What are worm infections?

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Parasitic worms are pathogenic organisms that attach to the internal structures of their hosts, including humans. Worm infections can range from mild discomfort to severe illness and death. Worms usually enter the body in the form of eggs or cysts; they then mature within the tissues they infect, including the intestines, liver, muscles, lungs, and brain, and in the bloodstream.
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Definition

Parasitic worms are pathogenic organisms that attach to the internal structures of their hosts, including humans. Worm infections can range from mild discomfort to severe illness and death. Worms usually enter the body in the form of eggs or cysts; they then mature within the tissues they infect, including the intestines, liver, muscles, lungs, and brain, and in the bloodstream.

Worm infections are rare in the United States and Europe but are common in developing nations in Asia, Africa, Central America, and South America. Parasitic worms are classified by their shape as either roundworms (nematodes and nemathelminths) or flatworms (flukes and platyhelminths).

Causes

A worm has three life stages: egg, larva, and adult. Some worms form cysts within a host’s body that can develop into larvae when circumstances are favorable. Eggs or larvae of parasitic worms can enter the body (or host) through the mouth or through skin surfaces. Some are transmitted through an insect bite. This intermediate insect host is known as a vector. Examples of worm infections include tapeworm, hookworm, liver fluke, trichinosis, filariasis, and pinworm.

Tapeworm. Tapeworm infections occur from eating the raw or undercooked meat of animals infected with the larvae of these flatworms. Tapeworm infections mainly occur in Southeast Asia, Africa, the Middle East, Mexico, South America, Russia, and areas of the former Yugoslavia. In the United States, some forms may be acquired by swallowing the infected fleas of infected dogs or cats.

Taenia saginata infects beef and T. solium infects pigs. The larvae attach to the intestinal wall and mature into adult tapeworms, which can grow to be more than twelve feet long and live many years. Segments of the worm, which contain eggs, are released and then pass from the body in feces. Animals eat the feces, the grass, or other contaminated foods, thus perpetuating the cycle.

Hookworm. Hookworms are roundworms existing as two types: Ancylostoma duodenale and Necator americanus . A. duodenale is found in North Africa, the Middle East, and India; in the past, the worm was present in southern Europe. N. americanus is present in the Americas, Southeast Asia, China, Indonesia, and sub-Saharan Africa.

Hookworm larvae live in moist soil. The larvae are able to penetrate the soles of the feet or other skin areas that contact soil. The larvae then enter small veins and the lymphatic system, work their way to the lungs, and enter the alveoli (sacs) in the lung. They are then coughed up into the infected person’s mouth and passed down into the intestines, where they mature into adults. The adult worms release eggs, which pass out of the person’s body in the feces. If the feces are deposited in the soil, the cycle repeats.

Liver fluke. Liver flukes (Fasciola hepatica ) are flatworms that infect the liver of humans and other mammals. They are present in temperate areas where sheep are raised because these flukes use sheep and humans as a host. Most human infections occur in Southeast Asia, northern Africa, South America, and Cuba. Occasional infections occur in the United Kingdom, southern Europe, and Australia. The flukes require an intermediate host such as Galba truncatula , an aquatic snail, to complete its life cycle. Larvae enter the snail and develop into what is known as metacercariae; these organisms leave the snail and attach to plants such as water chestnuts and watercress. When eaten, further development of the parasite occurs; these organisms then burrow through the intestinal wall and into the liver; they then enter the bile ducts and mature into the adult form. The adults lay eggs, which pass through the bile duct into the intestines; they are then excreted, completing the life cycle.

Trichinosis. Trichinosis is caused by the roundworm Trichinella spiralis . Infections were once common in the United States; however, they are now quite rare. Infections are more common in developing nations and in eastern Europe. Pigs become infected with trichinosis when they eat infected rodents or meat from other pigs. Humans become infected when they eat infected pork. The infection is passed in the form of cysts within skeletal muscles. When infected muscle is eaten, the cysts develop into adults and mate in the small intestine. Their eggs develop into larvae, which pierce the small intestine and enter the bloodstream and migrate to other structures such as the heart or eye, or to the lymphatic system. The skeletal musculature is the only place where they can survive and form cysts.

