Ascaris lumbricoides - Introduction, Classification, History, Habitat, Morphology

Introduction to Ascaris lumbricoides

Ascaris lumbricoides, also known as the common roundworm, is a large parasitic worm that causes ascariasis in humans. It is the most common intestinal nematode infesting humans – with estimates of parasitic infection at some point in their lives to be one-sixth of the human population. Ascariasis is much more common in tropical and subtropical countries.

Classification of Ascaris lumbricoides

Scientific classification of Ascaris lumbricoides is as follows:

Kingdom: Animalia

Phylum: Nematoda

Class: Chromadorea

Order: Ascaridida

Family: Ascarididae

Genus: Ascaris

Species: A. lumbricoides

History of Ascaris lumbricoides

Ascaris lumbricoides infection has been known in humans since antiquity and for many centuries, they were thought to arise by spontaneous generation. The scientific name Ascaris lumbricoides was coined by Linnaeus in 1758.

Henry Ransom found Ascaris eggs in human feces in England. Just two years later, French Casimir-Joseph Davaine described the parasite in the literature.

Although attempts to infect animals with Ascaris lumbricoides by giving them eggs were unsuccessful, Italian man Salvatore Calandruccio successfully infected a boy by giving him 150 roundworm eggs. However, Battista Grassi was the one to publish this information without crediting Calandruccio for his work.

The studies on the life cycle of the parasite were first reported by Francis Stewart in Hong Kong in 1916. He fed parasite eggs to rats, later to mice, and found infective larvae in the lungs and the feces. No mature Ascaris lumbricoides were found in test subjects.

Sadao Yoshida, in 1918, ingested roundworm larvae recovered from the trachea of a guinea pig and 76 days later demonstrated eggs in his own stool.

The life cycle of eventually confirmed by Shimesu Koino in 1922. He ingested 2,000 Ascaris lumbricoides eggs and tested his own sputum to find larvae a few days later. He took an anthelmintic drug a few days later and recovered 667 immature Ascaris lumbricoides.

Habitat of Ascaris lumbricoides

The adult Ascaris lumbricoides, which is one of the largest parasitic intestinal nematodes, habitats the small intestine of humans – specifically the jejunum and middle part of the ileum.

Morphology of Ascaris lumbricoides

The significant morphology of Ascaris lumbricoides includes:

1. Adult worm (male, female)

2. Egg (fertilized egg, unfertilized egg)

Adult worm

• the adult Ascaris lumbricoides parasite is pink in color resembling an earthworm

• large, cylindrical with tapering ends – anterior end thinner, more attenuated than the posterior end

• smallmouth surrounded by three lips – one dorsal and two ventral

• body cavity contains digestive and reproductive organs

• short-lived – lifespan for around 6 months

Adult male

• the adult male of Ascaris lumbricoides measures 15cm to 30 cm in length and 3mm to 4mm in diameter

• ventrally curved tail with a conical tail forms a hook-like structure

• male genitals include testes, vas deferens, ejaculatory duct in a single coiled tubule

• the anus opens into the cloaca from which a pair of copulatory spicules arises

Adult female

• adult females are larger than male Ascaris lumbricoides – measuring 20 to 40 cm in length and 2mm to 6mm in diameter

• straight and conical tail

• on the mid-ventral area of the parasite called the waist, a minute vulva opens at the junction of the anterior and middle third of the body

• the subterminal part contains the anus, a transverse slit on the ventral surface

• one gravid female Ascaris lumbricoides can lay 240,000 eggs per day

Infective form

The egg of Ascaris lumbricoides is the infective form of the parasite.

Egg

The host stool may contain both fertilized and unfertilized eggs of Ascaris lumbricoides.

Fertilized eggs

• fertilized eggs are released by fertilized female Ascaris lumbricoides

• oval to sub-spherical in shape

• measures 45 µm to 70 µm in length and 35 µm to 50 µm in breadth

• bile-stained, golden brown in color

• surrounded by a thick shell with the light brown and mammalian albuminous outer coat

• some eggs lack the mammalian albuminous outer coat and are called decorticated eggs

• each egg has a large conspicuous unsegmented ovum with a clear crescentic area at each pole

• floats in saturated salt solution, which is a distinguishing character of fertilized eggs

• freshly passed fertilized eggs are not infectious to man

• only become infectious to man after 10 to 40 days of development to become embryonated eggs

• embryonated eggs are infective to humans

Unfertilized eggs

• unfertilized eggs are brown, thin-shelled, ellipsoidal

• measures 78 µm to 105µm in length and 35 µm to 50 µm in breadth

• heaviest of all helminthic eggs; do not float in saturated salt solution

• presence of only unfertilized eggs indicates adult male is absent in the particular host or absence of mating between the male and female parasite