High prevalence of S. Stercoralis infection among patients with Chagas disease: A retrospective case-control study

Background We evaluate the association between Trypanosoma cruzi infection and strongyloidiasis in a cohort of Latin American (LA) migrants screened for both infections in a non-endemic setting. Methodology Case-control study including LA individuals who were systematically screened for T. cruzi infection and strongyloidiasis between January 2013 and April 2015. Individuals were included as cases if they had a positive serological result for Strongyloides stercoralis. Controls were randomly selected from the cohort of individuals screened for T. cruzi infection that tested negative for S. stercoralis serology. The association between T. cruzi infection and strongyloidiasis was evaluated by logistic regression models. Principal findings During the study period, 361 individuals were screened for both infections. 52 (14.4%) individuals had a positive serological result for strongyloidiasis (cases) and 104 participants with negative results were randomly selected as controls. 76 (48.7%) indiviuals had a positive serological result for T. cruzi. Factors associated with a positive T. cruzi serology were Bolivian origin (94.7% vs 78.7%; p = 0.003), coming from a rural area (90.8% vs 68.7%; p = 0.001), having lived in an adobe house (88.2% vs 70%; p = 0.006) and a referred contact with triatomine bugs (86.7% vs 63.3%; p = 0.001). There were more patients with a positive S. stercoralis serology among those who were infected with T. cruzi (42.1% vs 25%; p = 0.023). Epidemiological variables were not associated with a positive strongyloidiasis serology. T. cruzi infection was more frequent among those with strongyloidiasis (61.5% vs 42.3%; p = 0.023). In multivariate analysis, T. cruzi infection was associated with a two-fold increase in the odds of strongyloidiasis (OR 2.23; 95% CI 1.07–4.64; p = 0.030). Conclusions T. cruzi infection was associated with strongyloidiasis in LA migrants attending a tropical diseases unit even after adjusting for epidemiological variables. These findings should encourage physicians in non-endemic settings to implement a systematic screening for both infections in LA individuals.


Principal findings
During the study period, 361 individuals were screened for both infections. 52 (14.4%) individuals had a positive serological result for strongyloidiasis (cases) and 104 participants with negative results were randomly selected as controls. 76 (48.7%) indiviuals had a positive serological result for T. cruzi. Factors associated with a positive T. cruzi serology were Bolivian origin (94.7% vs 78.7%; p = 0.003), coming from a rural area (90.8% vs 68.7%; p = 0.001), having lived in an adobe house (88.2% vs 70%; p = 0.006) and a referred contact with triatomine bugs (86.7% vs 63.3%; p = 0.001). There were more patients with a positive S. stercoralis serology among those who were infected with T. cruzi (42.1% vs 25%; p = 0.023). Epidemiological variables were not associated with a positive strongyloidiasis serology. T. cruzi infection was more frequent among those with strongyloidiasis (61.5% vs PLOS

