Fertilized and unfertilized Ascaris lumbricoides eggs are passed in stool of the infected host. Fertilized eggs are are rounded and have a thick shell with an external mammillated layer that is often stained brown by bile. In some cases, the outer layer is absent (known as decorticated eggs). Fertile eggs range from 45 to 75 µm in length. Unfertilized eggs are elongated and larger than fertile eggs (up to 90 µm in length). Their shell is thinner and their mammillated layer is more variable, either with large protuberances or practically none. Unfertile eggs contain mainly a mass of refractile granules. Complete development of the larva requires 18 days under favorable conditions (moist, warm, shaded soil). However, eggs of Ascaris lumbricoides may continue to develop and are infectious even when preserved in formalin.
Unfertilized egg of A. lumbricoides in an untained wet mount,
C: A. lumbricoides fertilized egg with embryo in the early stage of development,
in a wet mount. Image taken at 200× magnification.
Decorticated, fertile egg of A. lumbricoides in an unstained wet mount,
A. lumbricoides with larva inside.
I: An adult
Ascaris lumbricoides worm. Diagnostic characteristics include tapered
ends and size (length 15 to 35 cm; the females tend to be larger).
This worm is a female, as evidenced by the size and genital girdle (the dark
circular groove at bottom area of image).