Endoparasites
Endoparasites
Endoparasites live inside the host. "Endo-" means inside.
They can live in the gut, lungs, blood, or almost any other area of the body.
Nematodes
Nematodes, or roundworms, are generally small, wormlike organisms with a tough skin (cuticle).
Life cycle:
Egg
Four larval phases
Adult
Adults may measure from 2 mm up to 30 cm long, depending on the species.
Nematode larvae grow to the next stage by molting.
Parasitic nematodes are endoparasites as adults.
Tapeworms
Tapeworms, or cestodes, are flat, ribbonlike worms with no digestive tract.
Tapeworm anatomy:
Head (scolex)
Body composed of segments called proglottids.
Life cycle:
Egg
Larva
Adult
Adult tapeworms may measure from 5 mm to 7 meters long, depending on the species.
All tapeworms are endoparasites with two hosts in their life cycle:
Intermediate host: larval tapeworm lives here.
Definitive host: adult tapeworm lives here.
The adult tapeworm always lives in its host’s digestive tract.
Trematodes
Trematodes, or flukes, are flat, leaf-shaped worms with a partial digestive tract.
They have a mouth and an intestine but no anus.
Life cycle:
Egg
Several larval stages (some are endoparasites of snails)
Adult
The adult trematode can be from 2 mm to 10 cm long.
Most are endoparasites.
A few adult trematodes are ectoparasites of fish. Gyrodactylus, which lives on the skin of fish, is an example of an ectoparasite trematode.
Protozoans
Protozoans are a large group of single-celled organisms made up of many different types.
Most protozoans are microscopically small.
Each type of protozoan has a different life cycle, ranging from one to many stages.
Because protozoans are only one cell, they have no adult or egg stages.
The feeding stage of a protozoan is sometimes called a trophozoite, and the resting stage is sometimes called a cyst.
Parasitic protozoans are endoparasites.
Endoparasites of Ruminants (Cattle, Sheep, and Goats)
The major endoparasites of cattle, sheep, and goats include a number of parasites that infect all three types of animals, so these hosts have been grouped together.
Nematodes
Abomasal Worms
Ostertagia, Haemonchus, and Trichostrongylus species are important endoparasites of the abomasum (stomach) of cattle, sheep, and goats.
Life cycles of these three nematodes are very similar.
First- and second-stage larvae are free-living organisms.
The host eats the third-stage larvae, starting the infection.
Adults of all three worms live on the surface of the mucosa (the lining of the stomach).
Both the larvae and adults of Haemonchus suck blood. A thousand Haemonchus adults can suck (milliliters) of blood per day, causing severe anemia.
A heavy Haemonchus infection ( to worms) can kill a sheep very quickly.
Sheep and goats of all ages are susceptible to Haemonchus infection; cattle over the age of 2 usually resist it.
Trichostrongylus larvae greatly damage the stomach’s mucosa.
The young adults may cause bleeding and small ulcers when they break out of the stomach lining.
Heavy Trichostrongylus infections cause diarrhea and weight loss. Even light infections can decrease growth and diminish appetite.
Ostertagia is probably the most important roundworm parasite of livestock.
Ostertagia larvae invade the stomach’s gastric glands. There, the worms molt and develop and then return to the stomach.
The larvae and young adults destroy some gastric gland functions, including acid production. Without acid, the stomach can’t digest food.
Severe diarrhea and a large loss of weight are signs of Ostertagia infection. Heavy infections may be fatal.
To survive harsh conditions outside their host, the fourth-stage Ostertagia larvae may slow their development. These are known as arrested larvae.
In the northern United States and Canada, Ostertagia larvae arrest just before winter and resume development in the early spring.
In the southern United States, the larvae arrest in the late spring and resume development in the autumn.
These patterns of arrest ensure that adults don’t lay eggs when free-living larvae would be unable to survive: in the cold winters of the north or the hot, dry summers of the south.
These developmental patterns result in two types of Ostertagia disease: Type I and the more severe Type II.
Type I disease is caused by unarrested larvae, a few at a time, destroying the gastric glands. Type I disease normally occurs during the first grazing season.
Type II disease is caused by a group of arrested larvae destroying the gastric glands upon resuming their development. Type II normally occurs in older animals during the northern spring and the southern autumn.
