An Example of Phylum Platyhelminthes: Schistosoma Haematobium

In this article we will discuss about Schistosoma Haematobium:- 1. Geographical Distribution of Schistosoma Haematobium 2. Habit and Habitat of Schistosoma Haematobium 3. Structure 4. Body Wall 5. Digestive System 6. Excretory System 7. Nervous System 8. Reproductive System 9. Life History 10. Other Trematodes.

Contents:

1. Geographical Distribution of Schistosoma Haematobium
2. Habit and Habitat of Schistosoma Haematobium
3. Structure of Schistosoma Haematobium
4. Body Wall of Schistosoma Haematobium
5. Digestive System of Schistosoma Haematobium
6. Excretory System of Schistosoma Haematobium
7. Nervous System of Schistosoma Haematobium
8. Reproductive System of Schistosoma Haematobium
9. Life History of Schistosoma Haematobium
10. Other Trematodes of Schistosoma Haematobium

1. Geographical Distribution of Schistosoma Haematobium:

The medically important human blood fluke, Schistosoma haematobium has a wide distribution in South Africa and it has been a scourge in Egypt since ancient times. Out­side Africa S. haematobium occurs in Portu­gal, Mauritius, Mesopotamia and Madagas­car. A few cases have been reported from Mumbai (India).

2. Habit and Habitat of Schistosoma Haematobium:

Unlike other trematodes, Schistosoma haematobium is dioeceous. The males lodge the females in their body folds. The adult flukes are found in the mesenteric branches of portal vein, vesicoprostatic, public and uterine plex­uses and in the vesical veins.

The disease caused by it is called Schistosomiasis, the clinical manifestation of which is Haematuria. The larval stages are spent in the inter­mediate host—the fresh-water snail, Balinus (in Africa) and Planorbarius in Portugal.

3. Structure of Schistosoma Haematobium:

The males are larger than the females and attain a length of 1-1.5 cm and a breadth of 1 mm. The males are flat while the females are cylindrical. The males possess a gynecophoric canal (Fig. 14.15) formed by a tube-like ventral infolding of the thin, post-acetabular body margin. The females and males are otherwise structurally almost simi­lar.

The oral and ventral suckers are present. In males the ventral sucker is more strongly developed. In females both the suckers are of the same size and weak. The genital aper­ture is located on the ventral surface imme­diately behind the ventral sucker.

4. Body Wall of Schistosoma Haematobium:

The body wall of Schistosoma haematobium is composed of a homoge­neous epidermis (formerly designated as cuticle) bearing backwardly directed spines or papillae. The spines are found all over the epidermis in males while in females the spines are restricted to the anterior and posterior ends.

The epidermis rests on a basement membrane which is followed by circular, oblique and longitudinal muscle fibres. The spaces among organs of the body of Schistosoma haematobium are filled with parenchyma cells of irregular shape.

5. Digestive System of Schistosoma Haematobium:

The digestive system of Schistosoma haematobium is similar in both the sexes. The mouth is sub-apical and leads to a funnel shaped buccal cavity. The buccal cavity is surrounded by the oral sucker. The buccal cavity leads to the oesophagus. The pharynx is absent.

The oesophagus on reaching the anterior margin of the ventral sucker bifur­cates into two. These two branches reunite in the middle portion of the body to be contin­ued as a single sinuous tube terminating blindly. The oesophagus remains, surrounded by an ‘hour-glass shaped’ digestive gland.

6. Excretory System of Schistosoma Haematobium:

Excretory system of Schistosoma haematobium consists of Protonephridium and is formed by a posterior bladder, lateral collecting tubules with anterior and posterior branches and terminal flame bulbs.

7. Nervous System of Schistosoma Haematobium:

The brain is bilobed and located in the dorsal side just above the middle of the oesophagus. Paired posterior nerve trunks are given out from the brain.

8. Reproductive System of Schistosoma Haematobium:

Male reproductive organs:

The male re­productive system consists of 4 or 5 testes. The testes are located close behind the ven­tral sucker and are tightly clustered. Vasa efferentia from all the testes is enter into a short vas deferens. The vas deferens at its posterior end has become enlarged to form the seminal vesicle which opens directly to the ventral surface immediately behind the ventral sucker.

