Assessment of seasonality in soil-transmitted helminth infections across 14 schools in Jimma Town, Ethiopia

Introduction Up to date, the frequency of preventive chemotherapy based on the prevalence is the only strategy in the control programmes of soil-transmitted helminths (STHs). However, prevalence of STHs may be affected by climatic and/or seasonal changes, particularly when these are important determinants of transmission of STH infections. Our objective was to describe the prevalence and infection intensity and seasonal variation (mainly dry vs rainy season) of any STHs among school age children. Methods Assessment of infection intensity and prevalence of STHs was carried out during dry season (February-March, 2012) and end of rainy season (September-October, 2012) across 14 primary schools in Jimma Town, Jimma, Ethiopia. A total of 1,680 school children (840 in each season) were included. All stool samples were processed by the McMaster egg counting method. Odds of infection and intensity was performed to assess any differences in prevalence and infection intensity between the schools and the two seasons. The pooled odd ratio and their 95% confidence interval was also computed and presented using the "metafor" package of the statistical software R. The level of significance was declared at p < 0.05. Results Infections with any STH were observed in 824/1,680 (49.0%) subjects. T. trichiura was the most prevalent (35.5%), followed by A. lumbricoides (23.4%) and hookworms (9.9%). Among the schools there were a huge variation in prevalence, ranging from 16.7% to 68.3% for any STH, 6.7% to 39.2% for A. lumbricoides, 10.8% to 55.0% for T. trichiura and 0 % to 28.3% for hookworms. A significant difference in prevalence (for T. trichiura) and in infection intensity (for A. lumbricoides and T. trichiura) across seasons was observed. Generally, STH infections were more prevalent in the dry season (52.4%) compared to the rainy season (45.7%) and as well intensity of all three STH infections was higher in the dry season. Conclusion Our data suggested that there were huge variation in STH prevalence among schools and a significant difference in infection intensity and prevalence across seasons. This in turn might limits how national governments and international organizations define and target resources to combat the disease burden due to STH infection. Long term studies are needed to confirm the influence of seasonal factors and related ecological, environmental and socio-economic factors.


Introduction
Over five billion people of the world are at risk of infection from soiltransmitted helminths (STHs), caused by the roundworm Ascaris lumbricoides, the whipworm Trichuris trichiura and the two hookworm species Ancylostoma duodenale and Necator americanus [1]. It is estimated that in 2010 approximately 1.4 billion people were infected worldwide, accounting for 20% of the disability-adjusted life years caused by NTDs [1,2]. The highest prevalence data are reported in tropical and subtropical regions from Sub-Saharan Africa, the Americas, China and East Asia [3][4][5].
Up to date, the frequency of preventive chemotherapy (PC) is based on the prevalence is the only strategy in the control programmes of STHs. Based on the STHs prevalence, either Albendazole or Mebendazole drugs being administered annually when the overall STHs prevalence is at least 20% but less than 50% or bi-annually when the prevalence exceeds 50% [6][7][8]. However, this STH prevalence may be affected by climatic and/or seasonal changes, particularly when these are important determinants of transmission of STH infections. Adequate moisture and warm temperature are essential for egg/larval development in the soil [1,8,9]. For example, eggs of A. lumbricoides and T. trichiura will not embryonate at low humidity, whereas higher humidity is associated with faster development of eggs [10,11] whereby in turn, these differences in development and survival will affect and influence parasite establishment in the human host and, hence, infection levels.
In wet seasons, pre-parasitic stages of worms might survive in the environment that favours and increases in transmission. This is in contrast with dry climatic conditions, which kills deposited infective stages on the soil surface making the dynamics for transmission to decline [9]. Previous studies reported that high STHs infection was detected in rainy season compared to summer season and comparatively lower incidence of STH was noticed in winter season [12,13]  hosting all eight grades of students were invited to participate. In each school subjects were stratified according to three age classes (age class A: age 5-9 years, B: age 10-13 years and C: age 14-18 years). For each age class at least 20 subjects were selected on a voluntary basis, resulting in a total of at least 60 subjects per school. The subjects were asked to provide at least 3 g of stool.
This quantity of stool was required to examine the samples individually (2 g) after processed with the McMaster egg counting method (analytic sensitivity of 50 eggs per gram of stool (EPG)) for detection and enumeration of STH eggs [16]. Figure

Results
Prevalence and infection intensity:

Discussion
Overall, the prevalence and intensity of STH infection is high among SAC in Jimma Town where nearly half of the children are infected at least with one STH species. Our data showed even within a defined geographic area located in less than 10 km radius (Figure 1) given that these environmental factors have been shown to play significant role in the transmission of STH [21][22][23][24][25], which is the limitation of this study, as we did not explore those factors.
Compared to end of rainy seasons (September-October) SAC were shown to have marginally higher prevalence of both T. trichiura and hookworms, but relatively uniform A. lumbricoides infections during dry seasons (January-February). In the same line, Nwoke et al, [25] have reported highest (18.7%) recovery of parasites including STHs from soil in dry season as compared to low recovery rate (12.0%) in wet seasons, indicating seasonal variation of soil contamination level by STHs. However, the present findings disagree with the findings of previous studies reported elsewhere [12,13] that showed highest prevalence of STH in rainy season compare to dry season. Looking at infection intensity across age classes, the observed trend revealed a concentrated EPG in the age class A (5-9 years) for both A. lumbricoides and T. trichiura and tend to decline across age class B (10-13 years) and C (14-18 years), which is consistent with findings of previous studies elsewhere [26][27][28]  Seasonal fluctuations of the environment may affect patterns and intensity of STH.