Ostertagia, Haemonchus, and Trichostrongylus infections very often occur simultaneously in an animal. Since their life cycles are similar, control measures for all three parasites are the same.
To prevent heavy infections, veterinarians recommend prophylactic doses of anthelmintics, timed to prevent heavy buildup of larvae on the pasture.
In the northern United States and Canada, for example, veterinarians prevent severe Type II disease by prescribing anthelmintics from late spring to midsummer. This ensures that adult worms die before they lay eggs—thus no arrested larvae hatch the following spring.
Another control method is to move the livestock to a different pasture every year so that arrested larvae in the vacated pasture won’t survive long enough to infect a host.
Intestinal Strongyles
The free-living portion of these nematodes’ life cycle (Cooperia and Nematodirus species) resembles that of the abomasal nematodes.
They lay eggs in the gut; the eggs pass out with the feces.
The first- and second-stage larvae are free-living organisms.
The host eats the third-stage larvae, starting the infection.
Adult nematodes live in the host’s small intestine.
These parasites don’t usually cause their hosts many problems, but heavy infections, especially in a host with other parasites, may cause diarrhea and weight loss.
Veterinarians diagnose strongyles by finding the eggs in the feces.
Anthelmintics treat the infection; the control measures and prophylactic treatments for abomasal nematodes also work for intestinal nematodes.
Lungworms
All three types of lungworms (Dictyocaulus, Protostrongylus rufescens, and Muellerius capillaris) live in sheep and goats, but only Dictyocaulus is found in cattle.
Adult lungworms live in the host’s lungs.
Dictyocaulus is a large nematode of up to 8 cm long.
Protostrongylus is a moderate-sized worm, up to 3 cm long. Both of these lungworms live in the bronchi (lung’s air passages).
Muellerius, at about 2 cm, is also a moderate-sized worm, but it lives in the lung tissue.
All these lungworms lay eggs that hatch in the host’s lungs.
The first-stage larvae migrate up the trachea (windpipe). When they reach the back of the mouth (the top of the trachea), the host swallows them.
The first-stage larvae then pass through the gut and exit the animal with the feces.
Dictyocaulus larvae grow and molt on the ground until they become third-stage larvae. Third-stage larvae climb up onto a blade of grass, to be swallowed by the host when it eats the grass.
Slugs or snails, while feeding on grass, eat the first-stage Protostrongylus and Muellerius larvae. The larvae grow and molt inside the slug or snail (the intermediate host) to the third stage. The sheep or goat accidentally eats the slug or snail while feeding.
Digestion frees the third-stage lungworm larvae from their intermediate host. They burrow into the intestinal wall and get into a blood vessel.
The blood eventually carries them to the lungs, where they break out of the blood vessel and develop to adult lungworms.
In cattle, light lungworm infections usually cause few problems. The cow may cough a little and breathe faster than usual. Heavy infections usually happen only to calves.
Heavily infected calves may have trouble breathing because the large worms may block up the air passages of the lungs. The calf will breathe rapidly and cough frequently. In fact, the calf may have so much trouble breathing that it doesn’t have time to feed.
Lungworm infections in sheep and goats are usually light and cause few problems. Goats sometimes suffer heavy Muellerius infections.
A goat with a heavy lungworm infection may have a persistent cough and trouble breathing.
Veterinarians diagnose lungworm infection by finding the first-stage larvae in the feces.
Unfortunately, in a heavily infected calf, the worms may block the air passages before they lay eggs. In these cases, diagnosis depends upon the signs (coughing and rapid breathing) alone.
The veterinarian can give the infected animals anthelmintics to kill the adult worms.
Control of snails and slugs on the pasture may prevent Protostrongylus and Muellerius infection. Protect calves from heavy lungworm infections by placing them in a pasture that hasn’t been grazed in the current year. Such a pasture should have few if any larvae.
Tapeworms
Tapeworms have two hosts in their life cycle:
The intermediate host, where the larval tapeworm lives.
The definitive host, where the adult tapeworm lives.
Ruminants are the definitive hosts for some tapeworms and the intermediate hosts for others.
