Understanding Potomac Horse Fever
Potomac Horse Fever has become a growing concern amoung horse owners in our region, especially over the last few years. To ensure that it is an illness that can be fully understood, we had Dr. Ashley Vanderburgh-Oakley on the blog this month to break down the syndrome that causes signs of colic, fever and diarrhea and can be life threatening.
What is Potomac Horse Fever?
Equine Monocytic Ehrlichiosis, more commonly known as Potomac Horse Fever (PHF), is a serious illness caused by a bacterium called Neorickettsia risticii. Infection with this bacterium can be fatal and therefore is the reason that knowledge about this condition is important for all horse owners!
How do horses get PHF?
Neorickettsia risticii is an obligate intracellular organism, meaning it can't survive on its own outside of a cell. Its life cycle is complex, but essentially horses can become infected by ingesting certain aquatic insects or snails while grazing near water or in swampy areas.
What are the signs and symptoms?
The severity can vary between individual horses, but depression/lethargy, a fever, and loss of appetite are the most common signs to look for. Diarrhea occurs in about half of cases, many of which also show signs of colic. In the most severely affected horses, this can result in dangerous dehydration with the need for hospitalization. Laminitis and abortions may also be consequences of infection.
How does my vet diagnose it?
Your vet will start with a physical exam and ask questions about your horse's history, current behaviour, and duration of symptoms to help narrow down a potential cause of illness. A PCR test on a horse's blood is available for confirming a diagnosis of PHF, however, when diarrhea is an exibting symptom, a full PCR diarrhea panel is typically run first to determine if the cause is due to an infectious agent.
How is it treated?
Intensity of treatment will depend on the severity of the horse's symptoms. Antibiotics are usually given intravenously, as well as other medications to improve comfort, reduce fever, and protect against laminitis. Intraveneous fluid therapy is often a neccesity to support a horse experiencing diarrhea and subsequent dehydration.
Is my horse contagious? Can they get it a second time?
Fortunately, PHF cannot be transmitted between horses. But horses who have already had PHF can certainly get it again!
How can I protect my horses?
Environmental management is an excellent strategy to mitigate the risk of your horse contracting PHF. These include eliminating standing water, cleaning out water troughs and buckets regularly, preventing access to natural water sources, and using insect traps near your horse's water source. In addition to environmental management, vaccination can also offer your horse protection. It cannot completely eliminate the risk of contracting PHF but can greatly reduce severity and duration of illness.
When should I vaccinate?
Because of the seasonal occurrence of PHF, strategic timing of vaccination is important to offer your horse the most protection. In Alberta, that means the ideal time is in late June, in anticipation of increased exposure through the end of July into September.
For a horse that has never been vaccinated previously, a series of two vaccines 3-4 weeks apart is recommended. After that, yearly boosters are required. Pregnant mares and foals can also be vaccinated in consultation with your veterinarian.
Dr. Ashley Vanderburgh-Oakley
Westhills Equine Veterinary Services