04/04/2025
Ascaris lumbricoides eggs observed on wet mount prep of stool sample.
A lumbricoides, a roundworm, is the largest nematode infecting humans. Disease caused by these is called ascariasis. Lab diagnosis can be made by demonstration of parasite (eggs, larvae or adult worm) in clinical specimens.
A gravid female roundworm can lay up to 2,50,000 eggs per day. This accounts for about three eggs per mg of f***s. At this concentration, the eggs can be readily seen by the microscopic examination of saline emulsion of faeces.
Eggs are oval-shaped, they have a thick, mammillated outer shell, often giving them a rough or bumpy appearance. This protective coating helps them survive harsh environmental conditions, making them highly infectious. Eggs tend to be bile stained and appear golden brown in saline mount.
These eggs are typically unembryonated when passed in f***s and become infective after a few weeks in soil. Ingestion of embryonated eggs leads to intestinal ascariasis.
It is interesting to note the differences between fertilized and unfertilized eggs
Fertilized eggs have a rounded shape and feature the thick shell with an external mammillated layer. These fertile eggs typically measure between 45 to 75 µm in length and 35 to 50 µm in the width. In some instances, the outer layer may be absent, resulting in what is known as decorticated eggs.
On the other hand, unfertilized eggs are elongated and larger than their fertile counterparts, reaching lengths of 80 up to 90 µm and 40 to 45 µm in the width. Their shell is thinner, and the mammillated layer shows greater variability, with either prominent protuberances or almost none. Unfertile eggs primarily consist of a mass of refractile granules.
On microscopy, the fertilized egg is seen surrounded by a thick smooth translucent shell consisting of three layers:
1. The outer coarsely mammillated albuminoid coat.
2. A thick transparent middle layer.
3. a thin inner lipoidal membrane.
Department of Parasitology, Sylhet Agricultural University, Bangladesh
