Milverton-Wellesley Veterinary Services
Potomac Horse Fever
As horse owners, most of you will agree that you often have things to worry about with regard to disease and injury, as horses seem to be experts at finding trouble for themselves and their owners! But sometimes trouble finds the horse, and there is nothing you as the owner can do about it. There are many diseases and infections out there, and thankfully there are several vaccines available to help protect your furry friends as much as possible against some of these infections.
There is an infection that has been quite common in the USA for many years, and until recent years has been seen relatively infrequently here in Ontario. It is however becoming more prevalent, and it warrants a discussion to make you all aware of it. It is called Potomac Horse Fever (PHF), and it is caused by a pathogen called Neorickettsia risticii. This disease is usually seen during the summer months of June, July, and August, but depending on temperatures, it can vary a bit and may be seen into the Fall.
The intermediate host that houses the pathogen is a trematode (or fluke) found in freshwater snails. These snails are accidentally ingested by horses grazing near freshwater sources such as streams and rivers. Other insects associated with these flukes include both the larvae and adults of the:
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Mayfly
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Damselfly
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Caddis fly
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Dragonfly
These insects can be found anywhere, including pastures that are not near water sources. They are ingested by horses while grazing. It is thought that many horses are exposed each year, but only about 30% of those exposed will develop disease.
This disease can potentially have some pretty serious clinical signs. Thankfully we still only see isolated cases each year. Some infected horses become quite ill, and clinical signs include:
Unfortunately, this disease can be fatal if not treated appropriately and immediately. Some horses may have only mild signs of disease, and a diagnosis is never sought due to quick recovery. Treatment consists of a specific antibiotic to combat infection, as well as aggressive IV fluid therapy, anti-endotoxic doses of anti-inflammatory drugs, gut protectants, cold therapy for feet, etc. Often this type of aggressive treatment is difficult to accomplish on farm and horses are frequently referred to a hospital setting.
There is a vaccine on the market that was created to help protect against this disease, but general consensus among experts is that the vaccine is not as helpful as once thought. The strain used to create the vaccine is not the same strain found in this area. The immunity generated by the vaccine is very weak, and very short lived (approximately 3 months), even with a booster series of two vaccines a month apart. Vaccinating once per year is not effective. Some experts do believe that while vaccinating will not prevent disease, it may prompt enough of a response to lessen the severity of clinical signs. It is recommended not to use the multivalent PHF vaccine that is available in combination with other antigens. The recommendation is to use the monovalent vaccine that contains only the PHF antigen. Owners can try to reduce the risk of infection by reducing nighttime grazing, as some of these insects are usually present in the grass in greater numbers at night. It has also been recommended not to leave outdoor lights on during times when these insects are more prevalent, as light is a big attractant.
This disease is worrisome for three reasons:
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Clinical disease can be very serious;
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There is not a great protective vaccine;
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The hosts that carry the pathogen are found in our environment.
Thankfully, most clinics in this area only see a small number of cases each year, and hopefully it will stay that way. The intention of this write up is not to cause excessive worry; the intention is to alert owners to the presence of this disease, and to urge owners to seek veterinary advice as soon as clinical signs are seen during the summer months, so that appropriate treatment can be implemented if PHF is suspected.