Seasonal and Spatial Dynamics of Freshwater Snails and Schistosomiasis in Mizan Aman, Southwest Ethiopia

Abstract

Introduction: Freshwater snails, particularly snails from the genus Biomphalaria, play a key role in the transmission of schistosomiasis, a parasitic disease prevalent in tropical regions. Schistosomiasis poses a significant public health challenge in these regions, leading to chronic illness, reduced productivity, and impaired childhood development, particularly in communities with limited access to healthcare and sanitation. Understanding the seasonal and spatial variations in snail populations and infection rates is crucial for controlling schistosomiasis, especially in areas like Southwest Ethiopia, where the disease burden is high. Methods: This study was conducted in Mizan Aman, Southwest Ethiopia, across two seasons, dry and wet. A total of 1150 snail samples were collected from 20 freshwater sites, and their species, abundance, and infection status were assessed. Environmental parameters, including temperature, pH, salinity, and conductivity, were measured to analyze their impact on snail populations. Results: Four snail species were identified, Biomphalaria pfeifferi, Biomphalaria sudanica, Lymnaea natalensis, and Bulinus globosus, with B. pfeifferi and B. sudanica being the most prevalent. Snail abundance varied by site and season, with 598 in the dry season and 552 in the wet season. Snail abundance and species composition showed significant spatial variation, with higher counts in sites like Sasin and Agu 1, while some sites had no snails. Biomphalaria snails, particularly B. pfeifferi, are the principal intermediate host for Schistosoma mansoni. The overall prevalence of Biomphalaria snails exceeded 85% in both seasons, and their average infection rate in Mizan Aman was 13.5%. This infection rate showed a strong correlation (r = 0.733, p < 0.001) with the incidence of schistosomiasis cases in the community. Seasonal variation in environmental factors, such as temperature and pH, had no significant effect on snail abundance; however, water salinity showed to be correlated with snail abundance during the dry season. Furthermore, community-led vegetation clearance at selected sites significantly reduced snail abundance. Conclusions: This study highlights the seasonal and spatial dynamics of freshwater snails, particularly Biomphalaria species, in relation to schistosomiasis transmission in Mizan Aman, Southwest Ethiopia. The findings confirm that B. pfeifferi species is the predominant intermediate host for schistosoma in this region and that schistosomiasis infection rates among snails significantly correlate with human cases in the community. While environmental factors such as temperature and pH showed no significant influence on snail abundance, water salinity had an impact during the dry season. Additionally, community-led vegetation clearance was an effective intervention in reducing snail populations. These results emphasize the need for targeted, site-specific control measures integrating ecological and community-based interventions to sustainably reduce schistosomiasis transmission.

1. Introduction

Freshwater snails, particularly of the genus Biomphalaria, are the principal intermediate host for schistosomiasis, a parasitic disease that significantly impacts public health in tropical and subtropical regions, including sub-Saharan Africa [1,2]. Schistosomiasis is caused by parasitic trematode worms of the genus Schistosoma, whose cercariae are released by infected Biomphalaria snails into freshwater bodies, posing a major threat to communities dependent on these water sources for daily activities [3,4]. Understanding the factors influencing snail populations and their infection rates is critical for effective and focused schistosomiasis control. Seasonal changes and environmental conditions, such as temperature, pH, and salinity, are known to affect snail distribution and infection dynamics [5,6]. Studies conducted in Uganda and Nigeria have shown that wet seasons typically support higher snail populations, which may correlate with increased transmission of schistosomiasis due to favorable environmental conditions for snail reproduction and the survival of Schistosoma cercariae [7,8,9].

Despite the crucial role of freshwater snails in the transmission of schistosomiasis, very little data exist on the seasonal and spatial variation in snail abundance and infection rates in different ecological zones within Ethiopia [3]. This study aims to address this gap by examining the distribution of freshwater snails across multiple sites in Mizan Aman where schistosoma prevalence was reported to be relatively high (32%) [10]. Examinations were conducted over the dry and wet seasons, focusing on species composition, abundance, infection rates, and the relationship between environmental factors and snail presence. This study builds on prior research examining snail populations during the dry season in Mizan Aman [11]. This research also includes data from the wet season, allowing for a comparative analysis of seasonal variations. By expanding on these findings, it provides valuable insights into schistosomiasis transmission dynamics in Ethiopia and the broader East African region, contributing to the growing body of research on the disease. Additionally, by identifying key seasonal and spatial patterns in snail abundance and infection rates, the study provides a deeper understanding of the ecological drivers of schistosomiasis transmission in Mizan Aman, with significant implications for public health management and policy-making in endemic regions.

