Ancylostoma Duodenale: Habitat, Morphology and Life Cycle

In this article we will discuss about Ancylostoma Duodenale:- 1. Historical Retrospect of Ancylostoma Duodenale 2. Geographical Distribution of Ancylostoma Duodenale 3. Habitat 4. Morphology 5. Life Cycle 6. Pathogenicity 7. Treatment and Control of Disease.

Contents:

1. Historical Retrospect of Ancylostoma Duodenale
2. Geographical Distribution of Ancylostoma Duodenale
3. Habitat of Ancylostoma Duodenale
4. Morphology of Ancylostoma Duodenale
5. Life Cycle of Ancylostoma Duodenale
6. Pathogenicity of Ancylostoma Duodenale
7. Treatment and Control of Disease Caused by Ancylostoma Duodenale

1. Historical Retrospect of Ancylostoma Duodenale:

Ancylostoma duodenale, the Old World hook worm is a very common nematode parasite (Fig. 15.13) in the small intestine of man. It causes “ancylostomiasis” in man, and until recently this hookworm ranked as the most important helminthic infection of man, but it has been brought under control in many countries.

A. Dubini, an Italian physician first dis­covered the parasite during the autopsy of a woman in 1838. Perroncito (1881) described the development of free-living larvae in the soil. Looss described the pathogenesis and mode of entrance of the larvae in the intes­tine of the man in 1898.

2. Geographical Distribution of Ancylostoma Duodenale:

The infection of the parasite has been reported among the rural people of the tropi­cal countries and it may also occur in tem­perate countries where the temperature and humidity are favourable for the develop­ment of the larvae in the soil.

Incidence of the hookworm has been re­ported from Europe, Egypt, India, Bangla­desh, Sri Lanka, Central and North China, and Pacific Islands. In India it is more pre­dominant in Punjab, Haryana, and Himachal Pradesh.

3. Habitat of Ancylostoma Duodenale:

The adult hookworms reside in the small intestine-of man particularly in the jejunum, less often in the duodenum, and rarely in the ileum. Thus, the name of the parasite ap­pears to be a misnomer. It has been found on rare occasions in pigs. The adult worms anchor the wall of the small intestine by their anterior ends.

4. Morphology of Ancylostoma Duodenale:

The detailed structure on morphology of A. duodenale (Fig. 15.13) has been discussed by Looss (1905).

Adult worms:

1. The mature worms are cylindrical in shape, plump, rigid and creamy-white in colour (Fig. 15.13).

2. The anterior end is bent dorsally like a hook (hence the name is “hook­worm”) and provided with the dorsally placed oral aperture.

3. The oral aperture is provided with 6 sharp teeth (cutting plates) on the ventral aspect, two teeth are found on each side, and two on the dorsal sur­face.

4. The buccal capsule is large and cons­picuous, and is lined with chitin-like substance.

5. The cuticle is with fine transverse striations.

6. A minute finger-like cervical papillae is present on each side, a little away from the anterior extremity.

7. Two cephalic glands, a small oesopha­geal gland and two pear-shaped cer­vical glands are connected to the oesophagus, the secretion of the oesophageal gland prevents clotting of the ingested blood.

But Thorson (1956) reported that the oesophageal gland opens near the cutting plates (teeth) in the buccal capsule and par­ticipate in the extracorporeal diges­tion. The function of cervical glands and cephalic glands is not known.

8. Sexual dimorphism is distinct. The female is slightly larger and has a straight and pointed candal end (Fig. 15.13). The male is characterized by bursa copulatrix (an invagination of the body-wall around the genital ap­erture) at its caudal end. The bursa is supported by 13 rays. It has two protrusible spicules (1 mm in length) which assist in sperm transfer during the copulation (Fig. 15.13).

9. The female worms measure about 10 to 13 mm x 0.6 mm while the males measure about 8 to 10 mm x 0.5 mm.

10. The female gonopore is separate and is located at the junction of the poste­rior and in the middle third.

11. The male has a cloaca where the ejacu­latory duct opens.

Eggs:

1. The eggs are oval and colourless with broadly rounded extremities, and measure about 60 µm x 40 µm.

2. Each egg has a thin outer shell and a very fine vitelline layer.

3. When freshly passed the egg has the segmented ovum surrounded by a clear space. It is usually segmented into 2 to 8 cells.

5. Life Cycle of Ancylostoma Duodenale:

The life cycle of Ancylostoma duodenale (Fig. 15.14) is completed within a single host (man), hence it is called monogenetic. No intermediate host is recorded in the life cycle of A. duodenale.

Copulation and Fertilization:

The male and female worms mate in the intestine of man. During copulation, the bursa of male is applied on the vulva of female and sperms are transferred in the seminal recep­tacles where fertilization takes place.

Passage of the eggs from the infected host:

A single female worm lays about 10 to 20 thousand eggs per day. Fertilized eggs come out of the hosts body alone with the faeces. The contained ovum undergoes segmenta­tion during the passage through the bowel. The eggs are not infective to man but the larvae hatched out of the eggs in soil repre­sents the infective stage.

