Eosinophilia and Select Parasitic Infections Among People Seeking Humanitarian Protections in the United States After Travel Through the Darién Jungle: A Multicenter Cross-sectional Study

Abstract

Background

People seeking humanitarian protections in the United States often have complex ecologic exposures during their migration, as in the Darién Jungle, a rainforest between Colombia and Panama. We characterized eosinophilia and select parasitic infections in this population.

Methods

We conducted a cross-sectional study of clinical evaluations of unhoused people seeking humanitarian protection in Boston and New York City. Patients underwent laboratory testing based on clinician discretion. We extracted data regarding migration history and testing for eosinophilia (>500 cells/μL), Strongyloides infection, and Chagas disease. We used χ² testing to evaluate the association between exposure to the Darién Jungle and test results.

Results

Our sample included 632 people (median age, 27 years; 55.0% female), primarily from Venezuela (32.0%), Ecuador (16.6%), Colombia (14.7%), and Haiti (13.3%). Of these, 411 people (65.0%) crossed the Darién Jungle. The median (IQR) duration in the Darién Jungle was 5 (4–7) days. We found that 71 people (11.2%) had eosinophilia. Of 438 people who completed testing for Strongyloides infection, 21 (4.8%) were positive, which occurred more frequently in people with eosinophilia (P = .006). Testing for Chagas was negative in all people. Children were more likely to have eosinophilia as compared with adults (odds ratio, 1.76; 95% CI, 1.04–2.95). There was no statistically significant association between crossing the Darién Jungle and eosinophilia (odds ratio, 1.13; 95% CI, .67–1.95).

Conclusions

Eosinophilia and Strongyloides infection are common among unhoused persons seeking humanitarian protections in the United States, particularly children. Clinicians should screen for these conditions and be vigilant for migration-related exposures.

There has been a global rise in forced displacement as people around the world travel through irregular and increasingly dangerous routes of migration to resettle in countries of asylum [1]. At the end of 2023, >110 million people were forcibly displaced, which includes refugees, asylum seekers, and internally displaced persons [1]. Reasons for migration include war, economic collapse, political crises, and climate change [1, 2]. Climate change, in particular, is an emerging reason for displacement as it can cause short-term stressors, such as extreme weather events, and long-term stressors, such as droughts, rising sea levels, food shortages, and depletion of life-sustaining resources [3–5]. In Central America, 6% of migrating people have cited the climate as a reason for migration [6].

Asylum seekers are displaced persons who flee their countries of origin due to a credible fear of persecution and seek protected status after arrival at their destination [1]. In contrast to other immigrant groups, asylum seekers and people under temporary protected status in the United States are not formally required to undergo predeparture medical screening and are not uniformly offered medical care or medical screening after arrival [7–9]. The number of asylum seekers arriving in the United States after travel through South and Central America is increasing: US Customs and Border Protection officials reported >2.4 million encounters with people irregularly crossing the México–United States border in 2023 [10]. Meanwhile, >2 million asylum applications are awaiting processing in the United States, which is the most in recent history [2].

Factors that can increase the risk of infections for people seeking humanitarian protections in the United States include poverty, detention and incarceration, living in poorly maintained migration encampments, sexual assault, malnutrition, and access to health care [11, 12]. Irregular migration through Colombia and Panamá has also introduced unprecedented human interaction with the ecologic environment of the Darién Jungle, a dense tropical rainforest connecting Colombia and Panamá. According to the Panamanian government, >500 000 people crossed the Darién Jungle in 2023, of whom 113 180 were children, many of whom experienced violence, abuse, and exposure to environmental elements [13]. Travel through the Darién Jungle is associated with physical and sexual violence, extortion, drowning, and a high risk of death [13–15].

Exposure to the diverse ecology in the Darién Jungle may heighten the risk for acquiring a variety of endemic infections, many of which are associated with eosinophilia. Migrating people frequently report traversing parts of the Darién Jungle barefoot, being immersed in rivers and streams, consuming contaminated drinking water, and being bitten by mosquitoes and other insects [15–17]. Older published reports link travel through or habitation in the province of Darién, Panamá, to a variety of important infections, such as leishmaniasis, mansonellosis, malaria, intestinal parasites (including infection with Strongyloides stercoralis), and Chagas disease [18–25]. However, it is not known whether the intensity and duration of exposure to the rainforest in Darién are sufficient to put migrants at a higher independent risk of parasitic infection. Our objective was to characterize the prevalence of eosinophilia, Strongyloides, and Trypanosoma cruzi infection in a population of asylum seekers in the United States, many of whom traveled through the Darién Gap.

