Clinical Testing and Diagnosis for Loiasis

Considerations

The diagnosis of loaisis should be considered in any patient with a history of travel in an endemic area who presents with eye worm, Calabar swellings, or unexplained peripheral eosinophilia.

Any patient in whom therapy for lymphatic filariasis or onchocerciasis is anticipated who comes from a region of Africa co-endemic for loiasis should be screened for infection before starting treatment. There is a risk of fatal encephalopathy after treatment with diethylcarbamazine (DEC) or ivermectin.

Recommended tests

The standard diagnostic test for the diagnosis of loiasis is demonstration of microfilariae on a daytime (10 AM – 2 PM) Giemsa-stained thin or thick blood smear. It is important to note that the timing of the blood smear should be adjusted to reflect local time at the point of origin of a traveler who is still experiencing jetlag. Concentration techniques such as Nuclepore™ filtration (Whatman Inc., Florham Park, NJ), Knott's 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.

Alternative tests

Identification of the adult worm on physical exam in a patient with significant exposure history or by a pathologist after its extraction from subcutaneous or subconjunctival tissue would also be sufficient to make the diagnosis of infection. However, a blood smear would still need to be performed for quantification of microfilarial load prior to the initiation of antiparasitic agents.

Blood tests are available to assist in making the diagnosis in amicrofilaremic patients, though they are not widely available. There is a general screen for any filarial infection (including Wuchereria, Brugia, Onchocerca, and Mansonella infections) that is available in some specialty diagnostic labs. Because the test is highly sensitive, it is useful in determining if an individual has had filarial infection, but it is not specific enough to identify which filarial infection. There are several Loa-specific serologic tests in existence, such as the tests for antibodies to the LlSXP-1 recombinant antigen, which can be used in both an ELISA and a luciferase immunoprecipitation systems (LIPS) assay. But these are currently available only in the research setting and are not approved for diagnosis in the United States.

As with any antibody test, the results indicate only that the patient has been exposed to the disease, but they do not indicate if the patient has an active infection. This distinction is less important in symptomatic travelers, but it limits the usefulness of the test in persons from endemic areas.

In general, diagnose L. loa infection with a blood smear. However, when blood smears are negative and clinical suspicion of infection is high, the general antibody test could exclude infection. If the general antibody test is positive, it might be necessary to consider seeking additional diagnostic information by enlisting the assistance of researchers who perform specific antibody and/or PCR tests.