Ascaris Worms: Parasitological Facts

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Ascaris species are large intestinal roundworms belonging to the family Ascarididae of the Phylum Nematoda. The family is among the largest of nematodes, some species reaching a length of 45 cm or more. The family contains one of the oldest associates of human kind: Ascaris lumbricoides, the intestinal large roundworm. Domesticated pigs have the Ascaris suum which is very similar morphologically and physiologically to A. lumbricoides.

Because of the great size, abundance, and cosmopolitan distribution of A. lumbricoides and A. suum, they are among the first parasites known to humans. Ancient Greeks and the Romans were very familiar with them, and they were mentioned in the Ebers papyrus. It is likely that A. lumbricoides was originally a parasite of pigs that adapted to humans when swine were domesticated and began to live in close association with humans – or perhaps it was a human parasite that we gave to pigs. (The physiologies of people and swine are remarkably similar as, on occasion, are their eating and social habits.) Today two populations of this parasite exist, one in humans and one in pigs. Poor sanitation is the major cause of the spread of Ascaris infection.

This article will focus on the biology, epidemiology, symptoms, diagnosis, and treatment of A. lumbricoides because it is the nematode that actually affects human medically.

Biology

A period of 9 to 13 days is the minimal time required for embryos to develop into active juveniles. Although embryos are extremely resistant to low temperature; desiccation, and strong chemicals, embryogenesis is retarded by such factors. High temperature and sunlight are lethal in a short time. Juveniles molt to the third stage before hatching through the indistinct operculum.

Infection occurs when unhatched juveniles are swallowed with contaminated food and water. They hatch in the duodenum, where they penetrate the mucosa and submucosa and enter lymphatics or venules. After passing through the right heart, they enter the pulmonary circulation and break out of capillaries into air spaces. Many worms get lost during this migration and accumulate in almost every organ of the body, causing acute tissue reactions.

Epidemiology

Worldwide, 1.27 billion persons, about one quarter of the world population, are infected. Indiscriminate defecation, particularly near habitations, “seeds” the soil with eggs that remain viable for many months or even years. Resistance of Ascaris spp. eggs to chemicals is almost legendary. They can embryonate successfully in 2% formalin, in potassium dichromate, and in 50% solutions of hydrochloric, nitric, acetic, and sulfuric acid, among other similar inhospitable substances. This extraordinary chemical resistance is a result of the lipid layers of their eggshell, which contains ascarosides.

Contamination, then, is the typical means of infection. Children are the most likely to become infected (or reinfected) by eating dirt or placing soiled fingers and toys in their mouths. Dogs can become infected and probably play an important role in environmental contamination in some areas.

Pathogenesis

Little damage is caused by penetration of intestinal mucosa by newly hatched worms. Juveniles that become lost and wander and die in anomalous locations, such as spleen, liver, lymph nodes, or brain, often elicit an inflammatory response. Symptoms may be vague and difficult to diagnose and may be confused with those of other diseases. Transplacental migration into a developing fetus is also known.

It is probable that A. lumbricoides occasionally sucks blood from the intestinal wall; its main food is the liquid contents of the intestinal lumen. In moderate and heavy infections, the resulting theft of nourishment can cause malnutrition, underdevelopment, and cognitive impairment in small children.

Diagnosis and Treatment

Accurate diagnosis of migrating juveniles is impossible at this time. Demonstration of juveniles in the sputum is definitive, provided a technician can identify them. Most diagnoses are made by identifying the characteristic, mammilated eggs in the feces or by an appearance of the worm itself. So many eggs are laid each day by one worm that one or two direct fecal smears are usually sufficient to demonstrate at least one.

Mebendazole is the drug of choice, with pyrantel pamoate as an alternative. Mebendazole binds to tubulin in the worm’s intestinal cells and body wall muscles. No effective treatment of migrating juveniles has been discovered. Nitazoxamide, a drug used to treat cryptospordial diarrhea, appears to be effective against a variety of helminthes, including A. lumbricoides. The drug ivermectin is also effective against it.

Reference

Meyer, Marvin. 1992. Essentials of Parasitology.