Should I Vaccinate My Horse Against Potomac Horse Fever?

As is true every year, we are starting to see horses that have Potomac Horse Fever in our practice. In some cases, the horses had received the vaccine this spring. Some commonly asked questions are: Why is the vaccine not 100% protective? Should I revaccinate my horse even if he got a vaccine in the spring?

Potomac Horse Fever causes signs of fever, mild to severe diarrhea, inappetance, colic and laminitis. Any combination of these signs may be present. Abortion may occur in pregnant mares if infected. It has a wide geographical distribution, occurring in most of the United States, Canada, South America and some European countries. It is most prevalent in the late spring through early fall. Most cases occur in July, August and September. The organism is an ehrlicia, Neorickettsia risticii. Horses become infected when they ingest infected aquatic insects such as mayflies, damselflies or caddie flies.  The disease has been associated with rivers and streams where these insects are in abundance. However, dead insects in the water buckets or hay may also be a source. The disease can be endemic on a farm in which case disease may be expected in future years.  But it is also known to occur sporadically.

A vaccine for Potomac Horse Fever exists. It is made from the killed organism.  Many horses that are vaccinated still develop the disease. It is a widely held view among veterinarians that the disease seen in vaccinated horses is less severe than in unvaccinated horses. However, there is no scientific evidence to support this view at this time.

Why is the vaccine not 100% effective? 1) There are several strains of Neorickettsia risticii. The vaccines on the market are based on one isolated strain. The vaccine may offer some cross protection against different strains.  2) The immunity is short lived with the vaccine. The immune response to this particular vaccine is poor compared to other vaccines. It peaks at 3-4 weeks post vaccination but may continue to provide immunity for 3-4 months.

Should I have my horse revaccinated even if he got a vaccine in the spring? There is not much scientific evidence to suggest that revaccinating horses a few months later offers them more protection against the clinical disease. But we do know that the antibody titers are low in response to the vaccine and do not last long. Owners that might want to consider revaccinating are those that have pregnant mares, especially those 90-120 days in foal and owners that have horses that live in an area in close proximity to water or on a farm where Potomac Horse Fever has been diagnosed in the past. In cases where revaccination is appropriate, we recommend revaccinating no sooner than 3 months after the first vaccine.  One of the Blue Ridge Equine veterinarians can talk to you about your horse’s risk and whether revaccination is in his or her best interest.

Keeping the barn lights off at night will prevent the attraction of flying insects that may carry Potomac Horse Fever. In barns that have had previous cases, we recommend taking all horses temperatures twice a day and calling the veterinarian when any horse has a temperature over 101.6o.