Wuchereria bancrofti - Pathogenesis, Pathology

Pathogenesis, Pathology of Wuchereria bancrofti

On the basis of the stage of infection

Pathogenicity of Wuchereria bancrofti is not achieved by infective third-stage larvae (L3) as they do not produce any pathological lesions while invading the host skin.

Similarly, circulating microfilariae larvae also are not pathogenic to the mammal host.

The pathogenesis, and pathology of Wuchereria bancrofti are achieved by the adult male, and adult female parasites as well as the fourth-stage larvae. They are the causative agent of filariasis.

Lymphatic filariasis occurs in the following stages:

1. Dilation of lymphatic vessels

2. Infection of the lymphatic vessels (lymphangitis)

3. Obstruction of lymph nodes

Dilation of lymphatic vessels

• caused by an inflammatory reaction

• inflammatory reaction provoked by the presence of adult Wuchereria bancrofti, developing larvae, metabolic products released during larval molt, and unsheathing of microfilariae during molting

• during the early stage of infection, inflammatory changes lead to the dilation of lymphatic vessels

• although the exact mechanism of dilation of lymphatic vessels is not known, the action of the host immune response or toxicity and toxic products from the parasite is suggested to be responsible

Infection of the lymphatic vessels (lymphangitis)

• as the Wuchereria bancrofti infection progresses, lymphangitis gradually develop

• characterized by the presence of dilated, inflamed, and thickened lymphatic vessels

• this condition is associated with edema, tender painful areas, erythema

• as lymphatics are dilated, secretion of proteinaceous material from lymphatics into the surrounding tissue is increased

• the result of this stage of infection is the formation of conspicuous lymphoedema and thickening of the endothelium

Obstruction of lymph nodes

• lymphangitis is followed by fibrotic degenerative changes in the lymphatic vessels proximal to the lymph nodes

• sclerosis, obstruction of the proximal lymphatic channels, and fibrotic necrosis are observed in lymph nodes

• valves close to the Wuchereria bancrofti parasite also undergo degeneration

The host lymph node obstruction is caused by the:

• presence of adult parasites in the lumen

• endothelial cell proliferation

• thickening of lymphatic vessels

• focal necrosis

• fibrosis, cellular changes, giant cell formation of the lymphatic vessels close to the residing parasite

Pathological changes due to obstruction of lymph nodes are elephantiasis. This classical feature involves affected tissues becoming grossly oedematous with the overlaying skin becoming highly stretched and thickened.

On the basis of the infected time frame

Acute filariasis

Acute filariasis caused by Wuchereria bancrofti includes:

1. Lymphadenitis

2. Lymphangitis

3. Lymphatic abscess

Lymphadenitis

• a typical feature of acute filariasis

• Wuchereria bancrofti attacks lymphadenitis at regular intervals

• precipitated by muscular exercise

• episodes of attack vary from 1 to 2 attacks per year

• typically occurs in the inguinal region and occasionally the axillary lymph nodes

• in some cases, lymphadenitis can occur in atypical sites such as popliteal lymph nodes or breasts

• rarely occurs simultaneously at more than one site

Lymphangitis

Lymphadenitis is followed by retrograde lymphangitis

Lymphatic abscess

• lymphatic abscesses are usually superficial

• infected lymph nodes may suppurate and form abscesses

• the ulcer is formed after an infected lymph node explodes

In chronic filariasis

Chronic filariasis caused by Wuchereria bancrofti infection, occurs mostly below the knee. In some cases, the arm below the elbow may also be involved. Chyluria and genital involvement are present.