Factors Associated with Surviving Potomac Horse Fever

Factors Associated with Surviving Potomac Horse Fever

The team concluded that the only independent factors associated with Potomac horse fever survival were the animals' blood serum chloride concentration and treatment with oxytetracycline.

Photo: Anne M. Eberhardt/The Horse

Editor's note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 American College of Veterinary Internal Medicine Forum, held May 30 - June 2 in New Orleans, La.

Although it's not prevalent in all corners of the nation, Potomac horse fever (PHF) can be scary when it hits. Veterinarians are still working to fully understand the disease, which is caused by the bacteria Neorickettsia risticii. The collective understanding of this disease took a step forward when a team of researchers identified several factors associated with survival in affected horses.

Sandra Taylor, DVM, PhD, assistant professor of large animal medicine at the Purdue University College of Veterinary Medicine, discussed factors identified as being associated with surviving PHF at the 2012 American College of Veterinary Internal Medicine Forum, held May 30 - June 2 in New Orleans, La.

Taylor explained that PHF, which has a fatality rate approaching 30%, causes colitis (inflammation of the colon) and sometimes laminitis. She noted that "there are many different strains of the bacteria, so immunity against one strain does not always protect against infection with another strain."

To identify survival factors, Taylor and colleagues evaluated the medical records of 50 horses diagnosed with PHF at two referral institutions over a 15 year span. The horses included in the study ranged in age from 4 months to 29 years, she said.

The team found that the most common clinical signs identified in affected horses were diarrhea, fever, anorexia, depression, colic, and lameness. Horses remained hospitalized for an average of six days, and 76% of horses survived to discharge, Taylor said. Laminitis was suspected in 36% of horses and confirmed with radiographs in 26%, she added; the majority (88%) of horses with laminitis were affected in all four feet. Cases were seen from June to December with the peak incidence in August, she said.

She also noted that a significant amount of information can come from bloodwork:

  • Nonsurvivors had significantly higher blood urea nitrogen and creatinine concentrations (called azotemia) and blood phosphorous levels (which collectively indicate kidney disease secondary to dehydration from diarrhea);
  • Anion gap (the difference in the measured cations and the measured anions in blood serum), blood glucose, and red blood cell numbers were also elevated and are reflective of dehydration and stress; and
  • Survivors had significantly lower chloride, sodium, and calcium concentrations in the blood serum, which reflect severity of colitis.

Additionally, she noted that oxytetracycline was effective in treating horses with PHF, and that vaccination did not prevent disease in this population of horses. Finally, she noted that horses diagnosed with laminitis had a higher likelihood of nonsurvival compared to horses without laminitis.

After performing logistic regression (a type of analysis used to predict the outcome of a categorical criterion variable based on one or more predictor variables) the team concluded that the only independent factors associated with PHF survival were the blood serum chloride concentration and treatment with oxytetracycline.

"Our findings indicate that horses with compatible symptoms in endemic areas during the summer months should be treated with oxytetracycline, even prior to a confirmed diagnosis," Taylor concluded. "Although this study population did not appear to be protected by vaccination, it is possible that vaccination decreases incidence and severity of disease. Development of a vaccine that can protect against all strains of the bacteria may eliminate the disease, but future research in this area is necessary and ongoing."

About the Author

Erica Larson, News Editor

Erica Larson, News Editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in three-day eventing with her OTTB, Dorado, and enjoys photography in her spare time.

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