Morphology: long worm-like organism in the background of peripheral blood smear.

Look alike: trypanosomes, other filarial organisms such as equine onchocerciasis (Onchocerca cervicalis) and canine heartworm (Dirofilaria immitis)

Clinical relevance: Setaria equina is a parasitic roundworm that infects horses. Adult worms live in the abdominal cavity and usually cause no clinical signs. Mild peritonitis may result from very heavy abdominal infestations. Blindness may result if worms develop in the eye.