Loa loa filariasis laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Kalsang Dolma, M.B.B.S.[2]

Overview

Diagnosis can be difficult in patients with low levels of larvae in the blood. The diagnostic work-up is usually begun after someone develops eye worm, Calabar swellings, or unexplained elevated levels of eosinophils on blood tests after travel to an affected area. The diagnosis can be made by identification of the adult worm by a microbiologist after its removal from under the skin or eye, identification of an adult worm in the eye by a healthcare provider in a patient with risk factors for infection,identification of the larvae on a blood smear made from blood taken from the patient between 10AM and 2PM and identification of antibodies against L. loa. Unfortunately these tests cannot distinguish between active infection and a history of exposure or past infection and they are not widely available in the United States.

Laboratory Findings

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. Additionally any patient in whom therapy for lymphatic filariasis and onchocerciasis is anticipated who comes from a region of Africa co-endemic for loiasis should be screened for infection prior to initiating treatment because of the risk of fatal encephalopathy after treatment with diethylcarbamazine (DEC) or ivermectin.

Microscopy

The standard diagnostic test for the diagnosis of loiasis is demonstration of microfilariae (larvae) on a daytime (10AM to 2PM) Giemsa-stained thin or thick blood smear or demonstration of an adult worm removed from subcutaneous or subconjunctival tissue. 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 person 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 would then be needed to direct treatment.

Identification of the Adult Worm

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