Introduction
In recent years, the significant increase in the number of Latin American migrants in Europe has meant the introduction of parasitic endemic infections, such as Trypanosoma cruzi infection and strongyloidiasis [1][2][3]. Both are neglected tropical diseases (NTD), sharing a similar epidemiological profile in Latin America (LA) and producing life-long infections, usually silent, leading to high morbidity and mortality [4][5][6]. Chagas disease is a zoonosis endemic to LA countries [6,7], caused by hemoflagellated protozoan Trypanosoma cruzi, usually after contact with faeces of blood-sucking triatomines [8]. Congenital, organ transplantation and transfusion-related transmission are other principal routes of T. cruzi infection, which have been described in non-endemic areas [9]. The acute infection is followed by an asymptomatic chronic stage during years. After 20-30 years, up to 30-40% of patients will develop the symptomatic chronic phase, with cardiac and/or digestive involvement [8,10,11]. Chagas disease diagnosis in the chronic phase is based on serological tests [10,11].
Strongyloidiasis is a highly prevalent (over 30-100 million people worldwide) nematode infection, with a unique life-cycle where Strongyloides stercoralis females reproduce parthenogenetically to produce an autoinfective cycle that can lead to life-long and barely symptomatic carriage [5,12,13]. Nonetheless, in the context of immunosuppression, it can cause severe forms with larvae dissemination to extraintestinal organs and high mortality rates [14,15]. The diagnosis of strongyloidiasis is challenging due to irregular larvae output resulting in low sensitivity of common parasitological methods. Serology is a very sensitive test (88-95%) and it may be useful in the follow-up, as titers usually decrease after successful treatment [16][17][18].
In the context of migration and the increasing use of immunosuppressive treatments (steroids, monoclonal antibodies. . .), T. cruzi infection and strongyloidiasis have emerged as an important public health problem in Europe, North America and other areas hosting Latin American population [3,19]. Some European countries including Spain, have implemented national programmes to control transfusional and mother-to-child transmission of T. cruzi [20,21], and recent recommendations for the screening and management of strongyloidiasis in non-endemic areas have been published [22]. However, European countries are far from achieving an adequate control of the morbidity caused by these two silent chronic infections. Besides, little is known about the association between both infections in LA migrants and whether this eventual association should prompt a joint screening strategy in tropical diseases clinics.
The main aim of the present study is to evaluate the association between T. cruzi infection and strongyloidiasis in a cohort of LA migrants screened for both infections in a non-endemic setting.

Study setting and design
This is a retrospective case-control study performed at the Tropical Medicine and International Health Department in Hospital Clínic, Barcelona. Hospital Clínic is a tertiary teaching hospital and a national reference centre in Spain for Tropical Imported Diseases. Systematic screening of T. cruzi infection (among others) is performed among adult individuals who have lived for more than a year in endemic countries, or who are born from mothers with LA origin. Similarly, strongyloidiasis testing was incorporated in the systematic screening of these patients in January 2013. Individuals are commonly sent to our outpatient clinic referred by friends or relatives, primary healthcare professionals or they come by their own initiative.

Participants
Eligible participants of the study were selected from the cohort of adult individuals screened for T. cruzi infection and strongyloidiasis between January 2013 and April 2015. Individuals were included as cases if they had a positive serology for S. stercoralis in the screening blood test. Individuals who had been diagnosed with strongyloidiasis prior to the study period or had previously been treated with ivermectin were excluded. Controls were randomly selected from the cohort of individuals who were screened for T. cruzi infection and tested negative for S. stercoralis serology. Two controls were randomly selected for each case included in the study.

Study procedures
Eligible participants were invited to participate in the outpatient clinic, after signing the informed consent form. Individuals were asked for clinical and epidemiological data, which was filled in a standardized questionnaire. This contained data on sociodemographic variables, area of origin, residence in rural areas or potential risks of Chagas disease transmission (contact with T. cruzi vector, history of maternal Chagas disease or blood transfusions). Blood samples were taken to perform serological tests for T. cruzi, S. stercoralis and HIV infection. Routine hematology and biochemistry tests (including liver and renal function) were performed in all cases. Eosinophilia was defined as >500 eosinophils/mm 3 or a percentage ! 5%.
Laboratory diagnosis of T. cruzi infection was established by two serological ELISA tests, following international recommendations [23]. One was a commercial ELISA with recombinant antigens (BioELISA Chagas, Biokit S.A., Barcelona, Spain), and the other was an in-house ELISA with whole T. cruzi epimastigotes antigen. Diagnosis of T. cruzi infection was defined by positivity in the two serological tests.
S. stercoralis serological screening was performed with the commercial test IVD-ELISA (IVD Research, Carlsbad, CA) which detects IgG antibodies by using somatic antigens from larvae of the parasite. A cut-off of the sample absorbance/0.2 (i)>1.1 is defined as positive. Individuals were also asked to provide three stool samples from different days for direct microscopic examination. Agar plate culture was also performed in at least one stool sample (when available) per individual.

