Potomac Horse Fever (equine monocytic ehrlichiosis) is an inflammatory gastrointestinal disease that affects horses of all ages. It is an infectious and contagious disease most common in spring and summer. This disease is transmitted to horses via a bacteria, Ehrlichia risticii which has been recently renamed Neoriketssia ristici. This bacteria is associated with a parasite of freshwater snails. The bacteria also infects the larvae of mayflies and caddis flies which live in fresh water.
Make sure to get your horse vaccinated for Potomac Fever and always turn lights off around water sources at night to avoid attracting flies that carry this and other diseases.