is a very simple technique in which the patientís 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.
Physician palpating a nodule.
Photo courtesy of WHO/TDR/Crump
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
Women having a portion of skin removed from hip for skin
Photo courtesy of WHO/TDR/Crump
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 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.
Man with the DEC patch test on his hip.
Photo Courtesy of WHO/TDR/Crump
(Enzyme-linked immunosorbent assay)
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
(Polymerase Chain Reaction)
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.
Photo courtesy of Progemics
Core Facility for Applied Proteomics and Genomics
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.