In many parts of the country, hot, humid summer weather means it’s time to take steps to protect horses against equine monocytic ehrlichiosis, more commonly called Potomac horse fever (PHF). The disease was named for the Potomac River Valley, where it was first recognized in 1979, but cases have been identified throughout the United States as well as Mexico and Canada.
“Equine neorickettsiosis is the better name for this disease because endemic areas occur all across the country, not just in the Potomac River area,” says Nicola Pusterla, DVM, PhD, of the University of California–Davis.
PHF occurs when horses ingest the bacterium Neorickettsia risticii. N. risticii infects a parasitic fluke (a type of flatworm), which under-goes a complex life cycle in which at different stages it can be found in a number of aquatic species, including freshwater snails as well as aquatic insects, birds and bats, and it can even be free-swimming in water.
But the threat to horses comes from flying aquatic insects. N. risticii has been found in more than a dozen species, including dragonflies, damselflies and stoneflies, whose larvae consume infected free-swimming flukes then continue to carry the bacterium-containing fluke when they emerge as adults. If horses consume the bodies of infected aquatic insects while grazing or drinking, they may develop Potomac horse fever.
The specific insect species that pose the greatest risk are those, like mayflies and caddisflies, that emerge all at once in swarms of thousands of individuals to mate and die within hours. These insects are attracted to bright lights and may fly for miles to swarm around outdoor lights—including those in stable yards and in barns—where they can die in large numbers and drop into feeders and troughs, to be consumed along with a horse’s hay, water, grass and feeds.
Once eaten by the horse, the bacteria are released by the flukes and invade cells in the bowel wall, weakening it and creating a cascade of inflammation and allowing the release of toxins into the bloodstream. Generally, the incubation period from ingestion of the bacteria to the onset of signs is a week to three weeks. The first sign of illness is high fever, as much as 107 degrees Fahrenheit, with a lack of appetite and lethargy. Over the next day or two, horses with PHF may also develop potentially severe complications:
• About 40 percent develop acute laminitis, a potentially crippling inflammation of the connective tissues inside the hoof. Laminitis can cause excruciating pain, and in worst-case scenarios, the coffin bone can separate from the hoof wall and sink or rotate downward, which is a crippling condition called founder. Humane euthanasia may be necessary for horses with serious laminitis resulting from PHF.
• About 60 percent develop loose stools or severe diarrhea. These horses may also become dehydrated and appear colicky.
• Pregnant mares infected with N. risticii may abort their fetuses.
PHF is not contagious nor does it pass between horses via casual contact. When multiple cases appear together on the same farm, it means that more than one horse consumed infected insects.
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Diagnosis and treatment
The most common signs of PHF are fever accompanied by loss of appetite and lethargy. Blood tests may show low white blood cell counts as well as decreases in electrolytes, proteins and other factors. Some veterinarians also test a horse’s blood for antibodies to N. risticii or for the organism itself with a polymerase chain reaction (PCR) test, which detects N. risticii DNA in the horse’s blood and feces.
The primary treatment for PHF is the antibiotic oxytetracycline, administered intravenously, along with anti- inflammatory medications and supportive care. Horses with PHF are often admitted to hospitals or referral clinics where they receive electrolytes and intravenous fluids. Medications that bond with toxins in the intestine and decrease the diarrhea may also be administered. Treatment is more effective when started as early as possible in the course of the illness, but many horses respond well within 24 hours.
Icing the feet is also a wise precaution to prevent laminitis. If you’re caring for a horse with PHF at home, your veterinarian will instruct you on how to feel your horse’s feet for heat and how to look for a “bounding” pulse behind the fetlock, which can be early indicators of inflammation of the soft tissues within the foot. If you notice these signs, you’ll want to get the horse’s feet on ice right away. Boots that are specially designed for this purpose are available, but you can also use any sturdy container deep enough to cover a horse’s feet with ice water up to the mid-cannon. You’ll also want to call your veterinarian immediately; this is an emergency.
When treated promptly, most horses —at least 70 percent—are able to make a full recovery from PHF. Those that don’t make it are usually euthanatized due to complications from laminitis.