Ruminant Tapeworm
The adult ruminant tapeworm (Moniezia species) lives in the small intestine of cattle, sheep, and goats. It’s a very large worm, up to 600 cm long and 2.5 cm wide.
Like most of the common tapeworms, its last proglottid, when full of developed eggs, breaks off the body and passes out with the feces. Often the proglottid breaks up, releasing its eggs, as the host passes the feces. The feces thus have both intact proglottids and eggs.
Free-living mites, which are the ruminant tapeworm’s intermediate host, eat the eggs on the ground. These mites live on the ground and on blades of grass, where a ruminant accidentally eats them as it grazes.
If the ruminant eats a mite with a larval tapeworm in it, the larva develops to the adult stage in the ruminant’s small intestine.
The ruminant tapeworm turns up in young animals during their first grazing season. This tapeworm usually causes its host no serious problems. Heavily infected young animals may be unthrifty and suffer diarrhea.
Veterinarians diagnose tapeworms by finding the eggs or proglottids in the ruminant’s feces. Anthelmintics treat tapeworms.
Farmers and ranchers with ruminant tapeworm problems often plow the pasture in the fall, killing many of its resident mites. The grass that grows back the following spring will have few infected mites to be ingested by the new calves, lambs, or kids.
Taeniid Tapeworms
Ruminants are the intermediate hosts for a number of taeniid tapeworms (Taenia species), whose definitive host is usually a dog.
The typical larval taeniid, about the size of a large pea, lives in a ruminant’s internal organ or muscle.
The beef tapeworm of humans is a taeniid tapeworm whose larvae reside in cattle muscles. As the name suggests, adult taeniid tapeworms can live in humans as well as dogs.
The taeniid tapeworm reaches its definitive host as follows: a ruminant accidentally eats the taeniid’s eggs or proglottids; then a dog (or a human in the case of the beef tapeworm) eats raw meat infected with the larvae.
Tapeworm adults develop in the definitive host’s small intestine. Proglottids and eggs pass out of the definitive host in the feces.
Although the larvae of these tapeworms cause ruminants no problems, meat inspectors condemn larva-infested meat, making it worthless.
Worse, a taeniid tapeworm of sheep has a larva that travels to the sheep’s brain, where it can grow to the size of a golf ball. This larva destroys brain tissue as it grows, eventually affecting the sheep’s movement. The circling and lack of balance that results is called true gid.
Since there’s no easy way to diagnose larval tapeworms before slaughter—and no treatment for the infection—the farmer or rancher must try to prevent the problem from happening in the first place.
Since the dog is the definitive host for most of these taeniid tapeworms, keep dogs not needed to herd sheep off the pasture; this limits the dog’s defecation in the field and prevents infection with tapeworm proglottids and eggs in the sheep.
Dogs needed for sheep herding respond well to prophylactic treatment for the adult tapeworm. So do humans.
Another prophylactic measure is to keep dogs from eating raw meat—cooking kills the larvae.
Hydatid Tapeworm
The hydatid tapeworm’s (Echinococcus granulosus) definitive host is the dog. Ruminants, horses, and humans that accidentally eat the hydatid tapeworm’s eggs all serve as its intermediate host. Sheep raised on pasture with sheep-herding dogs are prone to larval hydatid infection.
This tapeworm is named for its larval stage: the hydatid cyst, a large (up to 10 cm) fluid-filled ball typically in the liver or lungs of the intermediate host. The small larvae grow on the inside wall of this cyst.
Ruminants with only a few cysts rarely show signs of infection, but their livers are condemned at slaughter. Hydatid cysts are difficult to detect before slaughter. An ultrasound procedure can find cysts in living animals but is too expensive to be practical except for small herds at high risk for the parasite.
The hydatid cyst, like the taeniid tapeworm, has no practical treatment in ruminants, but hydatid cysts can be prevented with the same measures that prevent taeniid tapeworm infections.
Trematodes
Liver Fluke of Ruminants
This large trematode (Fasciola hepatica) can grow up to 3 cm long. Liver fluke adults, as their name suggests, reside in the liver, specifically the bile ducts, of a ruminant, horse, or human.
