Plasmodium and soil-transmitted helminth co-infection : epidemiological interaction and impact among children living in endemic areas of Bagamoya, coastal region of Tanzania

Background: Parasitic infectious agents rarely occur in isolation and multiparasitism is a norm among population living in poor resource settings of developing countries. Soil transmitted helminths (STH) and Plasmodium falciparum are among the most common parasitic infections contributing the highest burden of morbidity and mortality in the tropical and sub-tropical countries including Tanzania. Children are the most affected suffering from higher prevalence and heavy parasitic load of both Plasmodium and STH. Despite the fact that co-infections occur and cause severe morbidities, less is known and invested on how the two common parasites Plasmodium and STH interact. Epidemiological evidence is lacking and intervention efforts are ongoing based on single parasite approach. The progress of intervention program coupled with epidemiological surveys to evaluate the disease burden at the individual level is lacking. New knowledge on interactions and attributed risk is relevant to better understand the disease epidemiology in relation to interventions for policy decision, better management and tailored integrated control measures.
The overall goal and specific objectives: This PhD thesis aimed to explore interactions between Plasmodium and STH infections in Bagamoyo, coastal region of Tanzania, an area where both parasitic diseases are prevalent and large scale National control programs to prevent infections are ongoing. The specific objectives pursued were as follows i) to investigate performance of the diagnostic methods used to detect helminth infections among adults and children enrolled within the TB and malaria arms of IDEA project ii) to investigate the relation of STH and Plasmodium parasite prevalence rates among children enrolled in a community cross sectional survey of IDEA project, malaria arm iii) to investigate the impact of STH on malaria clinical presentation and treatment outcome among children enrolled in a case control study of IDEA project, malaria arm.
Methods: These studies were conducted as the malaria component of the IDEA project. This is a global research program funded by the European Union (EU) designed to study the immunological interplay between helminth infections and HIV, tuberculosis (TB), and malaria. The studies were conducted in four villages situated in the rural western area about 20 to 60 km from Bagamoyo town, Tanzania. To investigate the sensitivity and performance of the diagnostic methods (objective 1), stool samples were purposely selected for methods comparison from the community survey of adults and children screened within the IDEA arm of TB and malaria respectively. Diagnostic accuracy of Kato-Katz, FLOTAC, Baermann and polymerize chain reaction (PCR) methods were analyzed for common STH, hookworm and S. stercoralis. To investigate the relation of STH and Plasmodium parasite prevalence rates (objective 2), a community cross-sectional survey was conducted among children aged 2 months to 9 years inclusively. This community-based survey was also used to recruit asymptomatic Plasmodium parasitemia children (controls) for the case-control study below. The prevalence of Plasmodium parasitemia was around 10% within the study area and thus around 1,000 children from the community were screened and enrolled. To investigate the impact of STH on malaria clinical presentation and treatment outcome (objective 3), a case-control study with a semi longitudinal follow-up was conducted. Cases were enrolled in two different groups, cases with severe malaria and cases with uncomplicated malaria. Controls were children with asymptomatic Plasmodium parasitemia. The longitudinal short term observational part of the study consisted of the assessment of response to anti-malaria treatment in the three groups according to World Health Organization (WHO) procedures (day 0, 1, 2, 3, 7, 14, 28, 42). The exposure in this case-control study was taken as presence or absence of an infection with at least one of the helminth species investigated. Included children were assessed at each visit (recruitment and follow up visits) by a qualified, trained study clinician for signs and symptoms of malaria and other common diseases using a structured questionnaire designed for the study.
In community survey and case-control study, stool, urine, adhesive tapes and blood samples were collected and examined using a broad set of quality controlled diagnostic methods for common STH (Ascaris lumbricoides, hookworm, Strongyloides stercoralis, Enterobius vermicularis, and Trichuris trichiura), schistosoma species and Wuchereria bancrofti. Blood slides and malaria rapid tests (mRDTs) were utilized for Plasmodium diagnosis. Stool, urine and adhesive tapes were collected at the initial screening (both studies) and during the day 28 follow up visit (case-control study). In order to investigate the impact of STH on malaria clinical presentation and treatment outcome, children diagnosed with STH received a delayed anti-helminthic treatment at the end of study follow up (day 42).
Principle findings: The recruitment of the study was done between June 2011 and November 2012, covering all year seasonal variations. Among the adults and children samples used to investigate the diagnostic performance of the different methods, hookworm (10.0%) and S. stercoralis (7.4%) were the most prevalent STH followed by T. trichiura (1.9%), A. lumbricoides (0.2%) and S. mansoni (0.2%). Generally, more than 80% were low intensity infections. Using a direct method comparison as reference FLOTAC had a significantly higher sensitivity (93.8%) than Kato-Katz. This was not the case for PCR. Sensitivity of PCR for S. stercoralis diagnosis (17.4%) was significantly lower than that of Baermann method (47.1%). The direct method comparison revealed an equal sensitivity of the PCR and Kato-Katz methods for hookworm diagnosis (73.0%). A significantly negative correlation was found between the PCR cycle thresholds (Ct) values and microscopic egg per gram (EPG) or larvae counts of hookworm and S. stercoralis respectively.
