Onchocerca volvulus - Host Immunity, Clinical Manifestation

Host Immunity of Onchocerca volvulus

Since adult worms of Onchocerca volvulus live in the nodules, they are hidden from most components of the host immune system. Microfilariae, however, are attacked by host immunity as they exit the nodules.

Complex antigens (soluble and surface) show cross-reactivity with several other filarial worms and are produced by microfilariae and infective J₃ juveniles are detected by the host immune system which triggers an immune response to clear the infection.

The soluble antigens released by Onchocerca volvulus induce an elevated level of antibody production (IgG, IgM, and IgE).

Although it is unclear if it provides a protective function, opsonizing antibodies that tag cells for destruction are also produced against the infective J₃ stage and microfilariae of Onchocerca volvulus.

Onchocerca volvulus microfilariae can escape host immune system. This is achieved by cleaving C3b to form iC3b and blocking the complement system pathway. IC3b is unable to reach the next step in the pathway and hence the microfilariae sees little to no attack by the immune system.

After the age of 40, the number of parasites carried decreases – suggesting that protective immunity develops over time.

Immediate hypersensitivity, immune complex deposits, products of activated lymphoid cells, autoantibodies, and cytotoxic T cells are suggested to be responsible for clinical manifestations of onchocerciasis.

In the sub-cutaneous nodules, host neutrophils, eosinophils, and macrophages can be found. Similar to other helminthic infections, these inflammatory cells are responsible for killing microfilariae in untreated cases.

Clinical Manifestation of Onchocerca volvulus

Clinical manifestations of Onchocerca volvulus include skin and ocular lesions. The frequency and intensity of clinical symptoms of the parasitic infection vary on the duration of exposure and geographical areas.

Skin lesions

• acute pruritic rashes may be present – which sometimes also include urticarial and vesicular lesions

• a condition known as sowda (in Yemen) or Erisipela dela costa (in Central America) occurs – which is characterized by hypertrophic, hyperpigmented, and thickened skin

• in chronic cases, atrophic depigmented and lichenified lesions are present

• one of the major clinical syndromes of Onchocerca volvulus is onchocercoma or the sub-cutaneous nodule

Onchocercoma

• also known as subcutaneous nodules

• typical pathological lesion in Onchocerca volvulus infection

• are firm, smooth, rubbery, and contain adult worms

• caused by microfilariae

• severe itching occurs

• the shape varies from round to elongated

• size ranges from a few millimeters to several centimeters in diameter

• can be freely moving or attached to the underlying tissue

• site of distribution of nodules depends upon geography

  * Africa – nodules found in the lower part of the host body such as the coccyx, sacrum

  * Central America, African Savannah – nodules found in upper body parts such as the head, neck, shoulders

  * Yemen – nodules are found in lower limbs

  * in children below the age of 9, sub-cutaneous nodules containing Onchocerca volvulus are found in the head

• in chronic cases, keratitis occurs which may result in blindness

Ocular lesions

As the Onchocerca volvulus infection progresses, ocular lesions lead to visual impairment or blindness. It progresses through the following stages:

1. Punctate keratitis

2. Sclerosing keratitis

3. Iridocyclitis, choroiditis, and optic atrophy

Punctate keratitis

• punctate keratitis is the most common lesion

• caused by dead microfilariae

• of whitish corneal opacity

• mostly found in the superficial area of the stoma

Sclerosing keratitis

• sclerosing keratitis due to Onchocerca volvulus infection occurs as a result of host's immune reaction in response to a large number of microfilariae in the cornea

• a serious complication is blindness

Iridocyclitis, choroiditis, and optic atrophy

Iridocyclitis, choroiditis, and optic atrophy are conditions that are common in heavy Onchocerca volvulus infections. It can result in blindness.

Complications of Onchocerca volvulus

In addition to blindness, secondary bacterial infections of the skin lesions are another important complication.

Prognosis of Onchocerca volvulus

The prognosis of Onchocerca volvulus infection is good in patients receiving proper and adequate therapy – before irreversible ocular lesions develop.