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. The disease is usually seen during the late summer months and early fall (July – October), but depending on temperatures, it can vary a bit and may be seen farther 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:
-
Mayfly
-
Damselfly
-
Caddis fly
-
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.
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. 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 not 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 hooves, 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 the 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 very 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 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. Generally, we are recommending an initial vaccine with a booster one month later, followed by another booster 3 months after that. Ideally, this vaccine series should start well before Potomac season begins and will need to be repeated annually.