Infectious Disease and Microbiology – Roundworms, Intestinal

Intestinal roundworms are clinically important nematodes that infect humans either after ingestion of eggs from contaminated food, water, or soil, or after infective larvae penetrate the skin. Important intestinal roundworm infections include trichuriasis, enterobiasis, ascariasis, hookworm infection, and strongyloidiasis.

Trichuriasis is caused by Trichuris trichiura, or whipworm, and occurs worldwide, particularly in communities with poor sanitation. Hundreds of millions of people are infected globally. Humans are the principal host, and infection is acquired by ingesting mature eggs from contaminated soil.

After ingestion, whipworm eggs hatch in the intestine, and the larvae migrate to the cecum. Adult worms reside mainly in the cecum and ascending colon and may survive for approximately one year. Diagnosis is made by stool microscopy showing characteristic thick-shelled ova with polar plugs.

Trichuriasis is commonly treated with albendazole 400 mg orally daily for 3 days in mild to moderate infection, with longer courses considered for heavier infection. Mebendazole is another effective option. Heavy worm burdens can cause chronic gastrointestinal symptoms and painful rectal prolapse.

Enterobiasis is caused by Enterobius vermicularis, commonly called pinworm. It occurs worldwide and is especially common among children, households, schools, and other crowded settings. Infection can spread rapidly among family members regardless of socioeconomic status.

Adult pinworms live in the terminal ileum and cecum. At night, female worms migrate to the perianal region to deposit eggs, producing intense itching. Scratching contaminates the hands and fingernails, allowing autoinfection and person-to-person transmission through contaminated clothing, bedding, and surfaces.

Diagnosis of enterobiasis is usually made using the cellophane or Scotch-tape test, which collects the characteristic elongated oval eggs from the perianal region. Stool microscopy is generally less useful because the eggs are deposited outside the intestinal lumen.

Treatment includes a single oral dose of albendazole 400 mg or mebendazole 100 mg, with the dose repeated after 2 weeks to reduce the risk of reinfection. Pyrantel pamoate is an alternative. Household members should generally be treated at the same time, and bedding and clothing should be thoroughly washed.

Complications of enterobiasis include perianal excoriations, secondary bacterial infection, and rarely migration of worms into the female genital tract or abdominal cavity.

Ascariasis is caused by Ascaris lumbricoides and is one of the most common human helminthic infections worldwide. It is especially prevalent in areas with poor sanitation and is commonly seen in children.

Ascaris is the largest human intestinal nematode, with adult worms reaching approximately 35 cm in length. Eggs are passed in feces and become infectious after maturing in soil. They can remain viable in the environment for years.

After infective eggs are swallowed, larvae hatch in the intestine, penetrate the intestinal wall, enter the bloodstream, and migrate to the lungs. They cross into the alveoli, ascend the respiratory tract, are swallowed, and then mature into adult worms in the small intestine. Adult worms may survive for 1–2 years.

Diagnosis is usually established by stool microscopy demonstrating characteristic thick-shelled eggs. Peripheral eosinophilia may develop during the pulmonary migration phase.

Ascariasis is usually treated with albendazole 400 mg orally as a single dose or mebendazole 100 mg twice daily for 3 days. Heavy infection can cause intestinal obstruction, impaired nutrition, and malnutrition, particularly in children.

Hookworm infection is caused predominantly by Necator americanus and Ancylostoma duodenale. It is common in tropical and subtropical regions and affects a substantial proportion of the world’s population.

Infective hookworm larvae in contaminated soil penetrate exposed skin, commonly through bare feet. The larvae enter the bloodstream, migrate to the lungs, penetrate the alveoli, ascend the respiratory tract, and are swallowed. They then attach to the mucosa of the small intestine and feed on blood.

Adult hookworms are approximately 1 cm long and may survive for many years. Chronic intestinal blood loss can result in significant iron deficiency, especially in individuals with poor nutritional reserves.

Diagnosis is made by stool microscopy showing characteristic thin-shelled, colorless eggs. Eosinophilia may occur during larval migration, while chronic disease may produce iron-deficiency anemia.

Treatment generally consists of albendazole 400 mg orally as a single dose or mebendazole for 3 days. Pyrantel pamoate is an alternative. Iron replacement may also be required in patients with significant anemia.

Severe hookworm infection may cause profound iron-deficiency anemia, malabsorption, poor growth, and failure to thrive.

Strongyloidiasis is caused by Strongyloides stercoralis and occurs mainly in tropical and subtropical regions. It is more common in rural communities, institutional settings, and populations with lower socioeconomic conditions.

Infective filariform larvae penetrate the skin following contact with contaminated soil. They enter the circulation, migrate through the lungs, penetrate the alveoli, ascend the respiratory tract, and are swallowed. Adult female worms then live in the upper small intestine.

A distinctive feature of Strongyloides infection is autoinfection. Rhabditiform larvae produced in the intestine can transform into infective filariform larvae before leaving the body. These larvae penetrate the intestinal wall or perianal skin and restart the migration cycle, allowing infection to persist for decades.

Diagnosis is based on demonstration of rhabditiform larvae in stool. Because parasite excretion may be intermittent, repeated stool examinations may be required. Eosinophilia is common in uncomplicated acute and chronic infection. Serologic testing can be useful when stool examinations are negative but clinical suspicion remains high.

Ivermectin is the preferred treatment for strongyloidiasis, usually given at 200 micrograms/kg orally daily for 2 days. A repeat course may be required. Albendazole is an alternative, although generally less effective. Hyperinfection or disseminated strongyloidiasis requires prolonged ivermectin treatment until parasitologic clearance is achieved.

Strongyloides hyperinfection is particularly dangerous in immunocompromised patients, especially those receiving corticosteroids. Large numbers of migrating larvae can cause severe gastrointestinal and pulmonary disease, intestinal microperforation, bacteremia, sepsis, and death.

General prevention of intestinal roundworm infections depends on proper sanitation, safe disposal of human waste, thorough handwashing after defecation and before food preparation, and careful washing, peeling, or cooking of fruits and vegetables. Wearing protective footwear is particularly important for preventing hookworm infection and strongyloidiasis when contact with contaminated soil is possible.

The differential diagnosis includes other intestinal helminthic infections and should be guided by exposure history, clinical manifestations, stool findings, eosinophilia, and geographic risk factors.

Follow-up stool examinations may be useful after treatment to confirm eradication and detect reinfection, particularly in persistent or high-risk infections. Enterobiasis requires attention to household transmission, while strongyloidiasis requires especially careful follow-up in immunocompromised patients.

The overall prognosis is excellent when intestinal roundworm infections are recognized and appropriately treated. However, untreated heavy infection can produce important complications, including rectal prolapse from trichuriasis, intestinal obstruction from ascariasis, severe anemia from hookworm infection, and potentially fatal hyperinfection syndrome from strongyloidiasis.