Enterobiasis and strongyloidiasis and associated co-infections and morbidity markers in infants, preschool- and school-aged children from rural coastal Tanzania: a cross-sectional study

Soil-transmitted helminth infections belong to the neglected tropical diseases [1]. The most common soil-transmitted helminths are roundworms (Ascaris lumbricoides), whipworms (Trichuris trichiura), and hookworms (Ancylostoma duodenale and Necator americanus). Infections are acquired via the ingestion of the parasites’ eggs, for example with contaminated food, or in case of hookworms via larvae penetrating bare skin [2],[3]. The adult worms live in the intestines of humans, and eggs or larvae are excreted with faeces and thrive in the soil. Hence, infections are particularly prevalent in areas, where hygiene and sanitation are low and where the environment favours a rapid transmission. It is estimated that globally around 1.45 billion people are infected with at least one intestinal helminth species, and a global burden of 5.2 million disability adjusted life years (DALYs) is attributed to these infections [4]-[6].

In addition to the four major soil-transmitted helminth species, there are two intestinal nematodes infecting humans, the threadworm Strongyloides stercoralis and the pinworm Enterobius vermicularis that are mostly neglected in prevalence reports and global burden of disease estimates due to their rather unpleasant and cumbersome diagnosis with the Baermann and/or Koga agar plate methods or adhesive tape test, respectively, and due to difficulties to assess associated morbidity [1],[2],[7]-[10].

S. stercoralis has an intra- and extra-human lifecycle during which the parasite can reproduce asexually and sexually, respectively. Larvae that hatched from eggs within the intestines are able to penetrate the intestinal wall and can perpetuate the lifecycle within the human host for decades after initial exposure [2],[11]. Infections are mostly mild and often asymptomatic in otherwise healthy individuals [12]. Skin lesions, pulmonary and gastro-intestinal symptoms, and blood eosinophilia are reported as unspecific disease markers [10],[13]-[17]. Chronically infected immunocompromised patients, however, are at high risk of developing a lethal hyper-infection syndrome, caused by proliferating tissue-invasive larvae that might carry bacteria from the intestines to organs, leading to systemic infections, multiorgan failure and systemic sepsis [1],[12],[18]. It is estimated that S. stercoralis affects between 10% and 40% of the population in tropical- and sub-tropical countries, but adequate information is lacking, particularly for the high-risk areas including Sub-Saharan Africa and Southeast Asia [19]. Global burden of disease estimates do not exist for this parasite [5],[19],[20] and strongyloidiasis remains an underestimated health problem [21]. Good reliable data on infection, morbidity, and indisputable causal links between them, are urgently needed to make the public health argument for a guided response against S. stercoralis [7].

E. vermicularis is transmitted via the ingestion of eggs contained in dust, water, or sticking on hands and food. The female adults live in the cecum and large intestines and migrate to the anus to deposit their eggs on the perianal skin [2]. Since eggs become infective within hours, autoinfection is possible [22]. Enterobiasis includes symptoms such as intense pruritus in the perianal area, which can lead to insomnia, restlessness, and irritability. Moreover, the adult pinworms can migrate into the appendix or genital tract, causing appendicitis and genitourinary complications [23],[24]. Ectopic infections of liver, lung, kidneys, and other organs occur infrequently [24],[25]. Enterobiasis is considered the most common helminth infection worldwide [26]. It is estimated that globally 4-28% of children are infected [3], but recent and profound prevalence and burden estimates are missing, particularly for Sub-Saharan Africa.

The neglect of strongyloidiasis and enterobiasis in most studies pertaining to soil-transmitted helminth infections results in a knowledge gap about their potential impact on co-infections and the nutritional status of the host. While it is widely acknowledged that helminth infections modulate immune responses and might influence the acquisition and outcomes of diseases such as malaria, tuberculosis (TB), and human immunodeficiency virus-acquired immune deficiency syndrome (HIV-AIDS), the exact mechanisms are not known [27]-[37]. Research on this topic has gained much interest over the past years, but results are inconsistent. Multiple dimensions seem to be involved, including helminth species, infection intensity, and the host’s age, genetics, and immunological and nutritional status [38],[39].

The IDEA research program is designed to intensively study the immunological interplay between helminth infections and malaria, TB, and HIV-AIDS (http://​ec.​europa.​eu/​research/​health/​infectious-diseases/​neglected-diseases/​projects/​014_​en.​html). Of course, to obtain reliable results, not only the presence of major helminth species, but also infections with S. stercoralis and E. vermicularis need to be taken into account. Here, we present, to our knowledge for the first time, the extend of strongyloidiasis and enterobiasis, besides other helminthiases, in infants, preschool-aged children (PSAC), and school-aged children (SAC) from rural coastal Tanzania that participated in a cross-sectional study implemented to recruit participants for the IDEA-malaria study arm in the Bagamoyo district, United Republic of Tanzania. We also show anthropometric and haematological characteristics of the study group and associations found between the individual helminth species infections, between helminth infections and asymptomatic Plasmodium infections and between helminth infections and anthropometric status, anaemia, and additional haematological parameters in different age-groups. With this report we aim to shed more light on the distribution and public health importance of two highly neglected soil-transmitted helminth species and therefore to contribute to inform yet missing global burden and prevalence estimates.