Trematode eggs travel with the bile to the intestine, eventually leaving the host in the feces. The eggs hatch only in water, where the first larval stage infects a snail. The larvae go through several stages in the host snail, multiplying at each stage.
Many larvae then leave the snail, find a plant growing in the water, and climb above the water level on its stem. There they change into a cyst. A ruminant feeding on plants at the pond’s edge eats the cysts along with the plants. The cysts hatch in the ruminant’s small intestine and burrow through the intestinal wall. They travel to the liver, penetrate it, and move through the liver until they find a bile duct, where they mature to adulthood.
A sheep can suffer fatal liver damage if it eats many cysts in a short time. Fortunately, this rarely happens. Most sheep and other ruminants suffer only minor damage as a few larvae at a time move through the liver.
Over time, however, many larvae move through the liver, leaving scar tissue that takes up much of the liver and reduces its function. The larval fluke can also transmit Black Disease, a bacterial liver infection.
Meat inspectors condemn fluke-infected livers at slaughter.
Veterinarians diagnose the liver fluke by finding its eggs in the feces. Anthelmintics kill liver flukes.
Fence off wet areas where the intermediate host snail lives to prevent ruminants from eating cyst-carrying plants.
Deer Liver Fluke
The deer liver fluke (Fascioloides magna) has the same life cycle as the liver fluke of ruminants. Though named for the deer, the deer liver fluke makes its home in deer, cows, and sheep, and behaves differently in each.
The very large (up to 10 cm long) adult flukes live in cysts attached to a deer’s bile ducts. In the cow’s liver, these cysts don’t attach themselves to the bile ducts. In the sheep, the larva wanders through the liver, growing but never forming a cyst or reaching adulthood.
While meat inspectors condemn fluke-infected cow livers at slaughter, these flukes cause deer and cattle no problems. A sheep, however, can die from even one deer liver fluke larva. This happens because the larva grows large and continually moves through the sheep’s liver, destroying much of the liver tissue.
It’s very difficult to raise sheep in areas where deer are infected with this fluke.
Veterinarians can’t diagnose deer liver fluke infections in cattle or sheep; the eggs don’t come out with the cow’s bile, and no eggs form in the sheep. Some anthelmintics kill the adult fluke in deer and cattle.
Control this parasite as you would the liver fluke of ruminants: keep the livestock from feeding on plants that may carry the cysts. Try also to keep deer from the livestock pastures.
Protozoans
Coccidia
The coccidia of ruminants (Eimeria species) live in the cells of their host’s intestine. There are several different species of coccidia, which can live in each of the different ruminants. Some of these coccidia cause disease; others don’t.
All coccidia are species specific—that is, an Eimeria species that infects cattle won’t infect any other host, and the same goes for the coccidia of other animals.
Coccidia have a very complicated life cycle. They go through many stages and invade many different cells of the host’s intestine. During most of these stages, the coccidia multiply, so a few coccidia can start an infection that grows to millions of parasites.
When the coccidia finish multiplying in the host, they form oocysts that pass out with the feces. The oocyst is the infective stage. When a new host accidentally eats it, the oocyst hatches in the new host’s small intestine and then invades the cells of the intestinal wall.
Coccidiosis usually turns up in younger animals, especially those under stress. Disease outbreaks commonly strike cattle in feedlots and sheep just after shipping. Severe winter weather may be all it takes to turn a calf with a light coccidia infection into a calf suffering from bloody diarrhea. Infected sheep and goats may develop diarrhea, but it’s usually bloodless. At greatest risk are young goats; heavy coccidia infections can kill them.
Veterinarians diagnose coccidial infection by finding the oocysts in the feces. Because the diarrhea may occur before oocysts are being shed, however, a veterinarian may diagnose coccidiosis based only on the age of the host and the diarrhea.
Several drugs treat coccidiosis; veterinarians give them to young ruminants about to be stressed (shipped or put into a feedlot, for example) to prevent disease.
Cryptosporidium
Cryptosporidium is a coccidia-like organism that infects many different mammals, including ruminants, horses, and humans. Like the coccidia, it has a very complicated life cycle during which it lives in cells of its host’s intestine.
Young Cryptosporidium-infected animals may suffer a very watery diarrhea. The oocysts that leave the host in the feces are so small that they’re hard to see even under a high-powered microscope.
