Potomac Horse Fever can cause very serious diarrhea, and other symptoms may include fever, laminitis, and colic.  It is caused by Neorickettsia risticii, a bacteria found in freshwater snails that is believed to be transmitted to horses via inadvertent ingestion of infected aquatic insects.  It is usually seasonal, seen most commonly in the hot summer months or early fall. If Potomac Horse Fever has occurred in a particular geographic area, it is likely that additional cases will occur in future years. However, vaccination against this disease has been controversial.  Evidence of protection against clinical disease is lacking, possibly because the vaccine may not stimulate a protective immune response, or potentially because multiple strains of the bacteria may exist, whereas only one strain is present in the available vaccine.