There is significant diversity among the worms that are associated with human disease; ranging from
barely visible roundworms (0.3 mm long) to huge tapeworms (25 meters long), and from oblong- and crescent-shaped to whip-like or elongate flat ribbons. Helminths can be grouped into three categories - nematodes (roundworms), trematodes (flukes), and cestodes (tapeworms).
The life cycle of helminths includes
fertilized egg (embryo), larval, and adult stages. In the majority of helminth species, the adult stage obtains nutrition and reproduces sexually in a host's body.
Differentiation of the sexes differs among the helminth groups. In nematodes (roundworms), the sexes are
separate and usually different in appearance.
Differentiation of the sexes differs among the helminth groups. In trematodes (flukes), the sexes
can be separate or hermaphroditic (male and female organs on the same worm).
Differentiation of the sexes differs among the helminth groups. Cestodes (tapeworms) are generally
To ensure survival of the species, parasites must transmit an infective form (usually an egg or larva) to another host which may
infect another organism of the same species or a different species. By definition, the host in which larval development occurs is called the intermediate (secondary) host, and the host in which adulthood and mating occur is called the definitive (final) host.
Nematodes have a smooth, protective
outer covering (cuticle) that is shed as the worm grows. The body is essentially headless and tapered to a fine point anteriorly. The sexes are separate and distinct. The male is often smaller than the female, and has a hooked posterior end with spicules used for coupling with the female during mating.
The human parasites can be divided into
intestinal nematodes which develop to some degree in the intestine, and tissue nematodes which spend their larval and adult phases in soft tissues other than the intestine.
Ascaris lumbricoides is a
large intestinal roundworm that probably accounts for the greatest number of worm infections. Most reported cases in the United States are in the southeastern states.
A. lumbricoides spends its larval and adult stages in humans and releases embryonic eggs in
feces, which may be spread to other humans through food, drink, or soiled objects that are placed in the mouth
Ascaris lumbricoides life cycle is
Ingested eggs hatch in the human intestine, then penetrate the intestinal wall and enter the lymphatic and venous drainage. The parasites move through the heart and eventually arrive in the capillaries of the lungs. The larvae migrate up the bronchial tree to the glottis where they are swallowed and returned to the small intestine. They reach adulthood and reproduce (up to 200,000 fertilized eggs per day). The fertilized eggs are shed in feces and may infect other hosts.
Male and female worms are not attached to the intestine, and may migrate to
other areas. They may invade the biliary channels of the liver and gallbladder, and occasionally emerge from the nose and mouth. Inflammatory and allergic reactions often occur along these migratory routes causing symptoms such as bronchospasms, asthma, and skin rashes. Heavy worm infestations can retard the physical and mental development of children.
Trichuris trichiura is often called a whipworm due to its likeness to a
small buggy whip. This organism has a transmission cycle similar to Ascaris with humans as the sole host. Trichuriasis has its highest incidence in tropical and subtropical regions with poor sanitation. Worms burrowing into the intestinal mucosa can cause local hemorrhage and a site for secondary bacterial infecion.
Enterobius vermicularis is often called a
pinworm. It is the most common worm disease of children in temperate regions. Freshly deposited eggs have a sticky coating that causes them to stick to fomites and to lodge beneath fingernails. After drying, the eggs become airborne and settle in household dust.
pinworm Organisms may be ingested from
contaminated food or drink, or the result of self-inoculation from one's own fingers. Eggs hatch in the small intestine releasing larvae that migrate to the large intestine, mature, and mate. The primary symptom is perianal itching due to the mature female emerging from the anus to lay eggs in the perianal region. Pinworm infestation is not fatal and is usually asymptomatic, but affected children can experience disrupted sleep, nausea, abdominal discomfort, and diarrhea
Hookworm infestations are caused by
two different organisms depending on the geographic region (old world vs. new world). The "hook" refers to the adult's oral cutting plates which help the worm anchor to the intestinal mucosa, and the curved anterior end of the body. The life cycles, transmission, and pathology of both organisms is similar. Hookworm larvae hatch outside the body and infect by penetrating the skin.
Hookworm larvae are deposited into the soil by defecation. They hatch into very fine
filariform larvae that climb onto grass and vegetation to improve the probability of contacting a host. Inoculation is usually by penetration of bare skin via sites such as hair follicles, abrasions, or soft tissues between the toes.
