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Adrian Gardner, Lindsay Hardy, Sigall K. Bell, Eosinophilia in a Returned Traveler, Clinical Infectious Diseases, Volume 50, Issue 5, 1 March 2010, Pages 792–793, https://doi.org/10.1086/650485
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Diagnosis: loiasis.
An increasing number of physicians are confronted with travel-related illnesses as a result of expanded worldwide travel. Among returned travelers with eosinophilia, a definitive diagnosis can be made in approximately one-third (36%) of patients; however, this figure increases to >60% of patients in cases in which eosinophilia exceeds 16% [1]. In a series of >14,000 travelers, reported by Schulte et al [1], schistosomiasis, hookworm, cutaneous larva migrans, and strongyloidiasis were the most commonly identified infectious causes of eosinophilia among returning travelers.
In our patient, the peripheral blood smear revealed microfilariae with both a compact column of nuclei extending into their tail (Figure 1) and a surrounding sheath (Figure 2), which are features that are diagnostic of Loa loa infection. L. loa is a filarial parasite that is spread by the Chrysops fly (which is endemic to parts of Western and Central Africa) and should be considered in the differential diagnosis of moderate-to-severe eosinophilia in travelers returning from an area of endemicity [1]. Larvae, which develop in the fly, are transmitted to humans at the time of the fly bite. Adult nematodes mature over a 3-month period, mate, and can live in humans for up to 17 years. Microfilariae are released into the peripheral blood 6–12 months after infection, which allows the diagnosis to be made on the basis of peripheral blood smear findings [2]. The microfilarial periodicity is correlated with the biting patterns of its vector: in the case of L. loa, microfilariae are released into the peripheral blood during the daytime, because this is when the Chrysops fly feeds. The optimal timing of a peripheral blood specimen is noon. Infected humans may experience transient Calabar swellings (localized subcutaneous edema that is thought to result from a hypersensitivity reaction to worms or their metabolic products), urticaria, or visible migration of the adult worm through the conjunctiva (“eyeworm”). Calabar swellings and urticaria are more common among individuals who are not long-term residents of areas of endemicity [3]. Our patient likely experienced Calabar swellings, which he described as “swelling of the forearms.” Unlike filariae that cause lymphatic filariasis (“elephantiasis”), L. loa microfilariae do not cause lymphatic obstruction.