METHODS

Study Design and Setting

We conducted a cross-sectional study of asylum seekers receiving emergency assistance shelter in New York City and the Greater Boston area. The study site in New York City was based in Ryan Chelsea-Clinton, a federally qualified health center that is part of a large network of primary care centers (Ryan Health) that provide care for many people in the city's emergency shelter sites. New York City provided emergency shelter services for >100 000 people in 2023 [26]. Patients being evaluated at this site were screened for their migration history. Patients were identified as seeking humanitarian protections (eg, asylum or temporary protected status) by patient navigators and provided with focused social services. Clinicians who conducted initial clinical evaluations of asylum seekers utilized a laboratory order set at their discretion, which included a complete blood count with white blood cell differential, Strongyloides serology (IgG), and T cruzi antibody (Chagas disease). We examined medical records for primary care encounters with people who reported their migration history and completed their initial visits between 1 January 2023 and 30 November 2023.

The site in the Greater Boston area was at Massachusetts General Hospital Chelsea HealthCare Center, a primary care center that serves socially vulnerable families in Chelsea, Massachusetts, where nearly 50% of its population was born outside the United States. In 2023, the state of Massachusetts provided emergency assistance shelter services for >7500 families [27], with several shelter sites near Chelsea. This clinical site worked with community-based organizations that were administering social services and shelter assistance to newly arrived families who were identified as seeking humanitarian protections. Laboratory and microbiologic testing were ordered per clinician discretion. We retrospectively examined medical records for screening encounters provided in this clinical site between 1 September 2022 and 30 September 2023. For both sites, we included people for analysis only if their initial clinical encounters were within 1 year of arrival to the United States, if information regarding their routes of migration was available for data collection, and if they had completed bloodwork with results available for review. Many patients were recommended to undergo additional testing outside of what we report here, but they had challenges in following up.

Data Collection

We collected demographic and clinical information on all patients evaluated during the study period. Clinicians asked about patient gender or gender identity using open-ended questioning to be as inclusive as possible. All patients in this sample self-identified as either male or female, which is how we report this variable. We abstracted information regarding migration routes, including the country of origin and whether the person traveled through the Darién Jungle. For people who traveled through the Darién Jungle, we abstracted information regarding the travel duration if recorded. We did not find any information in medical records to determine the amount of time that transpired between crossing the Darién Jungle and undergoing medical evaluation. All patients in the analysis had a complete blood count with a white blood cell differential, which included an eosinophil blood count. We defined eosinophilia as an absolute eosinophil count >500 cells/μL [28]. Past studies have suggested that eosinophilia occurs between 3 and 11 weeks after parasitic infection, peaking between 9 and 30 weeks depending on the infection [29–31]. Given that most migrating people spend several months in transit prior to entering the United States and that there are typically delays between arrival in the United States and assessment in our clinics, we assumed that enough time would have elapsed to observe eosinophilia from exposures incurred prior to departure or while in transit.

Laboratory Testing

Testing for Strongyloides employed the GSD Strongyloides ELISA IgG test kit. Testing for Chagas disease employed the ORTHO T cruzi ELISA test, both according to the manufacturers’ specifications.

Statistical Analysis

We categorized the age of the patients as children (0–12 years), adolescents (13–21 years), and adults (>21 years). Place of origin was classified into the following regions: North America (consisting exclusively of Mexico), the Caribbean, Central America, South America, Africa, and Asia (including Russia). Our primary outcome of interest was peripheral eosinophilia, and our primary exposure of interest was travel through the Darién Jungle. We also determined the presence of positive serologies for Strongyloides and T cruzi for those who completed testing. Categorical variables were compared by χ² testing. We used logistic regression to model our primary outcome (eosinophilia) and exposure (travel through Darién) while adjusting for the patient's age (model 1). We adjusted for age given our a priori expectation that children would be more likely to have parasitic infection [32, 33]. We also created a separate regression model (model 2) that included age group, travel through Darién, and country or region of origin. For model 2, we grouped countries into categories that included people with and without the exposure of Darién. We conducted a secondary analysis by restricting the exposure variable to those who reported spending >1 week in Darién to investigate the possible association between a longer exposure to Darién and the outcome of eosinophilia. We also conducted an analysis by modeling the duration of time in the Darién Jungle as a continuous variable. P = .05 was the threshold for significance, and R version 4.4.1 was used for all analyses.