The larva hatches out of the egg in 24 to 48 hours in damp soil with warmth (27 °C) and sufficient oxygen. It is to be mentioned that very little development takes place in diluted faeces due to its high acidic pH (4.8-5).

Development of larva in soil:

The newly hatched larva is called rhabditiform larva or first stage juvenile owing to its rhabditic oesophagus. It meas­ures about 250 µm in length and possesses a rounded anterior end and a sharp caudal end.

It is very active and feeds voraciously on the organic matters present in the stool and soil. It grows rapidly and moults twice in one week: the first on the 3rd day and the second on the 5th day. By this time the larva attains a length of 500 to 600 µm.

After the second moult, the oesophageal bulb disap­pears and the organ becomes simple and muscular. The shedded cuticle is retained as the protective sheath. It is now called a filariform larva or third stage juvenile which is infective to man. Although actively mo­tile, the larva ceases to feed and does not grow further. Development from egg to filari­form larva takes about 8-10 days.  

Biology of the larvae:

The larvae live in the upper 1/2 inch of the soil. In tropical climates, the life span is short; 90% of them die within 2 to 3 weeks, and the rest by 6 weeks. Filariform larvae are readily destroyed by direct sunlight. They are very sensitive to desiccation and extremes of temperature. They also move towards the source of oxygen.

Entrance into a new host:

The filariform larvae gain entrance into the body of the host (man) by penetrating the skin. They cast off their sheaths and bore through the skin directly or through the hair- follicles.

The warmth of sole of legs and hands is believed to attract the filariform larvae and hence the infection is more com­mon to the bare-footed subjects. Infection may also happen from handling faeces-soiled clothing’s if they are left damp for 4 or 5 days. There has been reported that intra-uterine infections are also possible.

Migration after entrance:

After several hours of infection, the lar­vae enter into lymphatics or small venules. The larvae unable to reach the vascular spaces usually die or are phagocytosed. They are passively carried to the right part of the heart and then to the lungs by way of pulmonary artery, on about 3rd day.

They then migrate in the alveoli by boring the wall of pulmonary capillaries where they are arrested owing to their large size. From the lungs the larvae ascend the bronchi, trachea, epiglottis to the back of the pharynx and are swallowed.

During the migration when they reach the oesophagus, the third moulting occurs with the formation of a temporary buccal capsule bearing 4 small teeth. Finally, they reach the jejunum via the stomach and the duodenum between the 7th and the 10th day. By this time, the larva grows rapidly and attains a size of 2 mm x 0.13 mm.

Establishment and laying of eggs:

The growing larva settles in the jejunum and undergoes the 4th moult and final moult on the 15th day. During this moult the defini­tive buccal capsule is formed in place of temporary buccal capsule with teeth. The worm becomes sexually mature in 3 to 4 weeks and the eggs begin to appear in the stool on the 6th week.

The wounds are afflicted by the sharp teeth of adult worms in the intestinal wall at the region of attachment. Through these wounds blood comes out and the worms then suck the blood by the action of suctorial pharynx.

Coagulation of the blood is pre­vented by the production of a secretion from the mouth which has an anticoagulatory property and thus blood is prevented from being coagulated. A. duodenale is recorded to suck profuse quantity of blood and the rate of sucking of blood is recorded to be 0.8 ml in 24 hours.

6. Pathogenicity of Ancylostoma Duodenale:

The infective larvae cause some kinds of eruption of the skin which may be pustu­lar, known as ground-itch. The hosts in­fected with adult worms become anaemic as a result of considerable loss of blood from the intestinal wounds.

Besides the anae­mic condition, the hosts become susceptible to all diseases, particularly tuberculosis be­cause of the damage done by the larvae during their transit from the heart to the final abode.

7. Treatment and Control of Disease Caused by Ancylostoma Duodenale:

The hookworm infection can be checked by administering tetrachlorethyl, hexylresorcinol, carbon tetrachloride and blephenium, etc.

Control Measures:

1. The most important is the sanitary disposal of human faeces to prevent pollution of the earth.

2. Borehole latrine should be introduced in the villages to reduce the spread of larva.

3. Boots and gloves should be weared while working in the garden.

Enterobius Vermicularis:

Popularly known as pinworm and infects children. The adult worms are very small in size and inhabit the intestine, caecum and appendix. Sexes are separate. The gravid females migrate to the rectum and produce itching sensation. Eggs are laid in the rectum and infection occurs by bodily transference of eggs.

Dracunculus Medinensis:

Commonly known as Guinea-worm and man it its final host. Young ones occur in a shrimp-like form of crustacea called Cyclops. The cyclops are ingested along with drink­ing water and the young parasites are liber­ated into the stomach from where they mi­grate to subcutaneous tissues.

Males meas­ure about 4 cm and die after copulation. The females are about 12 cm in length and gravid females have two uteri full of fertilized eggs. For laying of eggs the females come to the superficial layer of the skin and die after discharging eggs.