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The Merck Manual of Medical Information--Home Edition
Section 17. Infections
Chapter 184

Parasitic Infections

A parasite is an organism, such as a single-celled animal (protozoan) or worm, that survives by living inside another, usually much larger, organism (the host).

Parasitic infections are common in rural Africa, Asia, and South America but are rare in developed countries. However, people from developed countries who visit developing countries can be infected by parasites and unknowingly return home with the infection, which may not be readily diagnosed because it is so uncommon.

Worms most commonly enter the body through the mouth, although some enter through the skin. Those that infect the intestine may stay there or may burrow through the intestinal wall and infect other organs. Worms that penetrate the skin often bore through the soles of the feet or enter the skin from infected water while a person is swimming.

If a doctor suspects that a person may have a parasitic infection, samples of blood, stool, or urine may be obtained for laboratory analysis. A doctor may also draw a sample of fluid from an organ or tissue that may be infected. Repeated examinations usually are necessary to find the parasites in these samples.

Parasites often reproduce in the host they infect, so their eggs are sometimes found in the host. If parasites reproduce in the digestive tract, the eggs may be found in the person's stool. To make the diagnosis of a parasitic infection, a doctor will usually collect three samples of stool at 1- or 2-day intervals. Sometimes stool samples are obtained with a sigmoidoscope (a flexible viewing tube used to examine the lower portion of the large intestine). (see page 485 in Chapter 100, Diagnostic Tests for Digestive Disorders) The person providing a stool sample shouldn't take antibiotics, laxatives, or antacids, because these drugs can reduce the number of parasites and make their detection in the laboratory more difficult.

Alternatively, to make the diagnosis, sometimes fluid is withdrawn from the duodenum (upper part of the small intestine), or a sample of the intestinal contents is obtained using a nylon string passed through the mouth.


Amebiasis is an infection of the large intestine caused by Entamoeba histolytica, a single-celled parasite.

Entamoeba histolytica exists in two forms during its life cycle: the active parasite (trophozoite) and the dormant parasite (cyst). Trophozoites live among the intestinal contents and feed on bacteria or on the wall of the intestine. When infection begins, the trophozoites may cause diarrhea, which expels them from the body. Outside the body, the fragile trophozoites die. When the person doesn't have diarrhea, the trophozoites usually become cysts before leaving the intestine. The cysts are very hardy and may spread either directly from person to person or indirectly through food or water.

Direct transmission, the more common route in the United States, occurs through contact with infected stool. Amebiasis is more likely to spread among institutionalized people with poor sanitation practices than among noninstitutionalized people and by sexual contact, particularly among male homosexuals, rather than by casual contact. Indirect transmission of the cysts is more common in areas where sanitation is poor, such as in migrant labor camps. Fruits and vegetables may be contaminated when grown in soil fertilized by human stool, washed in polluted water, or prepared by someone who is infected.


Most people who are infected, particularly those who live in temperate climates, have no symptoms. Sometimes, the symptoms are so vague that they are barely noticed. Symptoms may include intermittent diarrhea and constipation, increased gas (flatulence), and cramping abdominal pain. The abdomen may be tender when touched, and the stool may contain mucus and blood. The person may have a slight fever. Between attacks, symptoms diminish to recurring cramps and loose or very soft stools. Wasting of the body (emaciation) and anemia are common.

Invasion of the intestinal wall by trophozoites may cause a large lump (ameboma) to form. The ameboma may obstruct the intestine and be mistaken for cancer. Occasionally, trophozoites perforate the intestinal wall. The release of intestinal contents into the abdominal cavity causes severe abdominal pain and an abdominal infection (peritonitis), which requires immediate medical attention.

Trophozoite invasion of the appendix and the surrounding intestine may cause a mild form of appendicitis. Surgery for appendicitis can spread the trophozoites around the abdomen. Therefore, surgery may be delayed for 48 to 72 hours while drugs are given to kill the trophozoites.

An abscess filled with trophozoites may form in the liver. Symptoms include pain or discomfort in the area over the liver, an intermittent fever, sweats, chills, nausea, vomiting, weakness, weight loss, and, occasionally, mild jaundice.