2. Materials and Methods

Study area and site selection: This study was conducted in Mizan Aman, located in Southwest Ethiopia (SWE), a region characterized by its diverse freshwater habitats and seasonal variations in rainfall, with distinct dry and wet seasons. Snails were sampled from 20 different locations: Agu 1, Agu 2, Sasin, Shonga 1, Shonga 2, Shonga 3, Kusha, Keker 1, Keker 2, Esine 1, Esine 2, Esine 3, Kosokol 1, Kosokol 2, Kosokol 3, Kosokol 4, Chorobay, Borini, Kabash, and Wugni. These encompass rivers and streams that were selected based on their potential to harbor snails and their proximity to human activity. The sites were selected based on local information about water-related activities, such as bathing, washing clothes and utensils, swimming, playing, and water collection, as well as their proximity to schools and the local community. A total of 20 sampling points across 10 rivers and 1 stream were selected for mapping.

Sampling and Data Collection

Snail collection and identification: Snail collection was carried out in two distinct rounds, each corresponding to a specific season to capture seasonal variations in snail abundance. The first round, conducted during the dry season, took place from December to January 2022. The second round, representing the wet season, was held from June to July 2023. This structured approach allowed for a comprehensive analysis of seasonal changes in snail populations across the study sites. At each site, snails were sampled using a standard scooping method with a long-handled scoop net for 30 min [11]. In shallow water, snails were collected by hand. All collected snails were identified to species level using morphological characteristics [12]. Snails were placed individually in vials containing clean water and exposed to artificial light to stimulate cercaria shedding. Shedding snails were identified as infected, and cercariae were classified using a microscope. Infection rates were recorded as the proportion of shedding snails to the total collected at each site.

Environmental and water quality analysis: Environmental data were systematically collected at each site during each sampling round to assess the influence of habitat characteristics on snail populations. Key water quality parameters were measured, including water temperature (°C), pH, salinity (g/L), total dissolved solids (TDS, mg/L), and conductivity (mS). Additionally, the substrate type and vegetation type at each site were documented. These measurements provided a comprehensive environmental profile, crucial for understanding the factors influencing snail distribution and abundance. Geographic coordinates and environmental features were recorded for each site to enable a comprehensive analysis of seasonal and spatial variations. Vegetation removal was conducted by the community after the initial snail sampling (dry season), therefore, during the wet season.

Schistosomiasis prevalence: Human infection data were collected from local community members who were actively using water sources during the snail mapping period. Stool samples were transported to the Parasitology Laboratory at Mizan Tepi University for detailed analysis. The Kato–Katz technique was employed to examine the stool samples for parasitic infections.

Statistical analysis: The data was collected through the mWater mobile data collection platform, a digital tool used for water, sanitation, and hygiene (WASH) data collection, monitoring, and mapping. The data was then exported to SPSS version 25.0 (IBM Corp. IBM SPSS Statistics for Windows, Armonk, NY, USA) for cleaning and analysis of data. Both descriptive and inferential statistical methods were employed to examine seasonal and spatial variations in snail abundance, species composition, and infection rates. Snail abundance and species composition were summarized as total counts and percentages, categorized by site and season. The Kruskal–Wallis H test was applied to assess variation in snail abundance and infection rates across different sites and seasons. It was also used to analyze the seasonal variations in environmental parameters, including temperature, pH, salinity, and total dissolved solids (TDSs), to assess statistical significance across different conditions. Chi-square tests evaluated the statistical significance of snail presence in different seasons and sampling sites. Spearman’s rank correlation analysis was conducted to examine the correlation between infected Biomphalaria snails and schistosomiasis prevalence. A p-value below 0.05 was considered the threshold for statistical significance in all analyses.

Ethical considerations: Ethical approval for the study was granted by the ethical review committee of the College of Medicine and Health Sciences, Mizan Tepi University. Additionally, letters of support and permission were obtained from the Bench Sheko zonal health department and the Mizan Aman district health office. Written informed consent was secured from local residents who participated in the stool examination.

