Deworming Dogs and Puppies
Although nearly all dogs are infested with parasites at one time or another, most develop an immunity that keeps the parasite population in check. This immunity can break down, however, under conditions of stress or ill health. When that happens, the worms increase in number and eventually produce signs of intestinal infection, including diarrhea, weight loss, anemia, and blood in the feces.
Dogs develop the highest level of immunity to worms that have a larval phase that migrates in tissue. These are the ascarids, hookworms, and threadworms. Whipworms and tapeworms do not have a migratory phase and thus produce little immunity.
Immunosuppressive drugs such as cortisone have been shown to activate large numbers of encysted hookworm larvae. Stressful events such as pregnancy, surgery, severe illness, trauma, and emotional upsets (such as shipping or going to a new home) can also activate dormant larvae.
Although some deworming medications are effective against more than one species of worm, no single medication is effective against them all. A specific diagnosis is necessary to choose the safest and most effective drug. This requires an examination of the dog's stool and determining whether the parasite is in the egg, larval, or adult stage. It is not advisable to deworm a dog suffering from an unexplained illness that is assumed to be caused by “worms.”
All anthelmintics (medications that act to expel or destroy parasitic worms) are poisons-meant to poison the worm but not the dog. Dogs debilitated by heartworms or some other infestation may be too weak to resist the toxicity of the dewormer. Be sure to check with your veterinarian before using any dewormer. It is also important to give the medication exactly as prescribed.
Most young puppies are infested with ascarids. Other worm parasites may also be present, but they're not as common. It is advisable to have your veterinarian check your puppy's stool before deworming for ascarids, because if other worms are present, a broad-spectrum deworming agent may be recommended.
Puppies should be dewormed at 2 weeks of age (before ascarid eggs are passed in the stool) and again at 4, 6, and 8 weeks of age. Then, it's prudent to put them on a monthly heartworm preventive for life that also protects the dog against other parsites, or at least continue monthly deworming for six months. This schedule kills all ascarids, whether acquired in utero, through infected mother's milk, or by ingesting the eggs. Pyrantel pamoate (Nemex or Strongid) is an excellent choice for ascarids and can be safely given to 2-week-old puppies. It is available as a liquid suspension or tablet.
Worm medications can be harmful to puppies that are ill from a respiratory infection, chilled, crowded in unsanitary surroundings or abruptly weaned from their mothers. Stressful conditions such as these should be corrected before administering the dewormer. Do not deworm a puppy who has diarrhea unless your veterinarian has determined that the diarrhea is caused by the parasite.
Deworming Adult Dogs
Most veterinarians recommend that adult dogs be dewormed only when there is specific reason to do so, such as when eggs or parasites are found during a fecal examination. Dogs can also be kept on a yearlong heartworm preventive that also protects against many of the intestinal parasites. All dogs should have a fecal examination done at least once a year.
Most dogs carry ascarids as encysted larvae, but intestinal infestation by the adult worm is rare in the healthy dog. Hookworms are likely to be a problem in adults only during periods of stress. Only milbemycin (Interceptor) is effective against encysted hookworm larvae.
Whipworms are a frequent cause of acute and chronic diarrhea in adult dogs. They are difficult to diagnose on routine fecal examination. Eradication requires the use of specific agents not commonly used for other worms.
Tapeworms are common in dogs but, fortunately, cause few symptoms. The worm segments are easy to detect in the stool. Threadworms are not common. Very few agents are effective against this parasite.