Occasionally, the trophozoites spread through the bloodstream, causing infection in the lungs, brain, and other organs. The skin may also become infected, especially around the buttocks and genitals, as may wounds caused by surgery or injury.


Amebiasis is diagnosed in a laboratory by examining samples of stool obtained from an infected person; three to six samples may be needed to make the diagnosis. A proctoscope (a flexible viewing tube) may be used to look inside the rectum and to obtain a tissue sample of any ulcers found there.

People with a liver abscess almost always have high levels of antibodies against the parasite in their blood. However, because these antibodies may remain in the bloodstream for months or years, high antibody levels don't necessarily indicate a current abscess. Therefore, if a doctor thinks a liver abscess has formed, a drug that kills amebae (an amebicide) may be prescribed. If the drug is effective, amebiasis is assumed to be the correct diagnosis.


Several amebicide drugs taken orally--such as iodoquinol, paromomycin, and diloxanide--kill the parasites in the intestine. Metronidazole or dehydroemetine is taken for severe disease and for disease outside the intestine. Stool samples are reexamined 1, 3, and 6 months after treatment to make sure the person is cured.


Giardiasis is an infection of the small intestine caused by Giardia lamblia, a single-celled parasite.

click on thumbnail to see full-size imageGiardiasis occurs worldwide and is especially common among children and in places where sanitation is poor. In the United States, giardiasis is one of the most common parasitic infections of the intestine. It is more common in male homosexuals and in people who have traveled to developing countries. It is also more common in people who have low stomach acidity, have had their stomach removed surgically, have chronic pancreatitis, (see page 507 in Chapter 104, Disorders of the Pancreas) or have an impaired immune system.

The parasite is transmitted from one person to another by cysts passed in the stool. Transmission may occur directly between children or sex partners or indirectly through contaminated food or water.

Symptoms and Diagnosis

The symptoms, which are usually mild, include intermittent nausea, belching, increased gas (flatulence), abdominal discomfort, bulky and foul-smelling stools, and diarrhea. If the infection is severe, the person may fail to absorb important nutrients from food, resulting in significant weight loss. The reason giardiasis interferes with nutrient absorption isn't known.

The symptoms suggest the diagnosis to the doctor. Laboratory test results that reveal the parasite in the person's stool or in secretions taken from the duodenum confirm the diagnosis. Because people who have been infected for a long time tend to excrete the parasites at unpredictable intervals, repeated stool examinations may be needed.


Taken orally, the drug quinacrine is very effective against giardiasis. However, it may cause gastrointestinal upset, and rarely it leads to extremely abnormal behavior (toxic psychosis). Metronidazole also is effective and produces fewer side effects, but it isn't currently approved for treatment of giardiasis by the Food and Drug Administration. Furazolidone is less effective than either quinacrine or metronidazole, but because it is available in a liquid, it can be given to children. A pregnant woman may be treated with paromomycin, but only if her symptoms are severe.

People who live with an infected person or who have had sexual contact with such a person should see a doctor for testing and, if necessary, treatment.


Malaria is an infection of red blood cells caused by Plasmodium, a single-celled organism.

Malaria is spread by the bite of an infected female Anopheles mosquito, a transfusion with contaminated blood, or an injection with a needle that was previously used by a person with the infection. Four species of parasites--Plasmodium vivax, Plasmodium ovale, Plasmodium falciparum, and Plasmodium malariae--can infect humans and cause malaria.

Drugs and insecticides have made malaria rare in the United States and in most developed countries, but the infection remains common in the tropics. Visitors from the tropics or travelers returning from those areas sometimes bring the infection with them, possibly causing a small outbreak.

The life cycle of the malarial parasite begins when a female mosquito bites a person with malaria. The mosquito ingests blood containing malarial parasites, which move to the mosquito's salivary glands. When the mosquito bites another person, the parasites are injected along with the mosquito's saliva. Inside the person, the parasites move to the liver, where they multiply. They mature over an average of 2 to 4 weeks, then leave the liver and invade the person's red blood cells. The parasites multiply inside the red blood cells, eventually causing the infected cells to rupture.

