Approach Considerations
While eye worms are pathognomonic for loiasis, skin and joint findings (urticaria, pruritus, swelling) may be more common than the ocular pathology for which loiasis is named ("African eye worm"). The diagnosis should be considered in individuals from endemic areas who have eosinophilia and no other findings. [25]
Definitive diagnosis can be made by visualizing L loa microfilariae in blood samples collected for Giemsa staining during peak transit times or by pathologic review of the morphology of worms recovered from tissue or the eye. [8]
Serologic testing is not readily or commercially available but can be performed at specialized centers (eg, US CDC), although these tests have some cross-reactivity with other tissue filaria (onchocerciasis and filariasis). Only one PCR test for loiasis is available in the United States, [25] although it may fail to detect infection during the prepatent period and may detect DNA from dead or dying microfilariae. [8] Owing to these persistent DNA remnants, a test of cure is not indicated after proper treatment.
Laboratory Studies
Pathological Diagnosis
The standard diagnostic test for loiasis is the demonstration of microfilariae on Giemsa-stained thin or thick blood smear using blood collected during the daytime (10:00 am to 2:00 pm). [25] The timing of the blood collection for smear should be adjusted to reflect the local time at the point of origin in a traveler who is still experiencing jetlag. Concentration techniques such as NucleporeTM filtration (Whatman Inc., Florham Park, NJ), Knott concentration, or saponin lysis may increase the diagnostic yield in persons with low numbers of circulating microfilariae. Identification of microfilariae on blood smear is sufficient for diagnosis of the infection. In addition, a definitive diagnosis can be made by identifying a migrating adult worm in subcutaneous tissues or the conjunctiva.
Quantification of the number of microfilariae per mL is needed to direct treatment. [25]
Immunodiagnosis
While serology tests exist to assist in the diagnosis of loiasis, they are not readily or commercially available, and these tests may cross-react with various other similar tissue nematodes such as onchocerciasis and filariasis. In addition, these serologic tests may indicate that the patient has been exposed to L loa but may not distinguish if they have active infection or prior exposure. For these reasons, they are typically less useful in endemic contexts, although negative results decrease the possibility of L loa infection. [25]
One L loa–specific Ig(G4) enzyme-linked immunoabsorbent assay (ELISA) test has been developed and deployed in some endemic contexts, revealing a sensitivity and specificity of 80% and 78%, respectively, in patients with microfilaremia, as well as significant levels of L loa–specific Ig(G4) antibodies in 55% of patients tested with no detectable microfilaria in blood, suggesting occult infection in this group. Despite the limited sensitivity and specificity of this method, it may serve as a useful tool to estimate the prevalence of loiasis in endemic regions. [26]
In addition, an Ig(G4) assay for loiasis has been developed, and it showed no reaction when used in noninfected individuals, including those with other forms of filariasis. However, the assay failed to distinguish between microfilaremia and amicrofilaremia (occult infection). [27]
Antigen Detection
Other serologic techniques and assays using recombinant L loa have been developed, including a highly sensitive and specific (96% and 100%, respectively) luciferase immunoprecipitation system (LIPS), which detects antibodies to the recombinant L loa antigen LISXP-1. [28]
Nucleic Amplification Tests
One polymerase chain reaction (PCR) test for loiasis has been approved for diagnosis in the United States. PCR-based techniques have been developed, with specificities and sensitivities as high as 100% and 95%, respectively. [29] However, these techniques yield negative results during prepatency and often return false-positive results in individuals with previous L loa infections, as they also detect DNA from dead microfilariae in blood. [8]
Real-time qPCR assays have also been developed with highly species-specific and -sensitive (96%) detection rates for L loa. [30] These have been adapted for loop-mediated isothermal amplification (LAMP), allowing the potential for point-of-care and in-field use to detect microfilariae. [31, 32]
Procedures
According to the CDC, the standard diagnostic test for loiasis is the demonstration of microfilariae on Giemsa-stained thin or thick blood smear using blood collected during the daytime (10:00 am to 2:00 pm). The timing of the blood collection for smear should be adjusted to reflect the local time at the point of origin in a traveler who is still experiencing jetlag. Concentration techniques such as NucleporeTM filtration (Whatman Inc., Florham Park, NJ), Knott concentration, or saponin lysis may increase the diagnostic yield in persons with low numbers of circulating microfilariae. Identification of microfilariae on blood smear is sufficient for diagnosis of the infection. Quantification of the number of microfilariae per mL is needed to direct treatment. [25]
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Giemsa stained blood showing microfilaria of Loa loa with white blood cells present. Courtesy of Wikimedia Commons [Stefan Walkowski].
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A fly of the Chrysops genus. Courtesy of Flickr [Judy Gallagher].
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Map of the estimated prevalence of eye worm history in Africa. Courtesy of PLoS Neglected Tropical Diseases.
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Live adult Loa loa worm in the anterior chamber of the eye. Courtesy of PLoS Neglected Tropical Diseases.