Veterinarians instead diagnose Cryptosporidium infection by staining the feces with special stains that reveal the small oocysts.
Treatment for Cryptosporidium involves supportive care until the animal clears the infection. Cryptosporidium is a protozoan parasite of the intestine of large animals. It causes severe diarrhea.
Tritrichomonas foetus
Tritrichomonas foetus is a protozoan parasite that lives in a cow’s vagina and uterus and on a bull’s penis. Like all protozoans, it multiplies in its host; even one organism can lead to a heavy infection.
Transmitted during sexual intercourse, this protozoan doesn’t harm the bull, but can cause the cow to suffer early abortion and infertility.
Veterinarians diagnose Tritrichomonas foetus by finding it on a culture of the bull’s prepuce, in the cow’s vagina, or in an aborted fetus’s tissues. The drugs that kill this protozoan may not always work in bulls.
Cows artificially inseminated with semen from an uninfected bull don’t contract this parasite since it’s a sexually transmitted disease. If artificial insemination isn’t an option, the rancher should use a young bull to service the herd. Young bulls are less likely to be infected.
Giardia
Giardia, described in more detail in the section on the protozoans of dogs and cats, can live in a ruminant’s small intestine and can cause diarrhea in young animals.
Sarcocystis
Sarcocystis is a protozoan that lives in the muscles of ruminants. It uses the ruminant as an intermediate host. The dog is the definitive host for Sarcocystis. This parasite is discussed more thoroughly in the section on the protozoans of dogs and cats.
Endoparasites of Swine and Pet Pigs
Nematodes are the major endoparasites of swine. The ascarid (roundworm), the whipworm, and the nodular worm of pigs are the most common endoparasites of pet pigs. Modern commercially raised swine have few parasites because they’re raised indoors on hard floors and their feces and urine are removed daily.
Nematodes
Pig Ascarid
Ascaris suum has a life cycle almost identical to the Parascaris equorum of horses. In heavy infections, the larvae migrating in the liver may cause white spots to appear on its surface. Adult worms are as large as the horse ascarid and can compete with the pig for food. A heavily infected pig doesn’t put on weight as easily as a healthy pig.
Though Ascaris suum causes its host little harm, the farmer trying to sell the pig for food wants to maximize its body weight and prevent spots on its liver. Prevention, then, is important from the farmer’s point of view. If the pigs are being raised on concrete, it’s easy to remove feces before the eggs become infectious.
Several anthelmintics kill the pig ascarid, which may be a problem for young pet pigs raised with older infected pigs. Older pet pigs are usually the only pig in the household; thus they’re unlikely to become infected since they don’t encounter the worm’s eggs.
Trichinella spiralis
Trichinella spiralis is another economically important nematode parasite of swine. This parasite causes the disease trichinosis in humans and other mammals.
Adult Trichinella live in the mucosa of the small intestine. Female worms give birth to first-stage larvae, which enter blood vessels, travel to the body’s muscles, and penetrate a skeletal muscle cell. There the larvae coil up and may live for many years. The larvae enter a new host’s digestive tract when the host eats this muscle. The host digests the muscle tissue, thereby freeing the larvae. The larvae molt four times and then become adults.
Pigs are infected when they eat uncooked meat containing first-stage larvae. Infection spreads through the herd in a number of ways. Infected meat can come from the uncooked garbage some swine are fed. Pigs raised in crowded conditions may bite off and eat each other’s tails. They may also eat other pigs that have died in the pen. Finally, pigs may eat live or dead infected rats.
Trichinella infections are normally light, causing the pig little harm. But farmers can’t afford to allow Trichinella into the herd, since pork containing Trichinella larvae isn’t fit for human consumption. Anthelmintics kill Trichinella, but infected pigs are hard to identify since the infection causes the pig no problems. Rat control helps prevent this infection; so does cooking garbage to kill any larvae it contains before pigs eat it.
Trichinella spiralis isn’t a problem in pet pigs.
Nodular Worm of Pigs
These small nematodes (Oesophagostomum species, about 12 mm long) live in the pig’s large intestine. The pig eats food off the ground and accidentally consumes the infective larvae. The larvae burrow into the wall of the large intestine and cause nodules to form in the wall. The larvae live in these nodules for about a week. Then they emerge from the nodules and develop to adults in the intestine.