Plasmodium vivax and Plasmodium ovale may remain in the liver cells while periodically releasing mature parasites into the bloodstream, causing attacks of malarial symptoms. Plasmodium falciparum and Plasmodium malariae don't remain in the liver. However, if the infection is untreated or inadequately treated, the mature form of Plasmodium falciparum may persist in the bloodstream for months, and the mature form of Plasmodium malariae may remain in the bloodstream for years, causing repeated attacks of malarial symptoms.

Symptoms and Complications

Symptoms usually begin 10 to 35 days after a mosquito injects the parasite into a person. Often, the first symptoms are a mild fever that comes and goes, headache, muscle aches, and chills, together with a general feeling of illness (malaise). Sometimes symptoms begin with shaking chills followed by fever. These symptoms last 2 or 3 days and are frequently thought to be symptoms of the flu. Subsequent symptoms and patterns of disease vary among the four types of malaria.

Symptoms and Patterns of Malaria

Vivax and Ovale Malaria
An attack may begin abruptly with a shaking chill, followed by sweating and a fever that comes and goes. Within a week, the typical pattern of intermittent attacks is established. A period of headache or of feeling ill may be followed by a shaking chill. The fever lasts 1 to 8 hours. After the fever subsides, the person feels well until the next chill. New attacks tend to occur every 48 hours in vivax malaria.

Falciparum Malaria
An attack may begin as chills. The person's temperature rises gradually, then falls suddenly. The attack may last for 20 to 36 hours. The person may feel more ill than with vivax malaria and have a severe headache. Between attacks, during intervals that vary from 36 to 72 hours, the person usually feels miserable and has a mild fever.

Malariae Malaria
An attack often begins abruptly. The attack is similar to that of vivax malaria but recurs every 72 hours.

In falciparum malaria, abnormal brain function may occur, a complication called cerebral malaria. Symptoms include a fever of at least 104° F., severe headache, drowsiness, delirium, and confusion. Cerebral malaria can be fatal. It most commonly occurs in infants, pregnant women, and travelers to high-risk areas. In vivax malaria, delirium may occur when the fever is high, but otherwise brain symptoms are uncommon.

In all types of malaria, the total white blood cell count is usually normal, but the numbers of lymphocytes and monocytes, two specific types of white blood cells, increase. (see page 810 in Chapter 167, Biology of the Immune System) Usually, mild jaundice develops if malaria is untreated, and the spleen and liver become enlarged. Low levels of blood sugar (glucose) are common and may be severe in people who have high levels of parasites. Blood sugar levels may drop even lower in people being treated with quinine.

Sometimes malaria persists when low levels of parasites remain in the blood. Symptoms include apathy, periodic headaches, a feeling of illness, poor appetite, fatigue, and attacks of chills and fever. The symptoms are considerably milder, and the attacks don't last as long as the first attack.

If the person is untreated, the symptoms of vivax, ovale, or malariae malaria subside spontaneously in 10 to 30 days but may recur at variable intervals. Untreated falciparum malaria is fatal in up to 20 percent of people.

Blackwater fever is a rare complication of malaria caused by the rupture of large numbers of red blood cells. The rupture releases red pigment (hemoglobin) into the bloodstream. The hemoglobin, which is then excreted in the urine, turns the urine dark. Blackwater fever occurs almost exclusively in people with chronic falciparum malaria, especially those who have taken quinine for treatment.


A doctor suspects malaria when a person has periodic attacks of chills and fever with no apparent cause. The suspicion is greater if within the previous year the person had visited an area where malaria is prevalent and if the spleen is enlarged. Identifying the parasite in a blood sample confirms the diagnosis. More than one sample may be needed to make the diagnosis because the level of parasites in the blood varies over time. The laboratory report identifies the species of Plasmodium found in the sample, because the treatment, complications, and prognosis vary depending on the species involved.

Prevention and Treatment

People who live in malaria-infested areas or who travel to them can take precautions. They can use long-lasting insecticide sprays in homes and outbuildings, place screens on doors and windows, use mosquito netting over their beds, and apply mosquito repellents on their skin. They also can wear enough clothing, particularly after sundown, to protect as much of the skin as possible against mosquito bites.

