Ascariasis

Ascariasis is a human disease caused by the parasitic roundworm Ascaris lumbricoides. Perhaps as many as one quarter of the world's people are infected, with rates of 45% in Latin America and 95% in parts of Africa.  Ascariasis is particularly prevalent in tropical regions and in areas of poor hygiene. Other species of the genus Ascaris are parasitic and can cause disease in domestic animals. Certain genes have been identified in human populations that may increase the susceptibility to infection.

Infection occurs through ingestion of food contaminated with feces containing Ascaris eggs. The larvae hatch, burrow through the intestine, reach the lungs, and finally migrate up the respiratory tract. From there they are then reswallowed and mature in the intestine, growing up to 30 cm (12 in.) in length and anchoring themselves to the intestinal wall.

Infections are usually asymptomatic, especially if the number of worms is small. They may however be accompanied by inflammation, fever, and diarrhea, and serious problems may develop if the worms migrate to other parts of the body.

Signs and symptoms

Patients can remain asymptomatic for very long periods of time. As larval stages travel through the body, they may cause visceral damage, peritonitis and inflammation, enlargement of the liver or spleen, toxicity, and pneumonia. A heavy worm infestation may cause nutritional deficiency; other complications, sometimes fatal, include obstruction of the bowel by a bolus of worms (observed particularly in children) and obstruction of the bile or pancreatic duct. More than 796 Ascaris lumbricoides worms weighing up to 550 g [19 ounces] were recovered at autopsy from a 2-year-old South African girl. The worms had caused torsion and gangrene of the ileum, which was interpreted as the cause of death.

Ascaris takes most of its nutrients from the partially digested host food in the intestine. There is limited evidence that it can also pierce the intestinal mucous membrane and feed on blood, but this is not its usual source of nutrition.^] As a result, Ascaris infection does not produce the anemia associated with some other roundworm infections^]

In Canada in 1970, a postgraduate student tainted his roommates' food with Ascaris lumbricoides. Four of the victims became seriously ill; two of these suffered acute respiratory failure

Mechanism

Life cycle

Adult worms (1) live in the lumen of the small intestine. A female may produce approximately 200,000 eggs per day, which are passed with the feces (2). Unfertilized eggs may be ingested but are not infective. Fertile eggs embryonate and become infective after 18 days to several weeks (3), depending on the environmental conditions (optimum: moist, warm, shaded soil). After infective eggs are swallowed (4), the larvae hatch (5), invade the intestinal mucosa, and are carried via the portal, then systemic circulation and/or lymphatics to the lungs . The larvae mature further in the lungs (6) (10 to 14 days), penetrate the alveolar walls, ascend the bronchial tree to the throat, and are swallowed (7). Upon reaching the small intestine, they develop into adult worms (8). Between 2 and 3 months are required from ingestion of the infective eggs to oviposition by the adult female. Adult worms can live 1 to 2 years.

First appearance of eggs in stools is 60–70 days. In larval ascariasis, symptoms occur 4–16 days after infection. The final symptoms are gastrointestinal discomfort, colic and vomiting, fever, and observation of live worms in stools. Some patients may have pulmonary symptoms or neurological disorders during migration of the larvae. However there are generally few or no symptoms. A bolus of worms may obstruct the intestine; migrating larvae may cause pneumonitis and eosinophilia.

Source

The source of transmission is from soil and vegetation on which fecal matter containing eggs has been deposited. Ingestion of infective eggs from soil contaminated with human feces or transmission and contaminated vegetables and water is the primary route of infection. Intimate contact with pets which have been in contact with contaminated soil may result in infection, while pets which are infested themselves by a different type of roundworm can cause infection with that type of worm (Toxocara canis, etc.) as occasionally occurs with groomers.

Transmission also comes through municipal recycling of wastewater into crop fields. This is quite common in emerging industrial economies, and poses serious risks for not only local crop sales but also exports of contaminated vegetables. A 1986 outbreak of ascariasis in Italy was traced to irresponsible wastewater recycling used to grow Balkan vegetable exports.^[7]

Transmission from human to human by direct contact is impossible.^[8]

Diagnosis

The diagnosis is usually incidental when the host passes a worm in the stool or vomit. Stool samples for ova and parasites will demonstrate Ascaris eggs. Larvae may be found in gastric or respiratory secretions in pulmonary disease. Blood counts may demonstrate peripheral eosinophilia. On X-ray, 15–35 cm long filling defects, sometimes with whirled appearance (bolus of worms).