Ethical Review and Patient Consent

This study was approved by the institutional review boards for Ryan Health and MassGeneral Brigham. Patient consent was not obtained due to the retrospective nature of this study.

RESULTS

This study included 632 people (Figure 1). The median (IQR) age was 28 (12–35) years, and 343 (54.3%) people were female. Places of origin were South America (n = 475, 75.2%), the Caribbean (n = 96, 15.2%), Central America (n = 31, 4.9%), Africa (n = 19, 3.0%), Asia (n = 9, 1.4%), and North America (n = 4, <1.0%). The most common countries of origin were Venezuela (n = 202, 32.0%), Ecuador (n = 105, 16.6%), Colombia (n = 93, 14.7%), and Haiti (n = 84, 13.3%). Of the 632 people in this sample, 411 (65.0%) crossed the Darién Jungle prior to arriving to the US-México border and entering the United States. Among those who crossed Darién, 259 people reported their duration in the jungle, and 62 people (24.0%) reported being in the jungle for >7 days. For those reporting their duration in the jungle, the median (IQR) duration in Darién was 5 (4–7) days, and the maximum was 30 days. Age and sex distributions were comparable between participants who did and did not travel through the Darién Jungle (Table 1).

Figure 1.

Flowchart of participants included in the study of eosinophilia and parasitic infections among newly resettled asylum seekers.

Table 1.

Study Characteristics of Unstably Housed Persons Seeking Humanitarian Protections Screened for Parasitic Infections in Boston and New York City, 2023

Characteristic	Crossed Darién Jungle (n = 411)	Did Not Cross Darién Jungle (n = 221)	P Value
Age, y	27 (11–34)	30 (12–37)
Age group, y			.67
Child, 0–12	113 (27.5)	56 (25.3)
Adolescent, 13–21	39 (9.5)	18 (8.1)
Adult, >21	259 (63.0)	147 (66.5)
Sex			.68
Female	226 (55.0)	117 (52.9)
Male	185 (45.0)	104 (47.1)
Place of origin			<.01
Africa	7 (1.7)	12 (5.4)
Asia	2 (0.5)	5 (2.3)
Caribbean	79 (19.2)	17 (7.7)
Central America	0 (0)	31 (14.0)
North America	0 (0)	4 (1.8)
South America	323 (73.2)	152 (68.8)

Data are presented as median (IQR) or No. (%).

Eosinophilia was present in 11.2% of the patients (Table 2). Among patients with eosinophilia, the median (IQR) absolute eosinophil count was 781 (596–1069) cells/μL. Among those with eosinophilia, 58 (82%) were from South America, 5 (7%) were from Central America, 4 (6%) were from the Caribbean, and 3 (4%) were from Africa.

Table 2.

Eosinophilia and Parasitic Infections in Unhoused Asylum Seekers in Boston and New York City, 2023

Test for Parasitic Infection	Study Population Prevalence
Eosinophiliaa	71/632 (11.2)
Absolute eosinophil count among those with eosinophilia, cells/μL	781 (596–1069)
Positive Strongyloides serology	21/438 (4.8)
Eosinophilia	7/21
Positive Chagas serology	0/468 (0)

Data are presented as median (IQR) or No. (%).

^aEosinophilia was defined as >500 cells/μL.

Of the 438 people who completed serologic testing for Strongyloides IgG, 21 (4.8%) were positive, of whom 7 had eosinophilia. People with eosinophilia were more likely to have positive Strongyloides IgG testing (15.6% vs 3.8%, χ² = 7.68, P = .006). T cruzi antibody was negative in all people who completed testing (n = 468).