Some Reminders About Malaria

  • Drugs taken for prevention are not 100 percent effective
  • Symptoms can begin a month or more after the infecting mosquito bite
  • Early symptoms are nonspecific and often are mistaken for those of influenza
  • Rapid diagnosis and early treatment are important, particularly for falciparum malaria, which is fatal in up to 20 percent of infected people

Drugs can be taken to prevent malaria during travel to a malaria-infested area. The drug is started a week beforehand, continued throughout the stay, and extended for a month after leaving. The most commonly used drug is chloroquine. However, many areas of the world have strains of Plasmodium falciparum that are resistant to this drug. Other drugs include mefloquine and doxycycline. However, doxycycline can't be taken by children under age 8 or by pregnant women.

No drug therapy is completely effective in preventing the infection. Travelers who develop a fever while in a malaria-infested area should be examined by a doctor immediately. Pyrimethamine-sulfadoxine, a combination of drugs, may be used for self-treatment until medical help is available.

Treatment depends on which type of malaria the person has and on whether the geographic area has strains of the parasite that are resistant to chloroquine. For an acute attack of falciparum malaria in an area known to have chloroquine-resistant strains, a person may take quinine or receive quinidine intravenously. For the other types of malaria, resistance to chloroquine is less common, and therefore a person usually takes chloroquine followed by primaquine.


Toxoplasmosis is an infection caused by Toxoplasma gondii, a single-celled parasite.

Sexual reproduction by this parasite occurs only in the cells lining the intestine of cats. Eggs (oocysts) are shed in a cat's stool. People become infected by eating raw or undercooked meat containing the dormant form (cysts) of the parasite or by being exposed to soil containing oocysts from cat feces. If a pregnant woman becomes infected, the infection can be transferred to her fetus through the placenta. The woman may then have a miscarriage, or the baby may be stillborn or born with congenital toxoplasmosis. (see page 1220 in Chapter 253, Infections in Newborns and Infants)


For children born with congenital toxoplasmosis, symptoms may be severe and rapidly fatal, or no symptoms may appear. Symptoms can include inflammation of the eyes, leading to blindness; severe jaundice; easy bruising; convulsions; a large or small head; and severe mental retardation. Very mild symptoms may appear shortly after birth, but more often they appear months or several years later.

Toxoplasmosis acquired after birth seldom causes symptoms and is usually diagnosed when a blood test reveals antibodies against the parasite. However, symptoms sometimes do appear. These vary, depending on whether the person has mild lymphatic toxoplasmosis, chronic toxoplasmosis, or acute disseminated toxoplasmosis. Toxoplasmosis in people with AIDS presents a different array of problems.

Toxoplasmosis: Symptoms and Problems

The symptoms of toxoplasmosis can vary, depending on what form the infection takes.

Mild lymphatic toxoplasmosis may resemble infectious mononucleosis. Symptoms can include enlarged lymph nodes in the neck and armpits that usually aren't tender, a feeling of illness, muscle pain, and a fluctuating low fever that can last for weeks or months but that eventually disappears. The person also may have mild anemia, low blood pressure, a low white blood cell count, an increased number of blood lymphocytes, and slightly abnormal results of liver function tests. Commonly, however, infected people have only enlarged, painless lymph nodes in the neck.

Chronic toxoplasmosis produces inflammation inside the eye. Often, the other symptoms are vague.

Acute disseminated toxoplasmosis can cause a rash, high fever, chills, and extreme exhaustion. This type of toxoplasmosis occurs primarily in people with an impaired immune system. In some people, infection causes inflammation of the brain and its lining (meningoencephalitis), liver (hepatitis), lungs (pneumonitis), or heart (myocarditis).

Toxoplasmosis in people with AIDS can spread throughout the body. Most often, brain inflammation (encephalitis) occurs, which may paralyze half the body, diminish sensation in specific areas, and cause convulsions, trembling, headache, confusion, or coma.


The diagnosis of toxoplasmosis is usually made by a blood test that reveals antibodies against the parasite. However, if a person's immune system is impaired, a doctor may instead depend on computed tomography (CT) and magnetic resonance imaging (MRI) of the brain to make the diagnosis.

