Filarial Worm: Histology, Habitat and Structure (With Diagram)

In this article we will discuss about:- 1. Histology of Filarial Worm 2. Systematic Position of Filarial Worm 3. Habitat and Structure 4. Periodicity 5. Pathogenicity and Clinical Features.

Histology of Filarial Worm:

The filaria worm, Wuchereria bancrofti is an endoparasite of the lymph glands, connective tis­sues of organs and lymph vessels. The genus Wuchereria contains the well-known species W. bancrofti, Seurat, 1921 which infects the lymphatic system of man in warmer parts of both the eastern and western hemis­pheres.

It produces inflammatory reactions lead­ing to fever, tissue swellings, lymphatic blockage and sometimes grotesque deformities of the limbs etc.. called elephantiasis. This filarial worm has almost a widespread distribution.

It is found in India, Korea, Japan, China, Arabia, East Indies, Brazil, South Pacific Islands, Central Africa, South America etc. In India it is distributed chiefly along the sea-coast and along the bank of rivers. It also occurs in Australia and Mediterranean areas between 40°N and 30°S.

Wuchereria bancrofti causes bancroftian filariasis, resulting in elephantiasis among human beings, causing revolting swellings often in the legs or genital system. The adult worm is found in tightly coiled nodular masses within the major lymphatic ducts.

The causative organism of filariasis was microfilaria and it was observed by Demanquay in 1863 in the hydrocoelic fluid of man. In 1876 Bancroft discovered the adult female in man. The adult male was first seen by Bourne (1888).

In 1878 Manson reported that a mosquito of the genus Culex acts as the transmitting agent for Wuchereria bancrofti. He (1887) also studied the life history of this parasite and suggested reasons for the marked diurnal periodicity of microfilaria in the peripheral human circulation.

Vogel in 1928 and Fain in 1951 gave accounts of the adult structure, The extensive literature on various aspects of filariasis has been the subject of numerous reviews of which the following are representatives: Mak (1983), Southgate (1984), Evered and Clark (1987), Ottensen (1990), Nelson (1990, 1991) etc.

Systematic Position of Filarial Worm:

Phylum – Aschelminthes

Class – Nematoda

Subclass – Phasmida

Genus – Wuchereria

Species – bancrofti

Habitat and Structure of a Filarial Worm, Wuchereria Bancrofti:

Habitat:

Filarial worm is a dreaded human parasite and found only in the lymphatic vessels and lymph nodes of man only. Wuchereria is digenetic i.e. it requires two hosts to complete its life cycle. Man is the definitive host harbouring the adult worms of this parasite, while intermedi­ate host is a blood sucking insect, usually a Culex mosquito specially C. pipiens and occasionally species of Anopheles and Aedes. Adult worm resides coiled up in the lymph glands and lymph passages of man.

Structure: Adult worms:

These are long hair like, filiform and cylindrical in shape. They are creamy white in colour and both ends are tape­ring, the head-end terminating in a slightly rounded swelling. Sexes are separate and there is a distinct sexual dimorphism.

The females are 65 to 100 mm long and only 0.25 mm in diameter. Males are smaller, 40 mm in length and 0.1 mm in diameter. The tail-end of male is curved ventrally and contains two spicules of unequal length and a number of genital papillae. The tail end of female is narrow and abruptly pointed. The female possesses a ventrally placed vulva disposed anteriorly and provided with pyriform ejector mechanism or ovijector.

Mouth aperture is simple without lips. Pharynx or oesophagus is divisible into an anterior muscular and a posterior glandular portion. There is no oesophageal bulb in the digestive tract and intestine is simple as in other nematodes. Males and females remain coiled together and can only be separated with difficulty.

Embryos (Microfilariae):

i. The live and active microfilariae are colourless, transparent and have elonga­ted bodies with blunt heads and rather pointed tails.

ii. The microfilariae of W. bancrofti measures about 290 μm in length by 6-7 μm in breadth.

iii. The live microfilariae are enclosed in a thin sheath which projects prominently beyond both the ends of the larvae.

