Diagnosis

Palpating

This is a very simple technique in which the patient’s skin is patted for nodules.  These nodules are very common in patients with Onchocerciasis because the microfilariae congregate near the skin surface and cause swelling.  Palpating only indicates swelling and does not provide concrete proof of Onchocerciasis, so other techniques are often used to provide an exact diagnosis.

Skin Snips

One of the most common diagnostic techniques is the skin snip.  It involves the removal of some skin from an inflammed area, placing the skin snip into saline to encourage microfilariae to leave the skin, and microscopic examination to determine microfilarial load.  There are a few drawbacks to this method of detection. Only trained personnel can perform the procedure, so there are additional costs for training and the actual microfilarial examination is not sensitive enough to detect an early-stage infection. Patients also find the invasiveness unpleasant and have concerns about the sterility of the surgical instruments.  

Mazotti Test

This test is rarely used anymore because it can cause a severe allergic reaction, possibly leading to death, and has been replaced for the most part by the DEC patch test. The test requires the oral administration of 6 mg of DEC and a positive result is indicated when pruritus (severe itching) and intense inflammation occur due to the death of microfilariae.

DEC patch test

The DEC patch test was created as an alternative to the Mazotti test because of the potential for serious side effects associated with the large dose of DEC. In the patch test, gauze pad is soaked in a 20% solution of DEC and placed on the hip.  The application site is later examined for skin inflammation due to DEC-induced microfilariae death.  Patients like the non-invasiveness of this procedure and it is the best method of testing for re-emergence of infection but the patch test is less sensitive than nodulectomies.

ELISA (Enzyme-linked immunosorbent assay)

The ELISA tests for the presence of antigens to the Onchocerca volvulus parasite. It only requires a small sample of blood from a simple finger prick that is examined with expensive lab equipment.  This test does have a high sensitivity for the antigens, but there are a few problems for using ELISA to diagnose Onchocerciasis.  One problem is the high cost of the procedure because of the need for trained lab personnel and for the equipment itself.  The other difficulty is the inability of the test to determine if the antigens are due to a current infection or a previous infection.  In endemic areas where the people are continually exposed to O. Volvulus, there is a strong possibility that their positive ELISA test is due to a past infection. 

PCR (Polymerase Chain Reaction)

PCR testing involves the removal of skin from a nodule and then extracting DNA from the microfilariae. PCR is very sensitive to low-level infections so it is the best method for diagnosing the start of a new infection. It is an improvement on the simple nodulectomy procedure, but it is very expensive due to laboratory costs and the patients dislike the invasiveness of a nodulectomy. 

PCR lab equipment.
Photo courtesy of Progemics Core Facility for Applied Proteomics and Genomics

Rapid-format Antibody cards

Antibody card testing requires a drop of blood from a finger prick in order to test for the presence of antibodies to O. volvulus. A positive test will result in a color change on the card surface.  This test is similar to the ELISA, but does not require lab testing and provides instantaneous results in the field.  It is just as sensitive as the ELISA and less expensive.