Toxocara canis (canine roundworm)

The life cycle of Toxocara canis, a common roundworm of dogs, is similar to that of Ascaris lumbricoides, but with a few additional frills. Assuming a dog has never been infected with Toxocara, the first time it is infected the worms develop as described for Ascaris. If a dog has been infected previously and ingests infective eggs, most of the larvae that hatch from the eggs do NOT develop into adults. Rather, they remain in the dog's tissues as "second stage somatic larvae." If these second stage somatic larvae are in a female and she gets pregnant, transplacental infection of the fetus will occur. This explains why, in some areas, up to 95% of puppies are born with infections of Toxocara canis. If another animal, such as a rodent, eats infective eggs of Toxocara, second stage somatic larvae will develop in this animal. If a dog then eats this animal, the dog gets infected. Humans can also be infected with second stage somatic larvae of Toxocara canis, as well as the larvae of other species of nematodes, resulting in a condition known as visceral larval migrans or VLM. The eggs of Toxocara are extremely resistant to adverse environmental conditions, and, once an area is contaminated with eggs (such as a playground, park, or your yard), its very difficult to sanitize the area. Thus, it's very important that dogs be checked for this parasite on a regular basis, and that dog's not be permitted to defecate indiscriminately.
Intestinal infections of Toxocara canis in dogs are diagnosed by finding characteristic eggs in the feces of the infected host. Infections of hosts with second stage somatic larvae of this parasite are more difficult to detect, and many probably go undiagnosed.