3. Results

Mapping was conducted at 20 selected sampling sites within Mizan Aman city (Figure 1), within a UTM Easting range of 778033–788162 and a Northing range of 763158–775972, at altitudes between 1282 and 1385 m above sea level. This is a region abundant in rivers and streams that play a vital role in meeting the water needs of the local community. The study was carried out over a dry season and wet season. The dry season extended from December 2021 to January 2022, while the wet season occurred from June to July 2023.

The sampling points were distributed across 10 rivers and 1 stream, selected based on human activities such as clothes washing, bathing, dishwashing, car/bike washing, water collection, swimming, and farming. Sampling sites differed by the ground composition, which was either rocky/sandy or muddy, as well as in the vegetation composition that varied slightly between seasons, with taro, banana trees, water grass, and insects consistently present at most sites (Supplementary Table S1).

3.1. Spatial Variation in Snail Abundance and Species Composition

Snails were collected from 20 sites along various water bodies, with some sites having multiple sampling locations within the same water body, each assigned a unique number. In Agu, a total of 514 snails were collected, with 388 from Agu 1 and 126 from Agu 2. Sasin, a river, contributed 356 snails. Shonga, a stream, accounted for 335 snails, with 165 from Shonga 1, 4 from Shonga 2, and none from Shonga 3. Kusha, a river, had 79 snails. Keker, a specific location within a water body, contributed eight snails, with no snails collected from Keker 2. Esine, a stream, yielded 118 snails, with 116 from Esine 1, 2 from Esine 2, and none from Esine 3. Kosokol, a water body, contributed 329 snails, with 126 from Kosokol 1, 1 from Kosokol 2, 101 from Kosokol 3, and none from Kosokol 4. Chorobay, a stream, accounted for 69 snails, while both Borini and Kabash, specific locations within a water body, had no snails collected. Finally, Wugni, a stream, contributed 75 snails. In total, 1150 snails were collected across all locations. Biomphalaria pfeifferi was the most abundant species, comprising 66.4% of the total sample, followed by Biomphalaria sudanica (20.7%), primarily found at Sasin and Agu 1. Statistical analysis revealed significant spatial variation in total snail abundance (p = 0.011), though species-level variation was only significant for Bulinus globosus (p = 0.040) (Figure 2A).

Biomphalaria snails are freshwater species recognized for their role as intermediate hosts in the transmission of schistosomiasis. Out of the 20 sites assessed, 12 were found to have Biomphalaria snails. The results revealed variability in Biomphalaria snail density across different sites. The highest relative abundances were recorded at Shonga 2 (100.0), Kusha (100.0), Keker 1 (100.0), Kosokol 2 (100.0), and Wugni (100.0), indicating these sites likely provide favorable environmental conditions for Biomphalaria snail populations. Moderate densities were found at sites such as Agu 2 (95.0), Agu 1 (93.5), Kosokol 1 (93.5), and Chorobay (92.2), while comparatively lower relative abundances were recorded at Sasin (85.0) and Esine 1 (84.2). There was statistically a significant difference in the relative abundance of Biomphalaria snails across sites (χ² = 32.238, df = 18, p = 0.021), indicating spatial variation in snail populations (Figure 2A). Furthermore, there was a significant difference in the number of snails found in the muddy substrate environment compared to the sandy one (Figure 2B), pointing out the muddy substrate as a supportive substrate for snails. Another environmental parameter that had a significant impact on snail abundance was the vegetation found around these water bodies. Our previous study identified vegetation types such as Taro (Godare) as significant predictors of snail presence (p < 0.001) [11].

To control snail populations, community-led vegetation clearance was conducted in four sites that had >50 snails in the dry season: Kusha, Sasin, Kosokol 1, and Shonga 1. Initially, these sites were surveyed without intervention, followed by vegetation removal efforts aimed at reducing snail habitats. This intervention led to variations in vegetation density and composition and resulted in a notable reduction in snail abundance (Figure 2C), particularly at Shonga 1, where collected snail numbers dropped from 124 in the dry season to 35 in the wet season, highlighting the effectiveness of vegetation clearance in disrupting snail habitats and potentially reducing schistosomiasis transmission risk.

