Parasites fall into two groups: internal and external. This page covers internal parasites and protozoa. For information on external parasites (fleas, ticks, and mites) go to our dermatology section.


Heartworms are the most serious of common parasites for dogs. They cause stress in the heart by restricting blood flow and damaging other internal organs. Without treatment, the disease can be fatal. Since heartworms are spread by more than 22 different kinds of mosquitoes, every dog is at risk. Weather variability means that heartworms pose a threat year-round. You must have each pet checked before any medication is begun or dispensed.

Treatments for heartworms:

Oral Monthly: Comes in the form of a pleasant tasting chewable tablet which also controls certain internal parasites. Tested for hearworm beforehand.

Topical Monthly: Applied to skin beneath coat monthly. Such as Revolution.

Pets testing positive for heartworm are treated with injectable heartworm adulticide only after extensive evaluation of blood chemistry values, chest x-rays, and electrocardiogram (ECG).


Hookworms are parasites that attach to the intestinal wall and suck blood. If left untreated, hookworms cause intestinal bleeding, anemia, and diarrhea. It only takes about 100 hookworms to be lethal to a puppy.

Hookworms are treated with an oral medication at repeated intervals. Remove feces from your pet’s environment.


Roundworms are the most common parasite. Most puppies are born infected with this parasite. The larvae penetrate the small intestine and are carried in the blood to the liver, lungs, and other internal organs where they may lie dormant. The larval migration eventually brings them into the intestinal tract where they mature to adults capable of reproducing and shedding eggs in the feces. In pregnancy, the roundworms migrate to the lungs of developing puppies and into the mother’s milk, repeating the life cycle. Older dogs can pick up an infection from contaminated soil.

Roundworms are treated with an oral medication at repeated intervals. Remove feces from your pet’s environment.


Whipworms are one of the most difficult parasites to eradicate in dogs, since female whipworms can produce up to 2,000 eggs a day. The eggs lie in the dog’s feces and can survive in the soil for years, even in the coldest climates. Approximately one out of every seven unprotected dogs is afflicted.

Whipworms are treated with an oral medication at repeated intervals. Remove feces from your pet’s environment.


There are many different species of tapeworms. Fleas are involved in the life cycle and transmission of the tapeworm known as Dipylidium Caninum in dogs. Rodents and rabbits are carriers of the Tenea species of tapeworms. Tapeworms are made up of many flat segments similar in appearance to grains of rice and can be seen intermittently in the bowel movements.

Tapeworms are treated with oral medication at repeated intervals, flea control, and avoiding wildlife consumption.


Giardia is a parasite that invades a dog’s gastrointestinal tract causing diarrhea, dehydration, and intestinal cramping. Drinking contaminated water and fecal exposure can lead to this infection in dogs.

Giardia is treated with oral medication. Remove feces from your pet’s environment.


Coccidia are protozoan parasites, the eggs of which are spread through fecal contamination. Both cats and dogs are susceptible to this parasite. Clinical signs include mucoid and bloody diarrhea.

Coccidia are treated with oral medication and removal of feces from your pet’s environment.


Toxoplasmosis is a protozoan parasite acquired either by eating raw meat or through fecal contamination. Clinical signs are varied and depend upon the organ system involved. Cat feces may harbor this organism. Accordingly, pregnant women should not clean cat litter boxes because infection by this organism can cause miscarriage.

Toxoplasmosis is treated with oral medication.