Introduction

Strongyloidiasis is a disease caused by soil-transmitted helminths, mainly by the species Strongyloides stercoralis. This intestinal nematode infects an estimated 300 million people worldwide, although this is probably underestimated. It is one of the most neglected of the neglected tropical diseases (NTD) and is widely distributed [1–2]. Although it generally occurs in subtropical and tropical countries, transmission is also possible in countries with temperate climates. Autochthonous cases have been described in Spain, possibly infected long time ago. It remains uncertain whether S. stercoralis is currently endemic in Spain. Still, some authors consider this country and some other southern European countries as endemic [3].

The life cycle of S. stercoralis is complex and follows multiple routes, including a complete life cycle outside the human host. The most frequent mechanism of infection is percutaneous entry of the filariform larvae. In healthy people, most of the cases are asymptomatic, although it can cause intermittent symptoms that mainly affect the intestine, the lungs or the skin [4].

About criterion used to establish the diagnosis of strongyloidiasis is not homogenized among the centers. The diagnostic laboratory criterion of strongyloidiasis is the observation of larval stages. However, in chronic infection, larvae excretion may be low and fluctuating, and microscopic observation is not sensitive enough and multiple stool specimens should be analyzed to increase the sensitivity of the method. The clinical criterion is a patient with epidemiological antecedents and any of the associated clinical manifestations, especially if it is an immunosuppressed patient.

These methods are laborious, time consuming, and in the case of fecal culture, requires well trained technicians in order to differentiate S. stercoralis. Several immunological tests have also been described (ELISA, IFAT and Western blot) with variable sensitivity and specificity depending on the population tested among other factors [1].

Alternative diagnostic methods, such as molecular biology techniques (mostly polymerase chain reaction, PCR) have been implemented. However, PCR might not be suitable for screening purpose, whereas it might have a role as a confirmatory test, since it still misses a relevant proportion of infected people [5].

Due to the subtle symptoms, low sensitivity of diagnostic techniques and the complex lifecycle that can cause asymptomatic autoinfection for decades, the prevalence of S. stercoralis is thought to be severely underestimated.

Typically risk factors for severe infection include immunosuppression, certain malignancies, human T-cell lymphotropic virus type 1 infection, and alcoholism. Likewise, S. stercoralis has been associated with agricultural or mining activities. In Germany, it was recognized as a parasitic professional disease in miners [6]. S. stercoralis infection has also been linked to low socioeconomic factors and infrastructure, indicating that it as a disease of disadvantage [7–8]. In recent years the number of diagnosed cases has been increasing in high income countries due to the growing number of immigrants, travelers and refugees [9–10].

To provide information on this topic, a systematic review of the cases of endemic strongyloidiasis in Spain was carried out, as well as the description of the epidemiological characteristics of these patients.

Methods

Aiming to assemble all scientific articles based on endemic strongyloidiasis diagnosed in Spain, a systematic review was carried out. Relevant articles were retrieved from PubMed, EMBASE, Scielo, ISI Web of Knowledge, and Cochrane Library databases using combinations of the search terms adapted to each database. Additionally, Gray Literature in the form of communications presented at national congresses was performed, as well as OpenGrey. As a secondary source, Google Scholar and free internet search was used for non-indexed articles. The keywords were “Strongyloides stercoralis”, “soil-transmitted helminthiasis”, “endemic”, and “Spain”. The following combinations of MeSH were used in PubMed: (Strongy* [MeSH] AND Spain), ("Strongyloidiasis" [MeSH] AND Spain NOT "imported" NOT "immigrant"), and ("Strongyloidiasis" [MeSH] AND "endemic" AND "Spain").

The selection criteria were articles published in any language until May 31st 2018 that contained the description of at least one human case of infection with S. stercoralis acquired in Spain without a history of travel to endemic areas. No restrictions were applied based on the study design or data collection. Human filter was applied. A manual search of the bibliographical references cited in the relevant articles was carried out.

All potential articles were analyzed by two researchers to assess compliance with the selection criteria. In situations of missed information, the corresponding author of the paper was contacted to gather the information. If the author answered the required information to fulfill the inclusion criteria, those articles were considered. If not, they were excluded because they could not ensure the endemic acquisition.

The exclusion criteria included: animal studies, cases in which endemic infection could not be assured, cases of foreigners from an endemic country for S. stercoralis, native people with trips to endemic or probably endemic countries in the past (e.g. Italy, France or Portugal), transplanted people in which this contagion route could not be excluded, and duplicated cases.

Based on these criteria the articles were reviewed in two stages. In the first stage, articles were selected by titles and abstracts according to selection criteria. In the second stage, the full text of the articles was analyzed. Finally, the articles that met the selection criteria were included in the study.

From each study the following data was extracted: the study period, year of publication and number of endemic cases described. The following epidemiological data from patients described in the studies was collected: age, gender, geographical origin, medical comorbidities and concomitant treatments, occupation (or hobbies if relevant), other risk factors, year of diagnosis, diagnostic technique used for diagnosis, presence of eosinophilia and clinical symptoms.

Results

Thirty-six studies were included describing a total of 1083 patients with endemic strongyloidiasis in Spain (see Tables 1 and 2) [11–46]. The average age of the described cases was 68.35 years, ranging from 17 to 100 years old. Two hundred and eight of the 251 (82.9%) patients in whom gender was reported were male, and most of them had current or past dedication to agriculture. The province in whom most cases were described was Valencia, with 1049 people diagnosed. Alicante had 13 and Murcia 5, eventually describing cases in provinces of coastal oceanic climate with abundant rainfall most of the year and temperatures below 22°C (Cantabria, Asturias, and Pontevedra). See Fig 1.

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Fig 1. Geographical distribution of autochthonous Strongyloides stercoralis infection in Spain.