The sheath is much longer than the larval body so that the larva can move forwards and backwards within it. The sheath is present as an investing membrane round the larva and it represents the chorionic envelope.

iv. The cuticle is thin and striated and is secreted by a single layer of sub-cuticular cells.

v. The anterior end of Mf. bancrofti is blunt bearing a distinct cephalic space and rudi­ments of adult buccal cavity with oral stylet (Fig. 13.3).

vi. The posterior end of this larva is pointed.

vii. The central axis of the larval body is pro­vided with somatic cells or nuclei of cells which extend from the head to the tail end. These nuclei of cells do not extend upto the tip of the tail and these are used as defi­nite landmarks in the identification of the species to which the microfilaria belongs.

viii. Practically the somatic cells or nuclei of cells appear as granules.

These granules are broken at definite places and they include the following:

(a) Nerve ring, an oblique space,

(b) Anterior v-shaped spot, represents the rudimentary excretory sys­tem and

(c) Posterior v-shaped spot or tail spot, represents anus or cloaca (i.e. termi­nal part of the digestive canal).

ix. A few G-cells (a series of usually 4 large celled—the genital rudiments) are present near the posterior end; while G-cells 2, 3 and 4 are just in front of the anus. G-cell-1 is present further in front.

x. The body column consists of somatic cells which are interrupted only by the aforesaid land-marks and a series of G-cells 1, 2, 3,4.

xi. Near the posterior end of the larva is the anal spot. The posterior end is devoid of nuclei of cells.

The microfilariae do not further develop in the human body unless they are taken up by their intermediate host (mosquito). The larvae remain latent till a mosquito sucks them up along with the body of the host. Unless this occurs within a reaso­nable time, the microfilariae degenerate and perish. The life span of microfilariae in the human body has been seen to be as long as 70 days.

Male:

i. Length 2-5 cm/0.1 mm in diameter.

ii. The tail end is curved ventrally.

iii. Spicules are unequal in sizes.

iv. The caudal end is provided with 12-15 pairs of small senile papillae.

v. Vulva aperture is absent.

Female:

i. Length 8-10 cm/0.2-0.3 mm in diameter.

ii. Tail end is narrow and abruptly pointed.

iii. Absent.

iv. Only 2 rows of small sessile papillae are present.

v. Vulva with ovijector present.

Periodicity of Filarial Worm:

Microfilariae of the most filarial worms exhibit a definite rhythmicity in their appearance in the peripheral blood, but appear periodically at night, mostly from 4 p.m. to 10 a.m. and show complete or partial hide out during the day time. This periodicity which represents one form of circadian rhythm, coincides with preferred bitting hours of the insect vector.

In 1879 Patric Manson discovered such a periodicity for W. bancrofti and thereafter it has been reported to occur in a num­ber of other filarial worms such as Loa loa, Brugia malayi, Dirofilaria immitis and D. corynodes (Fig. 13.7).

The following kinds of periodicities have been recognised among the various filarial worms:

(a) Nocturnal periodicity:

The microfilariae of different filarial worms like W. bancrofti, B. malayi, D. corynodes, appear in large numbers in the peripheral blood by night time, which corresponds with the preferred biting hours of the insect vectors.

(b) Diurnal periodicity:

The microfilariae of Loa loa are more numerous in the peripheral blood by daytime that synchronizes with the biting hours of Chrysops which transmit them to man.

(c) Semi-nocturnal periodicity:

The micro­filariae of Dirofilaria immitis appear in large num­bers in peripheral blood by evening or night and are sparsely occurred in the early morning. The microfilariae of D. immitis are transmitted by mosquitoes which bite in the evening.

(d) Non-periodic periodicity:

A definite strain of W. bancrofti has been found to be present all through the 24 hours in the peripheral blood but it is observed somewhat more numerous in the afternoon hours.

