Harnessing crayfish, Procambarus clarkii, to eliminate Schistosome transmitting snails in the Mwea irrigation scheme, Kenya

About 100 kilometers northwest of Nairobi, in the Mwea district of Kirinyaga county in central Kenya, all canals and streams were initially surveyed for the study (Fig. 1) (Allan et al., 2020). Kirinyaga County is located at the summit of Mount Kenya and between 1158 and 5380 meters above sea level. On average, it receives between 1200 and 1600 millimeters of rainfall annually. Based on data from the 2019 census, the county’s estimated total population was 610 411 KNBS census, 2019 (Gouvras et al., 2017). Since crayfish inhabit specific canals (Brown, 1994) and the sickness is very common in children and adults alike (Kibuthu et al., 2016), the Mwea research region was chosen for this investigation.

Fig. 1.

Snail collection points in Mwea Area Kirinyaga Count.

Two experienced collectors scooped snails for 30 minutes per sampling site using long-handled scoops (Steel sieve with a mesh size of 2×2 mm, supported on a triangle iron frame) (Gouvras et al., 2017). Canal shorelines had emergent and submerged vegetation that was combed by the scoop for snails. All snails were taken to the Kimbimbi sub-county hospital lab, where they were sorted into species based on shell morphology characteristics, using standard taxonomic identification keys, counted, and screened.

Three crayfish sampling and trapping techniques were employed. Meat-baited traps in water deeper than one meter were used in the initial method. Plastic 1 cm mesh was used to cover a cylindrical wire frame that was around 50 cm in length and 20 cm in diameter. The trap was submerged to remain on the bottom for a predetermined amount of time, typically two hours, after being baited with meat (Appendix 1). When the trap was lifted, Procambarus clarkii were manually removed via the opening after creeping in at both ends and being unable to crawl out. Snail scoops were utilized in the second trapping technique along the canals. Shallow littoral layer of emergent and sub-emergent vegetation (Appendix 2). Third, the community used ripped and frayed mosquito nets, which were dragged down the canal bed for ten minutes, then folded to capture the caught crayfish and taken out (Appendix 3).

Appendix 1.

Crayfish trap with few caught P. clarkii.

Appendix 2.

Snail scoop used to catch crayfish in littoral shallow water.

Appendix 3.

Torn and tattered mosquito net invented by villagers to trap crayfish.

Six months after the baseline survey, crayfish were added to 3 of the sites where they were originally absent or in low density. The stock crayfish population originated from the Mwea canals (Fig. 1).

Dump gunny sacks were placed at the top and bottom of perforated buckets containing crayfish (H: 90 cm, top radius: 60 cm, bottom radius: 45 cm) for temporary confinement. Every container contained about 120 crawfish. The same day, the crayfish were transported in a covered double-cab pickup to the experimental locations. For every 100 meters of canal length, 200 crayfish were released. For the next two years, a monthly monitoring of crayfish performance and their impact on schistosome-transmitting snails was conducted. The abundance measurements, the observation of egg broods under female tails, and the appearance of juvenile and young adults were used to assess how well crayfish populations were doing.

Following stocking, crayfish samples were taken every month until the study’s conclusion. Thirty meat-baited traps were submerged in water for two hours during each sampling session. The traps were made of 2.5mm medium tensile plain flexi wire and covered with a 45 by 25 cm long nylon mesh. The simple wire is divided into multiple 45-cm pieces to build crayfish traps. The joints of the three sections are welded after they are folded into circles. At equal intervals along the three circles’ length, three straight wires were riveted at each intersection with the circular cables. At intervals of fifteen minutes, the traps were inspected. After being weighed, sexed, and tallied, the captured crayfish were sent back into the water. Because they offer the strongest indicator of crayfish abundance, the analysis concentrated on the males that were taken (Fornstrom et al., 1997).

Snail and crayfish populations were expressed as percentages and proportions. Snail abundance and infection rates were compared using percentages and proportions. In addition, regression analyses were performed to determine the association between the P. clarkii and the snail population for all study sites.

A total of 15 study habitats were sampled for schistosome-transmitting snails and the presence/absence of crayfish at the baseline. We settled on 5 sites for follow-up due to their ease of accessibility, financial implications, and suitability for both emergent and submerged vegetation. Follow-up was done in both dry and wet seasons between August 2021 and December 2022. GPS coordinates and abiotic parameters were recorded (Table 2).

The pH of Mokou Canal was 7.97, the highest, and the pH of the Nineveh environment was 6.88, the lowest. Fertilizer use is the primary factor that contributes to Mwea’s acidity. Perhaps as a result of its close proximity to the Nice shopping centre, Nice has the highest water turbidity, measuring 0.15 NTU (Nephelometric Turbidity Units). Since water is periodically let into the canal by activating the main canal gate valves, Mianya had the lowest turbidity, at 0.05 NTU. Mokou had the lowest average water temperature, at 21.9 °C, while Nguka had the highest, at 25.7°C. At the 0.5 m/s/S, Mokou had the fastest water speed, compared to 0 m/s in Nineveh and Nice districts.

