An Example of Phylum Nematoda: Wuchereria Bancrofti

In this article we will discuss about Wuchereria Bancrofti:- 1. History of the Discovery of Wuchereria Bancrofti 2. Geographical Distribution of Wuchereria Bancrofti 3. Morphology 4. Life Cycle 5. Pathogenicity 6. Treatment and Prevention of Disease.

Contents:

1. History of the Discovery of Wuchereria Bancrofti
2. Geographical Distribution of Wuchereria Bancrofti
3. Morphology of Wuchereria Bancrofti
4. Life Cycle of Wuchereria Bancrofti
5. Pathogenicity of Wuchereria Bancrofti
6. Treatment and Prevention of Disease Caused by Wuchereria Bancrofti

1. History of the Discovery of Wuchereria Bancrofti:

In Paris Demarquay first reported the larval form of the parasite in the fluid of hydrocele in 1863. Then Ott Wecherer, a Portuguese physician reported the occurrence of the parasite in the chylous urine in man in 1866.

Bancroft was also found in the human blood in 1876. Cobbold gave the scientific name—Filaria bancrofti in 1877 but Seurat changed its genus and gave the scientific name—Wuchereria bancrofti, in 1921.

2. Geographical Distribution of Wuchereria Bancrofti:

The filarial worm is widely distributed in the coastal areas of tropical and subtropi­cal countries including India, Bangladesh, Philippines, Indonesia, and North-eastern Aus­tralia, Formosa, Southern China, Japan, many Pacific Islands, Central Africa, Caribbean Islands, Coasts of Arabia and parts of South America.

A Pacific strain occurs throughout the Pacific Islands except Hawai. In India, the parasite is found mainly along the coastal areas and along the banks of big rivers.

3. Morphology of Wuchereria Bancrofti:

1. The adult filarial worms (Wuchereria bancrofti) are long and hair-like, often creamy-white in colour (Fig. 15.11).

2. The sexes are separate and they show a distinct sexual dimorphism. The males measure 40 mm in length and 0.1 mm in diameter.

3. The females are about 80-100 mm in length and 0.24-0.3 mm in diameter.

4. The tail of the male is curved and there are two spicules of unequal length. It also contains a number of genital papillae and caudal alae.

5. The body tapers to a slightly swollen head and bearing mouth as a simple pore without lips.

6. The oesophagus is partly muscular and partly glandular.

7. The vulva or genital pore of female opens a little behind the middle and is provided with a pyriform ovijector.

8. The female is ovoviviparous or lays eggs which contain embryos.

The embryo is known as microfilaria (Fig. 15.10). Though they commonly referred to larvae (microfilaria larva) but they should appropriately be termed embryos be­cause their internal organization rep­resents an early developmental stage and they are also not comparable to other nematode larvae.

4. Life Cycle of Wuchereria Bancrofti:

The life cycle of Wuchereria bancrofti (Fig. 15.11) is completed through the two hosts (digenetic), man and mosquito. Man is the definitive host and the adult worms and the live embryos (microfilariae) inhabit in the lymphatic system and blood stream.

Mos­quito, the species belonging to genus Culex, Aedes and Anopheles, all act as intermediate host. Culex quinquefasciatus = Culex fatigans and in some places a closely related species, C. pipiens play a leading role as vectors of periodic form in different parts of the world.

Life cycle in man:

When both sexes are found in the same afferent lymphatic vessel of human beings, the copulation takes place and each gravid female gives rise to 1000 minute sheathed larvae called microfilaria (Fig. 15.10). These larvae are 127-320 µm long. They can move both with and against the blood stream.

Each larva is colourless, transparent and cylindrical with blunt head. The body is encased in a hyalin sheath and cuticle is thin, striated bearing underneath a single layer of flattened sub-cuticular cells or epi­dermal cells. The head bears a clear cephalic space and rudiments of adult buccal cavity with oral stylet. Various structures are seen towards backwards next to oral stylet.

These are nerve ring band, renette cells, a darkly stained interior mass, four large cells and future anus. The caudal end is pointed and without nuclei. The body column is pro­vided with somatic cells being interrupted at intervals by cellular and nuclear land­marks.

