The roundworm of the dog, also known as Toxocara canis, is a common intestinal parasite found in dogs.
The life cycle of Toxocara canis goes like this:
1) Eggs, which may be found in cockroaches, rats, earthworms, are ingested
2) Lavae hatch and migrates through the intestinal wall and to the liver
3) Lavae then migrates to the lungs via the blood stream and is then coughed out and swallowed
4) Adults then develop in the intestines once again, laying eggs that are excreted
5) This cycle takes about 4-5 weeks to complete
However, most of the lavae actually end up encysted in muscles where they lay dormant instead of developing into adults in the small intestine. In the bitch, these encysted lavae become reactivated during late pregnancy and the fetuses are infected. The worms can also be transmitted to pups via milk. Worms transmitted to pups in this manner take 2 weeks to develop into adults and start producing eggs.
This has great significance on the treatment regime of pups. Infected pups require treatment with an appropriate anthelmintic (eg. Benzamindazoles, macrocyclic lactones, pyrantel) every 2 weeks starting from 2 weeks of age till they are weaned. The reason for this is due to the transmammary route of infection. Once they are weaned, treatment will need to me administered once a month till 6 months of age. The reason for this is that by 6 months of age, most of the lavae will be encysted in muscles, instead of completing their life cycle. Thereafter, treatment will only be required 2 - 4 times a year.
Toxocara canis also poses a zoonotic threat, especially to young children. The reason for this is two-fold: Children have poorer hygiene practices as well as a relatively immature immune system.
Toxocarasis in humans typically have 2 presentations:
1) Visceral lava migrans - lavae migrate to other organs in the body, causing signs and symptoms referable to where the lavae localise
2) Ocular lava migrans - lavae migrate into the eye
PS: This is what the adults look like
And this is what the egg looks like. Note the L2 stage inside the egg on the right hand side.