In 1951 Hawking and Thurston demonstrated that during the daytime the microfilariae agglo­merate in blood capillaries of the lungs. The microfilariae are not very much active between the peripheral blood and lungs, instead they are passively carried by the blood into the lungs where during the daytime they tend to agglome­rate.

Again they are released from the lungs into the peripheral circulation during night time. Probably the accumulation of larvae at the junc­tion of the arterioles and capillaries include the increase in oxygen tension and decrease in carbon- dioxide tension.

The periodicity seems to be of immense sur­vival value to the larvae as it not only enhances the changes of larvae being sucked up the Culex mosquito (insect vector) but also assists to make a good compromise between the two entirely diffe­rent environments of their vertebrate hosts and the insect vectors.

Pathogenicity and Clinical Features of Filarial Worm:

Pathogenicity and Clinical features:

The morbid change caused by W. bancrofti is essentially confined to the lymphatic system. The disease ini­tiated by this parasite is called ‘wuchereriasis’, commonly called ‘filariasis’.

Filarial worm live in the lymphatic system of human being, where they obstruct the flow of lymph, causing a severe condition termed as ‘elephantiasis’ in which the limbs or other body parts grow to enormous size.

The term ‘filariasis’ is normally used to denote the morbid changes produced by the lymphatic dwelling of Wuchereria and Brugia. The symptoms are fever, chills, block­age of lymph channels and painful localised swellings in the limbs (Fig. 13.8).

The injurious influence excited by the adult worm, W. bancrofti and its developing larva on the host is an inflammatory reaction of the lymphatic system, lymphangitis which causes the basic lesion in classical filariasis. Lesion in occult fila­riasis is caused by microfilariae which is observed not only in the lymph nodes but also in the lungs, spleen, liver etc.

Clinical considerations:

(a) Classical filariasis:

It is caused by developing worms and adults and the microfilaria pre­sent in the peripheral blood:

i. Lymphangitis i.e., inflammatory reac­tions in the lymphatic vessels. The parts usually involved are the lymphatics of the testicular and epididymis, the lymphatics of the spermatic cord, abdominal lym­phatics and the lymphatics of the upper and lower extremities. This is by allergic reactions caused by toxic substances emanating from dead and degenerating parasites or by gravid females dischar­ging microfilariae.

The favourite site for the adults of W. bancrofti is the globus major of the epididymis. The visible inflamed lymph vessels may be seen as red streaks underneath the skin. On pal­pation they are found to be painful and appear as cord-like swellings. An acute abdominal symptom may arise as a result of involvement of the retroperitoneal lymphatics.

ii. Lymphatic obstruction:

This involves physical obstruction of lymph vessels by one or more adult worms. This is partly due to the inflammation of the walls of the lymphatics and the consequent hyperpla­sia and partly due to mechanical blockage by the worms. Lymphatic vessels may show extensive fibrosis on account of recurrent lymphangitis.

iii. Filarial fever:

This is usually accompa­nied by a rise of temperature ranging from 103°F to 104°F which may prevail for 3-5 days. This crisis will coincide with sweating. The fever is associated with inflammation of lymphatics harbouring the adult. Examination of blood shows a transient leucocytosis with an increase of neutrophils and microfilariae in blood.

(b) Occult filariasis:

This happens when the microfilariae are present in the tissues and not in the peripheral blood circulation. It is a con­dition in which there is a massive eosinophilia, enlargement of lymph node, spleen and liver and pulmonary complications. The adult worms produce microfilariae conti­nuously but they do not reach peripheral circulation because they are destroyed in the tissues. It is an unusual host-reaction to fila­rial antigen, resulting in the development of an eosinophil granuloma enmesh microfila­riae as their remnants.

This condition leads to tropical pulmonary eosinophilia and is characterised by low fever, loss of weight, violent bout of dry cough emitting blood tinged sputum in small amount, dyspnoea and enlargement of spleen. Chest radiography shows increased broncho-vascular markings or mottling’s of lung.