Causes of Roundworms
The two most common species of roundworms found in domestic dogs are Toxocara canis and Toxascaris leonina. A related roundworm that usually infects raccoons, called Baylisascaris, can also infect dogs if they lick or eat raccoon fecal matter containing roundworm eggs. Adult roundworms live and reproduce mainly in the stomach and small intestines of dogs. They can become quite large, growing up to 6 or 7 inches in length. Adult females can lay several hundreds of thousands of eggs in a single day, which ultimately are passed out in the dog’s stool. Roundworm eggs are protected by a hard, shell-like substance that enables them to survive for months or even years in the environment, after they are excreted in an infected dog’s feces. Dogs of any age, and even people, can become infected by ingesting roundworm eggs or larvae from the soil or contaminated fecal matter. Dogs can also develop roundworm infection if they eat the carcass of an infected animal, which typically is a mouse, rat or other small rodent.
Unborn puppies can become infected with migrating roundworm larvae that pass through their mother’s placenta. In fact, this is probably the most common route of infection. Puppies that become infected with large numbers of these larvae transplacentally often die before or shortly after they are born. Newborns can also become infected by ingesting roundworm eggs or larvae through their mother’s milk. When a young dog becomes infected with roundworms through whatever route, the eggs usually hatch in its stomach. From there, the larvae migrate through the puppy’s digestive and circulatory tracts, ultimately lodging in tiny blood vessels of the lungs, called “capillaries.” The larvae are coughed up, or crawl up, the inside of the puppy’s windpipe (trachea) into its throat. Once they arrive there, the larvae are swallowed and return to the puppy’s stomach and small intestines, where they mature into adults. After this, the cycle begins again.
After about 6 months of age, most dogs develop some degree of resistance to roundworms. The larval parasites tend to form cysts in the tissues of older puppies and adult dogs, rather than completing their normal life cycle. In the encysted form, roundworms are fairly immune to most de-worming medications and, for the most part, cannot be detected or removed by the dog’s immune system. They also do not cause clinical symptoms when they are in the cyst form. However, encysted roundworm larvae become activated in pregnant females, where they migrate to and through the placenta and into the mammary milk glands. This is how most puppies become infected.
One of the best ways to prevent roundworm infection in puppies is to de-worm the bitch before breeding her, and again several times after day 40 of her pregnancy. Unfortunately, some encysted larvae probably will escape these treatments and will infect the puppies either across the placenta or in the mother’s milk. Once the puppies are born, they can be de-wormed starting at about 3 weeks, under current veterinary recommendations. A number of medications are safe and effective at killing roundworms and other internal parasites, even in young dogs. Some of the canine de-worming medications that owners may have heard of are pyrantel pamoate (Strongid or Nemex), Drontol Plus, Telmintic, Panacur, Interceptor, Heartgard Plus and Vercom, among others. Some de-wormers are tablets, while others come in liquid or paste forms. Because each medication is effective against a different parasite or combination of parasites, a dog’s owner should consult with a veterinarian for the best de-worming protocol. Most roundworm preventions for puppies involve giving a series of dosages over several weeks, to increase the chances of killing all adults and also killing the migrating and maturing larvae. A general rule of thumb is to de-worm puppies at 3, 6 and 8 weeks of age, and to continue treatment if a fecal examination shows that internal parasites are still present.
Most roundworm eggs are extremely hardy and highly resistant to environmental factors. They can survive outside of their host and remain infective for years. It is almost impossible to remove roundworm eggs from soil that is heavily contaminated.
Roundworm larvae can cause a very serious condition in people called “visceral larva migrans,” which is particularly dangerous to young children. Visceral larva migrans develops when a person ingests roundworm eggs, particularly those of Toxocara canis or the raccoon roundworm, Baylisascaris. People can also become infected when the parasites penetrate their skin directly. Infants and toddlers most commonly develop this condition from poor hygiene, eating dirt and/or walking barefoot in areas where dogs, raccoons or other mammals frequently defecate, such as on soil, in grassy areas and in sandboxes in rural areas or city parks.
When taken in by a person, roundworm eggs hatch and develop into larvae in the stomach and small intestine, as they would in their canine host. However, they do not mature into adults there, as they normally would in a dog. Instead, they migrate out of the digestive tract by tunneling through the intestinal wall. They can end up in any number of places, including the liver, kidneys, spleen, heart, lungs, skin, eyes and/or brain. In infants and toddlers, roundworm larvae often migrate to the eyes and brain, causing blindness. Affected eyes may need to be removed surgically, through a procedure known as “enucleation”.