Treatment and Prognosis

Toxoplasmosis in newborns and in people who have an impaired immune system is treated with spiramycin or sulfadiazine plus pyrimethamine. Toxoplasmosis in people with AIDS tends to recur so frequently that treatment is usually continued indefinitely. Treatment during pregnancy is controversial because the drug could potentially harm the fetus. Because the disease disappears on its own in most adults with a normal immune system, pregnant women usually aren't treated with drugs unless a vital organ--such as the eye, brain, or heart--is infected or the symptoms are severe and persist throughout the body.

The prognosis for people with toxoplasmosis acquired after birth is good--except in those with an impaired immune system, such as people with AIDS, in whom toxoplasmosis is often fatal.


Babesiosis is an infection of red blood cells caused by Babesia parasites.

Hard-bodied ticks--the same deer ticks that transmit Lyme disease--transmit Babesia parasites. Although infection in animals is common, people are rarely infected. Symptoms include fever and anemia caused by the breakdown of red blood cells.

In people whose spleen has been removed, the risk of death is high. In these people, the infection closely resembles falciparum malaria; it produces a high fever, anemia, hemoglobin in the urine, jaundice, and kidney failure. A person with a functioning spleen has a milder illness that usually disappears on its own within weeks or months. Most cases of babesiosis in the United States (which are acquired on the offshore islands of New York and Massachusetts) are mild.

The diagnosis is made by identifying the parasites, which resemble those that cause malaria. Treatment consists of taking the drug clindamycin.


Trichuriasis is an infection caused by Trichuris trichiura, an intestinal roundworm.

This parasite occurs mainly in the subtropics and tropics, where poor sanitation and a warm, moist climate provide the conditions needed for the eggs to incubate in the soil.

Infection results when a person swallows food containing eggs that have incubated in the soil for 2 to 3 weeks. The larvae hatch in the small intestine, migrate to the large intestine, and embed their heads in the intestinal lining. Each larva grows to about 4½ inches. Mature females produce about 5,000 eggs a day, which are passed in the stool.

Symptoms and Diagnosis

Only a heavy infection causes the symptoms of abdominal pain and diarrhea. Very heavy infections may cause bleeding from the intestine, anemia, weight loss, and appendicitis. Occasionally, the rectum may fall through the anus (a condition called rectal prolapse), especially in a child or a woman in labor.

The barrel-shaped eggs are usually visible in stool samples examined under a microscope.

Prevention and Treatment

Prevention depends on using sanitary toilet facilities, maintaining good personal hygiene, and avoiding uncleaned vegetables. No treatment is needed for light infections. When treatment is needed, mebendazole is the preferred drug, but it can't be used in pregnant women because of its potentially harmful effects on the fetus.


Ascariasis is an infection caused by Ascaris lumbricoides, an intestinal roundworm.

The infection occurs worldwide but is more common in warm areas with poor sanitation, where it persists largely because of indiscriminate defecation by children.

The life cycle of the Ascaris parasite resembles that of the parasite that causes trichuriasis, except that the larvae also migrate through the lungs. Once a larva hatches, it migrates through the wall of the small intestine and is carried by the lymphatic vessels and the bloodstream to the lungs. There it passes into the air sacs (alveoli), ascends the respiratory tract, and is swallowed. The larva matures in the small intestine, where it remains as an adult worm. Adult worms range from 6 to 20 inches in length and from 1/10 to 2/10 inch in diameter. Symptoms may be caused by the migration of the larva through the lung and by the presence of the adult worm in the intestine.

Symptoms and Diagnosis

The migration of larvae through the lungs can cause fever, coughing, and wheezing. A heavy intestinal infection may cause abdominal cramps and, occasionally, intestinal obstruction. Poor absorption of nutrients may be caused by a heavy concentration of worms. Adult worms occasionally obstruct the appendix, the biliary tract, or the pancreatic duct.

