Roundworms
Most puppies are born with roundworms. There are 4 ways the infection occurs: across the placenta (embryonic development, this is the most common), mother’s milk while nursing, eating worm eggs (usually during grooming) and eating rodents who are carrying developing worms.
The life cycle of the roundworm is very complicated. The eggs contain infective larvae. The infective larvae hatch in the stomach and small intestine then migrate into the wall of the small intestine to enter the blood vessels. The blood allows the larvae to migrate to the muscles, the liver and the lungs where they can encyst (become walled off) for long periods of time. In puppies, the larvae move from the blood to the lungs, are coughed up, swallowed and reenter the stomach to develop into adult worms.
If a pregnant bitch has encysted larvae in her tissues, the larvae can reactivate; migrate through the uterus and the placenta to infect the fetal pup. The larvae enter the lungs, are coughed up and mature into adult roundworms. The larvae may also migrate to the mammary glands and the puppies are infected through the milk while nursing.
This is a common cause of diarrhea in young puppies. The worms can also be vomited when they are in the stomach. Because the worms consume a large amount of the puppy’s food, the puppy is commonly thin and has a pot-belly appearance. Occasionally, the worms migrate through the lungs causing pneumonia.
Fecal testing for parasites is always a good idea. A relatively recent study showed 1 out of 4 dogs tested negative for roundworms were actually positive because only eggs are detected in fecal exams. I now recommend all puppies be routinely treated, rather than screening for roundworms and only treating if they are found.
There are several medications available to treat roundworms. The medication is only effective on the adult worms. About 3-4 weeks is needed for the egg to develop into the adult roundworm, therefore several accurately timed doses of medication are recommended.
In rare cases, roundworms can infect humans (visceral larva migrans), usually children from contacting contaminated soil. Humans are not an appropriate host, so the worm dies and causes a severe immune reaction. When this occurs in the eye, it can cause blindness.