*2.1.4 Dissolved oxygen and BOD5*

The BOD5 had a positive relationship with the freshwater snail infection. These might be due to the presence of organic pollution which increases BOD5 and subsequently increases the infection of freshwater snails. Whereas dissolved oxygen concentration had a negative relationship with snail infectivity. The low dissolved oxygen was an indication of the presence of organic pollution in the aquatic environment [43]. This organic pollution is beneficial and expands the habitat of freshwater snail hosts and subsequently, more snails might be present. And also, it might have a trematode egg in the waste. With the high availability of freshwater snail hosts and trematode eggs, the probability of a miracidium searching and infecting a snail is higher. As a result, the cercarial infection in the freshwater snail host could be increased [44].

#### *2.1.5 Hardness of water*

Freshwater snails need calcium for shell hardening, egg production and are unable to get it in the water with less than 2 mg/l calcium. The ratio of calcium to magnesium is significant for calcium uptake by the snails. The freshwater snail that lives in the water with a low concentration of calcium was highly infected than in high concentration calcium [45].

#### **2.2 Biotic environmental factors**

#### *2.2.1 Predators*

The prevalence of freshwater snail infection is also affected by biological factors. The predators including Belostomatidae, Naucoridae, Nepidae, Gomphidae, and

Gyrinidae were prey on the freshwater snail and consume trematode larva miracidia [46, 47]. As a result, the prevalence of freshwater snail infection with trematode might be reduced. And also, the snail infectivity might be indirectly influenced by competitors like Thiaridae, Physidae, and Lymnaeidae [46, 48]. This results in an abundance of infected snails were decreased in the presence of predators and competitors [49].

Furthermore, biotic factors such as toxins produced by hosts, non-hosts, predators, or decoy organisms may act in concert with abiotic factors to expose free-living endohelminth stages to a diverse range of hazards on their way to the next host, further modulating transmission dynamics and infection patterns in a host population [27, 50].

The functioning of aquatic relationships is influenced by predatory and competitive forces. Predators often cause trophic cascades in which resource dynamics and energy balances in the environment are affected [51]. Such trophic cascades could be detected in Zambia [52] and Malawi [53] as a general reduction in aquatic biodiversity due to strong fishing pressure. Overfishing has reduced the predation pressure on snails, and fish mortality from metals, pesticides, and other chemical pollutants has reduced the top-down regulation of trophic interactions [32]. These all conditions reduce the abundance of snails in the aquatic environment and subsequently results in low snail infection. This is due to the high abundance of freshwater snails that could promote cercaria transmission as the access distance for the miracidia is reduced as a result more snails get infected, which could be because why the area with the lowest freshwater snails abundance had a low infection in snail [37].

#### *2.2.2 Competitors*

The non-host snail had the most significant influence on miracidia, causing them to enhance their host-finding behavior and penetration attempts. They discovered that failed penetration attempts resulted in severe mage and tiredness, making infecting the host snail more difficult. In addition, failed attempts caused them to shed penetration glands, making infection more difficult. When the media was infected with sew, the rate of miracidial infection in host snails decreased. This could be because of the fact that the sew contained a harmful material discharged by the none host snails, triggering a host snails regulatory response [3].

Snail introductions produce comparable outcomes, as they can infest wide regions of new suitable habitat and establish dominance. These have both bad and positive consequences since they cause economic damage and disruption to the biodiversity of the area in question, as well as positive results as a result of their competitive and predatory behaviors against hosts of dangerous parasites like schistosomes [32, 54].

#### **2.3 Habitat conditions**

Some habitat conditions like silt, organic, chlorophyll-a, canopy cover, riparian vegetation and freshwater snail abundance have an association with snail infectivity. These factors may be contributing to the development of the hotspot include the vulnerability of snails to infection, probability of interaction with viable trematode eggs, and suitability of snail habitat [55]. This study shows the habitat with silt, organic, shadow, and muddy grass-grown highly favored by the snails and the majority of infected snails were found in silt substrates, and greater than 50% riparian vegetation. The high abundance of snails that could promote cercaria transmission as the access distance for the miracidia is reduced as a result

more snails get infected, which could be because of why the site with lowest snail abundance had a low infection in snail [37].
