The present study investigated the prevalence of trematode infections among ruminants and their snail vectors across three study areas characterized by different agro-ecological conditions. The findings revealed an overall prevalence of ruminant trematodes, determined through coprological examination, at 35.5%. This figure surpassed a previous finding from Wolaita (36), which reported a prevalence of 26.04%. However, our finding shows a lower prevalence compared to previous reports by (21), (37), and (20), who recorded 61%, 60.42%, and 47.1%, respectively. The observed variations in trematode occurrence among the studies may be attributed to differences in study seasons, agro-ecological conditions across the study areas, animal management practices, and the availability of suitable habitat for the snail intermediate host.
In this study, Fasciola (20.5%) showed the highest occurrence, followed by Paramphistomum (11.7%), with Schistosoma being the least prevalent (6.3%). A similar pattern of trematode occurrence was also noted in another study (36). However, our findings contradict the observations of (20) and (21), where a higher prevalence of Paramphistomum compared to Fasciola and Schistosoma was reported. These two studies were conducted in lowland agro-ecology where suitable vectors for Paramphistomum are more abundant, potentially leading to an increased prevalence of rumen fluke. Generally, trematode prevalence varies considerably based on the abundance of intermediate hosts, as well as the ecological and climatic factors of the study areas. In our study, we considered different agro-ecological conditions, and Fasciola prevalence was notably high in the highland area of Asela, which may explain its relatively higher prevalence.
At species level, our study findings showed that fasciola was prevalent in 24.5%, 10.7%, and 21% of sheep, goats, and cattle, respectively. The overall prevalence of ovine fasciolosis closely agreed with previous research conducted in various regions of Ethiopia, such as 29.2% in Butajira and Gilgel Gibe (17), albeit lower figures reported by others (38), (39), and (22). Similar to a previous finding (17), our study indicated a low prevalence of caprine fasciolosis in comparison with other ruminants. This could be attributed to goats' browsing behavior, leading to less consumption of grass in marshy areas. Additionally, a disproportional sample size occurred due to fewer caprine species in our study population. Fasciolosis prevalence in Asela (26%) outweighed that of Batu (19%) and Hawassa (11.5%). Asela possesses shallow water bodies and wet areas that provide favorable conditions for the survival and reproduction of the intermediate host, Lymnaea truncatula. Known for its amphibious nature, this snail species thrives in such environments, thereby contributing to the higher prevalence of fasciola in the area.
The prevalence of Paramphistomum in the current study was 11.7%. Notably, a higher prevalence was observed in cattle (15.7%), followed by sheep (7.7%) and goats (3.6%) similar to previous studies (20, 40, 41), which also reported bovine Paramphistomum as being more common. Interestingly, Batu had a high prevalence of Paramphistomum infection (17.5%) compared to other study areas. The ecological characteristics of the area such as lowland and presence of lake may contribute to this heightened occurrence of paramphistomes. The presence of Batu (Ziway) Lake in this vicinity could provide a suitable habitat for the survival of the intermediate host, aquatic snails. Paramphistomes use a diverse range of freshwater snail vectors, including Bulinus, Biomphalaria, Ceratophallus spp., and play role in the transmission of trematodes (42).
The overall prevalence of Schistosoma in our study (6.3%) was found to be lower than that previously reported (43), (21), (44), (45), (46), and (47). In terms of its suitable agro-ecology, a significant portion of the study animals, approximately 12.5% (95% CI: 8.3–17.9%), were found to have Schistosoma infections in the Batu area, which is characterized by lowland area. The lack of Schistosoma detection in the Asela area is in line with our expectations; but, the result from Hawassa is contrary to what we anticipated. Several factors may account for the observed variations among studies, including differences in study areas, climatic and ecological variations among study sites, and variations in animal husbandry practices. Additionally, Schistosoma eggs were not detected in Asela and Hawassa, which affect the overall prevalence. In contrast to some previous reports (45, 46), no significant differences were found between the prevalence of ovine (3.6%) and bovine (3.5%). The prevalence of Schistosoma species in our study showed no significant difference between ovine (3.6%) and bovine (3.5%) infections, which contrasts with the findings of some earlier studies (45). As grazing animals, both sheep and cattle are likely to have similar exposure and susceptibility to trematode infections if the prevailing Schistosoma species is not so species-specific.
