What is Potomac Horse Fever?
Potomac Horse Fever (PHF) is a disease caused by the microorganism Neorickettsia risticii, that results in severe colitis and diarrhea in affected horses. This organism is commonly found in wet and marshy areas such as near ponds, rivers, or lakes. At this time, the exact mode of transmission and infection is not completely understood. However, it is believed that horses are exposed to this organism when they come into direct contact with infected bodies of water, flooded pastures, or most commonly through ingestion of infected small trematodes. These trematodes are carried by many water insects and small snails that can live in your horse’s pasture. Potomac horse fever has become an endemic disease in northern Indiana and is frequently seen in the late summer and early fall months. Horses that are affected by PHF typically will present with a fever, signs of colic, rapid and severe colitis/diarrhea, edema, and anorexia. Some horses, as many as 30-40%, will also suffer from secondary laminitis.
When a horse is infected with PHF, the organism will begin by infecting white blood cells called monocytes, making the disease difficult to detect by the horse’s immune system. N. risticii then infects the cells lining the horse’s large colon, causing damage to the finger like projections called villi. These villi are essential for the absorption and movement of fluids, electrolytes, protein, and other important nutrients within the large colon. Inflammation, edema, and thickening of the large colon lead to decreased intestinal absorption, resulting in severe watery diarrhea. This diarrhea is quickly followed by dehydration, electrolyte abnormalities, protein loss, and secondary pitting edema. Diagnosis of PHF is made through a combination of history, physical exam findings, and blood work evaluation. Horses with PHF will typically show elevated white blood cells, severe dehydration, and severe hypoproteinemia (low protein levels). These abnormalities paired with diarrhea and a high fever help to differentiate this disease from other cause of colitis/diarrhea.
Why is Potomac horse fever so dangerous to horses?
When a horse experiences diarrhea due to PHF several things are happening:
1) They are losing large volumes of water and are unable to reabsorb the needed fluids to maintain their hydration status. When left untreated, this fluid loss results in severe dehydration, which can ultimately lead to hypovolemic shock and death.
2) Horses with diarrhea are also losing large amounts of essential electrolytes such as sodium and potassium. Due to villous damage, horses with PHF are unable to absorb more electrolytes from their diet to replace the ones lost. Severe electrolyte imbalances can also lead to shock and ultimately death.
3) Protein loss is another major indicator that a horse has been infected with PHF vs. other forms of colitis. The loss of large volumes of protein, paired with the gut’s inability to absorb new protein from the diet, results in an oncotic pressure imbalance. Oncotic pressure is the delicate balance between the vasculature and surrounding tissues that maintains fluid levels. Proteins are responsible for helping maintain this pressure by attracting more fluid towards the area of higher protein levels. When proteins are lost due to diarrhea and not replaced, fluids will begin to leave the vascular system and moves into the surrounding tissues. This accumulation of fluid is what we call pitting edema and can be seen as swelling under the horse’s belly or in its distal limbs. The greater the protein loss, the more severe the resulting edema will be. This loss of fluid due to diarrhea as well as in the form of edema causes horses to become severely dehydrated even quicker.
Potomac horse fever is a very rapid and aggressive disease. These life threatening changes can all occur within days after of the onset of diarrhea. However, there is good news! Despite being a very devastating disease, when treated early, PHF is very responsive to select antibiotics.
Our major goals for treatment of PHF include several days of IV antibiotics, anti-inflammatories, and supportive care via IV fluid, electrolyte, and protein supplementation. It is vital that we provide the necessary supportive care in addition to antibiotic therapy to correct the systemic imbalances caused by this disease. We begin by replacing lost protein and electrolytes in order to prevent further loss of fluid into the surrounding tissues and stabilize the patient. We then slowly rehydrate them with IV fluids. Horses are typically very responsive to treatment and can make dramatic improvements within just 24-72 hours. Most horses when treated early, aggressively, and in the absence of concurrent laminitis have a good prognosis with full recovery!
What can you do as an owner to prevent your horse from getting PHF?
Currently, vaccines are available for the prevention of PHF! Although this vaccine is not a 100% guarantee that your horse will never get this disease, it is an excellent preventative and can decrease the severity of disease in the event your horse does become infected. At Conley and Koontz this vaccine is given bi-annually as a part of our recommended spring and fall vaccine series. We also recommend this vaccination for young horses and pregnant mares as a part of their pre-foaling vaccine series at 10 months of gestation. Be aware that this vaccination is typically NOT a part of the 4H required vaccine series, however it is a part of the Conley and Koontz Equine Hospital recommended vaccine protocol.
Jordan Flewellyn, DVM