Introduction

Eosinophils are known to be associated with many conditions including parasitic infections, allergic or autoimmune diseases, and several primary hematologic disorders. In immigrant or refugee populations – newly arrived from developing countries and/or from tropical areas – most of the eosinophilia identified with or without symptoms can be considered a reflection of parasitic infection (particularly helminth [worm] infections).

Epidemiology

Among immigrant and refugee populations, the prevalence of peripheral blood eosinophilia defined as > 450/mm³ or > 500/mm³ has been shown to range from 12% to 53% of those screened.^2–10

Although immigrants and refugees may have parasitic helminth infection, the absence of eosinophilia does not exclude parasitic infection.³ It is known that eosinophilia has a relatively poor predictive value (both negative and positive)^4,11,12 for parasitic infection in returning travelers. However, eosinophilia in asymptomatic refugees and/or immigrants should draw attention to possible parasitic infections, as those with lifelong exposure to parasitic infections are more often asymptomatic compared to short-term travelers. In one study of Southeast Asian refugees with persistent eosinophilia and a negative initial screening evaluation, 95% were found to have gastrointestinal parasites with one or multiple organisms (55% with hookworm and 38% with Strongyloides).⁶ Indeed, many of the tissue-invasive helminth infections (strongyloidiasis, filariasis, schistosomiasis) are associated with clinically asymptomatic conditions in chronic infections.^13–16 Moreover, in a study from a parasite-endemic region of Brazil, high-grade eosinophilia was more likely to be associated with parasite infection, and higher eosinophil counts were associated with having more than one infecting species of parasites in the same host (polyparasitism).¹⁷

Even in immigrant populations that have emigrated in the distant past, it must be remembered that some parasites (filarial parasites) have life spans that can exceed 10–15 years, and others have autoinfective cycles (Strongyloides) that enable lifelong parasite persistence. Indeed, the chronicity of infection, one of the hallmarks of helminth parasites, has been well documented, as can be seen in Table 21.1. Immigrants can also acquire new infections from visits back to their home country (so-called ‘visiting friends and relatives’).

Table 21.1 Documented length of infection in selected parasites

Pathophysiology

It has been suggested that eosinophils play an important role in the immune response to helminth infec tion, based on the observation of eosinophils surrounding dying parasites in an animal infection model of Nippostrongylus brasiliensis.¹⁸ Eosinophilia and increased serum IgE levels are characteristic of helminth infection which reflects the production of cytokines such as interleukins (IL-4 and IL-5) from CD4+ lymphocytes. In addition to IL-5 – the main cytokine responsible for the specific development of eosinophils – other cytokines (e.g. IL-4 and IL-13) and chemokines (e.g. RANTES and eotaxin) are important for the recruitment of eosinophils from the blood to the tissues.¹⁹

The degree of eosinophilia may also vary based on the parasites’ tissue distribution pattern, their migration route, the maturation stage of the parasites and the actual parasite burden.^20,21 Eosinophilia is often particularly pronounced in the early phase of infection, when developing larvae migrate through the lungs (e.g. Ascaris or Strongyloides) or through other tissues (as in hookworm and Trichinella infection). Because of the long prepatent period characteristic of some helminth infections, eosinophilia may occur at a time when parasitologic diagnosis is difficult (prior to egg secretion or antibody positivity). In most instances, eosinophil levels from helminth parasites gradually diminish over time (even without anthelmintic treatment), although they rarely return to normal levels.

The kinetics of parasite infection-associated eosinophilia varies widely depending on the particular species of infecting parasite. In a human volunteer study with Necator americanus, it was shown that blood eosinophil counts began to rise from baseline 2–3 weeks after infection and reached a peak between 38 and 64 days.²² In several experimental human filarial infections, peak eosinophil counts occurred 11–30 weeks after the initiation of infection.²³ In both types of infections there was a natural modulation (diminution) of the eosinophil levels over time.

Therefore, in immigrants or refugees who have been chronically exposed to helminth infection for an extended period of time, eosinophilia may be less prominent than in travelers infected from relatively recent exposure.¹³

Etiology

Many of the causes of eosinophilia resulting from infectious and noninfectious processes are detailed in Table 21.2.

Table 21.2 Symptoms and etiologies associated with eosinophilia

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