Parasites often live inside animals without causing trouble (in fact, it’s better for them to keep their meaty landlords alive and kicking, since it ensures a consistent food source and a place to call home!). The mystery picture in What’s this? #4 is from the liver of a horse, and correctly identified by a number of people (well done!) as the cystic stage of a tapeworm called Echinococcus granulosus. This is actually a parasite which has evolved to cycle through dogs and sheep, rather than horses. The dog is its Final host, where the adult lives and lays eggs in the intestines. The eggs are pooped out, hatch into larvae and wriggle free onto pasture, where sheep eat them. Dogs are then re-infected by eating sheep organs. Alas, the best laid plans of mites and men (and tapeworms) often go awry, and other animals like horses and humans can become infected accidentally. In humans it can form fatal cysts within the brain and other organs, which is a no-win for the parasite.
Another tapeworm which can be found in the horse, is called Anoplocephala perfoliata and looks like little leaves. They are often found in the small intestine where they can cause ulceration and inflammation of the intestine. Generally they do not cause the horse any problems, but sometimes they dig too deep and lead to rupture of the intestine, peritonitis and death.
A worm called Strongylus vulgaris (also known as a large strongyle) can be found in the large intestine and caecum (appendix) of horses. It is particularly nasty as it digs in and invades the wall of the intestine so it can travel along the arteries. Eventually it migrates to a branch of the aorta (the large artery leaving the heart). It can damage the blood vessels so much that the intestines die, which can kill the horse.
Another strongyle, commonly known as a small strongyle (Cyathostomum) lives in the large intestines and caecum of horses. Like other worms, enough of them can cause diarrhoea and weight loss. However, a cystic stage can also occur, with thousands of larvae in tiny cysts within the lining of the intestine. Sometimes, these will all emerge at once, bursting the intestinal wall and causing huge injury. Gut toxins can then leak into the blood and cause toxic shock, which is often fatal.