Parascaris equorum, the horse roundworm. A large yellow to white parasite (females may be up to 15 inches long). Ascarids may appear in the feces of infected horses. Adult horses can develop resistance to this parasite; thus, roundworms primarily infect young horses less than 2 years of age.
Clinical signs include: unthriftiness, potbelly, rough hair coat, and slow growth. Some young horses develop nasal discharge accompanied by a cough as a result of larvae migration.
See Ascarids in Horses for more information.