PHF Vaccine Efficacy: Better Safe than Sorry?

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.

Our group (at the University of California, Davis) and others have shown horses become infected via infected insects that have hatched from water (a stream, river, creek, ditch, or other source) and are infected with the agent. The horse consumes the insect while grazing or when the insect lands in a feeder.

So when testing the ability of a vaccine to protect a horse, the horse should be vaccinated, then challenged by the natural route of exposure. This has not been done for any equine Potomac horse fever vaccine.

Does the vaccine provide partial protection in some regions of the country where the infection has been conclusively diagnosed? No one knows with a degree of scientific certainty. So the use of any of the vaccines has to be decided based on the owner choosing to use a vaccine that "might" help, versus choosing to use no vaccine.

More work clearly needs to be done on this important infection of the horse.

About the Author

John Madigan, DVM, MS, Dipl. ACVIM

John E. Madigan, DVM, MS, Dipl. ACVIM,is a professor in the department of Medicine and Epidemiology at the School of Veterinary Medicine at the University of California, Davis. His areas of expertise include equine welfare, equine helicopter rescue, trigeminal mediated headshaking, potomac horse fever, anaplasma infection in horses and humans, veterinary emergency response, and equine veterinary education. He is also interested in neonatal medicine; his book, Manual of Equine Neonatal Medicine, is available in the fourth edition of the text.

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