08/19/2025
🐌 It is time to address the snail in the room! Recently, we posted about a number of positive Potomac Horse Fever (PHF) cases in our area. A few people commented that they had never heard of PHF in all of their years of horse ownership. We are here to squash the misinformation surrounding PHF. Let’s jump right in at a snail's pace.
🐌 When was PHF first identified?
Originally PHF was recognized in 1979 near the Potomac River in Maryland.
🐌 Where is PHF?
Originally it was thought that PHF was only found in the eastern United States; however, it has been identified throughout North America. And it has been described in South America and Europe too. Typically, PHF is seen in late spring through early fall.
🐌 How is it transmitted?
PHF has a complicated life cycle. It is transmitted by fresh water insects, not directly from horse to horse. Neorickettsia risticii is the bacterium causative agent. It has been identified in snails and various species of cadisflies. These aquatic insects, found near water, contain the rickettsia-infected trematodes which are ingested by the horse. The trematodes cause inflammation of the intestines often leading to an acute enterocolitis.
🐌 What are the symptoms?
Symptoms include fever, diarrhea, lethargy, colic, and laminitis. It can cause abortion in pregnant mares.
🐌 How do we test for PHF?
Submit EDTA whole blood and/or f***s to the lab for PCR testing. Per the Cornell Animal Health Diagnostic Center, they have seen a rise in positive PHF PCR tests over the years in NY state, with a higher number of cases diagnosed in Albany, Dutchess, Saratoga and Tompkins counties.
🐌 When did the PHF vaccine become commercially available? Is it effective?
In 1987, PHF vaccines were available. The monovalent PHF vaccine has been shown to provide better immunity than the PHF/rabies combination.
🐌 What can you do to protect your horse?
We recommend horses receive their first vaccination dose in late spring followed by boosters every three months through late fall. Following this schedule will give your horse continuous immunity until temperatures dip below freezing and the vector (disease-spreading) aquatic insects die for the year. While the vaccine is not 100% effective, it can decrease the severity of the illness and increase survival.
🐌 What can you do at the barn?
A simple tip is light at night attracts insects.
To help protect your horses, we advise to eliminate light sources near their drinking water. And provide clean drinking water.
🐌 While early treatment of PHF can be successful, the disease is frequently fatal.
📸: A previous patient diagnosed with PHF and survived.
