Factors Associated with Survival in 50 Horses with Potomac Horse Fever

The severity of colitis in horses with Potomac horse fever is the best predictor of survival, and oxytetracycline is efficacious in the treatment of horses with Potomac horse fever.

1. Introduction

Neorickettsia risticii causes Potomac horse fever (PHF). The objective of this retrospective study was to describe the signalment, clinical signs, clinicopathologic values, antimicrobial use, and survival status in horses affected with PHF and to identify variables for prediction of survival.

2. Materials and Methods

Horses diagnosed with PHF at 2 referral institutions over a 15-year period were included. A diagnosis of PHF was based on the presence of positive blood or fecal polymerase chain reaction (PCR), an indirect fluorescent antibody (IFA) titer >1:2,560 in nonvaccinated horses with compatible signs, or a 4-fold increase between acute and convalescent titers.

3. Results

Horses ranged from 4 months to 29 years of age. The most common clinical signs included diarrhea (66%), fever (48%), anorexia (42%), depression (40%), colic (38%), and lameness (18%). The median duration of hospitalization was 6 days, and 76% of horses survived to discharge. Laminitis was confirmed in 26% and suspected in 36% of cases. Serum urea nitrogen and creatinine concentrations, anion gap, and hematocrit were significantly (p < 0.01) higher in nonsurvivors, whereas serum chloride, sodium, calcium, and magnesium concentrations were significantly lower in nonsurvivors. Logistic regression indicated that serum chloride concentration on admission and antimicrobial treatment with oxytetracycline were independent factors associated with survival. 

4. Discussion

Severity of colitis as reflected by electrolyte loss and secondary prerenal azotemia is a predictor of survival. Treatment with oxytetracycline is associated with increased survival.