Filariasis. Lymphatic filariasis, also known as elephantiasis, is caused by the roundworms Brugia malayi , B. timori, and Wuchereria bancrofti . About one-third of infected persons live in Africa, about one-third in India, and the rest in other parts of Asia, the Americas,and the Pacific Islands. The disease is transmitted from human to human by some species of mosquitoes. Mosquito species that can transmit the disease are Culex quinquefasciatus and some Anopheles species; Brugia roundworms are primarily transmitted by Mansonia mosquitoes. Another type of filariasis is a tropical skin and eye disease, Loa loa filariasis, which is also known as African eye worm. It is contracted through the bite of a deer fly or mango fly.

In filarial infections, the parasites enter the bloodstream as microfilariae and develop into adults. The adults mate and produce more microfilariae. If a person is bitten by a host insect, the microfilariae infect that insect and, thus, complete the life cycle. The microfilariae travel from the bloodstream into the lymphatic system, which is a network of vessels that maintain a delicate fluid balance between body tissue and the bloodstream. They lodge in the lymphatic system, where they mature into adult worms. These worms live for four to six years and produce millions of immature microfilariae that circulate in the blood. The adult worms block the normal flow of lymphatic fluid, damaging the lymphatic system. Adult Loa loa worms often travel to the conjunctiva (just below the surface of the eyeball); the worms transit the eyeball in about fifteen minutes, causing much pain; however, the transit does not usually affect vision.

Pinworm. The pinworm (Enterobius species) is a roundworm that is a common intestinal parasite worldwide. It is most common in children. Persons are infected by eating food contaminated with the eggs. The eggs hatch in the duodenum (the first portion of the small intestine). The eggs develop into larvae and migrate toward the colon (the large intestine). The larvae become adults in the ileum (the lower part of the small intestine) and mate. After mating, the males die and pass from the body in the stool. The pregnant females attach themselves to the ileum and the first portion of the large intestine, where they feed on the intestinal contents. The female body becomes filled with eleven thousand to sixteen thousand eggs. The females then migrate to the infected person’s anus, where they release the eggs and die.

Risk Factors

The major risk factor for many parasitic worm infections is living in an area where the worms are endemic (always present). Infections are more common in slums and poor communities. Poor personal hygiene and poor sanitation increase the risk of infection. Contact with fecal material increases the risk of infection with intestinal worms. In developing nations, humans defecate on the soil or use fecal material (night soil) as fertilizer. Going barefoot in contaminated areas can result in a hookworm infection. Eating raw or incompletely cooked pork increases the risk of trichinosis. Pinworms are more common among people in close contact (such as in a classroom or in crowded living conditions). Immigrants from areas where parasitic infections are endemic can expose the persons of the new country or region to these infections.

Symptoms

The symptoms of worm infection vary, depending on the type of worm involved.

Tapeworms. Often, tapeworm infections do not have any specific symptoms. Weight loss and anemia can occur. Sometimes, a person will notice tapeworm segments in the stool. Rarely, tapeworms can cause intestinal obstruction, which produces nausea, vomiting, and severe abdominal pain. On occasion, cysts can form in the brain and the meninges (the tissues surrounding the brain). If this occurs, the patient may experience neurologic symptoms such as headaches, seizures, and confusion. On rare occasions, cysts can form in the eyes, which can cause blindness, or in the spinal cord, which can result in muscle weakness or paralysis.