Prevention

Prevention includes: use of toilet facilities; safe excreta disposal; protection of food from dirt and soil; thorough washing of produce; and hand washing.

Food dropped on the floor should never be eaten without washing or cooking, particularly in endemic areas. Fruits and vegetables should always be washed thoroughly before consumption.

Pharmaceutical drugs that are used to kill roundworms are called ascaricides and include:

• Mebendazole (Vermox) (C₁₆H₁₃N₃O₂). Causes slow immobilization and death of the worms by selectively and irreversibly blocking uptake of glucose and other nutrients in susceptible adult intestine where helminths dwell. Oral dosage is 100 mg 12 hourly for 3 days.
• Piperazine (C₄H₁₀N₂.C₆H₁₀O₄). A flaccid paralyzing agent that causes a blocking response of ascaris muscle to acetylcholine. The narcotizing effect immobilizes the worm, which prevents migration when treatment is accomplished with weak drugs such as thiabendazole. If used by itself it causes the worm to be passed out in the feces. Dosage is 75 mg/kg (max 3.5 g) as a single oral dose.
• Pyrantel pamoate (Antiminth, Pin-Rid, Pin-X) (C₁₁H₁₄N₂S.C₂₃H₁₆O₆) Depolarizes ganglionic block of nicotinic neuromuscular transmission, resulting in spastic paralysis of the worm. Spastic (tetanic) paralyzing agents, in particular pyrantel pamoate, may induce complete intestinal obstruction in a heavy worm load. Dosage is 11 mg/kg not to exceed 1 g as a single dose.
• Albendazole (C₁₂H₁₅N₃O₂S) A broad-spectrum antihelminthic agent that decreases ATP production in the worm, causing energy depletion, immobilization, and finally death. Dosage is 400 mg given as single oral dose (contraindicated during pregnancy and children under 2 years).
• Thiabendazole. This may cause migration of the worm into the esophagus, so it is usually combined with piperazine.
• Hexylresorcinol effective in single dose, mentioned in : Holt, Jr Emmett L, McIntosh Rustin: Holt's Diseases of Infancy and Childhood: A Textbook for the Use of Students and Practitioners. Appleton and Co, New York,11th edition
• Santonin, more toxic than hexylresorcinol, mentioned in : Holt, Jr Emmett L, McIntosh Rustin: Holt's Diseases of Infancy and Childhood: A Textbook for the Use of Students and Practitioners. Appleton and Co, New York,, 11th edition
• Oil of chenopodium, more toxic than hexylresorcinol, mentioned in : Holt, Jr Emmett L, McIntosh Rustin: Holt's Diseases of Infancy and Childhood: A Textbook for the Use of Students and Practitioners. Appleton and Co, New York, 11th edition

Also, corticosteroids can treat some of the symptoms, such as inflammation.

Native Americans have traditionally used epazote (Chenopodium ambrisioides) for treatment, which was not as powerful as pharmaceutical compounds, but spontaneous passage of Ascarids provided some proof of efficacy

Trivia

• Ascariasis may result in allergies to shrimp and dustmites due to the shared antigen, tropomyosin.
• Ascaris have an aversion to some general anesthetics and may exit the body, sometimes through the mouth.

Genus and Species	Ascaris lumbricoides
Common Name	Giant Intestinal Roundworm
Etiologic Agent of:	Ascariasis
Infective stage	Embryonated Egg
Definitive Host	Human
Portal of Entry	Mouth
Mode of Transmission	Ingestion of Embryonated egg through contaminated food or water
Habitat	Small Intestine
Pathogenic Stage	Adult, Larva
Mode of Attachment	Retention in the mucosal folds using pressure
Mode of Nutrition	Feeding of Chyme
Pathogenesis	Larva – pneumonitis, Loeffler’s Syndrome; Adult – Obstruction, Liver abscess, Appendicitis. With Blood-Lung Phase along with Hookworms and Strongyloides stercoralis.
Laboratory diagnosis	Concentration methods and Direct Fecal Smear: Kato-Katz
Treatment	Albendazole, Mebendazole, or Pyrantel pamoate
Diagnostic Feature – Adult	Female – prominent genital girdle
Diagnostic Feature – Egg	Coarse mammilated albuminous coating