Infection with the adult worm is usually diagnosed by identifying eggs in a sample of stool. Occasionally, laboratory tests reveal adult worms in the stool or vomit or larvae in the sputum. In the blood, the number of eosinophils, a type of white blood cell, may increase. Signs of the migration may be seen on a chest x-ray.

Prevention and Treatment

Prevention requires using adequate sanitation and avoiding uncleaned vegetables. Treatment consists of taking pyrantel pamoate or mebendazole. However, mebendazole can't be taken by pregnant women because of its potentially harmful effects on the fetus.

Hookworm Infection

Hookworm infection is caused by an intestinal roundworm, either Ancylostoma duodenale or Necator americanus.

About one fourth of the world's population is infected with hookworms. Infection is most common in warm, moist places where sanitation is poor. Ancylostoma duodenale is found in the Mediterranean area, India, China, and Japan; Necator americanus is found in the tropical areas of Africa, Asia, and the Americas. These hookworms are now rarely transmitted in the southern part of the United States.

click on thumbnail to see full-size imageIn the life cycle of either hookworm, eggs are discharged in stool and hatch in the soil after incubating for 1 to 2 days. In a few days, larvae are released and live in the soil. A person can become infected by walking barefoot through a field contaminated by human feces because the larvae can penetrate the skin. The larvae reach the lungs through the lymphatic vessels and bloodstream. Then they climb the respiratory tract and are swallowed. About a week after penetrating the skin, they reach the intestine. The larvae attach themselves by their mouths to the lining of the upper small intestine and suck blood.

Symptoms and Diagnosis

An itchy, flat, raised rash (ground itch) may develop where the larvae penetrated the skin. A fever, coughing, and wheezing may be caused by the migration of the larvae through the lungs. Adult worms often cause pain in the upper abdomen. Iron deficiency anemia and low levels of protein in the blood can result from intestinal bleeding. In children, slow growth, heart failure, and widespread tissue swelling may result from prolonged, severe blood loss.

If the infection produces symptoms, the eggs usually are visible in a sample of stool. If the stool isn't examined for several hours, the eggs may hatch and release larvae.


A doctor's first priority is to correct the anemia, which usually improves with oral iron supplements but may require iron injections. In severe cases, a blood transfusion may be needed. When the person's condition is stable, an oral drug such as pyrantel pamoate or mebendazole is taken for 1 to 3 days to kill the hookworm. These drugs can't be taken by pregnant women.


Trichinosis is a parasitic infection caused by Trichinella spiralis.

Trichinosis occurs in most parts of the world but is rare or absent in regions where pigs are fed root vegetables, as in France. In the United States, it has become rare.

Infection results from eating raw or inadequately cooked or processed pork or pork products. In rare cases, infection can result from eating the meat of bears, boars, and some marine mammals. Any of these animals may contain a cyst form of the larvae (trichinae). When the cyst wall is digested in the stomach or duodenum, it releases larvae that penetrate the wall of the small intestine. Within 2 days, the larvae mature and mate. The male worms play no further role in causing infections. The females burrow into the intestinal wall and by the seventh day begin to discharge living larvae.

Each female may produce more than 1,000 larvae. Production continues for about 4 to 6 weeks, after which the female worm dies and is digested. The tiny larvae are carried around the body by the lymphatic vessels and bloodstream. Only larvae that reach skeletal muscles survive. They penetrate the muscles, causing inflammation. By the end of the third month, they form cysts.

Certain muscles, such as the tongue, the muscles of the eye, and the muscles between the ribs, are particularly likely to be infected. Larvae that reach the heart muscle are killed by the intense inflammatory reaction that they provoke.


The symptoms vary, depending on the number of invading larvae, the tissues invaded, and the general physical condition of the person. Many people have no symptoms at all. Sometimes, intestinal symptoms and a slight fever begin 1 to 2 days after eating infected meat. However, symptoms from the larval invasion usually don't start for 7 to 15 days.

Swelling of the upper eyelids is one of the earliest and most typical symptoms, appearing suddenly about the eleventh day of the infection. Bleeding in the whites of the eyes and at the back of the eyes, pain in the eyes, and sensitivity to bright light come next. Muscle soreness and pain, together with a skin rash and bleeding under the nails, may develop shortly afterward. The soreness is pronounced in the muscles used to breathe, speak, chew, and swallow. Great difficulty in breathing may follow, sometimes even causing death.