Eggs pass out with the feces and hatch on the ground. The larva goes through several free-living stages before it becomes infective.
Light to moderate infections of the nodular worm cause the pig few problems. Heavy infections can cause diarrhea and anorexia (the pig doesn’t want to eat). Young pigs with heavy infections may show severe signs; some may die. This worm can infect any livestock or pet pigs raised outside.
Veterinarians diagnose nodular worms by seeing the clinical signs and finding the eggs in the feces. The infection responds to anthelmintics. The same anthelmintics prevent infection in pigs sharing an area with infected pigs.
Pig Whipworm
The whipworm (Trichuris suis) is a very common parasite of livestock and pet pigs raised outside. The adult whipworms are moderate-sized nematodes (5 cm) that live in the large intestine. The adult whipworm is so named because its thick back (one-third of its body) and very narrow front (the remaining two-thirds) make it resemble a whip: the back is the whip’s “handle,” the front its “lash.” The adult whipworm burrows into the intestinal wall until only its “handle” remains in the intestine. The whipworm feeds on blood from the intestinal wall.
The whipworm’s eggs, which pass out in the feces, are very easy to recognize: they’re brown, lemon-shaped, and have a plug on either end. The eggs mature on the ground until the infective larvae develop inside the egg. When the pig accidentally eats the egg off the ground, the egg hatches in the intestine and the larva develops into an adult in six to seven weeks.
Whipworm infections can cause diarrhea and blood in the feces. The infection responds to anthelmintics. Because the eggs can survive on the ground for several years, the pigs may need regular retreatment.
Lungworm of Pigs
The adult pig lungworm (Metastrongylus species) is a moderate-sized nematode (3 to 5 cm long), which lives in the air passages of the pig’s lung. There the lungworm lays eggs, which the pig coughs up and swallows. They pass out of the pig with the feces.
The larvae in the eggs on the ground develop only when eaten by the lungworm’s intermediate host, an earthworm. In the earthworm, the larvae develop to the infective stage. The infective stage larvae can live in the earthworm for many years. When the pig eats the infected earthworm, the larvae enter the intestinal wall and get into the blood. The blood carries them to the lungs, where they break out of the blood vessels and go to the air passages.
Pig lungworm disease is usually mild. The pig may cough and become unthrifty. The lungworm, since it needs an earthworm intermediate host, turns up only in pigs raised outside on dirt.
Veterinarians diagnose lungworm infection by finding the eggs, each of which has a larva, in the feces. Anthelmintics kill lungworms. To prevent lungworm infections, keep the pig away from earthworms.
Stomach Worm of Pigs
The adult stomach worm (Hyostrongylus rubidus) is a small nematode (7 to 10 mm long) that lives in a pig’s stomach, where it lays eggs that pass out with the feces. The eggs hatch on the ground. The larvae undergo two free-living stages and then become infective. The pig accidentally eats the infective larva, which enters the wall of the pig’s stomach and develops to adulthood. The adult worm comes out of the stomach wall and lives in the stomach, where it sucks blood from the stomach wall.
The feeding adults and embedded larvae may damage the stomach lining and cause ulcers. Heavily infected pigs may be unthrifty and may have diarrhea. The infection responds to anthelmintics. Prevent stomach worm infections by raising pigs off dirt and removing the feces regularly.
Kidney Worm of Pigs
The adult of the kidney worm (Stephanurus dentatus) is a moderate-sized nematode (3 to 5 cm), which lives in the kidney and walls of the ureters, the vessels that drain the urine from the kidney. Kidney worm eggs pass out of the pig in the urine.
The free-living stages are the same as those of the stomach worm. The infective larva may enter the pig by accidental ingestion or by burrowing through the skin. No matter how they enter, the larvae travel to the pig’s liver. There they move around for more than three months while they mature. Once mature, the larvae travel from the liver to the peritoneal cavity and then on to the kidney and walls of the ureters, where they mature to adulthood. This process may take anywhere from 6 to 19 months because of the long time the larvae spend moving through the pig.