Little is known about the epidemiology and public health importance of S. stercoralis and E. vermicularis infections in Sub-Saharan Africa. We found that strongyloidiasis and enterobiasis were predominant to other helminthiases in our study population. Noteworthy, already infants, an age-group mostly ignored in studies pertaining to soil-transmitted helminthiases, presented with S. stercoralis (5.8%) and E. vermicularis infections (4.2%). Prevalences increased with age, and 7.5% and 7.1% of PSAC and SAC, respectively, were infected with S. stercoralis and 16.7% and 26.3%, respectively, with E. vermicularis.

The prevalences for all helminth infections determined in our study population are relatively low compared to those reported from other settings in the United Republic of Tanzania [36],[55],[56]. The following considerations are offered for explanation: Firstly, over the past decade, Tanzania has successfully implemented a series of helminth control interventions [57],[58], which might have reduced the infections in the population. Indeed, our under five year old children might have received Mebendazole in the frame of the biannual interventions of the Expanded Programme on Immunisation (EPI), which provides vitamin A supplementation, measles vaccination, and deworming in the Bagamoyo District, and the SAC might have been targeted by school-based deworming organized by the Ministry of Health. Secondly, we included infants, PSAC, and SAC aged two months to ten years in our study. The age-prevalence-curve for the major soil-transmitted helminth infections and schistosomiasis increases with age and peaks in children between 8 and 15 years [3],[59]. Our children were hence not yet in the peak age for infection, but an increase in the prevalence of all investigated helminth species with age was indeed observed. Thirdly, the conventional parasitological diagnostic methods that we applied lack sensitivity, particularly when infection intensities are low and only a single faecal sample is examined [9],[60],[61]. While we used duplicate Kato-Katz thick smears and the FLOTAC dual technique on a single stool sample to detect soil-transmitted helminth and S. mansoni infections and duplicate urine filtrations from a single urine for S. haematobium diagnosis, the true helminth prevalences in our study were likely considerably higher.

S. stercoralis infections were associated with eosinophilia in our study population, a condition that is generally attributed to strongyloidiasis [13],[17]. Our study also confirmed results from the neighbouring Zanzibar island with regard to a positive association between S. stercoralis and T. trichiura infections [55]. Novel is our finding that strongyloidiasis was associated with asymptomatic Plasmodium parasitaemia in infants. While the expected prevalence of this co-infection in infants was only 0.5%, the observed prevalence was 2.1%. However, due to the very low number of co-infected infants and multiple testing, these findings must be interpreted with care. If future studies with a larger number of co-infected children confirm our results, it might be reasonable to assume that this tissue invasive helminth species in particular is modulating immunological pathways related to the acquisition, replication, and pathologic sequelae of Plasmodium infections. In the worst case, S. stercoralis infections in infants may be a contributing factor to the delayed acquisition of clinical immunity to malaria in this highly vulnerable age-group. More indepth-studies on the immunological interplay of strongyloidiasis and malaria are clearly needed.

To date, leading reviews on the global distribution and risk factors for S. stercoralis do not mention malaria, and publications summarizing the epidemiology and immunological interplay of helminth and malaria co-infection ignore S. stercoralis, respectively. The only study we found assessing S. stercoralis infections and malaria was on pregnant women from Uganda, and revealed no association [62]. Moreover, there are only very few publications mentioning S. stercoralis infections in infants and PSAC in sub-Saharan Africa, and there are no recent reports assessing strongyloidiasis in any age-group on mainland Tanzania [10],[19],[63]-[65]. Our findings that S. stercoralis was infecting a considerable number of young children of our study population from the age of ten months onwards, and was associated with asymptomatic Plasmodium parasitaemia, underline the importance of diagnosing this disease. Inclusion of S. stercoralis into helminth control programs, as suggested by other research teams, needs to be considered [7],[19],[21].

Enterobiasis was not associated with any specific haematological marker or helminth co-infection, but children of all age-groups with a reported anthelminthic treatment history over the past six months and SAC with an increased temperature had higher odds of an infection. This finding suggests that children with enterobiasis suffer, likely from anal itching and ill-being, and sought treatment for relief. The higher odds of infection might be a sign that either the treatment as administered did not cure the infection or that children got rapidly reinfected, likely due to transmission from additional infected family members [25]. Similarly, infants with reported anthelminthic treatment history had higher odds of hookworm infection in our study. Also hookworm infection can considerably impact on a child’s health and wellbeing [10] and therefore treatment might have been sought. However, infants with hookworm infection who had a reported treatment history might either not have been cured by the treatment or live in a particularly unhygienic environment that favours rapid reinfection. Cure might not have been achieved since, in case mebendazole was administered, cure rates for hookworm are very low [66] or, in case no liquid formulation was available, crushed tablets were difficult to administer to very young children and might not have resulted in complete clearance of infection.

