Q. Has there been new research completed on the available vaccines for Potomac horse fever (PHF) or are they what veterinarians gave years ago? A person told me that there has been no new research on Potomac horse fever vaccines that we use today. Are our vaccines up-to-date as far as preventatives? Do our horses need a PHF vaccine, or are we wasting our money giving it?

Ruth, via e-mail

A. All existing PHF vaccines are based on an original single isolate of the agent from the blood of a horse. Since that time, several "strains" of Neorickettsia risticii have been identified. Researchers have isolated different strains from the blood of horses that were clinically ill with Potomac horse fever that had a full and current PHF vaccine course. So, the vaccine is not completely protective.

An additional factor is that the way the vaccine was tested for efficacy during initial licensing of the vaccine was probably not relevant to how horses acquire the infection in nature. In challenge studies, horses were given the agent intravenously