3.2. Snail Composition Shows Mild Shifts Between Seasons

A total of 1150 snails were collected, with 598 during the dry season and 552 in the wet season. A total of four species of snails were identified: Biomphalaria pfeifferi, Biomphalaria sudanica, Lymnaea natalensis, and Bulinus globosus. B. pfeifferi was the most prevalent species, comprising 358 (59.9%) in the dry season and rising to 406 (73.6%) in the wet season. B. sudanica decreased from 147 (24.6%) in the dry season to 91 (16.5%) snails in the wet season, while L. natalensis dropped from 90 (15.1%) to 48 (8.7%). B. globosus remained low, comprising just three (0.5%) in the dry season and seven (1.3%) in the wet season (Figure 3A). The survey showed a higher relative abundance of Biomphalaria snails in the wet season (90%) compared to the dry season (84.4%), indicating greater prevalence in wetter conditions. The presence of total Biomphalaria snails exhibited significant seasonal variation, indicating fluctuations in their occurrence across seasons (Figure 3B).

3.3. Spatial Variation in Biomphalaria Snail Infection Rates Across Sites

A total of 1002 Biomphalaria snails were collected and examined for cercaria by the cercaria shedding assay (Figure 4A). A total of 135 snails (13.5%) were found positive for cercaria. Snail infection rates varied significantly across sites, with the highest at Agu 2 (30.4%) and Kusha (28.4%), suggesting that ecological factors may drive higher infection in these areas. Moderate rates were observed at Agu 1 (16.6%) and Chorobay (14.9%), while lower rates were seen at Sasin (8.2%) and Shonga 1 (7.9%). Several sites, including Shonga 2, Keker 1, Esine 2, Esine 3, Kosokol 3, and Kosokol 4, had no infected snails. There was no significant variation in the number of infected Biomphalaria across sites (χ² = 16.509, df = 11, p = 0.123). However, the distribution of infected snails showed a significant variation spatially (χ² = 33.862, df = 19, p = 0.019), indicating that their presence varies across sites (Figure 4B). In terms of seasonal fluctuations, during the dry season, 13.1% of Biomphalaria snails were found to be infected, while in the wet season, 13.9% were infected. These findings indicate a slightly higher prevalence during the wet season compared to the dry season. However, the statistical analysis showed that there was no significant difference in the total number of infected Biomphalaria snails between the dry and wet season (χ² = 0.318, p = 0.573) (Figure 4C).

There was minimal influence of water properties on the snail abundance and infection rates, except for salinity in the dry season. Water quality parameters are known to play a critical role in shaping freshwater snail distribution, abundance, and infection dynamics, particularly in the context of schistosomiasis transmission [13]. Therefore, water was collected from each sampled water body and analyzed for temperature, pH, salinity, total dissolved solids (TDSs), and conductivity. The results show a significant difference in temperature, with the dry season averaging 24.6 ± 2.7 °C, significantly higher than the wet season’s 22.1 ± 0.9 °C (p-value < 0.001). Similarly, pH was significantly lower in the wet season (6.9 ± 0.24) compared to the dry season (7.2 ± 0.66), with a p-value of 0.010. However, no significant differences were found in salinity (p-value = 0.160), total dissolved solids (p-value = 0.213), or conductivity (p-value = 0.204) between the two seasons (

Table 1. Change in water quality parameters between seasons in Mizan Aman. * is statisticaly significant.

Water Quality Parameter	Season		Change p-Value
	Dry [11] (December 2021–January 2022)	Wet (June–July 2023)
Temperature	24.6 ± 2.7	22.1 ± 0.9	<0.001 *
pH	7.2 ± 0.66	6.9 ± 0.24	0.010 *
Salinity (g/I)	187.2 ± 182.2	156.1 ± 158.8	0.160
Total dissolved solid (mg/I)	58.3 ± 54.1	44.0 ± 29.9	0.213
Conductivity (mS)	120.2 ± 110.8	88.1 ± 59.9	0.204

).

The relationship between water parameters and Biomphlaria snail abundance was assessed across seasons. As previously noticed, salinity had a significant effect on snail abundance during the dry season (r = 0.719, p = 0.013) [11]. However, seasonal changes in temperature and pH showed no significant influence on snail abundance, possibly because they remained within the optimal range for Biomphalaria snail proliferation, which includes temperatures between 20 °C and 30 °C and slightly alkaline pH levels [14]. While there was seasonal variation in environmental parameters, only salinity showed a significant connection to snail abundance (

Table 2. Association between environmental parameters and snail abundance across sampling seasons in Mizan Aman, Southwest Ethiopia.

