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Internal Parasites in Dogs and Cats

Parasites that live in the internal organs of dogs and cats can infect many different organ systems. Some parasites reside in only one area, such as the bowel or heart, whereas others can infect many different organ systems simultaneously throughout the body.

Learn about internal parasites based on the specific internal organ system that they infect:

- Bowel (gastrointestinal) parasites
- Heart parasites
- Parasites of the blood
- Lung parasites
- Parasites that infect various abdominal organs (i.e., kidneys, liver)
- Parasites that infect the whole body

Look up the individual theme’s in this section

Parasites of the Dog and Cat: Diagnosis

  • Parasites can be diagnosed in many different ways that are partly dependent on where the parasites live on your pet and the parasite's life cycles. Some parasites are easy to find and others require a multi-step process.

  • External parasites are most commonly diagnosed by a visual inspection. Ticks are not fast moving and once they are attached, they stay put. Ticks will appear as brownish gray or white growths on your pet's skin. Lice can also be seen on visual inspection and their eggs will be attached to hair shafts.

    Tick on a dog

  • Fleas are very fast moving, as they run and jump. They may be spotted as brown flashes darting to safety, especially if you roll your pet over and check the belly. The fleas run away from the light. Fleas are often detected by the debris they leave behind as well. This debris will be a salt and pepper-like grit, consisting of flea fecal material, eggs, and dander. To be sure this material is "flea dirt," put a small amount of it on a white paper towel and add a drop of water. If it is "flea dirt," the grit will turn reddish brown because there is digested blood in the debris. A fine-toothed comb called a flea comb can often pick up errant fleas and flea debris as well. On rare occasions your veterinarian will not be able to find any evidence of fleas. If fleas are still suspected, a skin biopsy to look for characteristic allergic changes may be done, or blood or skin tests, to check for reactions to flea saliva antigens.

    Flea debris on dog’s back

  • A simple piece of sticky tape can be used to detect Cheyletiella mites. These mite are nicknamed "walking dandruff." The tape picks up some dander off your pet, and a check under a microscope will reveal any mites.

    Scaling and inflammation at the skin surface in a dog with cheyletiellosis. Small white objects on the hairs are adult mites.
    Source: Stefanie Peters, Tierklinik Birkenfeld, Germany

  • If ear mites are suspected, your veterinarian will take a cotton swab or two from each ear. The debris collected is checked for mites under the microscope. Ear mites are usually easy to detect.

    Brown debris in a dog’s ear. This can be a sign of an earmite infection

    Otoscopic imageof a dog’s ear infested with ear mites (all the round, white objects)

  • Some of the skin-dwelling ear mites are harder to find. Demodectic mites live in hair follicles. To find them, your veterinarian will pinch the skin to express the contents of the hair follicles and gently scrape the cells off with a sharp blade or scalpel. These cells are inspected under a microscope to see if there are any mites. Demodectic mites are generally easy to find.

    A dog with a severe demodex infection

    Demodectic mites under a microscope

  • Sarcoptic mange mites are much harder to detect. Even a few of these mites can cause a great deal of itching and discomfort for your pet. Many skin scrapes need to be done to find these mites; a range of 20 to 40 skin scrapes is often recommended. The edge of the ear flaps is a favored site. With any luck, a few eggs or a mite will show up quickly. Even if no mites are found, if your veterinarian strongly suspects sarcoptic mange, your pet will be treated.

    A dog with a severe sarcoptic mange

    Sarcoptic mite under a microscope

  • The lowly stool or fecal sample can be invaluable for diagnosing internal parasites in your pet. Samples should be submitted to your veterinary clinic yearly. The stool sample is mixed with a special solution that allows parasite eggs to float to the top. The top fluid is then inspected under a microscope to identify any eggs. The diagnosis for most intestinal parasites is made when their eggs are passed in the stool, not by seeing the adult parasites themselves. This standard flotation method works for most nematode type parasites.

  • Many protozoal parasites also live in the intestines. To diagnose these creatures, a fresh fecal smear is needed. Your veterinarian or a veterinary technician will take a fresh sample from your pet during a rectal exam. This sample is then mixed with saline (a specific concentration of salt water) and checked under a microscope for any parasites. Some parasites, such as Giardia, can be notoriously difficult to find and multiple samples on different days may be required to positively diagnose it. Immunodiagnostic tests are now available to test for some intestinal parasites such as Giardia. These tests are based on identifying a protein (antigen) within the stool sample specific to that parasite by binding it to an antibody that has been combined with a coloring agent and incorporated into a test such as the Enzyme-Linked Immunosorbent Assay, or ELISA. If the parasite protein (antigen) is present, the antibody-antigen binding results in a color change that gives a positive diagnosis.

    Giardia parasite under a microscoop

  • Fecal samples may also be checked for occult blood. This test is not parasite specific, but some parasites such as whipworms cause bleeding into the intestines. Testing for occult blood is used for other health problems as well.

  • Lungworms are diagnosed through the identification of their larvae. As lungworm larvae are often coughed up, swallowed and passed in the stool, fecal examination can be helpful in their identification, but must be performed through a special method called the Baermann technique. This examination involves placing a sample of stool in cheesecloth and suspending it partially submerged in a water-filled container. The larvae will leave the stool and fall to the bottom of the container. Samples of the water are then examined under the microscope for evidence of these larvae. These larvae can also be diagnosed by collecting a sample of sputum or tracheal fluid by way of a tracheal wash or lavage. This test involves passing a long, small catheter down the trachea and injecting some sterile saline that is then sucked or aspirated back and looked at under the microscope.

    Metastrongyle larva (lungworms)

  • Blood samples can be used for parasite testing too. Heartworms are detected most commonly by blood samples. Blood may be checked by an ELISA test for antigens or antibodies related to the heartworm or even carefully examined under the microscope for microfilaria, which is an immature stage of the heartworm that circulates in the bloodstream.

  • Blood smears may be dried and stained on slides to look for some of the elusive blood parasites. Blood titers can also be used to detect parasite infections such as certain protozoal infections.
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