Onchocerca volvulus - Epidemiology, Reservoir, Transmission, Laboratory diagnosis, Treatment

Epidemiology of Onchocerca volvulus

Epidemiological studies of Onchocerca volvulus have revealed 17–25 million people infected with Onchocerca volvulus and 0.8 million showing some impairment of vision due to the filarial nematode. It is estimated that 187 million people are at risk of infection. Although the parasite has not been recorded as a cause of a single death, it is the major cause of disability.

About 99% of Onchocerca volvulus infections occur in 31 countries in sub-Saharan Africa while in Latin America (Brazil, Venezuela,), the disease is believed to have been brought by the slave trade.

The infection rate is high along the banks of rivers in the rainforests as well as in Savannah areas.

Reservoir, Source of Onchocerca volvulus

Infected humans are the only source and reservoir of Onchocerca volvulus infection as it lacks any animal reservoir.

Transmission of Onchocerca volvulus

Transmission of Onchocerca volvulus occurs via the bite of infected female black flies of the genus Simulium during a blood meal.

Laboratory diagnosis of Onchocerca volvulus

The laboratory diagnosis of Onchocerca volvulus is done by demonstration of microfilariae in the skin snips.

Sample

• skin snips

  * skin snips from buttocks, iliac crests, leg calves (Africa)

  * skin snip from behind shoulders, truck (Mexico)

  * skin snip from lower limbs (Yemen)

• skin scratches

• biopsy tissue (from sub-cutaneous nodule)

Microscopy

Microscopy of the skin snip specimen to observe the presence/absence of microfilariae is done.

Bloodless skin snips are collected with the help of a sterile needle and a razor blade.

Procedure of skin snip:

• around 3mm of skin is raised with the tip of a needle and cut off with a sterile razor blade

• skin snip is placed on a slide or in the well of a microtitration plate

• the specimen is immersed in the physiological saline and left at room temperature for 30 to 60 minutes

  * during this time, Onchocerca volvulus microfilariae emerge out of the skin into the saline

• the slide is examined under a microscope for the presence/absence of microfilariae

• if microfilariae are not seen, the slide is left overnight and re-examined

This method is specific and accurate but cannot differentiate between early or mild infections.

Skin scratch samples are obtained by careful removal/scrapping of the superficial layer of the epidermis with the blunt edge of the disposable lancet.

Serodiagnosis

If Onchocerca volvulus microfilariae are not observed in skin snip microscopy, serological tests can be used for serodiagnosis. However, a major disadvantage is that it cannot differentiate between past and present infections.

Some serological tests used include:

• ELISA (uses microfilarial antigens)

• rapid-format antibody card test (uses Ov16 antigen to detect immunoglobulin G4)

Molecular methods

PCR is the most common molecular method for the diagnosis of Onchocerca volvulus. This test can also detect the migration of microfilariae back to the host skin following treatment.

Imaging Methods

Ultrasound can detect vitreous changes in the eyes as well as deep onchocercoma.

Other tests

Other tests for diagnosis of Onchocerca volvulus include:

• histological diagnosis – demonstration of adult worms in the biopsy tissue from subcutaneous nodule

• slit-lamp examination – if present, live microfilariae is observed on the cornea

• DEC patch test – also known as Mazzotti patch test which is specific but less sensitive

  * DEC patch produces a local reaction to dead microfilariae at the patch site

Treatment of Onchocerca volvulus

Onchocerca volvulus is treated by oral administration of Diethyal carbamazepine (DEC), and Ivermectin.

Surgical removal of nodules is also done.