Statistical analysis
Stata version 13.1 (Stata Corporation, College Station, TX, USA) was used for statistical analyses. Categorical variables were described by counts and percentages, whereas continuous variables were expressed as means and standard deviations (SD) or medians and interquartile ranges (IQRs). The chi-square Pearson test was used to compare the distribution of categorical variables. The Mann-Whitney U test or the t-student test were used to compare the distribution of continuous variables.
To analyze the association between exposure variables and strongyloidiasis, logistic regression models were built to estimate unadjusted or adjusted odds ratios (ORs) with their 95% confidence interval (95% CI). Results were considered statistically significant if the two-tailed p-value was <0.05. The likelihood ratio test was used to obtain p-values.

Ethical considerations
The Ethics Committee of Hospital Clínic approved this study. Data collection forms were completely anonymous. Written informed consent was obtained from participants for collecting clinical and epidemiological data.

Cohort characteristics
During the study period, 392 patients were screened for T. cruzi infection in our center. Of these patients, 361 (92.1%) were also screened for strongyloidiasis, and were then eligible for the study (see Fig 1). Overall, 52 (14.4%) patients had a positive S. stercoralis serological result and were then included as cases and 104 out of 309 participants with negative results were randomly selected as controls by simple randomization. Table 1 shows the baseline characteristics of the cohort. The median age of the patients was 36 years (IQR 29-43) and 100 (64.1%) were women. The vast majority came from Bolivia (135 patients, 86.5%), and the other patients came from several different LA countries. Mean time in Spain prior to screening was 8.46 years (SD 3.64). There were 124 (79.5%) patients who came from rural areas, 123 (78.8%) had lived in an adobe house and 115 (74.7%) referred having had contact with triatomine bugs. Most patients were asymptomatic and the most common complaints were abdominal bloating (19.9%), heartburn (11.5%) and abdominal pain (9%). Absolute and relative eosinophilia were present in 30 (19.2%) and 50 patients (32.1%), respectively.
There were 76 (48.7%) patients with a positive T. cruzi serology. Most were women (52; 68.4%) and the median age was 37 years (IQR 30-43). Among those with a positive S. stercoralis serology, there were 34 (65.4%) women and the median age was 38 years (IQR 31-44). Mean serology titers were 5.3 (IQR 1.9-8.7), and 28 patients (53.8%) had titers greater than 2.50. None were positive for HIV. From the 114 (73.1%) patients who had provided at least one stool sample for examination, 2 (1.8%) had one positive sample for S. stercoralis and 6 (5.3%) had two positive samples. No other helminths were isolated from stool samples, although 25 (16%) patients had other microorganisms isolated from stool: 3 Giardia lamblia, 10 Blastocystis hominis and 12 Entamoeba sp. Table 2 shows the main characteristics of patients regarding to whether they had T. cruzi infection and/or strongyloidiasis.

Evaluation of risk factors for T. cruzi infection and S. stercoralis infection
Gender was not found to be associated with T. cruzi infection (p = 0.273) nor strongyloidiasis (p = 0.813). The proportion of patients aged 35 or older was also similar among T. cruzi (p = 0.307) and S. stercoralis (p = 0.302) infected and non-infected participants.

Association between Chagas disease and strongyloidiasis
After adjusting for sex, age, country of origin and rural area, T. cruzi infection was associated with a two-fold increase in the odds of strongyloidiasis (OR 2.23; 95% CI 1.07-4.64; p = 0.030) ( Table 3). We decided not to adjust for relative eosinophilia as this factor could cause collinearity with strongyloidiasis. Similarly, triatomine bug contact and living in an adobe house were also not included, since these variables could cause collinearity with rural area.

