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Diagnosis & Understanding Your Results

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Filariasis is diagnosed using specific blood tests, skin snips, or ultrasounds to detect the parasites. Because some worms are only active at night, blood draws must be timed carefully. Screening for a Loa loa co-infection is critical before treatment to prevent severe brain inflammation.

Key Takeaways

  • • Lymphatic filariasis is often diagnosed using a Filarial Test Strip or a night-time blood smear when parasites are most active.
  • • Doctors can sometimes see live adult worms moving in the lymphatic vessels on an ultrasound, a finding known as the Filarial Dance Sign.
  • • Onchocerciasis, also known as River Blindness, is typically diagnosed using a microscopic skin snip test.
  • • Patients who have traveled to Central or West Africa must be screened for a Loa loa co-infection before starting filariasis treatment to prevent severe neurological complications.

Diagnosing filariasis is a specialized process because the parasites behave differently depending on their species and the time of day. Your doctor will use a combination of blood tests, imaging, and physical exams to determine which parasite is present and how active the infection is.

Testing for Lymphatic Filariasis (LF)

Because the worms that cause LF often release their offspring (microfilariae) into the blood only at night, timing is everything.

  • Circulating Filarial Antigen (CFA) Tests: These are the most common diagnostic tools for Wuchereria bancrofti. They use a simple finger-prick of blood to detect proteins (antigens) shed by adult worms [1][2]. The Filarial Test Strip (FTS) is a modern version of this test and is highly sensitive [1].
  • Microscopy: A “blood smear” involves looking at a sample of your blood under a microscope to find the actual worms [3]. In many regions, this blood must be collected between 10:00 PM and 2:00 AM when the parasites are most active in the bloodstream [4]. However, in some regions like the South Pacific, the parasites are “sub-periodic” and do not follow this strict nocturnal schedule, meaning your doctor may adjust when your blood is drawn.
  • Antibody Tests: These tests look for your body’s immune response (IgG4 antibodies) to the parasite [5]. While helpful, a positive antibody test can sometimes mean you were exposed in the past, rather than having an active infection today [5][6].

The “Filarial Dance Sign”

Ultrasound is an incredibly useful tool for finding live adult worms [3]. Doctors look for what is called the Filarial Dance Sign (FDS) [7]. This is the visual detection of live adult worms moving within your lymphatic vessels on the ultrasound screen [7][3]. It is most commonly found in the scrotal lymphatics of men but can be seen in other areas of the body where lymph nodes are close to the surface [8][9].

Testing for Other Filariasis Types

  • Onchocerciasis (River Blindness): The gold standard for diagnosis is a skin snip [10]. A tiny piece of skin is removed and placed in fluid; if the larvae (Onchocerca volvulus) are present, they will emerge from the tissue and can be seen under a microscope [10][11].
  • Loiasis (African Eye Worm): This is often diagnosed by directly observing an adult worm moving across the eye (subconjunctival migration) [12]. Blood smears for Loa loa are typically done during the daytime, as these parasites are most active in the blood during daylight hours [12][13].
  • Mansonelliasis: Like other forms, this is usually found by examining blood smears or using molecular tests like PCR to find the parasite’s DNA [14][15].

A Critical Warning: Checking for Co-Infections

If you have lived in or traveled to West or Central Africa, it is vital that your doctor screens you for Loa loa before starting treatment for other filarial infections [16].

If a person has a very high “load” of Loa loa in their blood and takes certain medications (like ivermectin), it can cause a Serious Adverse Event (SAE) [16][17]. This happens because the medication kills too many worms at once, which can lead to life-threatening inflammation in the brain (encephalopathy) [18][19]. Your doctor will use a “test-and-not-treat” strategy if your Loa loa levels are too high, choosing alternative ways to safely lower the parasite count first [18].

Next step: Read about Standard of Care Treatment to see how these results guide your medication plan.

Frequently Asked Questions

Why do I need to get my blood drawn at night for a filariasis test?
The parasites that cause lymphatic filariasis often release their offspring into the blood only at night. Drawing blood between 10:00 PM and 2:00 AM ensures the parasites are active in the bloodstream and can be seen under a microscope.
What is the Filarial Dance Sign on an ultrasound?
The Filarial Dance Sign is the visual detection of live adult worms moving inside your lymphatic vessels. Doctors look for this distinct movement on an ultrasound screen to confirm an active infection.
How do doctors test for Onchocerciasis or River Blindness?
Doctors typically use a skin snip test to diagnose Onchocerciasis. A tiny piece of skin is removed and examined under a microscope to look for the parasite larvae emerging from the tissue.
Why is it important to test for Loa loa before starting treatment?
If you have a high level of Loa loa parasites and take certain medications, it can cause life-threatening brain inflammation due to a rapid die-off of worms. Doctors must screen for this co-infection to safely manage your treatment and avoid serious side effects.
Does a positive filariasis antibody test mean I have an active infection?
Not always. While a positive antibody test shows your immune system reacted to the parasite, it can sometimes indicate a past exposure to the infection rather than a current, active presence of worms.