Environmental Parameters	Biomphalaria Snail Abundance
	Dry [11]	Wet
Temperature	0.821	0.894
pH	0.873	0.819
Salinity (g/I)	0.013 *	0.117
Total dissolved solid (mg/I)	0.979	0.071
Conductivity (mS)	0.21	0.071

* Statistically significant.

).

3.4. Seasonal and Spatial Distribution of Schistosomiasis Infection

A total of 206 community members were tested for schistosomiasis across both rounds to examine its correlation with snail abundance and infectivity. Their inclusion in the study was based on their water-related activities during the survey period and the proximity of their homes to the sampling sites. Out of 20 sample sites surveyed for schistosomiasis infection, 13 sites showed evidence of infection, with notable seasonal and spatial variations. Higher infection rates were observed during the wet season at sites like Agu 1 (40%), Agu 2 (30.8%), Sasin (40%), and Chorobay (25%), while other sites, such as Shonga 1 (0%), Keker 1 (0%), and Kosokol 1 (0%), showed no infection or lower rates in the wet season. Spatially, infection prevalence varied across sites, with Esine 1 (35.3% dry, 23.5% wet) and Keker 2 (11.1% dry, 11.1% wet) showing consistent infection rates, while sites like Kosokol 1 (30% dry, 0% wet) displayed marked differences between dry and wet seasons. The results indicate a significant variation in schistosomiasis cases across sites (χ² = 31.242, df = 19, p = 0.038) (Figure 5).

A strong, positive, and statistically significant correlation (Spearman’s rho = 0.733, p = 0.000) was observed between the total number of infected Biomphalaria snail species and the number of schistosomiasis-positive cases in both (Figure 5B). This indicates that as the number of infected snails increases, the number of schistosomiasis cases also increases.

4. Discussion

4.1. Snail Abundance and Species Composition

This study mapped the spatial and seasonal distribution of freshwater snails in Mizan Aman, Southwest Ethiopia, and analyzed their association with environmental factors and schistosomiasis transmission. The results of this study provided insights into spatial and seasonal variations in snail abundance and species composition and the transmission dynamics of schistosomiasis.

Seasonal variation in snail abundance was observed, with key implications for the transmission dynamics of schistosomiasis. Notably, the abundance of snails was highest during the dry season, which is consistent with findings from other studies in tropical regions, where dry periods tend to favor increased snail populations due to reduced water flow and more stable habitat conditions, while wet periods, with higher water volume and habitat disturbances, may hinder their survival [9]. Regarding species composition, the survey identified four different snail species, Biomphalaria pfeifferi, Biomphalaria sudanica, Lymnaea natalensis, and Bulinus globosus, with B. pfeifferi being the dominant species across both seasons, comprising 358 (59.9%) in the dry season and 406 (73.6%) in the wet season. Biomphalaria pfeifferi is known to be an important intermediate host of Schistosoma mansoni, and its abundance in the study area underscores the potential for schistosomiasis transmission. The higher prevalence of B. pfeifferi aligns with findings from studies conducted in Kenya suggesting that B. pfeifferi snails can sustain schistosomiasis transmission effectively [15,16].

Spatial variation in snail abundance was significant, with the highest counts recorded at Sasin, Agu 1, and Shonga 1. The abundance of Biomphalaria snails was particularly high at sites with dense vegetation and frequent human activities, such as washing and bathing. These findings are consistent with other similar studies, who reported that snail density is highest in areas with dense aquatic vegetation, which serve as breeding grounds and refuges for snails [17].

