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This 9 year old mare, Canella, recently suffered a fatal case of the equine disease Potomac Horse Fever. Canella was owned by Brandie Johnson of Woodland. 

On Thursday, November 16, Brandie Johnson of Woodland noticed that her 9 year old mare, Canella, seemed a little under the weather. On Friday she ran a slight fever, and by Saturday she was not eating and developed explosive diarrhea. After a visit with her local veterinarian, Johnson transported Canella to Oregon State University veterinary hospital in Corvallis, Oregon for evaluation. 

Her illness remained a mystery until lab results came back on Monday morning. Canella was suffering from Potomac Horse Fever, an illness little known in the Pacific Northwest. After briefly improving, Canella suffered neurological symptoms and had to be humanely euthanized on Thanksgiving morning, only a week after the onset of her illness. 

Reports of cases of PHF are circulating in Clark County, including several incidences of horses who have died of the disease.  

PHF is a reportable disease in the state of Washington, according to Dr. Thomas Gilliam, Field Veterinarian for Washington State Department of Agriculture, and in 2017 three cases have been confirmed across the state. This is the most he has seen in his 40 years of professional practice, said Gilliam.  

But for Clark County, the picture is not as clear. Ill horses are typically transported to OSU because it is the closest veterinary hospital. PHF is not a reportable disease in the state of Oregon, according to Dr. Brad LeaMaster, State Veterinarian for the Oregon Department of Agriculture, so he was not able to provide information about the number of cases confirmed in that state.  

Dr. John Schlipf Jr., Assistant Professor of Large Animal Internal Medicine at OSU, said the veterinary hospital has positively identified 6 cases of PHF admitted there since September. One of those cases was from Clark County and one was from Cowlitz County; the remainder were from Oregon. He said that is about the usual number he sees treated at the hospital. 

Dr. Schlipf did share his impression that while overall numbers were steady, the horses admitted with PHF seemed to be sicker than in years past, and outcomes were more likely to be fatal. 

Numerous stories of Clark County horses contracting PHF have surfaced across social media in recent weeks, causing concern and questions about this previously little known disease.  

Dr. Meg Brinton of Ridgefield Equine Clinic in Ridgefield confirmed 2 cases in her practice, and has heard of others. Brinton referred one equine patient to OSU, where the horse died. Amazia Veterinary Service in Brush Prairie confirmed 3 cases of PHF in their practice and 1 suspected case pending tests. The three horses with PHF all died of the illness. Other area veterinarians weren’t reached for comment. 

It is unclear why the numbers of confirmed cases from various sources do not align. 

Dr. Brinton echoed the impression shared by Gilliam, that the number of confirmed and anecdotal reports of PHF are unusually high this year. The disease has appeared very sporadically in this area in past years. 

PHF was first identified in 1979 in the Potomac River Valley in Maryland, and is now spreading throughout the United States. PHF is caused by a bacterium called Neorickettsia risticci. Much is still unknown about this bacterium and its role as a vector for PHF, but recent research shows that it infects a certain type of fluke that parasitizes aquatic slugs and snails. These bacteria are also found in aquatic insects like mayflies and caddis flies, and it is most recently believed that horses are infected by ingesting the insects. It is a seasonal affliction, appearing most commonly in summer and fall. 

Once infected, it takes one to three weeks for a horse to exhibit the disease. 

It’s very important to recognize the disease early, said Dr. Brinton, because that is when it is most treatable.  

High fever is the most consistent sign. It can appear as a “biphasic fever,” which spikes, returns to normal, and spikes again. 

Approximately 60% of horses develop visible diarrhea. Others will have an internal inflammation of the colon causing fluid loss, though they don’t produce much diarrhea. In either case, the horse becomes severely endotoxic and dehydrated. 

Laminitis (founder) is seen in about 40% of PHF cases. The laminitis can be severe and progress despite the improvement of other clinical signs, and can be life-threatening even after diarrhea is resolved. The fatality rate for PHF is between 5-30%. Pregnant mares infected with PHF may later abort. 

The best protective action is to take a horse’s temperature daily if they exhibit fever, lethargy, or diarrhea, said Dr. Brinton. If PHF is suspected, the horse will be referred to OSU for supportive care, IV fluids, and intensive monitoring. 

“Horses can go south so fast from diarrhea and toxicity,” said Dr. Brinton. “They need IV fluids right away.” 

PHF is not contagious from horse to horse, though cluster outbreaks do occur. However, other diseases with similar symptoms, such as salmonella, are contagious. Therefore, it is important to practice biosecurity precautions such as separating the sick horse from other horses, washing hands after working with each horse, and not sharing grooming equipment between horses until the cause of the illness is known. 

Because of its transmission by aquatic snails and flies, PHF is believed to be a greater risk to horses living near bodies of fresh water. PHF typically occurs during warm seasons. Once PHF occurs in a particular geographic area it tends to recur, some studies suggest. 

The diagnostic test for PHF is a polymerase chain reaction (PCR) test on a blood or fecal sample. Treatment with Banamine, to control fever, and oxytetracycline to combat the bacterium is typically very effective if started early. 

However, a definitive diagnosis is important before beginning treatment, emphasized Dr. Brinton, because giving oxytetracycline could make other diarrheal diseases much worse.   

A vaccine for PHF is available, though Dr. Schlipf characterized it as “essentially worthless.” The vaccine has been reported to protect 78% of experimentally infected horses, but it is marginally protective in the field. This may be because the vaccine is manufactured to combat a single strain of the organism, and does not provide protection against diverse naturally occurring organisms. 

The vaccine is given as an initial series of 2 shots, 3 weeks apart, and takes up to 6 weeks to become effective. Immunity only lasts for 90-120 days, said Dr. Schlipf, and he recommended administering it twice per year, in spring and again in mid-summer to early fall, to offer its best protection.  

Administering the vaccine separately from other vaccinations may provide a better immune response, said Dr. Brinton. 

Because of the seasonal incidence of PHF and the time required to develop immunity, administering the vaccine at this time is unlikely to be protective to horses during the current seasonal PHF outbreak, said Dr. Brinton.  

Perhaps the best defense is to limit a horse’s exposure to PHF vectors. This includes restricting grazing near bodies of water, turning off barn lights when not in use, placing food and water in unlit areas, keeping stored feed covered, and cleaning water sources regularly. 

Leaving trees, bushes and grasses that grow up around water sources can provide natural barriers to encourage insects to stay near the water, and out of pastures and barns. 

Symptoms of PHF warrant an emergency call to a veterinarian, even on nights or weekends, emphasized Dr. Brinton.

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(1) comment


My gelding was boarded in Clackamas County. He was treated at OSU for PHF the same day as this mare and he was also put down within a few days.

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