In a logistic regression analysis of factors associated with eosinophilia, children were more likely to have eosinophilia as compared with adults (odds ratio [OR], 1.76; 95% CI, 1.04–2.95). There was no statistically significant association between crossing the Darién Jungle and eosinophilia (OR, 1.13; 95% CI, .67–1.95; Table 3). A regression model that included country or region of origin also found no statistically significant association between crossing the Darién Jungle and eosinophilia (OR, 0.67; 95% CI, .32–1.42). However, this model showed that eosinophilia was associated with children as compared with adults (OR, 1.81; 95% CI, 1.04–3.14) and origin from Venezuela (OR, 2.63; 95% CI, 1.07–6.57) or Ecuador (OR, 3.82; 95% CI, 1.62–9.55) as compared with other countries in South America. There was also no statistically significant association between duration in the Darién Jungle >7 days and eosinophilia (OR, 0.73; 95% CI, .42–1.25) and when time in Darién was modeled as a continuous variable (OR, 1.06; 95% CI, .99–1.14, for each additional day).

Table 3.

Factors Associated With Eosinophilia Among Study Participants

	Eosinophilia (>500 cells/μL), OR (95% CI)
Characteristic	Model 1	Model 2
Age group, y
Adult, >21	1 [Reference]	1 [Reference]
Child, <13	1.76 (1.04–2.95)	1.97 (1.14–3.37)
Adolescent, 13–21	0.32 (.05–1.09)	0.31 (.05–1.06)
Exposure to Darién Jungle
Did not cross Darién Jungle	1 [Reference]	1 [Reference]
Crossed Darién Jungle	1.13 (.67–1.96)	0.67 (.32–1.42)
Country or region of origin
Other South Americaa	…	1 [Reference]
Venezuela	…	3.76 (1.60–9.38)
Ecuador	…	2.70 (1.10–6.74)
Haiti	…	1.06 (.26–3.61)
Mexico, Central America, or the Caribbeanb	…	1.64 (.49–4.83)
Asia or Africa	…	3.05 (.78–10.01)

Abbreviation: OR, odds ratio.

Model 1 analyzed the association of eosinophilia with age group and exposure to the Darién Jungle. Model 2 included country or region of origin with the categories shown.

^aOther South America: countries in South America with the exception of Venezuela or Ecuador.

^bMexico, Central America, or the Caribbean: countries in these regions with the exception of Haiti.

DISCUSSION

In this multicenter study, we characterize the presence of eosinophilia and select parasitic infections among people seeking humanitarian protections in the United States with unstable housing. To our knowledge, our study is the first to evaluate the journey through the Darién Jungle, which has emerged as an important migration route, as a risk factor for eosinophilia or parasitic infections among people seeking humanitarian protection in the United States outside the refugee resettlement system. Prior studies in displaced persons were largely in refugees, who have different exposures and different access to care during and after resettlement. We have 3 major findings. First, of the unhoused asylum seekers that we evaluated who were receiving emergency shelter in Boston and New York City, 65.0% reported travel through the Darién Jungle on their way to the United States, which highlights the growing importance of this region and the need for US clinicians to become familiar with it. Second, the prevalence of eosinophilia was high, representing 11.2% of this sample. Third, while younger age and specific countries of origin were associated with eosinophilia, traversing the Darién Jungle was not.

The population of displaced persons who resettle in the United States is heterogeneous, as they have diverse and multifaceted reasons for migration and different countries of origin, as well as variable access to medical care prior to, during, and after migration. Refugees are perhaps the best studied population of displaced persons who resettle in the United States. Refugee status is formally designated prior to resettlement and is associated with at least a minimum level of social supports and health evaluations through the process of resettlement [2, 34]. Most refugees in the United States receive overseas presumptive (ie, predeparture) treatment for soil-transmitted helminths, administered by panel physicians who are mostly from the International Organization for Migration [35, 36]. This practice dramatically reduced the prevalence of parasitic infections diagnosed on the initial refugee health evaluation [35, 36]. Studies among the refugee population have found the prevalence of peripheral eosinophilia to be between 12% and 18% and highest in people migrating from sub-Saharan Africa [37–39]. Organisms identified on stool microscopy in those studies were most often Blastocystis species, Giardia duodenalis, Entamoeba species, and Trichuris species [37, 39].

