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Loiasis

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[Loa loa]

Causal Agent

Loa loa, a filarid nematode commonly referred to as the African eye worm.

Life Cycle

lifeCycle

The vector for Loa loa filariasis are flies from two species of the genus Chrysops, C. silacea and C. dimidiata. During a blood meal, an infected fly (genus Chrysops, day-biting flies) introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound The number 1. The larvae develop into adults that commonly reside in subcutaneous tissue The number 2. The female worms measure 40 to 70 mm in length and 0.5 mm in diameter, while the males measure 30 to 34 mm in length and 0.35 to 0.43 mm in diameter. Adults produce microfilariae measuring 250 to 300 µm by 6 to 8 μm, which are sheathed and have diurnal periodicity. Microfilariae have been recovered from spinal fluids, urine, and sputum. During the day they are found in peripheral blood, but during the noncirculation phase, they are found in the lungs The number 3. The fly ingests microfilariae during a blood meal The number 4. After ingestion, the microfilariae lose their sheaths and migrate from the fly’s midgut through the hemocoel to the thoracic muscles of the arthropod The number 5. There the microfilariae develop into first-stage larvae The number 6 and subsequently into third-stage infective larvae The number 7. The third-stage infective larvae migrate to the fly’s proboscis The number 8 and can infect another human when the fly takes a blood meal The number 1.

Geographic Distribution

Loa loa is found in Africa.

Clinical Presentation

Loiasis is often asymptomatic. Episodic angioedema (Calabar swellings) and subconjunctival migration of an adult worm can occur.

Microfilariae of Loa loa.

 

Microfilariae of Loa loa are sheathed and measure 230-250 µm long in stained blood smears and 270-300 µm in 2% formalin. The tail is tapered and nuclei extend to the tip of the tail. Microfilariae circulate in the blood.
Figure A: Microfilaria of L. loa a thick blood smear from a patient from Cameroon, stained with Giemsa. Note the nuclei extending to the tip of the tail to the left of the image.
Figure E: Microfilaria of L. loa in a thin blood smear, stained with Giemsa.
Figure B: Microfilaria of L. loa in a thick blood smear, stained with Giemsa.
Figure F: Microfilaria of L. loa in a thin blood smear, stained with Giemsa.
Figure C: Microfilaria of L. loa in a thick blood smear, stained with Giemsa.
Figure G: Microfilariae of L. loa captured by the Knotts concentration technique. Image taken at 100x magnification.
Figure D: Microfilaria of L. loa in a thin blood smear, stained with Giemsa.
Figure H: Higher magnification of the microfilariae in Figure G, taken at 500x oil magnification.
Adults of L. loa.

 

Adults of Loa loa are often isolated from the subconjunctiva. Adult females are 40-70 mm in length and 0.45-0.60 mm wide; males are smaller at 30-34 mm long and 0.35-0.40 mm wide. The exterior of the cuticle lacks ridging seen in many Dirofilaria but contains irregularly-spaced elevations called “bosses”.
Figure A: Adult of L. loa removed from the eye of a patient. Image courtesy of the Georgia State Public Health Laboratory.
Figure B: Adult of L. loa removed from the eye of a patient. Image courtesy of the Georgia State Public Health Laboratory.

Diagnostic Findings

Loa loa is usually diagnosed by the finding of microfilaria in peripheral blood smears or adults in the subconjunctiva. The blood films may be thick or thin and stained with Giemsa or hematoxylin-and-eosin. For increased sensitivity, concentration techniques can be used. These include centrifugation of the blood sample lysed in 2% formalin (Knott’s technique), or filtration through a Nucleopore® membrane. Microfilariae of L. loa exhibit diurnal periodicity and a diagnosis is best made from blood collected during the mid-day (10 AM-2 PM). The presence of Calabar swellings can aid in the diagnosis.

Treatment Information

Treatment information for loiasis can be found at: https://www.cdc.gov/dpdx/loiasis/index.html

DPDx is an educational resource designed for health professionals and laboratory scientists. For an overview including prevention, control, and treatment visit www.cdc.gov/parasites/.