Worm Parasites Cause Variety of Problems

By Edward L. Roberson.

Ascarids of dogs, commonly called "roundworms" and scientifically named Toxocara canis, are large worms which reach 4 to 8 inches in length when mature. As egg-laying adults, they live in the small intestine of young dogs less than 6 months old.

Probably 75 percent of pups and young dogs in the United States are infected with this parasite which passes eggs in the dog's feces (excrement). The adult worm is seen only rarely in older dogs because these dogs develop resistance to the adult worm. Older dogs, however, can harbor the small larval stage of this parasite.

The major means of infection of dogs with T. canis is by ingestion (eating) of infective ascarid eggs or transmission of infective larvae from a mother dog to her unborn or nursing pups. Ascarid eggs which are passed in the feces of a dog harboring adult worms are not immediately infective to another dog; they require a couple of weeks of warm temperature to develop a larval stage in the egg which is infective to any age dog.

If a young dog ingests the infective egg, the small larva which hatches in the dog's stomach will grow to a large adult worm which begins to produce eggs about 4 weeks after the infective stage was originally ingested.

When an older dog, however, ingests an infective egg, the hatched larval stage does not grow to an adult worm in the intestine. Instead, the larva migrates to the adult dog's musculature and there remains inactive. A dog throughout its life can acquire hundreds or even thousands of such quiescent larvae in its musculature.

Migrate to Fetus

In male dogs these ascarid larvae have no escape. In female dogs which become pregnant, however, the larvae become active and migrate via the umbilical cord to the liver of the fetuses during the final third of pregnancy (after the 42nd day). Thus newborn pups can be infected with T. canis ascarids from the start.

From the pup's liver the larvae make their way to the lungs, are coughed up, swallowed, and in the intestine will grow to large egg-laying adults in 4 weeks. While this intrauterine route of transmission is by far the most prevalent means of bitch-to-pup transmission of T. canis larvae, the parasites also can pass to pups through the mother dog's milk. Less than 5 percent of larvae, however, are transmitted by the latter route.

If the mother dog's initial supply of quiescent larvae is large, she can infect three successive litters of pups even in the absence of acquiring additional larvae in her musculature.

Clinical signs of Toxocara infection in pups depend on the number of larvae the pups have acquired from their mother or on the number of infective eggs ingested from contaminated environment where the pups live.

With but a few worms (10 or so), there may be no clinical evidence of infection. However, if the pup has several hundred worms which are growing to mature adults, the worm mass will distend the abdomen and give the pup a potbellied appearance. These worms deprive the pup of nutrients so that heavily infected pups are malnourished and slow growers. The pup's feces often are loose and frothy.

Diagnosis of ascarid infection is made by the above clinical signs and by microscopic examination of the feces for ascarid eggs. A veterinarian uses a salt or sugar flotation procedure to isolate the eggs from the feces.

Treat Pups Early. It is important to treat pups so that ascarids are expelled before they become egg-laying adults, that is, before pups are 4 weeks of age. Preferably Pups should be treated at 2, 4, 6, and 8 weeks of age. The initial treatment will expel intrauterine acquired ascarids. Subsequent treatments will expel ascarids acquired through the mother's milk.

Two drugs are available to veterinarians for treating young pups. They are pyrantel pamoate (Nemex) and fenbendazole (Panacur). Both are in suspension form and thus easy to administer to pups not yet eating solid food. The drugs also are effective in treating hookworms which are discussed below.

Treatment of the mother dog to destroy the larval ascarids in her musculature before the pups are born is difficult but possible. Only one drug, fenbendazole (Panacur), has been extensively tested for this purpose. It reduced ascarid burdens by 90 percent in pups born to treated bitches while pups born to unmedicated control bitches harbored an average of 400 worms each.

The regimen of treatment for the pregnant bitch must begin, however, about the 40th day of pregnancy and continue daily until 2 weeks after whelping that is, about 37 consecutive days.

Hookworms of Dogs

Hookworms are bloodsucking parasites. The most common species is Ancylostoma caninum. While the adult worm is only about an inch in length, it has a large mouth by which it attaches to the wall of the small intestine and digests a plug of tissue. This causes a small bleeding site. When 50 to several hundred worms are present, the total loss of blood can be dangerous even to an older dog but especially to small pups.

Dogs with heavy hookworm infections become anemic. The loss of blood into the intestine results in dark tar-colored feces. Gums of the animal will be blanched white instead of pink and the animal is weak. These clinical signs plus a fecal examination for eggs give a positive diagnosis of hookworm infection.

Both young and adult dogs can have infections with adult hookworms which shed eggs in the dog's feces. Once passed in the feces, hookworm eggs develop an infective larval form which hatches from the egg shell and can infect dogs either by penetrating the skin when dogs lie down in damp contaminated places or by being ingested with the dog's contaminated food or water.

Thus, dog food should never be placed directly on the ground. Use of food and water bowls which are cleaned daily will help minimize a yard-confined dog's exposure to infective hookworm larvae.

Older dogs, besides harboring adult hookworms in their intestines, also accumulate the immature larval stage of A. caninum in their body tissues (muscles, mammary glands). These larvae do not infect the fetuses of a pregnant dog as did Toxocara larvae, but they do pass in the milk of the mother to her nursing pups. About 60 percent of the larvae which do so will pass during the first week of nursing, but some larvae are transmitted every week as long as the mother dog is lactating.

These larvae will grow to egg-laying adult hookworms about 2 weeks after infecting the pup. So a pup as young as 2 weeks old can be contaminating the environment with hookworm eggs. The milk-borne route is the principal means by which young pups become infected with A. caninum hookworms.

If the number of hookworms is great, pups will begin showing clinical signs of anemia about 2 weeks of age and may die before 3 weeks of age.

Treatment. The regimen of treating pups for ascarids at 2, 4, 6, and 8 weeks of age is also ideal for eliminating hookworms as they are constantly acquired through the mother's milk. The two drugs, pyrantel pamoate and fenbendazole, mentioned earlier for use against ascarids, are effective also for hookworms of nursing pups.

Several additional drugs are available for treatment of hookworms in older dogs. Older dogs generally are given a single 1-day deworming once or twice per year as the owner seeks routine veterinary care. These treatments eliminate only the parasites which occur in the digestive tract.

Treatment of the pregnant bitch to kill larval hookworms and to prevent heavy mammary transmission of hookworms to her pups can be done as described earlier for ascarid larvae that is, treatment with fenbendazole daily from the 40th day of pregnancy until 2 weeks after whelping.

Worms in Cats

Cats also can be infected with ascarids and hookworms but the common species in cats are not the same species of parasites which occur in dogs. The common ascarid (Toxocara cati) and hookworm (Ancylostoma tubaeforme) of cats do not infect dogs and vice versa.

Ascarids and hookworms do not occur as frequently in cats as they do in dogs. This is probably related to the cat's burying its feces (excrement) while feces from dogs remain on the ground surface where ascarid eggs and hookworm larvae can be scattered more readily by rainwater, thus contaminating a wider area.

Cats acquire ascarid infections from ingesting infective eggs which have been passed in the feces of a cat harboring adult worms. Both young and older cats may harbor such infections. Nursing kittens may also acquire ascarid larvae through the milk of the mother cat, but intrauterine infection of the fetus during pregnancy is not known to occur.