There is an anterior shiny spot, called rudimentary excretory pore. The relative positions of these landmarks are definite in their body and they cause no trouble to man even if numerous.

The microfilariae are unable to develop further in the human host unless they are ingested by the mosquito. If not sucked up by the mosquito, the larvae will die.

Nocturnal and diurnal periodicity:

Noctural periodicity is the appearance of microfilariae at night in the peripheral circu­lation of the human host.

In the common strain of W. bancrofti the nocturnal periodi­city is the most common phenomenon but in Polynesian islands of South Pacific except Hawai the microfilariae appear in large num­bers in the pulmonary vessels (i.e., blood vessels of the deep tissue of the body) during the day, so it shows a certain degree of diurnal periodicity.

It has been reported that Aedes pseudoscutellaris, a mosquito vector of the Pacific strain of W. bancrofti, sucks blood during daytime from the body of human’s host. During night the microfilariae are found in the peripheral circulation between 10.00 pm to 2.00 am.

Reasons for periodicity:

The exact reasons for nocturnal and diur­nal periodicity are not exactly known. In man, the decrease in body temperature, in­creased O₂ tension, increase in CO₂ pressure, increased body acidity and relaxation of the host during sleep induce the microfilariae to migrate in the peripheral circulation during night. There is also chemotactic between the microfilariae and the saliva of mosquito hosts (vectors) which induces the microfilariae to be more plentiful in the peripheral circula­tion at night.

Life cycle in mosquito:

When the microfilariae are sucked up by the vector mosquito (Culex fatigans) along with the blood meal, the larvae pass to the midgut of the insect and lose their sheaths within 2-6 hours. Then the larvae penetrate the gut wall and migrate to the thoracic muscles. Inside these muscles the organisms become shortened into sausage-shaped forms measuring 124 to 250 µm in length and 10 to 17 µm in breadth.

This stage of development represents the first larval stage in which the first moult occurs. In the first larval stage the tail portion becomes atrophied and possesses the well-defined intestinal tract. Within three to seven days, the larvae grow rapidly, moult once or twice and attain the size which measure about 225-380 µm in length. These represent the second stage larvae.

On the tenth and eleventh day the metamorphosis of the larvae becomes complete inside mus­cles. The tail portion atrophies and the diges­tive system, body cavity and genital organs are fully developed. These forms are recog­nised as third stage filariform larva measur­ing 1500 to 2000 µm in length and 18-23 µm in breadth.

Then the larva migrates into the proboscis sheath of the mosquito’s mouth parts and this stage is infective to man (third stage filariform larvae). It is to be noted that after the second moult third larval stage begins.

Infection to man and development into adult worms:

When the infected insect bites the man, the third stage larvae enter the human host through the site of puncture wound on the skin. Then the larvae reach to the lymph vessels via blood stream and settle down in inguinal, scrotal or abdominal lymphatics.

Within five to eighteen months the larvae undergo two additional moults to attain the sexual maturity and start new generation of microfilariae. In the life cycle of W. bancrofti, four moults and five stages are recorded of which fifth stage is the adult.

Conditions for development:

The development of the infective stage (3rd stage larvae) takes a minimum of 8-10 days but more frequently 2 weeks or more. The optimum conditions are 80 °F and 90% humidity.

5. Pathogenicity of Wuchereria Bancrofti:

Due to heavy infection, the accumulated living or dead adult worms block the lymph circulation resulting the immense enlarge­ment of the affected organs, such as limbs, scrotum, vulva, mammary glands. Ultimately elephantiasis or filariasis or Bancroft’s filariasis is produced. It causes filarial fever, headache and mental depression.

6. Treatment and Prevention of Disease Caused by Wuchereria Bancrofti:

No proper or satisfactory treatment is yet known.

(i) Oral administration of Hetrazan and compounds of antimony and arsenic may be effective by eliminating microfilariae from the blood circula­tion.

(ii) Administration of Diethylcar- bamazine (DEC) mixed in common salts may be more effective under the National Filaria Control Programme.

(iii) There is no effective drug for elephan­tiasis.

(iv) Cortisone injection can reduce the swelling.

(v) Large swellings can be removed by surgery.

(vi) Protection from mosquito bites and control of mosquito vectors may be helpful in preliminary level.