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Diseases from the South

A closer look at Potomac Horse Fever and Equine Lyme Disease


With the warmer Months coming up Its important to highlight two diseases that have been working their way up to Canada in the past few years: Potomac Horse Fever (PHF) and Equine Lyme. In this newsletter we'll be providing you with the information you need to stay informed and help protect your equine companions this summer!


Potomac Horse Fever (PHF)


Although PHF stated in the Potomac River Valley in West Virginia, it has now spread to the majority of the states in the US and Several Cases are being recorded in Canada every year. The Disease itself is caused by bacterium known as Neorickettisia risticii which is first carried within certain microscopic fresh-water worms and then maintained within the environment in hosts such as water snails. When the worms are secreted within the water your horse can ingest them and become infected. The bacteria then invade the walls of the large intestine and immunity cells known as monocytes. This results in inflammation in the GI tract, Diarrhea and decrease immune responses, making the horse more susceptible to concurrent infections.

The main clinical signs associated with PHF are fever, Diarrhea and colic. Laminitis is also a common finding in horses with PHF, often occurring earlier than with other diseases. Low blood protein levels are also very common, leading to increased diarrhea, water loss and dangerous dehydration. Beyond clinical signs, PHF can usually be diagnosed with a blood test.


The first thing YOU can do to protect your horse from this disease is to contact your veterinarian if you're seeing any of the above signs in your horse(s). The earlier this disease is diagnosed the better, as both the dehydration from the profuse diarrhea and the risk of laminitis can quickly become fatal conditions if left untreated. A very specific antibiotic will also need to be used and may require special ordering. Additionally if you're in an endemic area where risk of exposure is high a vaccine is available through your veterinarian.


Lyme Disease


Like PHF, Lyme disease is caused by a bacterium. this time Borrelia burgdorferi (how many times can you say that one fast?!). Instead of ingesting it in their water however horses can become infected the same way that dogs or humans can: Through ticks. As tick populations continue to increase in Canada with the warming of the seasons, Tick-borne diseases are also becoming more prevalent. While Lyme disease in horses is still considered fairly rare it is important for owners and veterinarians alike to be aware of the signs, treatments, and preventative options available.


Unfortunately the symptoms associated with Lyme disease can be vary non-specific. These include:

  • Lethargy

  • Behavior changes

  • Intermittent lameness of one or several limbs

  • low-grade fever

As these signs can occur with many other diseases, a history of exposure to ticks or tick-heavy areas is important for your vet to make the diagnosis. A simple blood test is available to confirm exposure to the bacteria however, it can't distinguish between the actual disease vs the protection gained through vaccination, making observation of clinical signs even more important.


As with PHF, treatment for Lyme disease generally revolves around treating the symptoms and the use of specific antibiotics. IF your horse tests positive but has NO clinical signs, treatment won't be required. The best defense against equine Lyme disease is diligent tick prevention : Searching for ticks on your horse regularly, use of tick sprays and having your horse tested if prolonged exposure to a tick is suspected. There is currently no equine vaccine available however recent research has found that the canine vaccine may give fairly effective protection in horses lasting about 6 months. Chatting with your veterinarian on the pros and cons of using a canine vaccine and the risk factors for Lyme are the best way to make an informed decision for you and your horse.


Take Home Message


Potomac Horse Fever and Equine Lyme Disease are two diseases that are well worth learning about. Please speak with your veterinarian if you have further questions!


Written by Dr. Emily Zakrajsek

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