Discussion
In this retrospective case-control study, we offer an evaluation of the clinical and epidemiological characteristics of LA migrants screened for both T. cruzi infection and strongyloidiasis in a reference unit for tropical diseases. The most important finding of our study is the association found between both strongyloidiasis and T. cruzi infection. Almost all individuals screened were young, with no comorbidities-probably reflecting the overall epidemiological characteristics of global population migrating for work.
The proportion of T. cruzi infection was found to be high, as in other series of imported diseases centers and known to be altered by a positive selection bias [24,25]. This figure is highly conditioned by the fact that the majority of patients came from Bolivia, which is known to be a highly endemic country for Chagas disease [6]. In addition, most of them had lived in rural areas where the high prevalence of the vector is associated to suitable conditions for transmission such as the presence of adobe houses. Expectedly, T. cruzi infection was associated with Bolivian origin, having lived in an adobe house and a referred contact with triatomine bugs.
For initially screened patients, strongyloidiasis prevalence was 14.4%. This prevalence rate seems accordant to that found in similar migrant populations in non-endemic areas [26,27], although these studies were mostly conducted in HIV patients. Nonetheless, global prevalence of strongyloidiasis is generally underestimated and data on Bolivian prevalence of this nematode infection is especially scarce [5,28,29]. Considering the potential negative impact on patients of this life-long infection [15], this high prevalence should prompt the inclusion of active screening strategies among susceptible populations from LA [22].
In our study, strongyloidiasis was neither associated to the epidemiological nor to the clinical variables recorded. It could have been expected to find an association between a positive serology and a rural origin or having lived in an adobe house [30,31]. A possible explanation for this is that the very high prevalence of these risk factors in the whole cohort (around 80%) could have masked a possible association, but it seems unlikely to be the only explanation.
We found that strongyloidiasis and T. cruzi infection were associated even after adjusting for the main epidemiological variables. Few formal studies had previously analysed the possible association between both infections [32,33]. A possible explanation is that these two infections share an epidemiological burden where they are highly prevalent, but also the fact that both diseases are strongly influenced by socioeconomical factors such as soil contamination, barefoot walking or poor healthcare systems. Moreover, Salvador et al [33] reported a co-infection rate of 18% in those already diagnosed with T. cruzi infection and, interestingly, coinfected patients were found to have a higher proportion of positive T. cruzi RT-PCR in peripheral blood. The authors suggested that strongyloidiasis induction of Th2-immune response may lead to suppression of Th1-mediated immunity and therefore it may predispose to T. cruzi infection [33,34].
A recent cost-effectiveness study has shown that screening for Chagas disease in asymptomatic Latin American adults living in Europe is a cost-effective strategy [35]. In light of the high prevalence of strongyloidiasis found in T. cruzi infected patients, and that both diseases are prevalent and silent among Latin American migrants, a combined screening should be considered. The potential strongyloidiasis related complications and the benefits from ivermectin therapy are additional reasons to introduce systematic screening in susceptible populations.
The strengths of this study are that serology was performed systematically on the first visit, minimizing a possible selection bias, and the fact that screening for both T. cruzi and strongyloidiasis was achieved in more than 90% of the patients. However, our study has some limitations that should be acknowledged. First of all, this is an observational retrospective study and had a relatively small sample size of patients with strongyloidiasis. Secondly, the diagnosis of strongyloidiasis relied solely in a positive serologic test. A limitation of serology is that it may have false-positive results due to cross-reaction with filariae and other helminthes [36], and that it does not certainly indicate current infection [16]. Though these are important issues, especially in migrant patients where multiple parasite infections are frequent [37], IVD-ELISA has shown reliable results in term of accuracy, with high positive and negative predictive values [38]. Actually, more than half of those diagnosed with strongyloidiasis had serology titers above 2.50, which was correlated with the highest positive predictive value in one study [38]. Moreover, no other helminths were isolated from stool samples and the longtime living in Spain at screening reduces the possibility of a potential cross-reaction. Lastly, another limitation of our study is that the clear predominance of Bolivian patients compels us to be cautious with the generalizability of our findings, and further studies with higher proportions of LA migrants from other countries would be necessary.
In conclusion, T. cruzi infection was found to be associated to strongyloidiasis in LA migrants attending a tropical diseases unit. These results suggest that both infections are prevalent in these individuals and increase the scarce knowledge about the possible relationship between these two parasites. Finally, our findings should encourage physicians to implement a systematic screening program for both infections in LA individuals. Further research is needed in order to explore this possible association and the underlying mechanisms.