Out of a total of 520 animals slaughtered and underwent examination for trematodes, 108 and 118 of them were infected by Fasciola and Paramphistomum, resulting in prevalence of 20.5%, 22.7%, respectively. The relatively high occurrence of paramphistomum could be partly attributed to the limited availability of effective drugs for its treatment. Common anthelmintic (e.g. Albendazole), used for regular deworming against significant nematodes and liver flukes in Ethiopia have little to no effect on paramphistomes (48). The biology of the parasite and the highly prolific nature of the intermediate host could also contribute to increased prevalence of the parasite (30).
In contrast to what was anticipated and reported in previous studies (47, 49–51), our abattoir survey found no evidence of schistosoma prevalence in our research. This could be related to the use of less sensitive diagnostic methods, such as visual inspection of veins, and variations in the geographic origins of the study animals, that include regions where Schistosoma is not prevalent.
In the present study, the prevalence of fasciola showed higher infection rates in Asela (45.8%) compared to Hawassa (18.5%) and Batu (11.3%), indicating distinct agro-ecological trends. This increased occurrence of fasciola along the altitude gradient suggests increasing altitude creates suitable conditions favoring its intermediate host. In highland agro-ecologies, where there is typically cooler temperatures and higher rainfall, fasciolosis due to F. hepatica tends to be more prevalent among ruminant populations (52). The highland environmental conditions, characterized by abundant moisture and suitable temperatures, create optimal habitats for the intermediate hosts of Fasciola hepatica, primarily freshwater snails Lymnae truncatula which were also abundant in Asela area. Indeed, these snails exhibit amphibious nature, often found in diverse aquatic habitats. They inhabit shallow water bodies, such as ponds, streams, and slow-moving rivers, as well as terrestrial ecology such as muddy substrates. (53) have forecasted the varying degrees of F. hepatica risk across the country, particularly in wet and humid areas. Their projections suggest that the central highlands, such as Asela, will experience the highest occurrence of F. hepatica during the major rainy season (July to September), which provide favorable conditions for snail proliferation and the consequent transmission of the parasite to grazing animals.
In our post-mortem examination, a higher prevalence of paramphistomum was observed in bovine (24.9%) than small ruminants (15.1%), which agrees with the f prevalence reported in previous studies. Paramphistomum prevalence figures vary widely across different regions of Ethiopia, as documented by various researchers (11). For instance, (41) reported prevalence of 40.1% in cattle, 28.9% in sheep, and 16.5% in goats from Bishoftu area. Similarly, (40) found prevalence of 65.3% in cattle and 23.7% in sheep from Ashenge area of Tigray region. Occurrences of paramphistomum among ruminants have been observed in various parts of the country, with approximately 45.83% prevalence in western Gojam, 28.6% in Bishoftu, and 6.7% in Hawassa areas. Notably, higher prevalence figures are often observed in cattle compared to small ruminants (11). Cattle graze closer to water bodies and on flat landscape besides being less selective and bite larger volume compared to sheep and goats. As a result, they are more likely to consume vegetation and water contaminated with larvae, which increases their exposure to trematode infections (54, 55). Cattle also possess a larger rumen capacity in comparison to small ruminants, enabling them to ingest larger volumes of contaminated forage, which likely increase the likelihood of ingesting infective larvae.
Our study identified a significant association between the prevalence of trematodes and the age of animals, both in fecal examination and in the abattoir survey. Similar findings were reported by other authors (41), (56), (36), (57), and (58). As animals age, their likelihood of exposure to fasciola and paramphistomum larvae increases due to prolonged grazing exposure and higher intake of forage compared to young animals.
Furthermore, the coprological examination demonstrated that the prevalence of Fasciola was higher in non-dewormed animals and lower in the dewormed animals. This underscores the effectiveness of deworming in reducing parasite burden in ruminants and minimizing pasture contamination (59). Conversely, this study revealed an association between Fasciolosis prevalence and body condition. Consistent with findings from (60) and (61), Fasciolosis prevalence was higher in poorly conditioned animals compared to those in good body condition. This suggests that animals with poor body condition, likely due to malnutrition or other health issues, are more susceptible to infections caused by parasites.
In our current investigation, we collected specimens from the study areas and snails belong to three genera were identified, a result consistent with a previous study (62). In contrast to another study findings (63), our study areas exhibited a higher abundance of Lymnaea species, followed by Biomphalaria species and Bulinus species. Lymnaea, a freshwater snail, serves as the intermediate host for Fasciola. Therefore, the observed higher prevalence of Fasciola in our study may be attributed to the large population of Lymnaeid snails in the study area. Conversely, our study revealed that the distribution of Biomphalaria and Bulinus species is restricted to Batu and Hawassa. This finding aligns with two previous studies (64)and (65), which suggest that the aquatic habitat and ecology of the Ethiopian Rift Valley favor these snail vectors.