When Potomac Horse Fever was first identified in the Potomac River region of Maryland, horse owners who lived outside that area weren’t too concerned. But the rickettsial disease soon spread to other geographic regions.
“Potomac Horse Fever (PHF) was first noticed in 1979 in the Potomac Valley region of Maryland,” said Cornell University veterinarian Dr. Joy Tomlinson. “Since then, it’s been recognized in almost every state in the U.S., as well as parts of Canada, South America and Europe.”
The causative organism for PHF is Neorickettsia risticii, which dwells in flukes that live in moist areas such as streams, rivers and ponds. “The life cycle is fairly complex,” said Tomlinson. “The bacteria lives in flukes, which are a type of worm. The worms pass through a variety of different animals including aquatic snails and aquatic insects such as caddisflies, mayflies, damselflies and dragonflies. They can also be carried by bats and swallows. Aquatic insects are the most common route that horses become infected, but there’s potential that other animals can bring it onto the farm.” Tomlinson says it isn’t known how frequently transmission occurs via bats or swallows.
Water sources within five miles of where horses are living can be home to the insects that serve as vectors for the disease. Most cases of PHF are diagnosed between early spring and fall, with peak occurrence in July, August and September. Horses that spend the summer months on pastures near streams could be especially at risk for the disease so it is important to keep a close eye on them.
Tomlinson says the current thinking is that stabled horses become infected primarily through contaminated hay, but it’s unclear whether the risk occurs during the baling process when insects may be baled with hay, or from stored hay that insects have fallen onto after being attracted by night lighting in or around the barn. Because of the known risk of contaminated hay, one of the recommended preventive measures is to keep hay and grain covered so that water flies don’t land on these feed sources, and to turn off lights near hay storage whenever possible.
If the insects that harbor PHF don’t have adequate water in a drought year such as the summer of 2016, why are numerous cases of PHF still reported? Tomlinson speculates that during a dry spell, perhaps pastured horses tend to graze closer to water sources such as streams because that’s where the grass is greenest, or maybe the infective insects are seeking water troughs as a water source.
Several days after ingesting the insect, the horse develops a low-grade fever. Tomlinson says that this low-grade fever has been detected in experimental studies, but it’s usually mild enough that the horse owner doesn’t pick up on it. The horse will appear to be normal for a week or so, then major illness sets in.
The most consistent clinical sign of PHF is a high fever, usually from 102 to 107 degrees. “When a horse has a fever that high, they feel sick,” said Tomlinson. “They don’t eat well, they feel hot to the touch, their head will be hanging down and they look depressed. A high fever can be one of the things that triggers the owner and vet to worry about PHF. Any time a horse has a fever over 104 degrees, the horse should be seen immediately by a veterinarian. There aren’t too many diseases that cause a fever that high and immediate intervention can make a big difference in many of these cases.”
Disease progression varies widely from horse to horse. “Some horses will have a mild disease,” said Tomlinson. “Maybe a mild colic or inappetence and loose manure. Other horses get severe general illness with diarrhea. About 30 to 40 percent of horses with PHF will get laminitis or founder, causing severe lameness. Laminitis is the breakdown of tissue that holds the coffin bone suspended inside the hoof capsule.”
One of the problems with PHF is that the early, mild clinical signs are easy to miss, and severe disease can hit fast and hard in some horses. A horse may look fine one evening and be severely ill the following day. “It doesn’t necessarily reflect that the horses are poorly observed,” said Tomlinson. “It’s just a really bad case of the disease. There are some cases that are really tough — the first sign you see is the horse is foundering. But fortunately, the warning is usually the high fever.”
Tomlinson says PCR (polymerase chain reaction) of a blood sample is the best test for Potomac Horse Fever. This test detects the bacteria directly. However, once a horse has had a dose of antibiotics, that test isn’t as reliable because the antibiotics can be very effective at eliminating the bacteria. “In those cases, we’ll often try to use antibody titers to confirm the diagnosis,” she said.
“We can look at antibody titers, but that isn’t as straightforward or reliable as we’d like for PHF,” said Tomlinson. “Usually when we use antibody titers to determine recent infection, we are looking for an increase in antibodies over the first couple weeks. For many diseases, when the horse is first sick, the antibodies are low or negative because the horse hasn’t had time to mount a strong immune response yet. A couple weeks later, they should have a robust immune response and high antibodies. For PHF, they don’t always follow this pattern. Prior vaccination can make the titer high, which cannot be distinguished from infection. Because of the approximately two-week delay between infection and disease in PHF, sometimes titers will already be high when the vet first comes out to look at them and won’t continue to rise. Also, many horses in areas with high incidence of PHF will have antibodies from being previously exposed, which might be a red herring for what is causing the horse’s current illness.
Additional tests your vet recommends will likely include a complete blood cell count and serum protein to help assess severity of disease and guide treatment recommendations. Salmonella infections can manifest similarly to PHF, so recommended diagnostics might include tests for that bacteria as well.
Tomlinson says if the disease is caught early and antibiotic treatment is started immediately, the antibiotic is highly effective. “If you catch it at the first sign of a high fever and have the horse seen and treated right away, they’ll often respond well to treatment,” said Tomlinson. “Fever should resolve within 48 hours of starting antibiotic treatment and diarrhea usually resolves within 72 hours.”
The treatment of choice for PHF is oxytetracycline administered in the vein, and fluids to replace fluid and electrolyte losses. “If we get them on the right antibiotic, get them rehydrated and give them fever and pain reducers, they feel a lot better,” said Tomlinson. “If you have a horse that gets diarrhea, early veterinary intervention and referral to a veterinary hospital is the best thing you can do. A lot of the medications we give for this disease can be harmful to the kidneys and the effect is exacerbated if the horse is dehydrated. With horses, the sheer volume of fluids required can be hard to manage at home, and they can get in trouble fast.”
One of the most concerning complications from PHF is laminitis. “In the early stages (of PHF), we believe that icing the limbs to keep them cold is the most effective way to prevent laminitis. If laminitis develops, owners should consult with their veterinarian and farrier about the best way to handle potential long term therapy.”
There are at least 28 strains of the bacteria that cause PHF, but the available vaccine only contains one variant of the bacteria. “It’s what we have,” said Tomlinson, describing the vaccine. “It can help prevent the disease from that strain and might provide some protection from other strains. We think that vaccinated horses that get PHF don’t get as sick.”
The vaccine also has a short duration of immunity, so Tomlinson suggests weighing the cost/benefit with the horse’s veterinarian. “Owners and vets need to keep in mind that a vaccinated horse doesn’t rule out that PHF is or is not the cause of disease. A vaccinated horse should still be treated aggressively if PHF is suspected.”