Additional symptoms may include thirst, profuse sweating, fever, chills, and weakness. Fever generally comes and goes--often rising to at least 102° F., remaining elevated for several days, and then falling gradually. As the immune system destroys larvae outside the muscles, lymph nodes as well as the brain and its membrane linings may become inflamed, and vision or hearing disorders may develop. The lungs or the pleura (the membrane layers surrounding the lungs) and the heart may also become inflamed. Heart failure may develop between the fourth and eighth weeks. Most symptoms disappear by about the third month, although vague muscular pains and tiredness may persist for months.


As long as the parasite remains in the intestine, no tests can confirm the diagnosis. A biopsy of muscle tissue (in which a sample of tissue is removed and examined under a microscope), performed after the fourth week of infection, may reveal larvae or cysts. The parasite is rarely found in the stool, the blood, or the fluid surrounding the brain and spinal cord.

Blood tests are fairly reliable, although false-negative results (results indicating no infection when one exists) can occur, particularly if testing is done within 2 weeks of the start of the disease. Levels of eosinophils (a type of white blood cell) usually begin increasing about the second week, reach their maximum about the third or fourth week, and then gradually decline. Skin tests are unreliable.

Prevention and Treatment

Trichinosis is prevented by thoroughly cooking raw pork, pork products, and other meats. Alternatively, larvae usually can be killed by freezing meat at 5° F. for 3 weeks or at -4° F. for 1 day. However, larvae from Arctic mammals seem able to survive colder temperatures.

Mebendazole and thiabendazole, drugs taken orally, are effective against the parasite. Bed rest helps relieve muscular pain; however, analgesics, such as aspirin or codeine, may be needed. Corticosteroids, such as prednisone, may be used to reduce inflammation of the heart or brain. Most people with trichinosis recover fully.


Toxocariasis (visceral larva migrans) is an infection that results from the invasion of organs by roundworm larvae, such as Toxocara canis and Toxocara cati.

The parasite's eggs develop in soil contaminated by the feces of infected dogs and cats. Children's sandboxes, where cats often defecate, pose a hazard. The eggs may be transferred directly to the mouth if a child plays in or eats the contaminated sand.

After being swallowed, the eggs hatch in the intestine. The larvae penetrate the intestinal wall and are spread by the blood. Almost any tissue of the body may be involved--particularly the brain, eye, liver, lung, and heart. The larvae may remain alive for many months, causing damage by migrating to tissues and causing inflammation around them.

Symptoms and Diagnosis

Toxocariasis usually produces a relatively mild infection in children ages 2 to 4, but older children and adults may be affected as well. Symptoms may start within several weeks of infection or may be delayed several months, depending on the intensity and number of exposures and on the person's sensitivity to the larvae. A fever, cough or wheezing, and liver enlargement may develop first. Some people have a skin rash, spleen enlargement, and recurring pneumonia. Older children tend to have no or mild symptoms, but they may develop an eye lesion that impairs vision and that may be confused with a malignant tumor of the eye.

A doctor may suspect toxocariasis in a person who has high levels of eosinophils (a type of white blood cell), an enlarged liver, inflammation of the lungs, a fever, and high levels of antibodies in the blood. Examination of liver tissue obtained by biopsy may reveal evidence of larvae or of inflammation resulting from their presence.

Prevention and Treatment

Infected dogs and cats, particularly those under 6 months old, should be dewormed regularly, starting before they are 4 weeks old. Covering sandboxes when not in use prevents animals from defecating in them.

The infection in humans usually goes away without treatment in 6 to 18 months. The effectiveness of any treatment is uncertain. Mebendazole is probably the best treatment, and diethylcarbamazine may be helpful. Prednisone sometimes is taken to control the symptoms.

Beef Tapeworm Infection

Beef tapeworm infection is an intestinal infection caused by the tapeworm (cestode) Taenia saginata.

click on thumbnail to see full-size imageThe infection is particularly common in Africa, the Middle East, Eastern Europe, Mexico, and South America. Although uncommon in the United States, the infection does occur in many states.