Out of the 218 crayfish captured at the six study locations, Nguka had the most (104), ahead of Nice (82) and Mokou (32), indicating relative abundances of 47.7 %, 37.6 %, and 14.7 %, respectively. During the sampling period, there were no crayfish in Mianya, Murinduko, or Nineveh (Table 3).

To initiate shedding, snails were briefly subjected to natural light between 10:00 AM and 12:00 PM. Mianya (12.3 %) had the highest infection rates, then Nguka (12.3 %), Murinduko (6.3 %), and Nineveh (5.0 %). On the other hand, Nice Stream showed no infection rates (Fig. 2). According to the chi-squared test, all five habitats are thought to be possible places for the spread of S. mansoni infections. The five study environments’ infection rates did not differ significantly (p = 0.105).

Fig. 2.

Snail abundance and prevalence of S. mansoni infection in the study habitats during the rainy season (November – December 2022).

Snails were much less common (p=0.001) during the research period following the emergence of crayfish, especially in Nice and Mianya. A collection of 306 crayfish was sampled on the research locations during the initial follow-up visit. Except for Nineveh and Murinduko, all study locations saw overall crayfish survival. Water velocity (p=0.014) and vegetation cover (p≥0.026) were substantially correlated with crayfish abundance. It’s interesting to note that, in comparison to the other locations, we found the most crayfish and the fewest snails in the Nice stream (Fig. 4).

Mianya canal, located beside a primary school, had almost 10 % of the 95 B. pfeifferi collected shedding S. mansoni. Nineveh Canal had the fewest snails, both infected and uninfected (Fig. 2).

Since there was no water in the Nguka and Nice habitats during the dry season, there were no snails (Fig. 3). Commercial structures were gradually encroaching on the study area in the Nice environment. There were 50 B. pfeifferi snails and a few shedders in the Mianya environment. To supply nearby rice crops, irrigation water flowed throughout the ecosystem, encompassing the Karira habitat, which was not included in the study, and produced the most significant number of snails.

Fig. 3.

Snail abundance and prevalence of S. mansoni infection in the study habitats during the dry season (March – April 2022).

The January 2022 sampling time point saw a peak of crayfish in the Mokou environment. In August 2021, there were a lot of crayfish in the nice habitat, and in April 2022, they peaked. Slow-moving water, minimal habitat disturbance, and favorable reed and hedge grass conditions were all present in this area. Few crayfish were found in the fast-moving canal that is Mianya’s habitat, most likely as a result of the water’s quick flow. There were also a few crayfish in Nineveh. December 2021, January 2022, and April 2022 saw three significant surges in the Nguka habitat (Fig. 4).

Fig. 4.

Ability of crayfish to survive and establish thriving populations in Mwea streams habitats after introduction.

Habitats experience seasonal changes in predator/prey relationships based on biotic and abiotic factors. After the crayfish introduction was completed in August 2021, there were twelve snails in the area. In September 2021, the snail population increased to 161. However, between October 2021 and January 2022, the crayfish successfully suppressed snail numbers by briefly establishing themselves. Crayfish numbers steadily decreased from March to December 2022, presumably as a result of the canal gate valves opening and letting a lot of water flow, which makes it impossible for crayfish to survive. In the littoral zones, snails can cling to plants; a snail population surge was noted in June 2022, but the snails were exterminated in December 2022 (Fig. 5).

Fig. 5.

Predation potential of P. clarkii in Mokou stream.

For the August 2021 sampling point, the Nice Stream near the Nice shopping complex had 20 crayfish and zero snails. The sampling point at 3 snails in June 2022 saw the sole high. The snails were eradicated mainly by crayfish, which peaked at 50 predators in the December 2022 survey point (Fig. 6).

Fig. 6.

Predation potential of P. clarkii in Nice stream.

The number of snails in Nineveh, a habitat near the Kimbimbi hospital, peaked in January 2022 at 64. In December 2021 and April 2022, a small colony of crayfish with one and six predators, respectively, was discovered at this research site. Due to a lack of water in the canal and a decrease in vegetation cover brought on by human activity, no snails were captured between April and December 2022 (Fig. 7).

Fig. 7.

Predation potential of P. clarkii in Nineveh stream.

Using 19, 17, and 26 predators, respectively, the crayfish population was sampled in December 2021 and again in April and August 2022. The snail population, however, remained stable between September 2021 and March 2022. Snail populations peaked at 88 in September 2021 and 57 in March 2022, respectively (Fig. 8). The predator population suppressed snails during the survey period; just 29 snails were sampled in September 2021. The crayfish population, on the other hand, peaked four times in December 2021, January, April, and August 2022, with 42, 50, 49, and 34 predators, respectively. The population was nil from August to October 2021.

Fig. 8.

Predation potential of P. clarkii in Mianya stream.

Water levels remained relatively constant, with only slight variations, throughout the research period, except in May 2022, when rains followed a minor rise in water levels. All study habitats, irrespective of habitat type, held water. Except for April and June, when it varied between 25 and 28°C, the water temperature remained relatively constant throughout the observation period, ranging between 22 and 24°C.

