Morphologic detection of microfilariae in stained smears is still thought of as the industry gold standard.
Specimen: EDTA (or sodium citrate 3.8%) blood specimens, fluids, or tissue/skin snips.
Method:
Blood - The direct detection method for diagnosing active infection is the identification of microfilariae in a peripheral blood smear (thin or thick) by microscopic examination. (Blood may be collected via fingerstick or in an EDTA tube.) Microfilariae may sometimes be observed in specimens collected at night (after 8 pm). The periodic form (or periodicity) refers to the time of day or night when microfilariae may exhibit a marked presence in the peripheral blood. Nocturnal periodicity refers to a nighttime presence, a trait of Wuchereria and Brugia (10 PM–2 AM), while Loa loa is diurnal - denoting daytime occurrence (10 AM - 2 PM).
In blood specimens: Mansonella perstans and Mansonella ozzardi do not exhibit periodicity.
- A thick smear should be made and stained with Giemsa (or hematoxylin and eosin).11 Thin spears may also be utilized, but recall that organisms in low concentration may be missed.
- Concentration procedures may be utilized (Such as Knott's concentration method. The goal is to help concentrate the microfilariae in peripheral blood specimens in order to provide a higher yield for viewing, especially when very few may be present. The sediment is smeared onto a slide, and the traditional Giemsa stain procedure is then used. Knott's concentration method does not affect the sheaths of microfilariae. Sheaths will be easier to see because only white blood cells and microfilariae will be seen in the resulting smear of sediment.)
- Blood films should be scanned entirely at 10x magnification.
- Marked eosinophilia is usually (but not always) seen in the complete blood count (CBC)
Note: Misidentification may occur due to shrinkage or complete loss of the sheath (if present) during the preanalytic stage. Causes may include the use of:
- Old blood specimens (delay in processing)
- An inappropriate blood specimen (incorrect tube/anticoagulant)
Tissue/skin - The standard of diagnosis using tissue is the microscopic examination of bloodless skin snips. A skin snip is a type of biopsy specimen. Skin snips are the preferred specimen type for recovering Mansonella streptocerca and Onchocerca volvulus. (There is no periodicity in skin snip specimens.) Wet mounts of the biopsy specimen may demonstrate motile microfilariae. Cytologic procedures may also be employed.
Body Fluids (Urine, sputum, hydrocele fluid) - Centrifugation with an examination of the sediment is recommended. Cytologic procedures may be employed.
12. CDC/CDC Connects. Image #20999. "Created in May 2015, by Centers for Disease Control and Prevention (CDC), Epidemiologist, Caitlin M. Worrell, MPH, this image depicted a Haitian child holding on to his blood tube during a transmission assessment survey (TAS) that was being conducted in the Dondon Commune in Northern Haiti." PHIL public domain. Created 2015. Accessed April 6, 2023. https://phil.cdc.gov/Details.aspx?pid=20999
