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Diagnosis & Treatment of Common Parasitisms
in Dogs & Cats

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October, 1997, Zambezi


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Two phyla of wormlike animals parasitologists often concern themselves with are 1) platyhelminths, (pla-tee-hehl-MINH- theez), otherwise known as flatworms, and 2) the phylum nematoda, (nee-mah-TOHD- ah) which consist of long thin worms called roundworms.

Parasitic Flatworms

TrematodeParasitic flatworms feed on blood, tissue fluids, or pieces of cells inside the bodies of their hosts. Some of these animals have a pharynx that pumps food into a pair of dead-end intestinal sacs where the food is digested, but in many parasitic flatworms the digestive tract is simpler than in free-living forms.

For example, tapeworms, a common type of parasitic flatworm, has no digestive tract at all... instead, they live inside the intestines of their hosts and feed by latching onto the intestinal wall with hooks and/or suckers and absorbing the food that passes by- food that has already been broken down by the host's digestive enzymes.

Flatworms lack any kind of speciallized circulatory or respiratory system. Because they are so flat, they can depend on diffusion to transport oxygen and nutrients to their tissues. And they can get rid of carbon dioxide and most other metabolic wastes by allowing them to diffuse out through their body walls.

Parasitic flatworms also often do not have much of a nervous system. As you can imagine, there is not much need for a nervous system in an organism that mainly hangs onto an intestinal wall and absorbs food! In fact, in tapeworms the nervous system has completely disappeared as  the worms have adapted to their parasitic lifestyle.

In contrast with free-living flatworms, parasitic flatworms do not reproduce asexually, but instead often have complicated life cycles.


The members of the class Trematoda are parasitic flatworms known as flukes. Some flukes are external parasites that live on the skin, mouth, gills, or other outside parts of a host, but most flukes, including the ones that affect humans, are internal parasites that infect the blood and organs.

schistomiasis parasite
The male Schistosoma is much larger than the female...

These flukes have complicated life cycles that involve at least 2 different host animals. Although many flukes are less than a centimeter long, the damage they cause to their host during their life cycle sounds like a script to a horror movie!

Blood flukes are found primarily in Southeast Asia, North Africa, and other tropical areas. As you might expect, blood flukes live in the blood- specifically the blood within the tiny blood vessels of the intestines.

Humans are the primary hosts of blood flukes that belong to the genus Schistosoma. (The primary host of a parasite is the host organism in which adult parasites are found and in which sexual reproduction of the parasite occurs.)

Most flukes are hermaphrodites and undergo sexual reproduction in a manner similar to that of free-living flatworms. (However, the sexes are separate in Schistosoma.) Flukes produce many more eggs than free-living flatworms- about 10,000 to 100,000 times as many! Blood flukes lay so many eggs that the tiny blood vessels of the host's intestine break open. The broken blood vessels leak both blood and eggs into the intestine. The eggs are not digested by the host and thus become part of the feces.

In developed countries, where there are toilets and proper sewage systems, these eggs are usually destroyed in the sewage treatment process, but in many undeveloped parts of the world human wastes are simply tossed into streams or even used as fertilizer.

Once the fluke eggs get into the water, they hatch into swimming larvae. When these larvae find a snail of the correct species, they burrow inside it and digest its tissues. The snail is an intermediate host for the fluke. Although sexual reproduction does not occur in an intermediate host, this host is still an essential part of the parasite's life cycle. In the intermediate host (in this case, a snail), the flukes reproduce asexually. The resulting new worms break out of the snail and swim around in the water. If they find a human, the worms bore through the skin and eat their way to the blood vessels. In the blood, they get carried around through the heart and lungs to the intestine, where they live as adults.

People infected with blood flukes get terribly sick. They become weak and often die- either as a direct result of the fluke infection or because they cannot recover from other diseases in their weakened condition. Blood flukes cause some of the most serious health problems in the world today- but because the species dangerous to humans live only in the tropics, most people in the United States know nothing about them, even though hundreds of millions of people suffer from blood flukes.