Female reproductive organs:

The female reproductive system consists of a single and elongated ovary located in front of the re-­joined gut caeca. The oviduct is narrow and it proceeds forward to open into the ootype. The ootype opens to the outside on the ventral surface just behind me ventral sucker.

The posterior half of the body is filled with alternately lobed vitelline glands which pour their contents into the common vitelline duct which opens into the oviduct.

9. Life History of Schistosoma Haematobium:

The schistosomes remain permanently wedded and monogamous, the uncoupled females remain as spinsters. At egg laying time the female leaves the male and moves into smaller veins of the bladder and intes­tine and lays eggs singly. It has been esti­mated that a female S. japonicum lays about 1200 eggs per day. Fig. 14.16 shows the life history of S. haematobium.

Egg:

The egg is oval in shape. The average size is 120-170 µm by 40-70 µm. It is provided with terminal spine. The eggs retain their position by these spines. The eggs presumably being aided by the histolytic secretions of the contained embryo work their way out of the vessel into the lumen of the bladder and intestine whence they es­cape with urine or faeces.

Miracidium:

Dilution of the faeces or urine causes the egg to hatch into Miracid­ium larva within a few minutes to several hours. In undiluted faeces or urine the egg does not hatch. The body of the miracidium is covered over with cilia. The miracidium possesses two pairs of flame cells. It has a short gut and anterior and lateral penetra­tion glands,

The sex of the future adult worm remains already determined in the miracidium. As the miracidium lives for 24 hours or less, it tries to find out a suitable snail host (inter­mediate host—Balinus). When the miracidia come close to a suitable host they become excited and dash for burrowing into the tentacle or other parts.

Sporocyst:

During penetration the cili­ated outer covering of the miracidium is shed. The miracidium elongates and becomes trans­formed into sporocyst. The sporocysts are tubular in appearance and make their way through the viscera to the digestive gland.

The sporocysts produce within their brood cavities a second generation of sporocysts. The daughter sporocysts burst free from the mother sporocyst. From the germ cell masses of the posterior end of these sporocysts cer­caria develop.

Cercaria:

Mature cercariae come out from a birth pore near the anterior end of the sporocyst. The sporocyst continues to pro­duce them for several months. The length of the cercaria is about 200 mm. It is provided with a forked tail about 75 mm long. The body of the cercaria bears backwardly pointed spines.

The ventral sucker is well developed. Digestive system remains in the rudimentary stage. A ‘head organ’ occupies the ante­rior one-third of the body. It is provided with two sets of penetration glands. The penetra­tion glands are unicellular with large nu­cleus. They escape from the snail into water in ‘puffs’, a number at a time.

The forked tailed Cercaria or Fercocercus Cercaria swims and rests alternately in water for two or three days. If they fail to reach a final host by this time they die. If successful, they either burrow through the skin or are ingested by human beings. While burrowing, they employ the histolytic or hyluronidase bearing secretions of the penetration glands.

If ingested the cercariae attach themselves to the mucous membrane of the mouth and throat and bore in leaving their tails. Even­tually they find their way into the blood stream and are carried to lungs via heart. From there they are carried to the liver and then migrate to mesenteric veins.

10. Other Trematodes of Schistosoma Haematobium:

More than 3,000 species of digenetic flukes live as parasites on hosts ranging from fish to mammals.

Important flukes which infect mankind are:

(i) Fasciolopsis buski:

Intestinal flukes, most prevalent in India and China. Larval stages occur in snail. Cercaria encysted on the leaves of water nuts. When these raw water nuts are consumed by human beings in the Orient the parasites enter the gut.

(ii) Clonorchis sinensis:

Reside in the bile duct of man, most prevalent in Japan, China and Vietnam. Primary larval stages occur in snails, secondary in the muscles of fish. In­fection results from the consumption of raw or ill-cooked fishes (Fig. 14.17A).

(iii) Paragonimus westermani:

Reside in lungs; wide distribution—Japan, China, Philoppine, India, Africa and New Guinea. Eggs come out through the sputum of host. Larval stages in snails and then on crabs or cray fishes. Enter into the human body with uncooked crustaceans (Fig. 14.17B).