Hookworms. General symptoms, such as itching on the soles of the feet, can occur soon after infection. Cough and pneumonitis (lung inflammation) can occur when the worms break into the alveolae in the lungs. When the larvae enter the small intestine, diarrhea and gastrointestinal discomfort can occur. Heavy infestations result in iron deficiency anemia from intestinal blood loss and malnutrition. Long-term blood loss can result in facial edema (swelling). Children with chronic hookworm infections can suffer from growth retardation and intellectual impairment.

Liver flukes. Infected persons suffer from abdominal pain, nausea, vomiting headaches, and anemia. Some develop jaundice (yellowing of the skin), hives, and muscle pains. Over time, blockage of the bile ducts can occur.

Trichinosis. Within a week after becoming infected, persons experience gastrointestinal symptoms such as nausea, heartburn, and diarrhea. After the larvae migrate from the intestines to the muscles, symptoms include fatigue, muscle pain, fever, and edema (swelling). A characteristic sign of a trichinosis infection is periorbital edema (swelling around the eyes). Splinter hemorrhages in the nails may occur. These appear as narrow, red to reddish-brown lines of blood beneath the nails. Occasionally, the worms invade the central nervous system (the brain and the spinal cord), where they can produce serious neurological conditions such as ataxia (a lack of muscle coordination), respiratory paralysis, and death.

Filariasis. In lymphatic filariasis, the adult worms block the normal flow of lymphatic fluid, damaging the lymphatic system. This blockage produces tremendous enlargement of the arms, legs, or genitals, which may swell up to several times the normal size. The worms also lodge in the kidneys, causing damage. Loa loa filariasis can also produce swelling of the extremities. Transit of the worm across the eyeball is a unique sign of this disease. Lymphedema (swelling of the limbs) can occur if the worms block lymphatic channels in the arms and legs. Intermittent swelling, known as Calabar swellings, of the arms can occur because of an allergic reaction. Calabar swellings may be accompanied by urticaria (rash) and pruritus (itching).

Pinworms. Itching around the anus is a common symptom. The skin around the anus may also be inflamed. Vaginal itching can occur in young girls if the worms enter the vagina rather than the anus.

Screening and Diagnosis

Many worm infections produce an allergic response. Often, these infections will cause an increase in the eosinophils in the bloodstream, causing a condition known as eosinophilia. A complete blood count (CBC), including a differential count, reveals the number of eosinophils present in the bloodstream. Eosinophils are a type of white blood cell, which increases with an allergic response. A blood test, which can be done in most medical laboratories, may reveal specific antibodies to the type of worm present. A more complex but highly accurate test can be done in a genetics laboratory that can conduct deoxyribonucleic acid (DNA) sequencing for parasite-specific DNA in a blood sample.

In cases of suspected tapeworm infection, a stool sample will reveal tapeworm eggs or body segments containing eggs. For hookworms, except for early infections, a stool sample will contain hookworm eggs. For liver flukes, a stool sample is often positive for eggs. Adult worms can sometimes be present in a sputum sample or in vomit.

Trichinosis can be diagnosed after learning the infected person has eaten contaminated meat. If a sample of the meat is available, microscopic examination will reveal cysts. The characteristic signs of periorbital edema and splinter hemorrhages in the nails aid the diagnosis. As in many other worm infections, eosinophilia is present.

For cases of filariasis, examination of a blood sample may reveal the presence of microfilariae. Their presence in the bloodstream is periodic; thus, the sample must be drawn when the microfilaria are likely to be present. Visualization of a worm transiting the eyeball is diagnostic for Loa loa filariasis. A simple card test detects antigens to lymphatic filariasis; however, antibody tests are not particularly helpful with Loa loa filariasis because cross-reactivity between Loa loa and other worm infections often occurs.

For pinworm infections, the worms are seen in the anus, particularly at night when they lay their eggs. Placing a piece of tape against the anus will collect eggs for microscopic examination.

Treatment and Therapy

A number of anthelminthic (antiworm) medications are available to treat worm infestations. Inasmuch as many worm infections can produce anemia, iron supplements are helpful.