After penetration, the larvae burrow
into the skin, then reach the lymphatic or blood circulation. They migrate to the heart and lungs, then proceed up the bronchial tree to the throat. Most larvae are swallowed and reach the small intestine and anchor, feed on tissues, and mature. Eggs are shed in the host feces.
Symptoms can include a localized dermatitis at the site of entry;
pneumonia and eosinophilia associated with migration of the organism to the heart and lungs; but more severe symptoms are associated with the intestinal phase. Heavy worm infestations may cause nausea, vomiting, cramps, and bloody diarrhea. Blood loss can be significant resulting in iron-deficient anemia. Infants are particularly susceptible to hemorrhagic shock. Symptoms worsen with chronic or repeated infections.
Strongyloides stercoralis is the causative agent of
threadworm infection. The life cycle and disease pathology of this nematode are similar to hookworms, but this organism can complete its life cycle within the human host or outside in moist soil.
S. stercoralis eggs can either be shed in
feces or may hatch into larvae in the colon and remain in the human host system to complete the entire life cycle.
Alternative life styles of S. stercoralis increase the chance of
transmission and likelihood of chronic infections. Symptoms can be similar to hookworm infection. Nematode activity in the intestine may cause bloody diarrhea, liver enlargement, bowel obstruction, and malabsorption. Immunocompromised individuals such as AIDS patients, transplant patients on immunosuppressant drugs, and cancer patients on radiation therapy are at risk for severe, disseminated infestation involving many organs.
Trichinella spiralis is the etiologic agent of
trichinosis. The organism's life cycle is spent entirely within the body of a mammalian host which is usually a carnivore or omnivore such as a pig, bear, cat, dog, or rat.
In nature, the organism exists in an encapsulated larval form in the muscle tissue of the animal host, and is transmitted to
predators that prey on the infected host.
Humans acquire trichinosis by eating the
meat of infected animals (usually wild or domestic pigs or bears). The cysts are digested in the stomach releasing the larvae. The larvae burrow into the intestinal mucosa, mature, and mate. The males die and are expelled in the host feces. The females incubate the eggs and shed live larvae that penetrate the intestinal wall and enter the lymphatic channels and bloodstream. All tissues are susceptible to invasion, but final development occurs in skeletal muscle tissue where the coiled larvae are encysted. At maturity the cyst is about 1 mm long and can be observed by careful inspection of meat.
Initial symptoms of trichinosis include
nausea, diarrhea, abdominal pains, fever, and sweating. The mass migration of larvae and their entrance into muscle tissue brings about secondary symptoms including puffiness around the eyes, intense muscle and joint pain, shortness of breath, and pronounced eosinophilia. The most life-threatening manifestations are heart and brain involvement.
Symptoms eventually resolve, but a cure is
not available once the larvae have encysted in muscle tissue. The most effective preventive measures for trichinosis are to adequately cook, freeze, or smoke all pork and wild meats.
Tissue nematodes complete their life cycles in
human blood, lymphatic vessels, or skin. The most important pathogens of this category are filarial worms that are characterized by elongate, filamentous bodies; and transmission by biting arthropods. These organisms are associated with slow, chronic, deforming diseases
Filarial nematodes produce
tiny microfilarial larvae that circulate in the bloodstream. Their life cycle alternates between humans and blood-sucking mosquitoes or flies.
Wuchereria bancrofti is the causative agent of
Bancroftian filariasis (also called elephantiasis). The disease is common in tropical regions where the vectors live (female Culex, Anopheles, and Aedes mosquitoes).
Infection begins when a (wuchereia bancrofti)
mosquito bite inoculates larvae into the skin. The larvae penetrate the lymphatic vessels, mature, and female worms release huge numbers of microfilariae directly into circulation. The most conspicuous sign of this disease is the chronic swelling of the scrotum and legs which is called elephantiasis. This edema is due to inflammation and blockage of major lymphatic vessels which prevents return of lymph to circulation causing fluid to accumulate in the extremities. This condition occurs in individuals in endemic regions only after one to five decades of chronic infection. Acute infection in young, healthy individuals is usually subclinical.
Loa loa (also called the African eye worm) is transmitted by the bites of
dipteran flies. Its life cycle is confined to subcutaneous tissues. This nematode is temperature sensitive and is often drawn to the body surface in response to cooler temperature. It can be felt slithering beneath the skin or observed beneath the conjunctiva. One method of treatment is to pull the worm from a small hole in the conjunctiva after application of local anesthesia.
