Internal parasites

There are different types of worms and other parasites that can cause infection and illness in your pet.  These parasites can live in many different areas of the body including the stomach and intestines, the lungs, and even the heart and blood vessels.  Parasite infections can cause serious diseases and even death.

Intestinal parasite

These are the most common kind of internal parasites and are a frequent issue for dogs and cats.  This group is made up of worms and protozoan (very small, single cell organisms) that live in the intestines.  Some of the most common are:

  • Round worms
  • Hook worms
  • Whip worms
  • Tape worms
  • Giardia
  • Coccidia


Pets get these parasites from eating animals outside, drinking pond or stream water, sniffing the poop of an infected animal and ingesting some, and as puppies and kittens from their mom.  Usually dogs and cats don’t show signs of having intestinal parasites until they are very sick.  Then we can see diarrhea, vomiting, weight loss, and even anemia (low numbers of red blood cells).  The worms can be seen passed in the poop if the infection is large enough.

Most of these parasites can be passed from animal to animal and also to humans.  In humans, infections can be serious and cause damage to the eyes, brain, and intestines. 

Intestinal parasites are diagnosed by testing the stool or seeing worms.  If infected, treatment is often two or three doses of deworming medication.  Even if not infected, all pets should receive deworming medication at least every three months to prevent infection. 


Heartworms are a parasitic worm that lives in the heart and blood vessels of cats and dogs.  Pets get infected with heartworm after being bitten by a mosquito which has the worms.  Both indoor and outdoor pets are at risk for heartworm infection as mosquito’s are often able to get inside buildings and houses.

Once a heartworm is in a pet’s blood vessels, it moves into the heart where it multiplies.  The worms spread out within the heart and into the lungs.  As they do this, they cause severe damage to the tissues.  Signs of heartworm include coughing, not being able to run or play for long, difficulty breathing, and death.

Heartworm is diagnosed by a blood test since the worms live within the blood stream.  Treatment for heartworm infection involves several medications given both by mouth and injection.  This treatment is expensive and can cause serious side-effects including death. Heartworm can be prevented by giving your pet a monthly preventative medication.  This is much safer and cost effective than treating an infection once it’s there!