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Miserable Little Parasites, Part 2

This month we’ll delve into the biological grossness of intestinal parasites. Yick!

This is a grotesquely big topic. Intestinal parasites divide themselves into four main categories: Nematodes (round-bodied worms), Cestodes (flat-bodied worms), Protozoa (single-celled animals), and Bacteria. These divisions are important because there are few similarities in life cycle and treatment between the categories.

Nematodes are the worms you think of when you think of worms. Most animals are infected by fecal-oral contamination (one step in a ‘muddy’ yard is all it takes to get these), but many nematodes have sneaky ways of circumventing mere clean feet and good hygiene. The typical roundworm (ascarids such as Toxocara and Toxascaris) infects a mother dog or cat when she cleans up after her infant babies, but how do the babies get worms? Roundworms form cysts in a dog’s body which remain in hibernation until, as a mature female, she is ready to have babies. The larval worms then activate and move into her offspring’s bodies before they are born. These puppies spread the infection back to the mother. Visceral larval migrans has severe health implications in animals and (rarely) children. Nematodes cause unthriftiness, weight loss and diarrhea from adult worms in the gut, and lung, skin, or eye inflammation from larval tissue migration.

If that’s not disgusting enough, there are other families of worms (hookworks, and whipworms) with even more imaginative ways of infecting an animal, such as ‘worming’ their way through intact skin when an animal walks through contaminated soil, or by transmission through mother’s milk.

We diagnose these parasitisms by microscopic testing of stool samples, although these tests are far from perfect. Fortunately, monthly heartworm prevention medications also eliminate this group of worms. Animals not on heartworm prevention should have their stool tested yearly.

Cestodes are tapeworms. These are two-host parasites, meaning they infest two different types of animal during larval and adult stages. Dipylidium is a common parasite of dogs and cats locally. The adult tapeworm inhabits the bowel of your pet and releases egg-filled segments that look like grains of rice that crawl out your pet’s, uh, orifice. These segments, called proglottids, are eaten by larval fleas, who become infected with the cyst-forming larval tapeworm. Grooming pets swallow infected fleas and the larva develop into adult tapeworms, some of which become quite long. Another common tapeworm cycles between cats and rodents in similar fashion.

Diagnosis of tapeworms is based upon finding proglottids on the animal’s coat. Treatment is easy and illness due to tapeworms is rare in animals whose owners pay even minimal attention. Animals cursed with fleas, and some active mousers will require worming two or three times yearly because of constant reinfection.

Protozoa include the flagellates Giardia and Tritrichomonas, which swim around using hair-like propulsive organs. Another group, the coccidia, includes the cyst-forming Toxoplasma. Isospora, and Cryptosporidium. These organisms are spread by fecal-oral contamination and often through contaminated water. They generally cause short-term diarrhea and unthriftiness, but Giardia may cause long-term problems. Toxoplasma may also be contracted by cats who eat infected rodents.

Toxoplasma represents a threat to pregnant women because the fetus cannot mount the immune response needed to eliminate the organism. An adult contracting Toxoplasma usually has only flu-like symptoms, but an unborn child may develop birth defects. Toxoplasma is the reason pregnant women should avoid raw milk and meat, avoid contact with cat feces, and be extremely cautious when gardening or consuming raw vegetables.

Diagnosis of protozoan parasites is based upon fecal testing, but the small size and low numbers of these organisms in stool often requires antibody-based testing and, most recently, DNA-based PCR tests. Treatment is highly variable and depends upon the exact organism causing disease. Some protozoan parasites do not respond to any known treatment.

Bacteria are usually friendly to the people and animals they reside within and provide valuable service by synthesizing necessary B-vitamins and helping with digestion. Some, like Salmonella, Campylobacter, and certain Clostridium species, can invade the gut and generate a wide range of disease states. The wrong bug in the wrong place can kill, but much of the disease-producing potential of these organisms is based upon the exact strain of bacteria and the state of the animal’s health. We test for the most common disease-producing bacteria by culture of stool and PCR tests.

Parasitic organisms can often be found in healthy appearing pets, but we have to look to find them. Yearly fecal testing is a smart investment in your pet’s health, particularly when there are children or pregnant women sharing the same living space, yet few animals are screened with regularity. This is something every pet owner should consider when choosing how best to spend their pet health care dollars.

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