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Ascariasis is a disease of human caused by the parasitic roundworm Ascaris lumbricoides.
Perhaps as many as one quarter of the world’s population are infected. Ascariasis is particularly prevalent in tropical and sub-tropical regions where hygiene is poor.
Other species of the genus Ascaris can cause disease in domestic animals, such as Ascaris suum which infects pigs. Some genes have been identified in humans that may increase susceptibility to infection.
Signs and Symptoms:
As larval stages travel through the body, they may cause visceral damage, peritonitis and inflammation, enlargement of the liver or spleen, and a verminous pneumonitis. The worms in the intestine may cause mal-absorption and anorexia which contribute to malnutrition. The mal-absorption may be due to a loss of brush border enzymes, erosion and flattening of the villi, and inflammation of the lamina propria.
The worms can occasionally cause intestinal blockage when large numbers get tangled into a bolus or they may migrate from the small intestine and block the bile duct or the pancreatic duct, all of which may require surgery. (More than 796 Ascaris lumbricoides worms weighing up to 550g. 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 some evidence that it can secrete anti-enzymes, presumably to protect itself from digestion by the hosts’ enzymes.
Life Cycle:
Infection occurs by swallowing food contaminated with Ascaris eggs from faeces. The larvae hatch in the intestine, burrow through the gut wall, and migrate to the lungs through the blood system. There they break into the alveoli and pass up the trachea and oesophagus where they are coughed up and swallowed.
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The larvae pass through the stomach for a second time into the intestine where they mature into adult worms. They maintain their position by swimming against the intestinal flow caused by peristalsis. Adult worms have a life-span of 1-2 years which means that individuals may be infected all their lives as worms die and new worms are acquired.
Infections are usually asymptomatic, especially if the number of worms is small, because the degree of disease is related to the number of worms in the intestine as well as to the size and health of the human host. It is typical to find that most individuals’ harbour a small number of worms, while a small proportion are heavily infected, something that is characteristic of many worm infections. The distribution of A. lumbricoides among human hosts is best described empirically by the negative binomial distribution.
Pathology:
Infection with Ascaris lumbricoides often causes no symptoms. Infections with a large number of worms may cause abdominal pain or intestinal obstruction. Adults feed on the contents of the small intestine and in heavy infections this may compound problems in malnourished individuals (especially children).
Migration of larvae may cause localized reactions in various organs. Penetration of the larvae from capillaries into the lungs can lead to Loeffler’s pneumonia, in which pools of blood and dead epithelial cells clog air spaces in the lungs. Resulting bacterial infections can be fatal.
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 faecal matter containing eggs has been deposited. Ingestion of infective eggs from soil contaminated with human faeces 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. Transmission from human to human by direct contact is impossible.
Diagnosis:
The diagnosis is usually incidental when the host passes a worm in the stool or vomit. The eggs can be seen in smear of fresh faeces examined on a glass slide under a microscope and there are various techniques to concentrate them first or increase their visibility, such as the ether sedimentation method or the Kato technique. The eggs have a characteristic shape. During pulmonary disease larvae may be found in fluids aspirated from the lungs. White blood cells counts may demonstrate peripheral eosinophilia, but this is common in many parasitic infections and is rot specific to ascariasis.
Prevention:
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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.
Treatment:
Ascaricide:
Pharmaceutical drugs that are used to kill roundworms are called ascaricides and include:
1. Mebendazole (Vermox) (C16H13N3O2). Causes slow immobilization and death of the worms. Oral dosage is 100 mg 12 hourly for 3 days.
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2. Piperazine (C4H10N2.C6H10O4). A flaccid paralyzing agent of ascaris. Dosage is 75 mg/kg (max 3.5 g) as a single oral dose.
3. Pyrantel pamoate (Antiminth, Pin-Rid, Pin-X) (C11H14S.C23H16O6). Dosage is 11 mg/ kg not to exceed 1 g as a single dose.
4. Albendazole (C12H15N3O2S) A broad-spectrum antihelminthic agent. Dosage is 400 mg given as single oral dose.
5. Thiabendazole. This may cause migration of the worm into the esophagus, so it is usually combined with piperazine.
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6. Hexylresorcinol effective in single dose.
7. Oil of chenopodium
Society and Culture:
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.