The adult worm lives in the human intestine and may grow 15 to 30 feet in length. Egg-bearing sections of the worm (proglottids) are passed in the stool and are eaten by cattle. The eggs hatch in the cattle and invade the intestinal wall. They are then carried in the bloodstream to skeletal muscle, where they form cysts (cysticerci). People are infected by eating the cysts in raw or undercooked beef.

Symptoms and Diagnosis

Although infection usually causes no symptoms, some people have upper abdominal pain, diarrhea, and weight loss. Occasionally, an infected person may feel a piece of the worm move out through the anus.

The diagnosis usually is made when a piece of the worm is found in the stool. A doctor may press the sticky side of cellophane tape against the area around the anus, place the tape on a glass slide, and examine it under a microscope for the parasite's eggs.

Prevention and Treatment

Beef tapeworm infection may be prevented by cooking beef at a minimum temperature of 133° F. for at least 5 minutes.

An infected person is treated with niclosamide or praziquantel taken orally. The stool is rechecked after 3 months and 6 months to ensure that the infection is cured.

Pork Tapeworm Infection

Pork tapeworm infection is an intestinal infection caused by the adult tapeworm Taenia solium. Infection with the larval stage of the worm causes cysticercosis.

Pork tapeworm infections are common in Asia, the former Soviet Union, Eastern Europe, and Latin America. Infection in the United States is rare, except among immigrants and travelers from high-risk areas.

The adult tapeworm measures 8 to 10 feet in length. It consists of a head armed with several small hooks and a body composed of 1,000 egg-containing sections (proglottids). Its life cycle resembles that of the beef tapeworm, except that pigs, rather than cattle, serve as the intermediate host. People also may act as intermediate hosts; the eggs reach the stomach either when a person swallows them or when proglottids are regurgitated from the intestine to the stomach. The embryos are released inside the stomach. They then penetrate the intestinal wall and travel to muscles, internal organs, the brain, and tissue under the skin, where they form cysts. Live cysts cause only a mild tissue reaction, whereas dead ones invoke a vigorous reaction.

Symptoms and Diagnosis

Infection with the adult worm usually doesn't cause any symptoms. Heavy infection with cysts may cause muscle pain, weakness, and fever. If the infection reaches the brain and its membrane linings, they may become inflamed. Seizures can occur as well.

In adult worm infections, eggs may be seen around the anus or in the stool. The proglottid or the head of the worm must be found in the stool and examined under a microscope to distinguish the pork tapeworm from other tapeworms. Live cysts in such tissues as the brain are best seen by computed tomography (CT) or magnetic resonance imaging (MRI). Sometimes cysts can be found by microscopic examination of a sample of tissue taken from a skin nodule. Blood tests for antibodies against the parasite also are available.

Prevention and Treatment

Thoroughly cooking pork prevents infection. The infection is treated with niclosamide or praziquantel taken orally.

Fish Tapeworm Infection

Fish tapeworm infection (diphyllobothriasis) is an intestinal infection caused by the adult tapeworm Diphyllobothrium latum.

Fish tapeworm infection occurs in Europe (particularly Scandinavia), Japan, Africa, South America, Canada, and the United States (especially Alaska and the Great Lakes region). Infection often is caused by eating raw or undercooked freshwater fish.

The adult worm has several thousand egg-containing sections (proglottids) and is 15 to 30 feet long. Eggs are released from proglottids inside the intestine and are expelled in the stool. The egg hatches in freshwater and releases the embryo, which is eaten by small crustaceans. Crustaceans in turn are eaten by fish. People are infected when they eat raw or undercooked infected freshwater fish.

Symptoms and Diagnosis

The infection usually produces no symptoms, although some people may experience mild intestinal upset. In rare cases, the tapeworm causes anemia by depriving the person of vitamin B12. Eggs are found in the stool.

Prevention and Treatment

Thoroughly cooking freshwater fish or freezing them at 14° F. for 48 hours prevents infection. The infection is treated with niclosamide or praziquantel taken orally.

(see box Other Worm Infections in this chapter)

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