Questions for Your Doctor

  • • Based on my travel history, should I be screened for Loa loa before we start any treatment?
  • • If my blood test for filariasis is positive, how do we determine if it is an active infection or a sign of past exposure?
  • • Can we use ultrasound to look for the 'Filarial Dance Sign' to confirm if adult worms are still present?
  • • Is the Filarial Test Strip (FTS) available here, and how does its accuracy compare to the older ICT test?
  • • Since I have skin nodules, will you be performing a 'skin snip' or a PCR test to check for Onchocerciasis?

Questions for You

  • • Have I spent time in Central or West Africa where Loa loa is common?
  • • Have I noticed any unusual movements under my skin or across the surface of my eyes?
  • • Was my blood drawn during the day or at night for my microfilariae test?

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References

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    Comparison of Immunochromatographic Test (ICT) and Filariasis Test Strip (FTS) for Detecting Lymphatic Filariasis Antigen in American Samoa, 2016.

    Sheel M, Lau CL, Sheridan S, et al.

    Tropical medicine and infectious disease 2021; (6(3)) doi:10.3390/tropicalmed6030132.

    PMID: 34287387
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    Development and Introduction of the Filariasis Test Strip: A New Diagnostic Test for the Global Program to Eliminate Lymphatic Filariasis.

    Pantelias A, King JD, Lammie P, Weil GJ

    The American journal of tropical medicine and hygiene 2022; (106(5_Suppl)):56-60 doi:10.4269/ajtmh.21-0990.

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    A rare clinical presentation of human Dirofilaria repens infection as a pseudo-tumour of the epididymis - Case Report.

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    Real time ultrasound diagnosis of ovarian and pelvic filariasis by filarial dance sign.

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    Journal of radiology case reports 2023; (17(7)):1-7 doi:10.3941/jrcr.v17i7.4675.

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    Unusual Localization of Blood-Borne Loa loa Microfilariae in the Skin Depends on Microfilarial Density in the Blood: Implications for Onchocerciasis Diagnosis in Coendemic Areas.

    Niamsi-Emalio Y, Nana-Djeunga HC, Chesnais CB, et al.

    Clinical infectious diseases : an official publication of the Infectious Diseases Society of America 2021; (72(Suppl 3)):S158-S164 doi:10.1093/cid/ciab255.

    PMID: 33909066
  11. 11

    Onchocerciasis (river blindness) - more than a century of research and control.

    Brattig NW, Cheke RA, Garms R

    Acta tropica 2021; (218()):105677 doi:10.1016/j.actatropica.2020.105677.

    PMID: 32857984
  12. 12

    Evaluation of a novel biplex rapid diagnostic test for antibody responses to Loa loa and Onchocerca volvulus infections.

    Campillo JT, Biamonte MA, Hemilembolo MC, et al.

    PLoS neglected tropical diseases 2024; (18(10)):e0012567 doi:10.1371/journal.pntd.0012567.

    PMID: 39388495
  13. 13

    Prevalence and intensity of Loa loa infection over twenty-three years in three communities of the Mbalmayo health district (Central Cameroon).

    Mogoung-Wafo AE, Nana-Djeunga HC, Domche A, et al.

    BMC infectious diseases 2019; (19(1)):146 doi:10.1186/s12879-019-3776-y.

    PMID: 30760228
  14. 14

    Identification and characterisation of Mansonella perstans in the Volta Region of Ghana.

    Opoku M, de Souza DK

    PloS one 2024; (19(6)):e0295089 doi:10.1371/journal.pone.0295089.

    PMID: 38848396
  15. 15

    Application of loop mediated isothermal amplification (LAMP) assays for the detection of Onchocerca volvulus, Loa loa and Mansonella perstans in humans and vectors.

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    PMID: 36684508
  16. 16

    Individual risk of post-ivermectin serious adverse events in subjects infected with Loa loa.

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  17. 17

    A call for loiasis to be added to the WHO list of neglected tropical diseases.

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  18. 18

    Operationalization of the test and not treat strategy to accelerate the elimination of onchocerciasis and lymphatic filariasis in Central Africa.

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  19. 19

    Encephalopathy in a patient with loiasis treated with albendazole: A case report.

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    PMID: 31676328

This page provides educational information about filariasis diagnostic tests. Always consult an infectious disease specialist or your healthcare provider to interpret your specific laboratory and ultrasound results.

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