Ascariasis

Etiology and pathogenesisTop

1. Etiologic agent: Ascaris lumbricoides (roundworm) is a nematode commonly classified as a soil-transmitted helminth (STH). The worm is found in the small intestines in humans. Adult worms are 15 to 35 cm long. Infection occurs through ingestion of eggs that have matured to the infective stage. The life cycle requires a period of development in moist soil. At an optimum temperature of 25 degrees Celsius, the maturation takes ~2 to 3 weeks. Once ingested, the eggs hatch and larvae migrate through the human body before adult worms mature in the small intestine. Eggs hatch in the stomach and in the small intestine, penetrate the gastrointestinal (GI) walls, and migrate through the portal circulation to the right atrium and ventricle, followed by the pulmonary circulation to be deposited in the lungs. After ~10 days in the lungs, the larvae break into the alveolar lumen and ascend the bronchial tree to the pharynx, where they are coughed up and swallowed. Swallowed larvae return to the small intestine to undergo final maturation. Male and female worms mate in the intestines and fertilized eggs are released with stool. Female worms may also produce unfertilized eggs. Mature forms may survive for 1 to 2 years.

2. Reservoir and transmission: Humans are the only host of A lumbricoides. Transmission is via the fecal-oral route. Infection occurs through ingestion of embryonated eggs in soil, typically through consumption of fruits and vegetables grown using human feces as a fertilizer or through dirty hands contaminated with soil containing the eggs.

3. Risk factors: Consumption of unwashed fruits and vegetables or geophagy (ingestion of soil).

4. Incubation and contagious period: The period from infection to the development of pulmonary manifestations is 4 to 16 days; parasites reach full maturity and eggs appear in stool after 2 to 3 months. The patient is not contagious for contacts. In soil the eggs typically remain viable for months but can remain viable for as long as 10 years and withstand freezing for as long as 40 days. They may be destroyed by prolonged exposure to direct sunlight and temperatures >50 degrees Celsius.

EPIDEMIOLOGYTop

Ascariasis occurs worldwide, with ~800 million people infected globally. The highest rates of infection are found in tropical and subtropical regions, particularly in Asia and sub-Saharan Africa. Locally acquired infections are rare in North America, most cases are imported from tropical and subtropical regions. School-aged children typically harbor the highest burden of infection within communities. The infection is typically more common in rural areas.

Clinical features and natural historyTop

The clinical course depends on the worm burden (the number of worms present in the host). With a small worm burden, the disease is usually asymptomatic. Occasionally, with high worm burdens, GI complications can occur, including intestinal obstruction and biliary ascariasis. The most common symptoms are nonspecific GI manifestations, such as abdominal discomfort and altered bowel movements.

1. Larval migration: Symptoms are more likely to occur with higher numbers of migrating larvae and are caused by the immune reaction to larval migration through tissues. Migration of larvae through the lungs can cause pneumonitis with symptoms including dyspnea, dry or productive cough, wheezing, and fever. Transient eosinophilia may be present. Plain chest radiography may reveal migrating opacities (corresponding to inflammatory infiltrates). The combination of transient infiltrates and eosinophilia is termed Löffler syndrome. In some patients urticaria may be observed. Charcot-Leyden crystals and rare larvae may be found in sputum and bronchoalveolar lavage fluids during the period of larval migration.

2. Intestinal ascariasis: Often asymptomatic or mildly symptomatic. Abdominal discomfort or pain, anorexia, nausea, diarrhea, and weight loss are most common in children and those with high worm burdens. High worm burdens in children have been associated with impaired physical and intellectual development as well as malnutrition. Rarely, if parasites penetrate the intestinal wall, peritonitis may develop. Worms may migrate from the small intestine, usually in response to alterations in the environment (eg, from anesthetics, fever, or other insults) resulting in hepatobiliary ascariasis or migration out of the anus, mouth, or nose.

3. Hepatobiliary ascariasis: Worms may migrate into the bile duct or pancreatic duct, causing cholangitis or acute pancreatitis. There are also reports of liver abscesses due to migration of adult worms into the liver, where they release eggs and die, invoking a granulomatous reaction.

DIAGNOSISTop

Diagnostic Tests

Standard ova and parasite examination of stool for microscopic detection of A lumbricoides eggs. For increased sensitivity ≥3 stool samples should be collected within ≤10 days (ideally every 48 h). Stool analysis does not detect immature worms or single male parasites (absence of eggs). In few cases ascariasis is diagnosed after the passage of an adult worm from the anus, mouth, or nose.

Differential Diagnosis

Symptoms consistent with Ascaris pneumonitis can be found in other migratory parasitic infections including toxocariasis, hookworm infection, strongyloidiasis, schistosomiasis, and tropical pulmonary eosinophilia.

The differential diagnosis for abdominal symptoms is quite broad and includes other GI helminths (eg, Trichuris, hookworms, tapeworms) and protozoa, as well as other causes of dyspepsia, abdominal pain, or biliary tract diseases.

Treatment

Treat all patients with parasitic infection, including asymptomatic individuals, given the worms’ ability to migrate.

Antiparasitic Treatment

1. First-line treatment: Oral mebendazole 100 mg bid for 3 days or 500 mg in a single dose; or a single dose of oral albendazole 400 mg.

2. Alternative treatment: A single dose of oral pyrantel pamoate 11 mg/kg (max 1 g; recommended in pregnant or breastfeeding patients) or a single dose of oral ivermectin 150 to 200 microg/kg.

Surgical treatment may be indicated in patients with complications (see above).

Pulmonary ascariasis: If antiparasitic agents are given in this phase, retreatment should be given 2 weeks later, as the drugs' efficacy against larvae in the lungs is not confirmed. A follow-up stool examination should be performed after several months and appropriate therapy should be introduced if intestinal ascariasis is confirmed.

prevention

1. Washing hands before meals and after contact with soil.

2. Preventing geophagy in children.

3. Thorough washing of fruits and vegetables.

4. At the community level prevention involves access to sanitation facilities, access to clean drinking water, and avoiding the use of human feces to fertilize fruit or vegetable crops.