There are only one or two kinds of blood flukes in lakes and streams of the United States. These flukes normally have fishes or water birds as their primary hosts. If these worms find human swimmers, they try to burrow through the skin and cause what is known as "swimmers itch"- but because they are not adapted as human parasites, the worms cannot live in human bodies. The itch goes away after a time and the body repairs the damage.


Tapeworm scolexMembers of the class Cestoda are long, flat, parasitic worms that live a very simple life. (They have a head called a scolex (SKOH-leks) on which there are several suckers and a ring of hooks. These structures attach to the intestinal walls of humans and other animals. Inside the intestine, these worms are surrounded with food that their primary host has already digested for them. The worms absorb this food through their body walls, allowing them to grow to impressive sizes- adult human tapeworms have been seen over 18 meters long! Tapeworms almost never kill their hosts, but they do use up a lot of food. For this reason, hosts may lose weight and become weak.

Behind the scolex of the tapeworm is a narrow neck region that is contstantly dividing to form the many proglottids (proh-GLAH-tihds), or sections, that make up most of the body of the tapeworm. Proglottids contain little more than male and female reproductive organs. Sperm produced by the testes, or male reproductive organs, can fertilize eggs in the proglottids of other tapeworms or of the same individual. Fertilized tapeworm eggs are released when mature proglottides break off the posterior end of the tapeworm and burst open. A mature proglottide may rupture either in the host's intestine or after it has been passed out of the host's body with feces. A single proglottide may contain over 100,000 eggs, and a single worm can produce more than half a billion eggs each year!

If food or water contaminated with tapeworm eggs is consumed by cows, pigs, fishes, or other intermediate hosts, the eggs enter the intermediate host and they hatch into larvae. These larvae grow for a time and then burrow into the muscle tissue of the intermediate host and form a dormant protective stage called a cyst. If a human eats raw or incompletely cooked meat containing these cysts, the larvae become active within the human host. Once inside the intestine of the new host, they latch onto the intestinal wall and grow into adult worms.

Parasitic RoundwormsHookworm

Members of the phylum Nematoda, which are known as roundworms, are among the simplest animals to have a digestive system with two openings- a mouth and an anus.Food enters through the mouth, and undigested food leaves through the anus. Roundworms, which range in size from microscopic to a meter in length, maybe the most numerous of all multicellular animals. It is difficult to imagine just how many roundworms there are around us all the time. A single rotting bucketful of garden soil or pond water may house more than a million roundworms.

Most roundworms are free-living. Free-living roundworms are found in virtually all parts of the earth- in soil, salt flats, and aquatic sediments; in polar regions and in the tropics; in fresh water, oceans, and hot springs. There are, however, many species of parasitic roundworms. Parasitic roundworms affect almost every kind of plant and animal.

All roundworms have a long tube-shaped digestive tract with openings at both ends. This system is very efficient because food can enter through the mouth and continue straight through the digestive tract. Any material in the food that cannot be digested leaves through an opening called the anus.

Free-living roundworms are often carnivores that catch and eat other small animals. Some soil-dwelling and aquatic forms eat small algae, fungi, or pieces of decaying organic matter. Some actuall live on the organic matter itself. Others digest the bacteria and fungi that break down dead animals and plants. Many roundworms that live in the soil attach to the root hairs of green plants and suck out the plant juices. These parasitic worms cause tremendous damage to many crops all over the world. Roundworms are particularly fond of tomato plants. For this reason, many tomato plants have been specially bred to be resistant to roundworms. Other roundworms live inside plant tissues, where they cause considerable damage.

Like flatworms, roundworms breathe and excrete their metabolic waste through their body walls. They have no internal transport system and thus depend on diffusion to carry nutrients and wastes through their body.

