Asymptomatic Peripheral Blood Eosinophilia Redux: Common Parasitic Infections Presenting Frequently in Refugees and Immigrants

Although the causes of peripheral blood eosinophilia are myriad, eosinophilia in refugees or immigrants from many parts of the world is most often a reflection of parasitic helminth (worm) infection. Soil-transmitted helminth infection affects more than 1.5 billion persons worldwide. If schistosome and filarial infections are added into the equation—even accounting for polyparasitism—this number reaches >2 billion persons. Thus, it stands to reason that many refugees and immigrants from areas where parasitic helminth infections are endemic present to clinicians with eosinophilia. The number of refugees (defined as individuals seeking refuge or asylum from their native country, typically because of persecution or fear of persecution) who came to the United States increased dramatically between 1980 and 2000 [1]. Although the number of refugees who come to the United States has diminished slightly in the past 5 years, the total number of all immigrants has increased, particularly the number of those from Africa and the tropical and subtropical regions of Asia. As a consequence, the health concerns of the immigrant and refugee populations have fallen on the shoulders of the community health systems in the cities and nearby regions that harbor these newly arrived people. The health problems likely to be encountered in this population include not only infectious diseases (e.g., latent tuberculosis, hepatitis B, intestinal parasites, and HIV infection), but also depression, other psychological disorders, anemia, dermatological conditions, dental problems, and trauma-induced injuries [2–6]. This information has been widely disseminated, and those who screen refugees and immigrants have become quite aware of which medical conditions they need to monitor routinely. Among immigrant and refugee populations, the prevalence of peripheral blood eosinophilia (typically defined as having an absolute eosinophil count of >450 cells/µL) has been shown to range from 12% to 53% of those screened [2–10]. Most of these studies focused on pathogen detection rather than the predictive value of the eosinophil levels, per se, for parasitic helminth infection. This relatively significant prevalence, coupled with the concept that many of the tissue-invasive helminths (e.g., Strongyloides stercoralis, filarial parasites, and schistosome parasites) are associated with clinically asymptomatic conditions in chronic infection [11], provides the rationale for evaluating eosinophilia as a marker of parasitic infection in the asymptomatic refugee and/or immigrant. Eosinophilia has been assessed as a marker for parasitic infection in returning travelers and has been shown to have relatively poor predictive values (both negative and positive) [8, 12, 13]. Because refugees and immigrants differ quite significantly from expatriate travelers, in that refugees and immigrants have had lifelong (and often intense) exposure to parasitic helminth infections, knowledge of the utility of eosinophil levels for predicting parasitic helminth infection in the refugee and/or immigrant population is important. This knowledge is particularly valuable in light of the possible life-threatening dissemination of Strongyloides stercoralis that can be seen in immunocompromised individuals [14]. Thus, Seybolt et al. [15] have, in this issue, revisited the topic of the evaluation of eosinophilia in clinically asymptomatic refugees in a retrospective analysis of 2224 refugees, of whom 266 (12%) had peripheral blood eosinophilia. Using stool examination for parasitic pathogens and targeted serological tests for Strongyloides species, Schistosoma species, and filarial species—presumably based on the geographic areas from which the refugees came—the authors demonstrated stool pathogens in 29% of the patients, and seropositivity ranged from 22% to 51%, depending on the serological test used. Unfortunately, many of the stool pathogens identified (e.g., Giardia lamblia and Entamoeba histolytica) are not associated with peripheral blood eosinophilia, and there apparently was little follow-up of the patients with eosinophilia who were serologically assessed [15]. Thus, the utility of the various serological tests remains a question. The major message of the study, however, is that stool examinations, although useful if a pathogen is identified, will miss a majority of the pathogens associated with eosinophilia, either because the pathogen is not found in the stool (e.g., filarial species and Schistosoma haematobium) or because the pathogen is notoriously hard to find (e.g., Strongyloides stercoralis). What is the optimal approach for the evaluation and treatment of the refugee or immigrant with eosinophilia and no symptoms? Were financial constraints no object, an extensive but selective evaluation that included multiple stool examinations, appropriate serological tests, and even better diagnostic tests (e.g., to detect parasite DNA or antigens), as well as follow-up after treatment, would be considered ideal. However, the experience in travelers and/or expatriates with eosinophilia, up to 50% of whom never have a cause of their eosinophilia identified, despite exhaustive evaluation [8, 16], suggests that we do not yet have a viable algorithm for systematically identifying the cause of eosinophilia in persons returning from tropical or subtropical regions of the world. Indeed, empirical anthelmintic therapy has been advocated on the basis of cost effectiveness [17]. For specific refugee populations (such as the Southeast Asian population) where infection with Strongyloides species and hookworm are most common [4] and in which the epidemiology of parasitic infection is known, single-dose therapy with ivermectin and/or albendazole has been mentioned, most often in the context of cost and efficacy [18]. The evaluation of unexplained eosinophilia in an asymptomatic individual (be they a traveler, a refugee, or an immigrant) is a challenging problem that requires knowledge about a wide variety of potential pathogens and their worldwide distribution. Nevertheless, the prevention of morbidity by the diagnosis and prompt treatment of parasitic helminth infection is an important task, and it requires sufficient insight to subvert our fellow wormy travelers. Potential conflicts of interest. T.B.N.: no conflicts.