Tapeworms can be treated with a single-dose oral medication. Niclosamide is the drug of choice; however, praziquantel and albendazole are also effective.

While still in the skin, hookworm infections can be treated by cryotherapy (localized freezing). During migration to the intestines and while in the intestines, albendazole (Albenza) or mebendazole are effective. Triclabendazole is the drug of choice to treat liver flukes. Resistant strains, however, have been reported in Ireland and Australia.

If given early, albendazole or mebendazole can eradicate the intestinal worms and larvae in trichinosis. These medications are less effective after cysts form; however, they are beneficial if the larvae enter the central nervous system, heart, or lungs. Analgesics (pain relievers) are given for muscular pain. Over time, the cysts often calcify; this destroys the larvae, and the muscle pain and fatigue resolve. Corticosteroids are given to reduce allergic reactions and inflammation when dead or dying larvae release chemicals within the muscles.

The drug of choice for filariasis (both lymphatic and Loa loa) is diethylcarbamazine (DEC); ivermectin is also effective. Doxycycline is under investigation as a supplementary agent to use with DEC. DEC is most effective against the microfilariae; it is less effective against the adult worms. Sometimes, after receiving a course of medication, the surviving worms are surgically excised. For pinworms, a single dose of either albendazole or mebendazole is effective. These medications are available by prescription and over the counter.

Prevention and Outcomes

Tapeworm infections can be prevented by avoiding raw meat and by cooking meat to a core temperature greater than 140° Fahrenheit for five minutes. Freezing meat to −4° F for twenty-four hours will also kill the eggs. Self-reinfection can be prevented by good hygiene and thorough handwashing after using the toilet.

Hookworm infections can be avoided by not walking barefoot in any area suspected of having infected soil, by defecating only into a toilet connected to a sewage system, and by avoiding the use of human feces for fertilization.

Liver fluke infections can be prevented by the avoidance of eating raw vegetables from any region inhabited by aquatic snails, such as G. truncatula. Filariasis infections can be prevented by avoiding the fly bites that spread Loa loa. Spraying homes with the pesticide dieldrin is an effective method of destroying the insect vectors.

Pinworm infections can be prevented by thorough handwashing before meals and after using the toilet, cleaning toilet seats daily, washing bed linens twice a week, keeping fingernails short and clean, and by not scratching infected areas around the anus.

Bibliography

Bogitsh, Burton J., Clint E. Carter, and Thomas N. Oeltmann. Human Parasitology. 4th ed. Boston: Academic, 2012. Print.

Bruschi, Fabrizio, ed. Helminth Infections and Their Impact on Global Public Health. New York: Springer, 2014. Print.

Gittleman, Ann Louise. Guess What Came to Dinner? Parasites and Your Health. Rev. ed. New York: Putnam, 2001. Print.

Kennedy, Malcolm W., and William Harnett, eds. Parasitic Nematodes: Molecular Biology, Biochemistry and Immunology. Boston: CABI, 2013. Print.

Leventhal, Ruth, and Russell F. Cheadle. Medical Parasitology: A Self-Instructional Text. 6th ed. Philadelphia: Davis, 2012. Print.

Matthews, Bernard E. An Introduction to Parasitology. New York: Cambridge UP, 1998. Print.

Morand, Serge, Boris R. Krasnov, and D. Timothy J. Littlewood. Parasite Diversity and Diversification: Evolutionary Ecology Meets Phylogenetics. New York: Cambridge UP, 2015. Print.

Muller, Ralph. Worms and Human Disease. 2nd ed. New York: CABI, 2002. Print.

Nagami, Pamela. The Woman with a Worm in Her Head, and Other True Stories of Infectious Disease. New York: St. Martin’s, 2002. Print.

Roberts, Larry S., and John Janovy, Jr. Gerald D. Schmidt and Larry S. Roberts’ Foundations of Parasitology. 8th ed. Boston: McGraw-Hill, 2009. Print.

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