Articles Index/Parasites
Posted 05/23/03


A Zoonotic disease is one that can be transmitted from animals to people. Although most dog owners have good control over parasites, particularly through use of heartworm preventatives such as Heartgard, Interceptor, or Revolution, it is nonetheless prudent to practice a program of strategic deworming. This accomplishes two goals: it rids dogs of existing parasites and also removes any parasites before they are able to reproduce and contaminate the environment.

Roundworms (Toxocara canis) are spread when a dog or cat ingests eggs containing infective larvae found in the soil or in vegetation contaminated with feces, or if a dog eats an intermediate host such as a mouse, rabbit, or bird that has infective larvae that have become encysted in body tissues. The ingested eggs hatch within the dog's stomach and the larvae from the eggs or from the encysted tissues of the intermediate host invade the bowel wall and are carried by the blood stream into the lungs.

Within the lungs the larvae may enter an alveolus, be coughed up in the trachea into the mouth, and swallowed. These larvae then mature into adult roundworms in the intestine. If the larvae do not enter the alveoli, they are carried by the blood to other tissues of the body, such as muscles, where they encyst and become dormant.

These arrested larvae are reactivated by the hormones of pregnancy and migrate to the placenta and into the lungs of developing puppies, or they migrate to the mammary glands of the mother where they are shed in the milk. Then, puppies ingest larvae when they nurse.

The larvae in the lungs of the newborn pups undergo trachael migration, are swallowed, and develop into adult roundworms in a puppy's intestine within approximately three weeks after pups are whelped.

Each adult female roundworm matures and passes as many as 200,000 eggs per day into the gut. These eggs are carried out with the feces and may remain infective in soil for many years. Humans ingest roundworm eggs via contaminated soil or sand via unwashed hands or through contaminated fruits and vegetables.

Tapeworms (Family Taeniidae). All taeniid parasites require an intermediate host and a definitive host. The definitive hosts are dogs or other wild canids and the intermediate hosts may be several species, including humans.

Typically, tapeworm eggs on contaminated soil or vegetation are ingested by an intermediate host, which may be an ungulate, such as a sheep, a rabbit or a rodent. Within the intermediate host, the eggs hatch in the intestine, releasing hexacanth embryos, which are first stage larvae. These embryos then migrate to the blood stream where they are carried to various organs. Within an organ, usually the liver, the embryo differentiates into second-stage larva.

When a dog eats the organs of an intermediate host, the second stage larvae develop scolices which attach to the intestine and become adult tapeworms. These worms range in size from 2 to 3 mm long. Once in the dog's gut, the adult tapeworm produces mature segments full of eggs. These eggs or segments are carried out through the intestine with the feces (those tell tale "rice-like" objects) and thousands of eggs contaminate the environment.

In addition to "rounds" and "tapes", dogs may also harbor hookworms, whipworms, and a variety of other microorganisms. While clean conditions, proper flea and tick control, and the aforementioned heartworm preventative go a long way in controlling parasites, making a fecal exam part of your dog's yearly physical is important. It is especially important to make sure your bitch is parasite free before she is bred.

ANTIC, March, 2003


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