It's virtually impossible to know the exact history of roundworm in dogs. Fortunately, science has learned much about this parasite, including its life cycle.
Dogs are vulnerable to two types of roundworm: Toxocara canis and Toxascaris leonina, which can infect cats as well. Both types of roundworm are treated the same way, however, the life cycle of Toxacaris leonina is more simple. Toxocara canis was identified in dogs in the 18th century but wasn't studied until 1908. The first case of human infection with Toxocara canis was reported in 1950. The life cycle of Toxocara canis was discovered in 1958 by J.F.A. Sprent.
Toxocara Canis Life Cycle - Part I
Understanding the life cycle of a roundworm is critical to eradicating this nasty parasite. The cycle begins when somewhere, a host - a mouse, another dog or other mammal - poops and includes toxocara eggs in his feces. The eggs are too young to infect a new host, but as the poo degrades into the soil, the eggs are loosened into the dirt, where they can remain infective for months or even years. The poop itself isn't infectious, but the contaminated dirt is. If your pup rolls or plays or walks through the contaminated soil, he'll ingest the eggs when he grooms himself.
Toxacara Canis Life Cycle - Part II
The eggs take up residence in the dog's intestinal tract where they hatch and turn into larvae, eventually burrowing their way out to live in other body tissues, usually the liver. Eventually the larvae move along to the lungs, developing into third stage larvae. They'll travel up to your pup's throat, triggering coughing; the coughing prompts him to swallow, where they return to his intestinal tract. In a pregnant dog, the larvae don't migrate to the lungs, but instead, head for the uterus to infect unborn puppies. The second stage larvae travel to the puppies' lungs to develop into third stage larvae. In a nursing mother, the second stage larvae will move to the mammary glands instead of the lungs, infecting the puppies drinking mother's milk.
Toxacara Canis Life Cycle - Part III
Back in the intestines, the larvae finish maturing and begin to mate, laying their first eggs about a week after their return to the intestine, and about four or five weeks after the initial infection. The cycle continues until the dog is treated for roundworm infection.
Toxascaris Leonina Life Cycle
Toxascaris leonina is much more straightforward in its development. Instead of developing and migrating through the body, Toxascaris leonina are ingested as second stage larvae and mature in the intestine. The process takes two to three months, and like Toxocara canis, this parasite can infect other species, such as mice, cats and other mammals. However, these larvae can develop into third stage larvae in other species. Only a dog can develop third stage larvae if he's ingested Toxocara canis.
If your pup has roundworm, don't fret. It's very common among dogs -- especially puppies -- and treatment is safe, simple and effective. Your vet will prescribe deworming medication to kill adult roundworms in the intestines, and some newer medications can affect larvae. Depending on the medication prescribed, you may need to administer as many as three doses to fully eradicate roundworms from your pup.