**1. Introduction**

Diseases resulting from zoonotic transmission of parasites are common [1]. Most parasitic zoonoses are neglected diseases despite causing a considerable global burden of ill health in humans and have a substantial financial burden on livestock industries [1]. Zoonotic trematodiasis are found worldwide and are responsible for some serious and debilitating helminthic diseases in people, particularly in rural and poor urban areas of low and middle-income countries [2, 3]. Many of the trematodes that infect humans are zoonotic or have zoonotic potential. Here we briefly discuss the most important zoonotic trematodes and focus on their first intermediate hosts, snails, and their control. Trematodes (Trematoda) belong to the phylum Platyhelminthes which also contains Turbellaria (mostly non-parasitic animals such as planarians), and three entirely parasitic groups: Cestoda, Trematoda, and Monogenea. Trematoda includes two subclasses of parasitic flatworms, also known as flukes, i.e., Aspidogastrea and Digenea. Here we focus on Digenea, which as adults are internal parasites of vertebrates. Trematodes have both sexual and asexual reproduction in different host species. Sexual reproduction occurs in the final vertebrate host, while asexual reproduction occurs in the first intermediate host, usually certain species freshwater or marine snails. Most trematodes have a second intermediate host where their infective stage (metacercariae) lodge. For the food-borne trematodes, various fish species, crustaceans, or

**Figure 1.**

*Eggs of various trematodes found in human feces or urine (source: Mae Melvin, public health image library (PHIL); Centers for Disease Control and Prevention).*

snails may serve as second intermediate host or in case of the Fasciolidae, cercariae encyst on aquatic or semi-aquatic plants (see more details below).

The Digenea contains about 20,000 species, within two orders, Diplostomida and Plagiorchiida. Only a few of these species infect humans, and some of the diseases they cause are briefly discussed below, i.e., schistosomiasis and several species of food-borne zoonotic trematodes (paragonimiasis, fascioliasis, clonorchiasis, opisthorchiasis, and others). Examples of eggs from these trematodes are shown in **Figure 1**. Some species of trematodes have a relatively narrow range of snail species that serve as intermediate hosts, while others have an apparently wide range (**Table 1**).
