Ascariasis is the most prevalent helminth infection on earth. Around one-fifth of human population is infected with Ascaris lumbricoides; the majority of infected live in rural or deprived urban settings in developing countries. In these endemic regions, disease prevalence is around 90% whereas in developed countries, the infection is rare. A. lumbricoides is the largest nematode (15-35 cm) parasitizing in the lumen of human small intestine.
Fertile eggs of ascaris become infective after an embryo moults twice within an egg (18 days to several weeks depending on the environmental conditions). After infective eggs are swallowed, the larvae hatch, invade the intestinal mucosa, and are carried via bloodstream to the lungs. The larvae undergo two moults in the lungs (10 to 14 days), penetrate the alveolar walls, ascend the bronchial tree to the throat, and are swallowed. Upon reaching the small intestine, they develop into adult worms which can live 1-2 years.
Daily ascaris egg production is around 200,000 eggs, which are shed in the feces. An infection can occur if a person swallows the microscopic eggs in contaminated food or water, or the eggs are transferred from hands to mouth after touching contaminated soil. Eggs can remain viable in the soil for up to 15 years.
The manifestations of ascariasis can be divided into acute and chronic. Patients experience acute lung inflammation, difficulty in breathing and fever as a result of larval migration through the pulmonary tissue (acute ascariasis). Abdominal distension and pain, nausea and diarrhoea are characteristic symptoms of adult worm invasion (chronic ascariasis). In small percentage of patients, entangled adult worms could lead to mechanical intestinal obstruction. The majority of invasions with A.lumbricoides are asymptomatic, and patients usually seek medical advice because they have seen a worm in their faeces.
A.lumbricoides infection is associated with poor personal hygiene, poor sanitation, and with use of human feces as fertilizer. A.lumbricoides has a tendency to aggregate in few heavily infected hosts. Children are known to harbor worms at generally higher intensity than adults. Both behavioral and genetic factors (immunity) are responsible for above-mentioned trends.
Both larval stage of A. lumbricoides and adult worms are known to induce strong specific immune responses involving all isotypes (IgM, IgG, IgA, IgE). Antibodies against larvae of A. lumbricoides appear already 10 days after infection. There is a number of observations of premature death of the helminths in the organism of the host due to effective immune response.
A strong tendency exists for specific immunoglobulin levels to track worm burden. The highest antibody concentrations were found in the 5-9-year age group, which also shows the highest intensity of infection.
In endemic and to a lesser extent in non-endemic regions there is a high proportion of seropositive individuals without clinical signs of ascariasis, indicating low-level exposure to migrating larvae or persistence of IgG antibodies after deworming of patients.
A. lumbricoides is known to induce vigorous total and specific IgE responses, as well as eosinophilia. The pulmonary phase can cause potentially lethal hypersensitivity responses in infected individuals, particularly children, and worm material is notorious for the allergic reactions that it provokes.
Around a half of ascaris proteins are immunogenic. Larval and adult stages are immunologically distinct, although they share some similarity. Both larvae and adult worms are used for preparation of somatic antigens for diagnostic purposes. Taking into account high level of cross-reactivity of many ascaris proteins with those of other nematodes (hookworms, Trichuris, etc.), mites, as well as human proteins, there are attempts at purification of somatic antigen and isolation of separate immunogenic proteins (34 kDa protein). Despite these attempts, somatic antigen is still widely used for diagnostic purposes.
In highly endemic regions, anthelminthic treatment, administered to high-risk groups without prior diagnosis, is a widely used strategy. Kato-Katz fecal smears is the most popular specific direct method for diagnosing infection with A.lumbricoides. Because of day-to-day variation in fecal egg output, particularly in areas with high proportions of light-intensity infections, this method demands collection of three or more fecal samples on consecutive days. Therefore, in non-endemic countries, antibody detection by an immunoassay is clearly the preferential, primary diagnostic approach.