Onchocerciasis

What is Onchocerciasis?

Onchocerciasis is an eye and skin infection caused by a parasite named Onchocerca volvulus (filarial worm). This disease is popularly known as “river blindness,” and black fly is the transmitting vehicle that spreads the infection through a bite. This insect breeds and survives near water streams and flowing rivers, especially neighboring remote areas and fertile land.

Onchocerciasis is a fatal disease after ‘trachoma’ that causes blindness. The parasite O. volvulus also causes filariasis lymphedema.

How does it occur?

When an infected female blackfly bites a person, it injects the parasite’s larvae into the host’s body. Further, the mature worms produce more larvae (microfilariae) that get manifested in the host body. When another blackfly bites the infected person, it ingests the larvae from the blood. During subsequent bites to other individuals, it gets transmitted.

About 17.7 million people are infected with this disease worldwide. It is estimated that currently, this infection is endemic in 30 African countries, Yemen and some parts of South America. As per the analysis, about 99% of cases occur in Africa. Males are at higher risk to get affected by this disorder than females.

In biological term, nematode is a phylum which describes the range and species of roundworms. So, nematode holds a mutual symbiotic relationship with Wolbachia pipientis. Wolbachia is a parasitic bacterium that infects phylum arthropods, i.e., insect’s species including roundworms. These parasitic worms live inside the filarial worm and get benefited by consuming nutrients from the host body. This mutual symbiotic relationship of worm and bacterium is fatal. Wolbachia plays a crucial role in the progress of the disease. By accelerating an immune response in patients, this bacterium increases the pathogenicity of the filarial worms. The filarial worms die when these microorganisms are separated from the parasite. However, this disease happens through the bite of an infected black fly.

The life cycle of Onchocerciasis

The initial phase in the life cycle of Onchocerca volvulus is the microfilaria. Numerous microfilariae flow in the blood of an infected human. A female blackfly of genus Simulium survive on the host’s blood and ingests microfilaria. The microfilariae invade the muscles of the stomach and thorax of the fly. This is the first larval stage marked as “J1.” The second larval stage is termed as “J2.” At the third stage (J3), the larvae immigrate to the mouth parts of the fly and go through maturation for seven days into the saliva.  In this Phase, if infected black fly bites any human, then the developed larvae are passed into the person ’s blood.  The larvae then enter the underlying tissues to produce a huge number of microfilariae every day. The microfilariae roam to the dermal layer, which is the most convenient spot for the female black flies to bite. Further, black flies repeatedly consume a new set of microfilariae to continue the cycle. In a short period, the immature worms occupy the eyes results in a visual loss. The symptoms of the Infection occur when the death of larvae occurs inside the skin or eyes.

Phases

The various manifestations, symptoms, and effects of infection gradually go through different phases which include:

Erisipela de la Costa: This is the first phase of the infection in which intense swelling of the face and itching is prominent. The skin undergoes various modifications, which differ from area to area. This acute phase of infection mostly reports in Central and South America.

Mal Morando: The skin layer or dermal layer often gets expose to severe inflammatory pain, and hyperpigmentation in which the infected area of skin gets dark due to the excessive production of melanin.

Sowda: This is the stage when skin condition becomes hyperactive. It demonstrates an initial level of River Blindness. Infected individuals go through distinct changes in their skin and various further stages of systematic modifications like:

·         Acute Papular Onchodermatitis: Small, raised solid abrasions or bruises called papules seems to be scattered all over the skin which produces itching and irritation.

·         Chronic Papular Onchodermatitis: The Lesions grow large and result in hyperpigmentation of the skin.

·         Lichenified Onchodermatitis: The skin becomes thickened and appears dark due to highly pigmented papules. The inflamed dermal layer gets masked with silvery scales called plaque. The swollen lymph nodes caused by a secondary bacterial infection called lymphadenopathy. It seems dry and leathery similar to the skin of an elephant.

