Several intestinal worms infest dogs. The most common intestinal worms are roundworms, hookworms, whipworms, and tapeworms. The one most commonly seen by the owner is the tapeworm. It looks like a small grain of rice. The dog is not actually passing the whole worm. It is passing an immature worm that breaks off the mature worm. The mature tapeworm can be several inches long and is flat.
Occasionally, if the worm burden is high enough, the owner may also see roundworms in the stool, or the dog may vomit up the roundworms. These are long spaghetti-shaped worms. However, most of the time the dog only passes the roundworm, hookworm, or whipworm eggs, which are microscopic. Therefore, you will not see the worms themselves.
Symptoms of intestinal parasite infestation include: weight loss (or failure to gain weight) and loose stools. Occasionally, in small puppies, high worm burdens can cause vomiting. Hookworms can also cause bloody stools.
Another common intestinal parasite is Giardia. This is not a worm, but is a protozoa. Dogs with Giardia infections usually have very loose stools to runny diarrhea.
Puppies will often have multiple infestations. It is common for them to have roundworms, hookworms, whipworms, and Giardia all at the same time. Therefore, it is very important to deworm puppies. Typically, your veterinarian will deworm your puppy at the same time he receives his vaccinations. Dog breeders will start deworming puppies as early as a couple of weeks of age.
Roundworms, hookworms, whipworms, and Giardia are all transmitted fecal-orally through the ingestion of the eggs. The infested dog will pass the eggs in his stool and then the next dog usually picks the eggs up on his feet. Then, when he groom himself, he ingests the eggs. The eggs then mature into adult worms.
Tapeworms are transmitted by the flea. Dogs typically get tapeworms from biting at the fleas that are biting them. They accidentally ingest the flea, and the immature stages of the tapeworm are released and mature into adult tapeworms. Cats often become infested with tapeworms by eating a rodent (rat or mouse) that has fleas.
Most heartworm medications also contain medication that will kill roundworms and hookworms. So, you are giving them a monthly heartworm and intestinal worm treatment with each monthly dose.
Tapeworms are not killed by most heartworm medications. Also, most over-the-counter medication do not kill tapeworms. If you see evidence of your pet passing tapeworms, it is important to make sure you are using a deworming medication that kills them.