The studied environments supported a diverse range of vegetation cover over the year, including papyrus, rice grass (weed), arrow roots, and grass. Both seasons of the year had different densities of vegetation cover, which corresponded to the times when rice was grown in the study environments. During rice regeneration, there was always vegetation covering. Nevertheless, at the first harvest in December, while the other farmers altogether collected the rice, all study locations had water, which helped to sustain the snail populations. Harvesting, clearing vegetation, and ploughing and levelling the rice fields were among the other human endeavors frequently seen during the current investigation.

In several study sites, abiotic factors such as the fast water flow amid both short and long-term rains were shown to affect the crayfish’s capacity to survive. For example, the Nineveh stream had a year-round fast water flow of up to 0.83 m/s. Furthermore, the stream’s lack of foliage covers leads to a number of human endeavors in the vicinity, such as washing motorbikes.

We found conflicting results here because of the heavy pesticide application in the paddies next to this stream and due to the higher water velocity (Gaynor et al., 2018; Venter et al., 2016). The snail population would decline shortly after the introduction of crayfish. Crayfish would progressively decrease as Biomphalaria snails increased after the rice cultivation, fertilizer application, and increasing water volume. Nevertheless, only in Nice and Mianya, where crayfish persisted, was this association noted. High densities of flora, including papyrus and arrow roots, were also present at the study sites. Crayfish abundance was substantially correlated with both the density of vegetation and water velocity (p≥0.007) (Fig. 10). In addition, regression analyses were conducted to unveil the existing association between the P.clakii and the snail’s intermediate host of schistosomiasis for all study sites. It was demonstrated that there were a negative association between the P. clakii and snails in all study sites, except Nguka. However, the association was not statistically significant at p-value ≥0.05 (Table 3).

Fig. 9.

Predation potential of P clarkii in Nguka stream.

Fig. 10.

Impact of water velocity and vegetation cover on predation potential of crayfish against snails.

A checklist of some of the fauna and flora found within the study area. Numbers of species was not taken into account across the study habitats. The most common emergent vegetation was arrow arum (Moore et al., 2016). The plant forms optimum breeding conditions for both crayfish and snails. (Table 4)

Specific biological control strategies have a likelihood of being effective, but they need extra human inputs either prior to discharge, through the crop environment’s natural colonization, or to promote post-release establishment. This is particularly the case with conservation biological control. Growers have been urged to plant specific nectar-producing flowers within field boundaries to make the crop environment more appealing to these natural enemies, such as adult parasitoids and hoverflies, which need sources of pollen and nectar to mature their eggs (Maldonado et al., 2019). There is also some experimental proof suggesting adding flowers to the crop may enhance the organic control of pests (Perez-Saez et al., 2019a; Sacchi et al., 2021).

Our findings provide credence to the hypothesis that, in complex environments where habitat variety may foster favorable conditions for compatibility between enhanced and resident enemies, boosting enemy numbers may have net positive impacts on vector control as well as health status (Bonds et al., 2010).

A native freshwater snail of the Río de la Plata basin in South America, the apple snail Pomacea canaliculata (Ampullariidae) (Parker, 1992; Tanner, 1989). This apple snail is regarded as a highly effective invasive species (Carlsson et al., 2009), which has spread feral populations across the globe, particularly in East and Southeast Asia. It has additionally caused significant ecological damage to wetlands and a significant loss of revenue in aquatic crops. In line with previous research by Djeddour et al. (2021), we found that this ampullariid snail invaded rice, canals, and both terrestrial and aquatic plants through our field surveys and sampling in the Mwea irrigation scheme.

According to a study conducted in Argentina by Maldonado et al., the presence of the apple snail significantly decreased the fertility of Biomphalaria peregrina and Physa acuta (Chandiwana et al., 1991; Cioli et al., 2014). It is unknown from our current investigation how the apple snail affects the effectiveness of planorbid snail repression and near extinction. It is worthwhile to investigate if the near extinction of B. pfeifferi was caused by crayfish alone or by a combination of crayfish and apple snail.

Several potential limitations and confounders in our experimental approach deserve consideration. The appearance of an exotic snail species, Pomacea canaliculata, may have played a role as an additive biological control (Maldonado et al., 2019). Second, crayfish are primarily nocturnal (Sacchi et al., 2021). Our crayfish sampling was done during the day, so our catch may not represent a fair and accurate abundance of the predators in the studied habitats. Third, this study was conducted during the COVID-19 pandemic, when lockdowns and the NTD program’s travel restrictions were suspended. Scooping is a labor-intensive exercise, and wearing masks makes it more challenging. Although quadrat snail sampling provides an absolute number of snails (Perez-Saez et al., 2019), the study habitat was primarily covered by cattail grass (Typha angustifolia) and green arrow Arum (Peltandra virginica), rendering quadrat sampling infeasible. Consequently, a timebased sampling technique was employed.

Since crayfish is an exotic invasive species (Albrecht et al., 2025), future studies should explore The use of sex hormones to generate mono-sex predators for release in Schistosomiasis endemic regions.