Trematodes, commonly called flukes, can infect your dog. They cannot transmit directly from one dog to another, though; they must go through other animals before they become infective. Trematodes are not common canine parasites, except in specific geographic areas, but they can cause severe disease, including liver failure.
Trematodes typically need one or more intermediate hosts before they are infective to dogs. Eggs are shed in feces and hatch in water. A miracidium hatches out and finds a snail host, which it infects. A cercaria develops in the snail. In one species, Paragonimus kellicotti, the cercaria stage leaves the snail to infect a crayfish. The next stage to develop, the metacerciae, infects dogs when they eat the host of the infective stage. The cycle repeats as infected dogs shed fluke eggs.
Each of the infective flukes causes disease and clinical problems in a different manner. Paragonimus kellicotti creates cysts in canine lungs, which can hemorrhage ,or a pneumothorax, air formation in the body cavity outside the lungs caused by lung tissue tearing. Heterobilharzia americana live in veins around the intestines; migration of eggs across the intestinal lining can cause intense inflammation leading to diarrhea, weight loss or lethargy.