Potomac Horse Fever - A Deadly but Preventable Disease 

During late summer and early fall, the veterinarians of Conley and Koontz Equine Hospital start to see horses with high temperatures, severe diarrhea, laminitis and sometimes death.  Every year dozens of horses contract Potomac Horse Fever (PHF).

PHF is caused by the bacteria Neorickettsia risticii (N. risticii), which live inside cells of other organisms and have a complicated life cycle.  N. risticii can live inside flukes, which are small slug-like parasites that live in or near water.  Once in water, aquatic insects such as Mayflies and Caddisflies can become infected. These insects only live for 1-2 days at a time and go through mass hatchings in the summer and fall, where large numbers will mate over water, reproduce, and then die over water or feed sources.

Because of this life cycle, Potomac Horse Fever can be transmitted to horses through contact with infected water or slugs, or through ingestion of infected insects in ponds, water buckets, pastures, or feed.  The disease is not directly spread from horse to horse.

You may think “why would my horse eat slugs or flies?”  What happens is the flukes are extremely small and the horse can swallow them when they drink or graze on wet grass.  Dead insects of course are common in a barn and horses can eat them when they eat grain, hay, or water.

Signs in affected horses can vary but usually horses will stop eating and become depressed, and then develop fever, severe diarrhea, dehydration, colic, and sometimes laminitis. Also, PHF can be transmitted to a fetus and may cause abortion in pregnant mares several months after infection. Horses that contract PHF must be treated aggressively to prevent severe dehydration, laminitis, and death.  Treatment includes aggressive fluid therapy, anti-inflammatory therapy, supportive care, and tetracycline antibiotics.

Management changes are very important in preventing PHF; the main goal is to prevent horses from ingesting the organism as much as possible. Wherever feasible, fill in small water collections and keep horses away from creeks and ponds, especially during summer and fall when the aquatic insects are hatching. Try to avoid anything that attracts these insects, especially lights over water sources. Move water troughs away from overhead lights in pastures, and don’t leave lights on unnecessarily in stalls. Also, check hay and grain periodically to be sure that they do not contain insect carcasses; discard any feed that has been contaminated.

PHF can be also prevented or at least the severity of disease can be reduced with strategic vaccinations.  N. risticii continually mutates and changes form so that the vaccine cannot specifically match the form that all horses will come in contact with. The vaccination is not guaranteed to prevent infection 100% of the time, but vaccinated horses will have less severe clinical signs and therefore a higher survival rate.  The vaccine does prevent disease in horses that come in contact with the strains in the vaccine and lessens the severity of disease if your horse comes in contact with other strains.

Due to economic considerations or the misguided belief that since their horse does not leave the farm they do not need to be vaccinated, many owners choose not to give “fall vaccines”.  Potomac Horse Fever and West Nile Virus are examples of diseases that are more prominent in the fall and diseases that horses do not need to be in contact with other horses to contract.  At Conley and Koontz Equine Hospital we recommend that horses in our practice receive a PHF vaccine twice per year.  The fall vaccine is particularly important. Also, pregnant mares should be vaccinated 4 to 6 weeks before foaling to provide protective antibodies to the foal when born.