·         Skin Atrophy or Lizard Skin: This is the condition when due to infection skin appears like dry, scaly, wrinkled, pigmented and depicts the loss of elasticity.

·         Depigmentation or Leopard skin: This illustrates the advanced stage of infection in which small pigmented spots with defined edges called macules are visible. After the unusual infection on the skin the causing parasite attack several parts of the eyes such as cornea, conjunctiva and inner colored layer uvea. The infection can also affect the retina and optic nerve. Keratitis can occur due to this infection, and in case of chronic infection, scarring slowly results in visual impairment and subsequently can cause blindness.

Symptoms

  • Itching
  • Severe hyperpigmentation & inflammatory pain (Mal Morando)
  • Intense swelling of the face (Erisipela De La Costa)
  • Skin Rashes
  • Papules formation on the skin (onchodermatitis)
  • Eye lesions
  • Patches and depigmentation on the skin termed as “leopard skin.”
  • Skin loses elasticity and gives the appearance of aging (called “lizard skin”)
  • Nodule formation under the skin
  • Swollen groin
  • Cataract
  • Vision difficulties
  • Swollen lymph (lymphedema)

Diagnosis

  • Skin snip biopsy: It is a diagnostic procedure where the skin gets seized from either Scapula (shoulder area) and iliac crest (near greater pelvis) or lower extremities such as hips, knee, leg, and toe. The further process consists of
    1. Incubation of skin tissue in saline solution at room temperature
    2. Identifying the embryos of the parasite with the help of a microscope
  • Nodulectomy: Applied in case of identifying the microfilariae (adult worms).
  • Slit lamp examination of the eye: Most common method to carry out a detailed physical evaluation of eye. Therefore, this test visualizes and identifies the free-floating intraocular microfilaria in the region such as iris, cornea, vitreous gel and lens.
  • Antibody tests: Two major antibody tests with a sensitivity of 97% and 86% are frequent in use these days. They are Enzyme-linked Immunosorbent Assays (ELISA) and rapid-format antibody card tests (Immunochromatographic test ICT).
  • Polymerase chain reaction (PCR): Biotechnological process to investigate and identify the parasite. However, it is commercially not viable.
  • Palpation: This is also a kind of physical examination of the skin and eyes. The healthcare practitioner can sense the nodules under the patient’s skin with his hands through this technique. However, this method doesn’t provide accurate indications or proof of the condition that’s why other techniques implemented for confirmation.
  • Mazzotti Test: It is an oral therapy for diagnosis in which a dose of diethylcarbamazine (DEC) prescribed to the patients. Then affected ones start reflecting acute skin rashes as a symptom and senses itching within few hours. However, this diagnostic technique has many side effects such as some chemical reactions and ocular complexities.

Treatment

The most extensively used treatment for infection is ivermectin. It’s an effective antiparasitic drug consumed once or twice a year for about 10-15 years. Diethylcarbamazine is another medicine but quite unpopular and insecure.

The treatment aims to destroy the microfilarial phase of the disease to control and recover the symptoms and inhibit further infection in the patient’s body. The above mentioned anti-parasitic drug ‘ivermectin’ is suggested to the patient to paralyze and slay the microfilariae larvae. As per the advice of doctors, two doses of this medicine are enough for an effective outcome. Doxycycline is a part of tetracycline antibiotic family which can destroy the Wolbachia bacteria, sustaining inside the macrofilariae (adult worms). Another treatment is nodulectomy, a surgical process to remove worms and hence, this technique helps in reduction of microfilariae numbers in the host’s body.

Prevention and Prognosis

There is no vaccination discovered yet that can prevent Onchocerciasis. One can adopt any of the following measures to prevent the parasite transmission:

  • Use Insect repellant
  • Wear full sleeved shirts and pants
  • Spray insecticides near water streams and blackfly prone areas
  • Use larvicide to control the vector

Early diagnosis and recognition of infection decide the outcome and treatment of the disease. Hence, regular follow-ups and checkups by the doctor are mandatory as it controls the worst consequences further.

References

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