Roundworms have simple nervous systems. They have several ganglia, or groups of nerve cells in the head region, but they lack anything that can really be called a brain. Although roundworms have several types of sense organs, these are simple structures that detect chemicals given off by prey or hosts. Several nerves extend from the ganglia in the head and run the length of the body. These nerves transmit sensory information and control movement. The muscles of roundworms run in strips down the length of their body walls. Aquatic roundworms contract these muscles to move like snakes through the water. Soil-dwelling roundworms simply push their way through the soil by thrashing around.

Roundworms reproduce sexually. Most species of roundworms have seperate males and females, but a few species are hermaphrodites. Fertilization takes place inside the body of the female. Roundworms that are parasites on animals often have complex life cycles. Two or three hosts may be involved in the life cycle of some roundworms. In other roundworms, such as Ascaris, the stages of the life cycle take place in different organs of one host.


Ascaris is a parasitic roundworm that lives in humans. Species that are closely related to Ascaris affect horses, cattle, pigs, chickens, dogs, cats, and many other animals. Ascaris and its relatives, which are collectively known as ascarids, have life cycles that are similiar to one another. One of the reasons puppies are wormed while they are young is to rid them of the ascarid that affects dogs.

Adult Ascarid worms live in the intestines, where they produce many eggs that leave the host's body in the feces. If food or water contaminated with these feces is eaten by another host, the eggs hatch in the small intestine of the new host. The young worms burrow into the walls of the intestines and enter surrounding blood vessels. Carried around in the blood, the tiny worms end up in the lungs. Here they break out into the air passages and climb up into the throat, where they are swallowed. Carried back into the intestines, they mature and the cycle repeats itself.



Hookworms are serious human intestinal parasites that are often found in the Southern United States and are common in tropical countries. As many as one fourth of people in the world today are infected with hookworm!

Hookworm eggs hatch outside the body of the host and develop in the soil. If they find an unprotected foot, they use sharp teeth and hooks to burrow into the skin and enter the bloodstream.

Like Ascaris, these worms travel through the blood to the lungs and then down the throat to the intestines. There, the adult worms dig into the intestinal wall and suck the blood of the host. These worms can devour enough blood to cause weakness and poor growth.


Trichinella RoundwormThe Trichinella roundworm causes the terrible disease known as Trichinosis (trihk-ih-NOH-sihs).  The adult worms, which are hard to see without a microscope, live and mate in the intestines of the host. Females carrying fertilized eggs burrow into the intestinal wall, where each releases up to 1500 larvae. These larvae travel through the blood stream, from which they eventually exit through small blood vessels, and then burrow into organs and tissues. This causes terrible pain for the host. The larvae then form cysts in the host's muscle tissues and become inactive.

The only way these encysted worms can complete their life cycle is if infected muscle tissue is eaten. This means that hosts for Trichinella must be carnivorous- animals that do not eat infected meat do not get Trichinosis. Two very common hosts for Trichinella are rats and pigs. (Rats eat any meat they can find, and may even eat each other. Pigs regularly catch and eat rats and other small animals.) Humans get Trichinosis almost exclusively by eating raw or incompletely cooked pork.

Filarial Worms

Filarial worms, which are found primarily in tropical regions of Asia, are threadlike worms that live in the blood and lymph vessels of birds and mammals such as humans. They are transmitted from one primary host to another through biting insects, especially mosquitoes.

In severe infections, large numbers of filarial worms may block the passage of fluids within the lymph vessels. This causes elephantiasis, a condition in which an affected part of the body swells enormously. Fortunately, extreme cases of elephantiasis are now rare.

Eye Worms

Eye worms are closely related to the filarial worms that cause elephantiasis. They are found in Africa and affect both humans and baboons. Eye worms live in and burrow through the tissues just below the skin of their host. In their travels, the worms occasionally move across the surface of the eye- hence the name eye worm.

(Most of the contents on this page is credited to the book: "Biology"- by Kenneth R Miller, Ph.D. and Joseph Levine, Ph. D. (Published by Prentice-Hall, Inc. 1991)




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