Vaccinate at-risk horses
A vaccine against PHF is available, and the American Association of Equine Practitioners recommends it for horses at greater risk of being exposed to N. risticii. Generally, this means horses who live on farms where PHF has appeared in the past as well as those who live in endemic areas within range of swarming aquatic insects.
However, says Pusterla, “It’s important to realize that vaccination does not prevent all cases of the disease.” The current vaccine is based on only one strain of N. risticii—horses may pick up others in natural settings. But even in these cases, vaccination may reduce the severity of the disease. “It may help prevent some of the detrimental effects, such as laminitis,” Pusterla says.
The current recommendation for an unvaccinated horse is a two-dose series, at a three- to four-week interval. Peak immunity develops three to four weeks after the second dose. For previously vaccinated horses, boosters are recommended every six to 12 months, although veterinarians may suggest administering boosters every three to four months for horses at greater risk of exposure to the disease. For pregnant mares, the recommendation is for a booster to be administered at four to six weeks prior to foaling.
Boosters are recommended in spring and again in midsummer to confer maximum immunity during peak seasons for the insects.
Other preventive measures
Although vaccination against PHF is important, says Pusterla, “prevention needs to rely on keeping the aquatic insects away from the horses in endemic areas, especially during big hatches.” Here’s what you can do:
• Turn off the lights. The flying insects that carry N. risticii are attracted to bright lights, including barn or outdoor fixtures that are left on at night. When the insects swarm and die off, they fall into hay feeders, water buckets, stalls, turnouts and other areas where they can be easily picked up and consumed by horses. Research has shown that horses stabled at the end of aisles nearer to open doors and outdoor lights are more likely to contract PHF.
If you often keep your interior barn lights on long after dark, consider installing screens to keep out flying insects, and turn off any exterior lights. If security is a concern, consider adding motion detectors to your outdoor lights so that they come on only when someone is present.
• Store hay indoors or under a tarp. Other measures to prevent dead aquatic insects from falling into your horse’s hay or water include placing outdoor feeders, buckets or troughs away from any overhead lights, and keeping all stored feeds covered and secured. Remember, too, that hay grown near rivers and other surface waters may contain dead aquatic insects. Even if you don’t live in an endemic area, monitor your hay for dead insects and remove any that you find.
• Keep horses away from natural water sources. Currently, ingesting the aquatic insects is the only known way for a horse to contract PHF in a natural setting. “Whatever is ingested goes through an acidic stomach,” says Pusterla, “so it needs to be resistant.”
When the flukes that carry N. risticii are infecting snails or other aquatic species or free-floating in the water, they are in a stage of their life cycle that is destroyed in a horse’s stomach. However, in the insects, the flukes develop into metacercariae, an encysted form that can survive the acidic stomach content and reach the intestine intact. But, says Pusterla, not every possible natural route of transmission has been tested.
Horses grazing near natural waterways may pick up aquatic insects as well as other species that may be carrying the infected flukes. If your pasture contains natural ponds or streams, consider fencing your horse away from these areas, especially in the hotter months when insect activity is at its peak. Allowing a buffer zone of high grasses and other vegetation to grow along the banks may encourage aquatic species to remain near the water rather than venturing into pastures and barns.
• Track local insect populations. The peak season—when the insects that carry N. risticii are most active— varies by region; your veterinarian or local extension agent will know when to be most alert for PHF in your local area. If you live in an area where aquatic insect swarms are common, you’ll also want to stay in touch with your local extension office or other sources of information that may be able to predict impending hatches. On those nights, you’ll want to take extra precautions to protect your horse by keeping him inside with the lights off.
• Be aware of weather-related risks. “Weather, temperature and water are the vector trinity that affects the life cycles of the aquatic insects,” says Pusterla. “If there is no water, for example, there is no disease.”
Even in endemic areas, conditions may fluctuate. “In California, we have ‘hot spots’ for PHF, but even here, we don’t see the disease every year—if there is a drought, for example.”
So it’s a good idea to keep in mind when the peak seasons are for aquatic insects in your area and how fluctuating temperatures and rainfall may affect them. And with growing changes to our climate, the peak seasons may be extending earlier or later than they ever have before. Your best bet for protecting your horse is remaining alert to changes in his demeanor during the warmest months and calling your veterinarian at the first sign of fever.
Originally published in EQUUS 490
