Answer to August 2019 Photo Quiz

Answer: Loa loa. Based on the morphology seen on the blood smear, the patient was diagnosed with asymptomatic Loa loa infection.

Loa loa filariasis is a parasitic infection endemic to the rainforests of Western and Central Africa, particularly common in Cameroon, Congo, and Nigeria. The parasite larvae are transmitted to humans through bite wounds from the deer fly, genus Chrysops. In humans, the larvae mature into adults, which produce microfilariae, which are ingested by the deer fly. In these areas of endemicity, 13 million individuals are estimated to be infected with Loa loa (1). However, a large majority of those infected are clinically asymptomatic.

Microfilariae of Loa loa, found in the peripheral blood, are 250 to 300 μm in length and 6 to 8 μm in width. The organisms possess a head and body as well as a tail that gradually tapers to a rounded end (see Fig. 1, top, in the photo quiz). Densely packed nuclei characteristically extend to the tip of the tail; this is visible in Fig. 1, bottom left, in the photo quiz. They also possess a sheath, which stains blue in hematoxylin but does not stain with Giemsa. The presence of the sheath is visible near the head in Fig. 1, bottom right, in the photo quiz. The presence or absence of a sheath as well as the arrangement of nuclei in the tail helps distinguish Loa loa from other filaria on microscopy.

Clinical manifestations include subcutaneous areas of swelling measuring 5 to 20 cm that occur most frequently on the face and extremities, known as Calabar swellings, secondary to migrating filariae or microfilarial release. The other major presentation is eye symptoms due to a filaria (3 to 7 cm long) migrating beneath the conjunctiva, which can often be seen as it crosses the eye. Conjunctival symptoms typically resolve after the worm has left the eye.

In those who are symptomatic, treatment of loiasis with ivermectin and diethylcarbamazine (DEC) has been associated with a potentially fatal encephalitis postulated to be due to inflammation caused by rapid decline in microfilarial burden (2, 3). In symptomatic individuals with <8,000 microfilariae/ml, DEC is the drug of choice. In those with symptoms and >8,000 microfilariae/ml, a course of albendazole is recommended prior to DEC treatment. Albendazole primarily has macrofilaricidal activity and has little activity against microfilariae, thus mitigating the likelihood of encephalitis.

In this case, the patient’s parasite load was estimated at 5 microfilariae/ml. His presenting symptoms stemmed from the GIST rather than the parasite. Thus, the finding of Loa loa infection was incidental. The patient had a low parasite burden without a risk for transmission to others, due to the absence of the vector in the United Sates. The benefit of treatment in an asymptomatic patient with a low microfilarial load is uncertain, but patients in resource-rich settings are likely to receive treatment. In the United States, DEC is not licensed for use and can be obtained only through the CDC for compassionate use. This patient was scheduled for treatment of the GIST with imatinib but was lost to follow-up. Interestingly, imatinib has been demonstrated to reduce microfilaremia (4) and perhaps would have been adequate to treat this patient’s asymptomatic Loa loa infection.

See https://doi.org/10.1128/JCM.01101-18 in this issue for photo quiz case presentation.