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. Ascariasis may exist as a zoonotic infection associated with pigs and use of hog manure, [3, 4] but, in most endemic areas, it is most likely transmitted from person to person. 
The image below depicts a roundworm that infects humans through soil contaminated by human feces.
See Common Intestinal Parasites, a Critical Images slideshow, to help make an accurate diagnosis.
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).
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. Some children appear to become very heavily infested, probably from multiple cumulative exposures over time and/or relative immunodeficiency. 
Ascaris lumbricoides suum, a swine nematode, has been thought responsible for zoonotic infection. Distinguishing this worm from A lumbricoides is difficult, as it differs by only 6 (1.3%) nucleotides in the first internal transcribed spacer (ITS-1) and by 3%-4% in the mitochondrial genome sequence.  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 persons have a low worm burden and may present with only cough, acute eosinophilia, or eosinophilic liver lesions visible on CT scans. However, a molecular genetic study from China casts doubt that infections in pigs are the cause of most 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 soil-transmitted 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.  In regions with soil-transmitted diseases, ascariasis tends to be more geographically dispersed than Trichuris or hookworm. 
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 1-2 years, 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 and might be equivalent to malaria or tuberculosis worldwide. 
Ascariasis is most common and intensive in children, who are more likely than adults to be symptomatic. In children, intestinal obstruction caused by heavy worm burden (≥60) is the most common manifestation of disease. An estimated 2 per 1000 infected children develop intestinal obstruction per year.  Among children aged 1-12 years who presented to a Cape Town hospital with abdominal emergencies between 1958-1962, symptomatic A lumbricoides infection was responsible for 12.8% of cases , with 68% of those due to intestinal obstruction, usually at the terminal ileum. The peak incidence was at age 2 years in a series from Colombia and age 4.8 years in a series from Turkey.
The prevalence of infection in Vietnam is estimated at 44.4%, more commonly in the northern peri-urban and rural areas of the country.  In Vietnam, vegetable cultivation using night soil fertilizer places adult women at especially high risk. Children with chronic ascariasis may experience decreased growth and development due to decreased food intake.
Adults with ascariasis are more likely to develop biliary complications due to migration of adult worms (see image below), possibly provoked by other illnesses such as malarial fever. In Damascus, of 300 adults referred for complications of ascariasis between 1988 and 1993, 98% had abdominal pain, 4.3% had acute pancreatitis, 1.3% had obstructive jaundice, and 25% had worm emesis. Twenty-one to 80% of patients had undergone previous cholecystectomy or endoscopic sphincterotomy.
Because of improved access to care and availability of ultrasonography, biliary ascariasis has been increasingly recognized and reported from endemic areas.  In Kashmir, it may account for up to 36% of patients presenting with biliary or pancreatic disorders and is reported to cause 20% of biliary disease in the Philippines. 
A review of biliary ascariasis suggests that this association may be causative as a result of dilatation of the common bile duct and elevation of cholecystokinin levels with resultant relaxation of the sphincter of Oddi. 
A report from India indicated that, of consecutive patients diagnosed with biliary ascariasis, 80% presented with recurrent abdominal pain, 30% with acute cholecystitis, 25% with obstructive jaundice, 25% with cholangitis, only 5% with pancreatitis, 5% with perforated viscus, and 5% with hepatolithiasis. Only 25% of the Indian patients required surgery, and conservative medical therapy with oral anthelminthics has recently been recommended. 
A postcholecystectomy syndrome of pain and jaundice is frequently due to ascariasis in endemic areas, presumably owing to enhanced patency of the biliary system after surgical or endoscopic sphincterotomy. 
Intestinal obstruction, usually of the terminal ileum in children, is the most commonly attributed fatal complication, resulting in 8000-100,000 deaths per year, according to the World Health Organization. Besides direct obstruction of the bowel lumen, toxins released by live or degenerating worms may result in bowel inflammation, ischemia, and fibrosis.
See International and Mortality/Morbidity.
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