In contrast to individuals formally resettled through the country's refugee resettlement program, asylum seekers and people seeking other forms of humanitarian protection frequently have a more precarious resettlement process, which can be characterized by detention or incarceration, unstable housing, and lack of mandated predeparture or postarrival medical screening evaluations [12]. Asylum seekers may also be at higher risk for ecologic exposures due to the extreme routes of migration currently being used [15]. In a study comparing refugees and asylum seekers in Washington, DC, 19% of asylees had evidence of intestinal parasites [40]. The majority of organisms identified in that study were nonpathogenic. A study of refugees and asylees at international clinical sites between 1997 and 2009 found that 10% had eosinophilia [41]. A recent multistate study found a lower prevalence of Strongyloides infection (1.9% vs 3.8%, P = .02) in adult asylum seekers vs refugees [8].

As migration journeys have become more unregulated, more reliant on traffickers, and more dangerous, there has been a rise in complex environmental exposures that put this population at risk for a variety of vector-borne and parasitic infections [15]. One important exposure for the millions of asylum seekers currently seeking resettlement in the United States is travel through the Darién Jungle between Colombia and Panama [15, 17]. Past studies have identified several parasitic infections of public health importance in this region, including malaria, leishmaniasis, Chagas disease, and soil-transmitted helminths [18–20, 22–24]. In our analyses, travel across the Darién Gap was not independently associated with eosinophilia, though many people who traveled through Darién had eosinophilia and evidence of a parasitic infection. Even if exposure to the Darién Gap were associated with eosinophilia or parasitic infections, it would only be suggestive as a site of infection, as we could not ascertain if infection occurred in the country of origin or in transit. The regression analysis in this study suggested that country of origin may be playing a role in either acquisition of eosinophilia or access to treatment prior to resettlement, as people originating from Venezuela and Ecuador had a higher odds of eosinophilia as compared with people from other parts of South America.

Our results suggest that the risk of eosinophilia and parasitic infection in migrating people is affected by a broader and more complex set of factors, such as long-term habitation in an endemic area before migration, especially in rural and impoverished environments. It is also possible that some migrating people receive empiric antiparasitic treatment along the migration route, although no formal studies on this have been published. Our study reinforces the recognition that there are multifarious risk factors for eosinophilia and parasitic infections in this population. In the absence of public health guidance for newly arrived asylum seekers in the United States, we recommend that clinicians undertake universal asymptomatic screening with a minimum of complete blood count and Strongyloides serology, with additional testing depending on specific risk factors and symptoms.

Sampling bias may influence these results, as the population that was able to access medical evaluation through these clinical visits and complete laboratory and microbiologic testing may not be representative of the larger forcibly displaced population. Additionally, other unmeasured migration-related environmental exposures for infectious diseases exist for this population, such as exposures in South and Central America, habitation in migration encampments in México, and living in crowded settings after entry into the United States [42]. While we strengthened the analysis by including 2 sites of medical evaluation, our statistical power may have been insufficient to detect a difference between the exposure groups. Finally, eosinophilia, particularly mildly elevated eosinophil counts, may not necessarily be related to infection, as hypersensitivities, autoimmune conditions, and hematologic disorders can all contribute to eosinophilia. Implementing testing algorithms and screening pathways for eosinophilia for this population and ensuring follow-up will be essential to optimize diagnosis and treatment. Future studies should also evaluate strategies to ensure completion of screening and treatment for those who test positive for parasitic infections.

In conclusion, clinicians in the United States are increasingly evaluating asylum seekers who use irregular routes of migration to enter the United States. These migration routes introduce new risks of infection related to human interaction with the environment. Eosinophilia and parasitic infections were commonly diagnosed in this population, particularly in children. The Darién Jungle was traversed by 65.0% of the people in our study, although it was not independently associated with eosinophilia, perhaps due to the brief duration in that region and the other risk factors for infection prior to or during migration. Clinicians caring for unhoused migrants should recognize environmental exposures and screen and treat their patients for parasitic infections. Public health officials and policy makers should emphasize protections and housing support for this population, rather than criminalization and detention, to reduce the risk of infection spread.