How is the giant liver fluke primarily transmitted to livestock?

The giant liver fluke, scientifically known as Fasciola hepatica, is primarily transmitted to livestock through the ingestion of infected intermediate hosts, specifically certain types of snails. In this life cycle, the adult flukes inhabit the liver of the definitive host, where they release eggs that pass through the liver and into the environment via the host's feces. The eggs then hatch in water, where they infect specific freshwater snails. Inside the snails, the flukes undergo development and ultimately leave the snail as free-swimming larvae. When livestock graze near water sources and inadvertently consume these infected snails or the encysted larvae present in vegetation, they become infected with the giant liver fluke.

While other methods like direct contact or contaminated water sources might seem relevant, they do not represent the primary mode of transmission for the giant liver fluke. Instead, the ingestion of the intermediate host—where the crucial developmental stage of the fluke occurs—forms the basis of transmission to livestock.