While there are several species of parasitic roundworms, the most common are Toxoccara cati and Toxocara canis, infecting cats and dogs respectively. Pets are infected in a variety of ways. They may ingest eggs from contaminated soil; they may consume a prey animal such as a mouse infected with Toxocara, they may be passed from a nursing mother to pups or kittens, and, in dogs, they may be passed to puppies in the uterus.
The roundworm lifecycle is quite remarkable. First, eggs from an adult worm are shed in the feces of its host. The eggs take about 1 month to become infective, but can remain infective for months or years in soil. Fresh feces are not infective. However, it is important to remove feces from the yard or litter pan, as it can contaminate the environment over time.
The infective egg is then ingested and hatches into a larva within the intestinal tract. Here, it burrows out of the intestine and migrates through the body, encysting in other tissues such as liver, muscle, and lungs. These second stage larvae may actually remain encysted for years, but some will migrate immediately to the lungs where they are coughed up and swallowed by the host animal.
Larvae swallowed by the host will then mature within the intestine, mate and shed new eggs thus repeating the cycle. It takes about one month for an egg to become a mature adult capable of producing more infective eggs.
Deworming a pet removes only those larvae in the intestine, not those encysted in other tissues. This is why periodic and repeated deworming is necessary to prevent parasitic infection. A female cat or dog who is nursing can infect her young, even if she is routinely dewormed, due to larvae that may be encysted in her mammary glands that are passed to the babies when they nurse. This is why it is so important to deworm all puppies and kittens and to do so multiple times.
Roundworms can cause disease such as vomiting, diarrhea and dull coat, poor growth or general unthriftiness in dogs and cats. Severe infestations may even cause pneumonia or intestinal blockage. However, in most cases, there are no outward signs of a pet being infected with roundworms.
Humans, usually children, can be affected by roundworms too. Kids may become infected by playing in contaminated soil where they may ingest eggs by putting their fingers in their mouths. The larvae can migrate through the body causing damage to organs like the liver or lungs, and, more seriously, can encyst in the eye and cause blindness. About 700 people per year are affected by ocular larval migrans (larvae within the eye).
Infection can be prevented by washing hands after playing outside in the dirt or handling pets, keeping yards free of dog feces on a regular basis and deworming pet dogs and cats on a regular basis with a monthly parasite control.