4.2. Biomphalaria Snail Infection Rates and Schistosomiasis Cases

Out of 1002 Biomphalaria snails examined, 135 (13.5%) were found to be infected with Schistosoma cercariae. Infection rates varied significantly across sites, with the highest rates observed at Agu 2 (30.4%) and Kusha (28.4%) showing consistently high infection rates across seasons. This is in line with studies in Ethiopia [18], which noted that certain locations within endemic regions serve as persistent transmission hotspots due to favorable environmental conditions, such as stagnant water bodies and abundant vegetation for snail habitation. It may also be attributed to differences in human behaviors on the riverbanks, and to local environmental conditions, such as water quality and vegetation cover, which influence snail susceptibility to infection. Similar findings were reported in China, where Schistosoma transmission hotspots were associated with specific ecological and environmental factors. This finding underscores the need for targeted surveillance and control measures at high-risk sites, reinforcing previous research that highlights the significant influence of environmental factors on the distribution of both snail populations and Schistosoma infection [19]. The seasonal variation in infection rates was minimal, with a slightly higher prevalence during the wet season (13.9%) compared to the dry season (13.1%). This marginal difference could be due to increased human–water contact during the wet season. Previous studies in Tanzania [20] and Uganda [21] also reported minimal seasonal variation in infection rates, emphasizing that transmission dynamics, as well as environmental factors such as vegetation and type of substrate, are more closely related to snail abundance and human exposure patterns than seasonal changes alone.

4.3. Correlation Between Infected Snails and Schistosomiasis Cases

The strong positive correlation observed between the number of infected Biomphalaria snails and schistosomiasis cases indicates a direct relationship between snail infection levels and human disease burden. This finding is consistent with a study conducted in Nigeria [22], which reported a similar correlation, highlighting the importance of snail control in reducing schistosomiasis transmission.

4.4. Environmental Factors and Snail Distribution

Water quality parameters, such as temperature and pH, showed significant seasonal variation, with higher temperatures and pH levels during the dry season. Previous studies have shown that temperature and pH influence snail reproduction and survival. A 1957 WHO report indicated that Biomphalaria snails thrive best at temperatures between 20 °C and 26 °C, with a lower limit of 18 °C and an upper limit of 30–32 °C. The optimal pH range for their development is between 7.0 and 8.0, which is slightly alkaline. When these factors fall outside the optimal conditions for Biomphalaria snails, reproductive success and survival rates may decline [14,23,24]. However, while there was seasonal variation in temperature and pH levels in this study, the ranges never extended far outside the optimal range, and thus the impact on the snail population appeared minimal in the study area.

The statistical analysis showed that environmental parameters such as salinity were shown to influence Biomphlaria snail abundance (r = 0.719, p = 0.013). The role of salinity in snail distribution has been noted in previous studies, which suggest that varying salinity levels can either promote or inhibit snail survival depending on the species [25]. In the wet season, TDS and conductivity were found to be more influential in determining snail populations, indicating that changes in water quality during these periods may have a more direct impact on the ecological conditions favorable to snails. The relationship between snail abundance and substrate type was also significant, with a strong positive correlation between muddy substrates and snail presence. Eleven of the twenty sites had a muddy substrate, corresponding with eight of the ten sites where infected snails were found. In fact, 132 of the 135 infected snails (97.78%) were found in sites with muddy substrates. This is consistent with earlier studies that have identified mud-rich habitats as ideal for snail habitation due to their stability and ability to support the necessary plant and microfauna communities [26].

4.5. Community-Led Vegetation Clearance

To control snail populations, community-led vegetation clearance was conducted at four sites with high snail populations: Kosokol 1, Kusha, Sasin, and Shonga 1. Initially surveyed without intervention, these sites underwent vegetation removal to reduce snail habitats, leading to changes in vegetation density and a notable reduction in snail abundance, especially at Shonga 1. The statistical analysis confirmed a significant decrease in snail numbers. This intervention highlights the effectiveness of habitat modification as a snail control strategy. Similarly, studies conducted in Senegal showed that vegetation clearance reduced snail density and schistosomiasis transmission [27]. Other studies in southern Morocco and West Africa reported significant snail population reductions following community-led vegetation removal, with no adverse effects on freshwater biodiversity or water quality [27,28]. These findings emphasize the importance of integrated snail control strategies combining environmental management and participatory health efforts [13].

4.6. Conclusion and Recommendations

This study identifies significant spatial and environmental variations in snail abundance, species composition, and infection rates in Mizan Aman, establishing a clear link to schistosomiasis transmission. Environmental factors such as salinity and substrate type were key determinants in shaping Biomphalaria snail populations and their infection dynamics. The findings highlight the need for site-specific, integrated control measures combining habitat modification, environmental management, and regular monitoring of snail populations to reduce disease prevalence. Effective habitat modification, such as minimizing muddy substrates and clearing vegetation, can serve as a critical strategy for controlling transmission.