Ascariasis (Ascaris lumbricoides)

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Chapter 283 Ascariasis (Ascaris lumbricoides)

Arlene E. Dent, James W. Kazura

Etiology

Ascariasis is caused by the nematode, or roundworm, Ascaris lumbricoides. Adult worms of A. lumbricoides inhabit the lumen of the small intestine and have a life span of 10-24 mo. The reproductive potential of Ascaris is prodigious; a gravid female worm produces 200,000 eggs/day. The fertile ova are oval in shape with a thick mammillated covering measuring 45-70 µm in length and 35-50 µm in breadth (Fig. 283-1). After passage in the feces, the eggs embryonate and become infective in 5-10 days under favorable environmental conditions. Adult worms can live for 12-18 mo (Fig. 283-2).

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Figure 283-1 Soil-transmitted helminth eggs.

(From Bethony J, Brooker S, Albonico M, et al: Soil-transmitted helminth infections: ascariasis, trichuriasis, and hookworm, Lancet 367:1521–1532, 2006.)

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Figure 283-2 Adult male and female soil-transmitted helminths.

(From Bethony J, Brooker S, Albonico M, et al: Soil-transmitted helminth infections: ascariasis, trichuriasis, and hookworm, Lancet 367:1521–1532, 2006.)

Epidemiology

Ascariasis occurs globally and is the most prevalent human helminthiasis in the world. It is most common in tropical areas of the world where environmental conditions are optimal for maturation of ova in the soil. Approximately 1 billion persons are estimated to be infected, with 4 million cases in the USA. Key factors linked with a higher prevalence of infection include poor socioeconomic conditions, use of human feces as fertilizer, and geophagia. Even though infection can occur at any age, the highest rate is in preschool or early school-aged children. Transmission is primarily hand to mouth but may also involve ingestion of contaminated raw fruits and vegetables. Transmission is enhanced by the high output of eggs by fecund female worms and resistance of ova to the outside environment. Ascaris eggs can remain viable at 5-10°C for as long as 2 yr.

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