Neither E. vermicularis nor hookworm infection was associated with asymptomatic Plasmodium parasitaemia in our study. While there are hardly any cross-sectional surveys assessing E. vermicularis infections with appropriate diagnostic methods in sub-Saharan Africa, let alone its potential association with malaria, there are multiple studies investigating associations between hookworm and Plasmodium infections. In line with our findings, there are studies that did not find an association between hookworm and Plasmodium infections [67],[68]. Other studies, however, indicate significant associations between these infectious diseases [38],[62],[69]. The heterogeneous results and potential consequences of co-infections are nicely summarized in recent reviews [27],[28].

Neither S. stercoralis, E. vermicularis, hookworm, nor any other investigated helminth infection was associated with wasting, underweight, thinness, stunting, or anaemia in our study population. While wasting, underweight, and thinness were rarely seen in the surveyed children and therefore no association could be determined, stunting occurred in 18.5% of SAC and anaemia affected more than half of the examined children. Previous studies conducted in the Kilombero district in Tanzania in the early 1980s had also revealed no association between wasting or stunting and intestinal helminth infection or Plasmodium parasitaemia [65]. In that particular time and place the observed substantial differences in the nutritional status of children were related to the lean and post-harvest seasons [70],[71]. The Bagamoyo district, however, is relatively rich in cash-crop production agricultural systems throughout the year, and was one of the four councils in Tanzania, that were not reported to have major food and nutrition insecurity problems in 2011/12 [72]. This might explain why no acute but only chronic signs of malnutrition like stunting were found in our study. Other studies indicated that particularly moderate-to-heavy intensity helminth infections were associated with reduced length-for-age z-score in young infected children from Peru [73], with decreased weight-for-age z-scores in SAC from Honduras [74], and with stunting in SAC from China [75]. The occurrence of mostly light helminth infection intensities in our study might hence explain why no association was found.

Anaemia was related with asymptomatic Plasmodium parasitaemia but not with helminthiases in infants, PSAC, and SAC in our study. It is widely acknowledged that Plasmodium infections, also if low-grade and asymptomatic as a result of semi-immunity, contribute to the development of anaemia [76]. Also hookworm infections have been shown to contribute to anaemia, particularly in children and women of childbearing age, and when infection intensities were moderate-to-heavy [3],[77]-[80]. Also here, the reason for not finding any association of helminth infections with anaemia might be due to the mostly light infection intensities found in our study population. Clearly, the low prevalence of helminth infections and specifically of moderate and high infection intensities, resulting in a lack of statistical power to determine effects on nutritional aspects including anaemia and stunting, are a limitation of this study. Moreover, we did not assess additional reasons for malnutrition and anaemia, such as restricted access to micro-nutrients, agricultural and dietary practices, food security, or social, political, and economic determinants, which might have biased our results.

Elevated eosinophil counts were strongly associated with hookworm (OR: 16.6) and S. stercoralis (OR: 4.0) infections in infants and with T. trichiura (OR: 3.2), hookworm (OR: 2.7), and S. stercoralis (OR: 2.2) infections in SAC. Marked eosinophilia is considered a common marker for early hookworm and S. stercoralis infections, and is explained by the immune reaction to the worm larvae, which migrate through the body tissues to reach their destination in the lungs and in the gastrointestinal tract [13]. The eosinophilia caused by T. trichiura is reported to be mostly mild [13]. The hematology values of our study population were only partially in line with reference values derived from a similar study population based in the Kilimanjaro district [51]. Considerably higher neutrophil counts were found in almost a quarter and elevated monocyte, eosinophil, and basophil counts in more than a third of all participants of all age-groups in our study children. While higher eosinophil counts were associated with specific helminth species infections and monocyte counts were associated with asymptomatic Plasmodium parasitemia in some age-groups, the latter being a common sign of acute malaria [81],[82], there might be additional underlying reasons such as viral or bacterial infections causing monocytosis or allergies and other diseases causing eosinophilia [13],[83]. Elevated basophil counts were not associated with helminth or asymptomatic Plasmodium infections in our study, confirming that it is not a useful clinical marker for the evaluation of suspected parasitic disease [84]. Moreover, considering the unpublished reference values used in the IHI-BRTC laboratory, the normal range of basophils at the IHI-BRTC is 0.0-0.3 per 10⁹/L and thus much higher than the Kilimanjaro values [51] and rather resembling the pediatric reference values reported for Uganda [85]. A limitation of our study is that no other infections or diseases that might have caused elevated or decreased blood cell counts were investigated and there is a clear need for the establishment of standardized hematological reference values for the Bagamoyo area.

The results of our cross-sectional study showed that E. vermicularis and S. stercoralis were moderately prevalent in young children from rural coastal Tanzania. A considerable number of infants were infected and prevalences increased with children’s age. Our data can contribute to inform yet missing global burden of disease and prevalence estimates for strongyloidiasis and enterobiasis. The association between S. stercoralis and asymptomatic Plasmodium parasitaemia found in infants of our study population warrants further investigations.