Schistosomiasis is caused by
blood flukes of several species of the genus Schistosoma. This disease is a very common parasitic disease affecting 200 million people at any one time. It is endemic to 74 countries in Africa, South America, the Middle East, and the Far East. The life cycle begins when infected humans release eggs into irrigated fields either by fertilization with fecal material, or by defecation or urination into the water.
Eggs hatch in the water yielding
ciliated larvae that instinctively migrate to snails and burrow into them. The larvae multiply and transform into larger, forktailed swimming larvae which are released into the water by infected snails.
Schistosome larvae that contact humans wading or bathing in the water attach themselves by
ventral suckers and penetrate hair follicles. They penetrate into small blood and lymphatic vessels and migrate to the liver. Schistosomes reach sexual maturity in the liver. Males and females remain entertwined permanently to facilitate mating. The pair eventually migrates to and lodges in small blood vessels at various sites including the mesenteric venules of the small intestine and venous network of the bladder. The actual site depends on the schistosome species involved
While attached at these intravascular sites, the parasites feed on
blood, and the females lay eggs that are eventually shed in feces or urine.
Initial symptoms of infestation include itching at the site of entry followed by
fever, chills, diarrhea, and cough. The most severe consequences of chronic infection are hepatomegaly and liver disease, splenomegaly, bladder obstruction, and hematuria. Eggs may occasionally invade the heart and central nervous system causing a severe granulomatous response.
The liver fluke Fasciola hepatica is a common parasite in
sheep, cattle, goats, and other mammals; and is occasionally transmitted to humans. The life cycle of this trematode is fairly complex involving the mammal as the definitive host, freshwater snails as an intermediate host, and encystment of larvae on a water plant. Mammals may ingest these cysts when eating the plants. These cysts release young flukes that migrate to the liver and lodge in the gallbladder, and mature. Humans usually develop symptoms such as vomiting, diarrhea, hepatomegaly, and bile obstruction only if they are chronically infected by a large number of flukes.
The small head of the worm (called the scolex) has suckers or hooklets for clinging to the intestinal wall, but no mouth because
nutrients are absorbed directly through the body of the worm. The head is attached to a long series of individual reproductive segments called proglottids. Proglottids are composed primarily of male and female reproductive organs and a primitive neuromuscular system that permits feeble movements.
The primary human parasitic cestode species are
Taenia saginata (beef tapeworm), Taenia solium (pork tapeworm), and Diphyllobothrium latum (fish tapeworm).
Taenia saginata is one of the
largest tapeworms (up to 2000 proglottids). Human infection begins with ingestion of contaminated raw or undercooked beef. The young tapeworms encysted in the beef muscle tissue that are the infective stage are called cysticerci (singular: cysticercus). When ingested, the cysticercus attaches to the intestinal wall by its scolex and matures into an adult tapeworm. Humans generally are not infected by ingestion of T. saginata eggs. For such a large parasite, tapeworms cause relatively few symptoms. Some patients discover proglottids in their feces, and some complain of vague abdominal pain and nausea.
Taenia solium is smaller than T. saginata. T. Solium is infective to humans in both the
cysticercus and egg stages. The cycle involving ingestion of cysticerci is nearly identical to that of the beef tapeworm. A different form of the disease (called cysticercosis) occurs when humans ingest pork tapeworm eggs rather than larvae. The eggs can hatch in the intestine, releasing tapeworm larvae that migrate to all tissues. The larvae settle in tissues and form peculiar cysticerci called bladder worms. These forms are enclosed in a small capsule resembling a little bladder. These cysticerci do the most harm when they lodge in the heart muscle, eye, or brain, especially if a large number of the bladder worms occupy the tissue space. Common symptoms may include seizures, psychiatric disturbances, and other serious neurological impairments.
The fish tapeworm Diphyllobothrum latum is common in the
Great Lakes, Alaska, and Canada. Humans do serve as a definitive host for this organism, so it can develop in the intestine and cause long-term symptoms. Questions regarding the safety of sushi consumption frequently arise. A good safeguard is to always choose a reputable establishment, because competent sushi chefs carefully examine the fish for readily visible larvae. Salmon and halibut are more often infected than tuna and octopus. Live parasites in fish can be eradicated by freezing for 5 days at -20 oC. (better still - cook the fish thoroughly)