An email came in a couple of days ago from a woman who wanted to talk with me about a blog post. A couple of years ago, Bridger Vet published a list of recommended equine vaccinations, and that list included the Potomac Horse Fever (PHF) vaccine as an optional, risk-based suggestion. She was curious as to why Bridger Vet recommended the vaccination when many veterinarians do not.

As a refresher, PHF is a potentially fatal febrile illness that is caused by the intracellular bacterium Neorickettsia risticii, which is found in flukes, freshwater snails, and aquatic insects. Horses ingest this bacteria by eating infected insects while grazing or drinking. Horses afflicted with PHF will exhibit mild colic, fever, and diarrhea or very soft manure; severe cases may also exhibit laminitis, possibly foundering and requiring euthanasia. If a pregnant mare contracts PHF, it is likely that she will abort.

It is worth pointing out that Potomac Horse Fever, which is sometimes called “ditch fever,” is not–NOT–limited to horses on the East Coast of the United States. The Potomac River is just where the first cases of PHF were documented and described. Currently, PHF has been documented in more than 40 states and Canada, and once PHF has been confirmed in an area, it is very likely that it will occur again. In Montana and Wyoming, PHF occurs during warmer weather, including spring, summer, and early fall in areas with pastures that border creeks, streams, and rivers. PHF has also been found in standing water in which carrier insects have fallen, and horses have ingested the bacteria from those insects.

Now that you’re up to speed on PHF, let’s go back to the vaccine. Currently, the available commercial vaccines are killed, adjuvanted products, meaning that pharmacological agents have been added to the vaccine to modify the horse’s immune response. The adjuvant boosts the immune response by giving a higher amount of antibodies and longer protection. Two of these PHF vaccines are also available as a combination with the rabies vaccine.

It is worth noting that veterinarians disagree about the efficacy of the PHF vaccination. The vaccine does not cover all strains of Neorickettsia risticii, and as such, it cannot be 100% effective. The PHF vaccine is similar to the rattlesnake vaccine for dogs in that while it cannot guarantee that your horse will not get PHF, it might decrease the severity of the infection and improve his odds of survival. “Anything you can do to protect your horse is worth it. The PHF can save your horse a lot of pain, and it can save you a lot of money. Even if the vaccine is only 50% effective, it is still worth it,” says Dr. Jane Undem, a veterinarian based in Lovell, Wyoming.

We suggest discussing the vaccine with your veterinarian, who will likely consider how the horse is being used. If you have a performance horse, meaning any horse that goes to horse shows, rodeos, reinings, cattle work at a neighbor’s ranch, endurance rides, trail rides, jumping events, and the like, the PHF vaccine will likely warrant an in-depth discussion. As an owner or exhibitor, you’ve put too much time, love, and money in to lose the season or the horse altogether to PHF.

Big Horn Basin Crud or Local Endemic Diarrhea

In the Big Horn Basin area of Wyoming, Dr. Undem has seen cases of what she calls “Big Horn Basin Crud” or “Local Endemic Diarrhea.” Big Horn Basin Crud or Local Endemic Diarrhea is similar to PHF in that the afflicted horse has violent, shooting diarrhea, but it does not have a fever. The cause of this illness has never been formally identified.

Treating Big Horn Basin Crud or Local Endemic Diarrhea can be expensive as it requires a large amount of diagnostic work, lots of intravenous fluids, and intensive, specialized care until the horse recovers. For those outside the Big Horn Basin, there is some good news: veterinarians have neither seen nor treated a case of Big Horn Basin Crud or Local Endemic Diarrhea in horses. For those in the Big Horn Basin, the endemic diarrhea has not been as prevalent the past few years, but that does not mean that it has been eradicated.

Treating Horses Diagnosed with Potomac Horse Fever

If you suspect that your horse might have PHF, it is important that treatment be started quickly. If PHF is caught early and laminitis has not set in, then your horse has a good chance of recovering. Because afflicted horses often experience severe diarrhea or a loose stool, intravenous fluids and electrolytes will be given. To counteract endotoxins and pain, antibiotics and non-steroidal anti-inflammatory drugs will be given. Treatment will likely be expensive and time-consuming.

Preventing Potomac Horse Fever

Here are some easy ways to help reduce the chances of your horse coming down with PHF:

  • Discuss your horse’s vaccination schedule with your veterinarian. If your horse is not getting the PHF vaccine, consider adding it in. PHF is at best expensive to treat and at its worst, lethal for your horse.
  • Move water buckets, water tubs, water troughs, and automatic water stations away from overhead lights and bug zappers. Dead insects can fall into the water, possibly infecting your horse.
  • Know your horses so that you can catch potentially fatal illnesses before it is too late. Walk your barns and pastures on a daily basis, taking stock of each horse and its habits. If you see worrisome behavior, contact your veterinarian immediately.

For questions about Potomac Horse Fever, the vaccine, and recommended equine vaccination protocols, please contact Bridger Vet at (406)-662-3335 for an appointment.

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