Ascariasis is the most common helminthic infection, with an estimated worldwide prevalence of 25% (0.8-1.22 billion people). Usually asymptomatic, ascariasis is most prevalent in children of tropical and developing countries, where they are perpetuated by contamination of soil by human feces or use of untreated feces as fertilizer. For more information on ascariasis in children, see the Medscape article Pediatric Ascariasis. Symptomatic ascariasis may manifest as growth retardation, pneumonitis, intestinal obstruction, or hepatobiliary and pancreatic injury. In developing countries, ascariasis may exist as a zoonotic infection in pigs, but little evidence has shown transmission of porcine ascariasis to humans.
The image below depicts a roundworm that infects humans through soil contaminated by human feces.
Adult Ascaris lumbricoides.
Ascaris lumbricoides is the largest of the common nematodes (roundworms) that infect humans. Adult A lumbricoides are white or yellow and 15-35 cm long (see first image below). They live 10-24 months in the jejunum and middle ileum of the intestine. Each day, female A lumbricoides produce 240,000 eggs (see second image below), which are fertilized by nearby male worms. A Chinese study showed that 45% of infected persons shed only fertilized eggs, 40% shed fertilized and unfertilized eggs, and 20% shed only unfertilized eggs. Unfertilized eggs accounted for only 6-9% of eggs shed. Fertilized eggs released into favorable soil may become infectious within 5-10 days. Eggs may remain viable in soil for up to 17 months. Infection occurs through soil contamination of hands or food, ingestion, and the subsequent hatching of eggs in the small intestine (see third image below).
Adult Ascaris lumbricoides.
Ascaris lumbricoides egg.
Life cycle of Ascaris lumbricoides.
Second-stage larvae pass through the intestinal wall and migrate through the portal system to the liver (4 d) and then the lungs (14 d). A significant exposure may produce subsequent pneumonia and eosinophilia. Symptoms of pneumonitis include wheezing, dyspnea, nonproductive cough, hemoptysis, and fever. Larvae are expectorated and swallowed, eventually reaching the jejunum, where they mature into adults in approximately 65 days.
Adult worms feed on digestion products of the host. Children with a marginal diet may be susceptible to protein, caloric, or vitamin A deficiency, resulting in retarded growth and increased susceptibility to infectious diseases such as malaria. Large and tangled worms may cause intestinal (usually ileal), common duct, pancreatic, or appendiceal obstruction. Mean worm burden varies from more than 16 to 4 and appears related to host factors, particularly age, geophagy, and immunity. Worms do not multiply in the host. For infection to persist beyond the 2-year maximum lifespan of the worms, re-exposure must occur.
Ascaris lumbricoides suum, a swine nematode, has been thought responsible for zoonotic infection. Distinguishing this worm from A lumbricoides is difficult. A suum appears to responsible for most ascariasis cases in well-developed countries with excellent sanitation (eg, Denmark, United States, UK ). In this setting, infected children have a low worm burden and may present with only acute eosinophilia or eosinophilic liver lesions visible on CT scans. However, a molecular genetic study from China casts doubt that infections in pigs are a significant reservoir for human infection.
In 1974, an estimated 4 million people, mainly in the southeast United States, had ascariasis. Recent estimates of ascariasis prevalence are unknown, but probably much lower. Immigrants from countries with a high prevalence of ascariasis comprise most recent cases.
The prevalence of ascariasis is highest in children aged 2-10 years, with the highest intensity of infection occurring in children aged 5-15 years who have simultaneous infections with other helminths such as Trichuris trichiura and hookworm. A recent Vietnamese study found that adult women living in rural areas, especially those exposed to human night soil and living in households without a latrine, were at surprisingly high risk for ascariasis. The Centers for Disease Control and Prevention (CDC) estimated that worldwide ascariasis rates in 2005 were as follows: 86 million cases in China, 204 million elsewhere in East Asia and the Pacific, 173 million in sub-Saharan Africa, 140 million in India, 97 million elsewhere in South Asia, 84 million in Latin America and the Caribbean, and 23 million in the Middle East and North Africa.
Because the lifespan of adult worms in the intestine is only one year, persistent infection requires frequent re-exposure and reinfection. The frequency and intensity of infection remain high throughout life in endemic areas and pose a risk to both elderly and young persons. In a recent study in rural southwest Nigeria, the intensity of excreted eggs per gram of feces among infected persons was 2,371 for Ascaris species, 1070 for hookworm, and 500 for Trichuris species, with only slightly lower rates among persons in urban areas. Estimates of disability-adjusted years of life due to ascariasis have fallen because of development and management programs during the 1990s, especially in Asia, but still constitute a significant burden in some countries.
See International and Mortality/Morbidity.
Endoscopic retrograde cholangiopancreatography (ERCP) has become a commonly used procedure for both diagnosis of ascariasis and removal of worms from the biliary tract. The ease of diagnosis and therapy in the same setting makes ERCP particularly valuable when used with real-time ultrasonography. The combined procedures yield a sensitivity of nearly 100%.
Because of the risk of complications, patients with ascariasis who have other concomitant helminthic infections should always undergo treatment for ascariasis first. Medical therapy is usually not indicated during active pulmonary infection because dying larvae are considered a higher risk for significant pneumonitis. Pulmonary symptoms may be ameliorated with inhaled bronchodilator therapy or corticosteroids, if necessary.
Conservative management of partial intestinal obstruction and biliary ascariasis is usually effective. The patient is maintained on nothing-by-mouth status, and the partial obstruction usually spontaneously resolves. Preventing oral intake decreases the risk of food compounding the obstruction while normal peristalsis redistributes or evacuates the worms. A controlled trial from Pakistan found that, in patients without peritonitis, hypertonic saline enemas relieved obstruction more quickly (1.6 d vs 3.4 d) and resulted in shorter hospital stays (4 d vs 6 d) than intravenous fluids alone. A recent study from India demonstrated that conservative therapy was successful in 19 of 22 (89%) children with intestinal obstruction. The regimen used consisted of no oral intake, intravenous fluids, antibiotics, piperazine salt per nasogastric tube, and glycerine plus liquid paraffin emulsion enemas.
The goals of pharmacotherapy are to eradicate infestation, to prevent complications, and to reduce morbidity.
Clinical Context: First DOC. A benzimidazole carbamate drug that inhibits tubulin polymerization, resulting in degeneration of cytoplasmic microtubules. Decreases ATP production in worms, causing energy depletion, immobilization, and, finally, death. Converted in the liver to its primary metabolite, albendazole sulfoxide. Less than 1% of the primary metabolite is excreted in the urine. Plasma level is noted to rise significantly (as much as 5-fold) when ingested after high-fat meal. Experience with patients < 6 y is limited.
To avoid inflammatory response in CNS, patient must also be started on anticonvulsants and high-dose glucocorticoids.
Well tolerated and does not appear to increase risk of worm obstruction.
Clinical Context: Well tolerated and does not appear to increase risk of worm obstruction. Causes worm death by selectively and irreversibly blocking uptake of glucose and other nutrients in susceptible adult intestine where helminths dwell.
Clinical Context: Neuromuscular blocking agent used to slowly paralyze worm to be eliminated from GI tract. May be DOC during pregnancy.
Parasite biochemical pathways are sufficiently different from the human host to allow selective interference by chemotherapeutic agents in relatively small doses.