The map was obtained from the open access website http://mapsvg.com/maps.

https://doi.org/10.1371/journal.pntd.0007230.g001

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Table 1. Main characteristics of included articles.

https://doi.org/10.1371/journal.pntd.0007230.t001

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Table 2. Clinical and epidemiological characteristics of patients with autochtonous strongyloidiasis in Spain.

https://doi.org/10.1371/journal.pntd.0007230.t002

Regarding the number of diagnosed cases per year, a decreasing trend is observed since the beginning of this decade. The year with higher number of diagnosed cases was 2003, with 82 patients. Since 2011, no more than 10 cases have been reported annually (Fig 2).

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Fig 2. Number of patients diagnosed with autochthonous Strongyloides stercoralis infection in Spain per year.

Only patients from articles that clearly specified the year of diagnosis of each case were included.

https://doi.org/10.1371/journal.pntd.0007230.g002

The technique that led to the diagnosis of strongyloidiasis was described in 743 patients from twenty-six different articles. In some cases, different techniques were used for the same diagnosis. In 692 patients (93.1%), the technique used for the definitive diagnosis of strongyloidiasis was the fresh stool examination, specific fecal culture, the Baermann test, the Ritchie technique or the Harada Mori technique. In 39 patients (5.2%) the diagnosis was made by the sputum or bronchoalveolar lavage examination. In 6 cases (0.8%) the diagnosis was made by serological techniques and in another 6 cases (0.8%) the diagnosis was made by histopathological analysis.

In 26 of the 37 patients individually described, comorbidities were reported. Out of those, most frequent were diseases that associate the use of corticosteroids such as: chronic obstructive pulmonary disease (COPD), asthma, and inflammatory bowel diseases or immunosuppressive conditions due to advanced HIV infection (AIDS stage) or malignancies. In all patients diagnosed with COPD, severity of airflow limitation (FEV1) was according to the Global Initiative for Chronic Obstructive Lung Disease (GOLD) criteria at least moderate GOLD 2 (50% ≤ FEV1 < 80% predicted) if not severe: GOLD 3 (30% ≤ FEV1 < 50% predicted).

Overall, 70.3% of these patients had at least one comorbidity. In patients in whom blood test results were reported, 41 out of the 50 (82%) exhibited eosinophilia. The median eosinophil count in patients with eosinophilia was 4,057 eosinophil/mm3; considering 24 individual reported counts.

Discussion

Strongyloidiasis prevalence may be underestimated in many countries. With the data provided by this review it is likely that underestimation could have been a reality for the last five decades in Spain. The main cause would be the lack of clinical suspicion. But also the subtle symptoms, the decades-long persistence of infection in untreated hosts and the absence of a diagnostic test of choice with high sensitivity and specificity would ultimately contribute.

An important finding of our work is that almost 97% of all published infections occurred in the province of Valencia. The fact that most cases diagnosed and published are in the province of Valencia, can respond to various reasons. Firstly, the area had the perfect combination of temperature and humidity, population exposed to S. stercoralis for occupational reasons such as rice farmers or irrigation ditch cleaners (activities that were characteristically carried out barefoot) and hygiene factors of rural areas during the 1960s (lack of drinking water and toilets in some homes). It is noteworthy that no cases of strongyloidiasis have been reported in other areas with similar climatology and population equally dedicated to the cultivation of rice fields, such as the Delta del Ebro in Tarragona province. We consider highly probable that there has been transmission in other areas outside those described. Secondly, health care professionals in the area of Valencia probably had a greater awareness of the infection, with a higher suspicion and therefore a higher number of diagnoses.

Although we concur that the estimated prevalence of S. stercoralis by one highly cited article is not representative of the entire country, we disagree that Spain should not be considered an endemic country [17]. However, autochthonous cases have been anecdotal in the last decade, as indicated by Martinez-Perez [47].

The results of the individuals diagnosed showed an average age close to 70 years old.

Given the known characteristics of the disease the contagion probably took place decades before the diagnosis, coinciding with the postwar period where hygienic conditions and infrastructure were affected. On one hand, factors of unavoidable mention that directly affect the transmission of this helminth are the improvement in hygienic conditions and the mechanization of agricultural work. On the other hand, the increase of awareness by health care workers, especially from the most affected communities, may have led to the diagnosis of new cases in recent years.

An overall higher incidence rate in male gender is described, which is consistent with previous studies [15, 17, 21, 27]. This might be explained due to a gender biased; since some articles focus on screening high risk population (farmers or smokers with COPD), traditionally associated with gender roles.

Regarding the diagnostic techniques used, there is great heterogeneity among the different studies. The sensitivity of techniques based on microscopy is not good enough, particularly in chronic infections. Serology is a useful tool but could overestimate the prevalence of the disease due to cross-reactivity with other nematode infections and its difficulty distinguishing recent and past (and cured) infections. However, current serological tests are specific enough and negativization or a decrease in the titers could be observed 6–12 month after treatment, making this tool very useful [48].

There are some limitations that have to be mentioned. Inevitably there are cases of strongyloidiasis that have not been written for publication. In addition, ten articles had to be excluded due to lack of information about travel history or did not comply with the minimum information required. Therefore, it is highly probable that there were more than 1083 cases. Lastly, given the characteristics of this review, it is possible that there are some duplicate cases in multiple description articles and described individually by another researcher.

In summary, there are still new diagnoses of autochthonous cases of strongyloidiasis in Spain every year, especially as occupational hazard in a specific Spanish region. Although the number of diagnoses is much lower than in the past decade, it is highly probable that the infection remains undiagnosed due to low clinical suspicion among Spanish population without recent travel history. Epidemiological studies in at risk areas based on serological techniques could give more information about the real situation of autochthonous cases of strongyloidiasis in Spain.

Supporting information