Out of 1,033 children included in the cross sectional community survey, 283 (27.4%) were infected with any of the helminth infection. The most prevalent helminth species were E. vermicularis (18.0%), hookworm (9.1%) and S. stercoralis (6.9%). Other types of helminth isolated were T. trichiura (2.5%), W. bancrofti (1.4%), S. haematobium (0.3%) and A. lumbricoides (0.1%). No child was diagnosed with S. mansoni. Helminth infection prevalence increased with age, from infants (10.2%), pre-school aged (25.0%) and school aged children (33.5%). S. stercoralis was the most common species affecting infants and significantly associated with a higher risk of asymptomatic Plasmodium parasitemia [OR=13.0 (95% CI of 1.3 – 127.2)]. Asymptomatic Plasmodium parasitemia was associated with mild [OR=1.8 (95% CI of 1.0 – 3.3)] , moderate [OR=5.4 (95% CI of 2.9 – 9.8)] and severe anemia [OR=11.2 (95% CI of 4.2 – 29.9)] in children below two years and those above two years presenting with moderate [OR=3.1 (95% CI of 2.0 – 4.8)] or severe anemia [OR=7.3 (95% CI of 2.0 – 26.0)]. Neither S. stercoralis, E. vermicularis, hookworm nor any other investigated helminth infection was associated with wasting, underweight, thinness, stunting or anemia in our study population.
Out of 992 children analyzed for dual helminth and Plasmodium infection, the prevalence of Plasmodium mono-infection was 8.1% (80/992), helminth mono-infection 23.5% (233/992) and co-infection with Plasmodium and any helminth species 5.0% (50/992). The prevalence rate of Plasmodium, specific STH species and co-infections increased significantly with age (p< 0.001) with school-aged children mostly affected except for S. stercoralis monoinfection and co-infections. There was a trend for STH infections to be associated with Plasmodium infection [OR adjusted for age group 1.4 (95% CI 1.0 - 2.1)], which was more marked for S. stercoralis [OR= 2.2 (95% CI of 1.1 - 4.3)]. Age and not schooling were risk factors for Plasmodium and STH co-infection.
In the case control study, there was a tendency for a protective effect of helminth on the development of clinical malaria [OR=0.6, 95% CI of 0.3 – 1.3] which was more marked for E. vermicularis species [OR=0.2, 95% CI of 0.0 – 0.9]. On the contrary, hookworm species tended to be associated with clinical malaria [OR= 3.0, 95% CI of 0.9 – 9.5]. In multiple conditional regression analysis, the overall protective effect was lower for all helminth infection [OR= 0.8, 95% CI of 0.3 – 1.9] but remained significantly protective for E. vermicularis species [OR= 0.1, 95% CI of 0.0 – 1.0] and borderline significant for hookworm species [OR= 3.6, 95% CI of 0.9 – 14.3]. Using ordinal logistic regression which better reflects the progression of asymptomatic Plasmodium parasitemia to severe malaria, there was a 50% protective effect with overall helminth [OR= 0.5, 95% CI of 0.3 – 0.9]. On the contrary, hookworm species was highly predictive of uncomplicated and severe malaria [OR= 7.8 (95% CI of 1.8 – 33.9) and 49.7 (95% CI of 1.9 – 1298.9) respectively]. Generally, children infected with STH had higher geometric mean time to first clearance of parasitemia.
Conclusion: Multiparasitism is common among children, but also in infancy, with school aged children bearing the highest dual burden of both Plasmodium and STH infections. The prevalent STH infections were E. vermicularis, hookworm and S. stercoralis, mostly of light intensity. Multiple diagnostic techniques should be performed including adhesive tapes and Baermann methods when evaluating the burden of helminth in children. Novel technologies with diagnostic assays that can be performed in a high throughput system on large number of population samples to detect all relevant parasite species in low intensity areas would be desirable. The findings of a protective effect of E. vermicularis should not deter at this stage deworming programs but rather foster implementation of integrated control program for common parasites to speed up the momentum of moving from morbidity and transmission control to elimination. This is especially true because of the enhancing effect of hookworm on malaria morbidity. Integrated multidisciplinary approach such as distribution of long lasting insecticidal treated bed nets and chemotherapy with dual effect (e.g. ivermectin) associated with health education and improvement of environmental sanitation and hygiene, improved housing and access to safe water should be implemented among school-aged children but also in the under-fives considering the pattern and types of infections within the area. The impact of E. vermicularis and S. stercoralis on Plasmodium parasitemia requires further investigation to better understand its risk, benefits and mechanism involved among children and other risk groups living in different transmission intensity in endemic areas.