Diagnosing Onchocerca volvulus, the causative agent of onchocerciasis, requires the combination of clinical presentations and laboratory techniques. Most times, physical signs, such as the presence of microfilariae in the eyes or swollen and hyperpigmented legs serve as initial diagnostic clues.
However, Laboratory tests are important for identifying and confirming active infections, assessing the intensity of the disease, and guiding treatment.
Among these, the skin snip biopsy remains the gold standard test and the mostly widely used method, supported by advanced tools like PCR and serological assays.
Table of Contents
Skin Snip method-
(from the nodules formed on the skin by Onchocerca volvulus)
Sample Collection
Preparation:
-
- Gather sterile instruments, including a scalpel or razor blade, forceps, needle, and sample tubes containing saline solution.
- Ensure the patient is informed about the procedure and consents to the process..
- Site Selection:
- Choose a skin site commonly affected by Onchocerca volvulus, such as the iliac crest, shoulders, calves, buttocks or lower limbs.
- Two to six skin snips are required to be taken from the nodules.
- *Avoid visibly inflamed or ulcerated skin*.
- Sample Extraction:
- Clean the skin surfaces with alcohol and allow it to dry.
- Raise the skin with a sterile needle and slice off the tip i.e. the superficial parts using a sterilized scalpel (approximately 2-3 mm in diameter and 0.5-1 mm deep).
- Ensure the biopsy does not penetrate deeper tissues to minimize discomfort and bleeding.
- Preservation:
- Place the biopsy samples into saline or distilled water on a slide or in a microtube.
Procedure for Analysis
- Incubation:
- Incubate the skin snip sample in the saline or distilled water at room temperature for 30 minutes or longer.
- During this period, microfilariae if present will migrate out of the skin tissue into the liquid medium.
- Microscopic Examination:
- From the tube place a drop of the incubated liquid onto a glass slide and cover with a coverslip
- Observe under a light microscope using x10 Obj to focus and x40 Obj to examine the slide.
- Identification:
- Use morphological features such as size, shape, and movement to confirm the presence of Onchocerca volvulus microfilariae.
- If no microfilariae is found, the preparation should be left overnight and then re-examined the next day.
Interpreting the Results
- Positive Result:
- Detection of live microfilariae in the incubated liquid confirms active onchocerciasis.
- Negative Result:
- Absence of microfilariae suggests no active infection, though this does not rule out past infection or inadequate sample collection.
Note:
Low microfilarial density in early infections or after ivermectin treatment may yield false negatives.
Also, cross-contamination and improper sample handling can compromise results.
Other Laboratory Examinations
Serological Tests:
These tests detect antibodies against Onchocerca volvulus antigens. used to identify exposure to the parasite, especially in children, aiding in mapping and surveillance efforts However, serological tests may not distinguish between past and current infections. For instance OV-16 enzyme-linked immunosorbent assay (ELISA) tests can yield positive results in early infections when skin-snip results are negative.
It is used to identify exposure to the parasite, especially in children, also in mapping and surveillance efforts.
OvH2 and OvH3 are also known to be sensitive to Onchocerca antigen.
But the limitation of serological test is that it only shows exposure to the parasite, not necessarily the presence of an active infection.
Polymerase Chain Reaction (PCR):
As always, PCR techniques will detect O. volvulus DNA in skin snips or blood samples. Highly sensitive and specific and useful for confirming low-level infections or in areas approaching elimination.
Conclusion
Skin snip biopsy, when performed correctly, provides definitive evidence of active infection, which is important in the fight against onchocerciasis. While newer techniques like PCR enhance detection sensitivity.
Combining clinical evaluations with laboratory findings ensures comprehensive diagnosis and supports global efforts to eliminate onchocerciasis.
