worm parasite in dogs and cats. There are two species that commonly infect dogs: Toxocara canis and Toxascaris leonina. Adult roundworms live in the stomach and intestines and can grow to 7 inches (18 cm) long. A female may lay 200,000 eggs in a day. The eggs are protected by a hard shell. They are extremely hardy and can live for months or years in the soil.

There are four ways dogs can become infected with roundworms. Prenatal infection occurs when the larvae migrate through the placenta in utero. Almost all puppies are infected in this manner before birth. Mother's milk can also transmit ascarids. In addition, puppies and adults can become infected by ingesting eggs in the soil. And finally, dogs can acquire the eggs by ingesting a transport or intermediate host, such as a mouse or other rodent.

The life cycle of T. canis in young puppies is as follows: Eggs entering through the puppy's mouth hatch in her stomach. The larvae are carried to the lungs by the circulatory system. Here they break through the capillaries into the air sacs, sometimes causing bouts of coughing and gagging. Once in the lungs, the larvae crawl up the windpipe and are swallowed. Back in the intestines, the larvae develop into adult worms. The adults pass eggs that become infective in soil in three to four weeks.

Dogs older than 6 months develop an acquired resistance to ascarids. Few, if any, larvae complete the life cycle. Most come to rest in various body tissues, where they encyst. While encysted, they are protected against the dog's antibodies and also the effects of most dewormers. (Interceptor is an exception. This dewormer has some effect on encysted larvae.) During pregnancy, however, encysted larvae are activated and migrate to the placenta and mammary glands. Deworming the dam before pregnancy reduces the burden of migrating larvae but does not eliminate all puppy infestations because there are still encysted larvae in the mother's body.

Ascarids rarely cause symptoms in adult dogs; in puppies older than 2 months, they usually produce only mild intermittent vomiting and diarrhea. Worms maybe found in the vomitus or passed in the stool. Typically, they look like white earthworms or strands of spaghetti that may be moving.

In very young puppies, a heavy infestation can result in severe illness or even death. These puppies often fail to thrive, have a dull coat and a pot-bellied appearance, and are anemic and stunted in growth.

Treatment: Nemex or Strongid (pyrantel pamoate suspension) is an excellent dewormer for nursing pups because it is safe and active against both ascarids and hookworms. Puppies should be dewormed by 2 weeks of age-before they begin to pass ascarid eggs and contaminate the environment. Repeat the treatment at 4, 6, and 8 weeks of age. The purpose of retreating is to kill worms that were in the larval stage during the first dewormings. Subsequent treatments are indicated if eggs or worms are found in the stool.

Drontal Plus, Vercom Paste, Telmintic, and Panacur have a broad spectrum of activity and are all highly effective against ascarids, hookworms, and whipworms. Drontal and Vercom are also highly effective against tapeworms. Panacur is only partially effective against tapeworms. Deworming the brood bitch with Panacur during the last two weeks of gestation and during lactation reduces environmental exposure and helps control puppy infection.

Prevention: Heartworm preventives such as Interceptor prevent and control ascarids as well as hookworms and whipworms. Heartgard Plus controls ascarids and hookworms but not whipworms.

Public health considerations: Ascarids can cause a serious disease in humans called visceral larva migrans. The infection is acquired when the eggs of T. canis are ingested by a human. Children 1 to 4 years old are most frequently affected, and often have a history of eating dirt.

When a human eats an ascarid egg, larvae develop as in the dog. However, because humans are not a definitive host, the larvae do not progress to adult ascarids. Instead, they burrow into the intestinal wall and migrate to the liver, lungs, and skin. Symptoms, which develop only when the infestation is heavy, include abdominal pain, cough, wheezing, itching, and a skin rash with papules. In a very heavy infestation, larvae may reach the heart, kidneys, spleen, brain, eyes, and other tissues. In young children, a syndrome called ocular larva migrans can lead to blindness and necessitate the removal of an eye.

Older children should be instructed not to put soil and sand in their mouths. Infants and toddlers require parental supervision to prevent this. Infants and young children should not be allowed to play with and handle nursing pups until they have mastered the discipline of washing their hands after petting an animal.

This article is excerpted from “Dog Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.