Though the adult worms usually cause no problem, the larvae can damage the liver while moving through it. Liver damage can lead to unthriftiness and, in heavy infections, to ascites (fluid in the peritoneal cavity). If mature worms are present, veterinarians can diagnose kidney worm infection by finding the eggs in the urine. The long developmental period and the damage done by the larvae, however, may result in unthrifty pigs with no eggs in their urine. In these cases veterinarians base their diagnosis on the history of the herd: If the kidney worm has turned up previously, it may still be in the pigs. This infection responds to anthelmintics. Kill free-living larvae by drying out the soil in which they live.
Threadworm of Pigs
The parasitic adult threadworm (Strongyloides ransomi) is a small (3 to 4 mm) nematode that lives in the small intestine. Parasitic adult threadworms are all female; they don’t need males to produce fertile eggs.
Threadworms have a very unusual life cycle for a parasitic nematode. Eggs pass out in the feces and hatch. Threadworms grow to male and female adults in moist soil. These free-living adults lay eggs that hatch in the soil and develop to infective larvae. The infective larvae burrow through the pig’s skin. Some of the larvae that enter the pig become dormant in the tissues; the rest burrow into a blood vessel and travel to the lungs. There these larvae break out into the air spaces and then migrate up the trachea. When they reach the back of the mouth, the pig swallows them. In the small intestine, the infective larvae mature into female adult worms.
Dormant larvae in a pregnant pig may “wake up” and find their way into the small intestine of the fetus, maturing to adults after the piglet is born. Dormant larvae in a nursing pig may also “wake up,” make their way to the mammary gland, and enter the piglet in the milk. Larvae passed in the milk go right to the small intestine and mature. The adult worms in the intestine can cause diarrhea and anorexia, especially in young piglets. Heavy infections can kill suckling pigs.
Veterinarians diagnose threadworm infection by finding the egg in the feces. The threadworm egg has a larva in it when it’s passed in the feces. There are anthelmintics that kill the adult threadworms in the small intestine, but currently, none that kill dormant larvae. If a pregnant pig receives certain anthelmintics just before she gives birth, however, they kill the larvae that have awakened and started moving towards the fetus or the mammary gland. Since free-living threadworms need moist soil, pigs raised on a dry surface won’t suffer from threadworm infection.
Tapeworms
There are no important tapeworms that live in the pig as adults.
Pork Tapeworm
One tapeworm of humans, the pork tapeworm (Taenia solium), however, uses the pig as an intermediate host. The pork tapeworm lives in the small intestine of humans as an adult. The larval stage lives in the muscles of the pig.
The pig gets the infection by eating the eggs or proglottids with eggs in them. The proglottids are found in the feces of infected humans, who get infected by eating raw pork. This infection in pigs is very rare in the United States since good sanitation facilities prevent the swine from encountering the proglottids. The infection in pigs causes them no problem. Veterinarians can’t diagnose larval infection in a living pig, nor is there any treatment for it. Meat carrying pork tapeworm larvae is condemned at slaughter.
Protozoans
The coccidia of pigs (Eimeria species and Isospora suis) have life cycles like the coccidia of ruminants. Also, like the ruminant coccidia, not all of the coccidia of swine cause disease. Isospora suis can cause severe diarrhea in newborn pigs.
Though veterinarians diagnose coccidia by finding the oocysts in the feces, sometimes a newborn’s diarrhea occurs before the oocysts are shed. In those cases, finding the oocysts in the mother pig’s feces, or in the feces of other pigs in the same area, may explain what’s happening to the newborn pigs. There are drugs that treat coccidia infections in pigs. Good sanitation and sufficient living space help to prevent severe disease.
Endoparasites of Horses
Nematodes
Ascarid of Horses
Parascaris equorum, the ascarid of horses, is very large. Males may be up to 30 cm, and females can grow to 50 cm. Adults of Parascaris equorum live in the horse’s small intestine, following a life cycle similar to that of dog and cat ascarids.
Eggs develop on the ground for 10 to 14 days. Eggs containing the second-stage larva can infect the horse that eats them. The roundworm eggs hatch in the small intestine; the larvae then burrow through the intestinal wall and into a blood vessel. They travel first to