Toxocariasis (also Toxocaral larva migrans) is a parasitic infection caused by the larvae of nematodes Toxocara canis (dog roundworm) and T. cati (cat roundworm). It is more commonly transmitted from dogs to humans, because cats' fecal matter is usually limited to one place (i.e. a litterbox). In dogs, puppies usually get infected from their mothers, either in utero or through breast milk after birth. The toxocara larvae mature in the small intestine of puppies (larvae in dogs that are older than five weeks do not undergo full development). The adult worms then produce eggs, which are excreted in feces. In soil, the noninfective eggs develop into their infective form, and become very hardy. They can then infect humans, although we are only paratenic hosts, because the larvae do not develop into adults in humans. The usual mode of transmission involves swallowing canine or feline fecal matter. However, there have been a few reports of toxocariasis acquired through the consumption of certain kinds of raw meat, like giblets and liver (Markell et.al, 1999). The parasite causes two kinds of clinical disease: visceral larva migrans (VLM) and ocular larva migrans (OLM). (image below is courtesy of DPDx)
The adult worm is a nematode, ranging in length from about 5-15 cm. The eggs are fairly rounded, with a thin shell and an albuminoid outer covering. The eggs of T. canis are approximately 85Ám by 75Ám, while those of T. cati are about 70Ám in diameter (Markell et.al, 1999). (image below is courtesy of DPDx)
In the United States, an estimated 10,000 cases of Toxocara infections occur yearly in humans. Between 4 and 8% of children have positive serology for toxocariasis. The disease is found worldwide, with varying rates from country to country.
Younger children (i.e. pre-school age) are at risk for the VLM form of the disease, while older children and adults usually contract OLM. Boys usually have a higher rate of positive serology than girls - this is probably because their playtime habits bring them in closer contact with soil and fecal matter.