Endoparasites are those that live within an animal's body.  The main parasites we see in veterinary practice are heartworms and intestinal parasites.  Luckily, as with ectoparasites, we have medications that can treat and prevent endoparasites in our pets.  Because there is a separate discussion focused solely on heartworms, here we will just focus on other internal parasites.

Worms.  At some point every pet gets them.  Below is a list of the common intestinal worms (nematodes), protozoa and other parasites we see affecting our pets' GI tracts and why they can be harmful.  There are more intestinal parasites out there that didn't make this list, but these are the most common ones we see on a routine basis.  The main reason we prevent intestinal parasites is actually not for the benefit of our pets (though we like that as well!).  It is so the people who live with the pets are not exposed to and infected by these common parasites.  To diagnose intestinal parasites, we look at a fecal sample to check for the presence of live protozoal parasites, or to see eggs indicative of a nematode infection.  Dewormers are then chosen based on what type(s) of parasite your pet has.

• Roundworms - in addition to causing diarrhea, bloating, and possible intestinal obstruction if too many worms, these worms can also cause pneumonia if present in high numbers.  In people, roundworms can cause a condition called visceral larval migrans, where the worm (in the wrong host) will migrate to various organs, cause an inflammatory reaction, and cause illness in the person affected.  Classically, children that are infected will have an ocular condition similar to that of retinoblastoma.  If your child is diagnosed with retinoblastoma and you have pets, be sure to tell your doctor that roundworms may be a possibility, especially if your pet is not on a routine deworming schedule.

• Hookworms - these worms cause severe anemias, especially in puppies and kittens, and if left untreated the pets can die from blood loss.  These pets are pale, weak and have pot-bellied appearances to their abdomens.  In people, hookworms cause cutaneous larval migrans, an itchy skin condition caused by migration of the larval hookworm within the skin.
  
• Whipworms - these worms usually cause a mucousy, bloody stool.  The eggs of these worms stay in the environment for years, making eliminating a whipworm infestation challenging.  The good news is that whipworms are not readily transmitted to people.
  

All of our nematode parasites are susceptible to common dewormers.  We routinely check fecal exams during dog and cat yearly exams, or any time a pet is having diarrhea.  This is to ensure that your pet is parasite free!  Most heartworm preventatives also include a dewormer that is effective against one or more of these parasites.  We make recommendations on which heartworm/dewormer combination is best for your pet on an individual basis. 

These are the most visible parasite - if your pet is infected with tapeworms you will likely see mucousy stool with possible segments of worm that look like pieces of rice and are eliminated along with the stool.   Pets acquire tapeworms only by eating fleas infected with larval worms.  The segments you will see in the stool are not infective and cannot transmit tapeworms to other animals or people.  Flea control is the most important treatment in controlling tapeworms.  We have a dewormer for tapeworms that is very effective at killing those within the intestinal tract, but reinfection is likely if the flea problem is not treated concurrently.

Don't think parasites are limited to the intestinal tract!  We see worms in the kidneys and bladder, lungs, liver, esophagus, and other organs.  We always keep these in mind when dealing with your pet's illness, but they are so uncommon it is not feasible to list them all here.