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. Immuno suppressive 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 traveling or going to a new home) can also activate dormant larvae.
Although some de-worming 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 de-worm 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.