POTOMAC HORSE FEVER

Horses contract Potomac horse fever (PHF) by ingesting the bacterium Neorickettsia risticii, which may be present in a number of species that inhabit ponds, rivers and other natural waterways. “We know that a big risk factor is proximity to a water source or river—usually within five miles, but that isn’t always the case,” says Michelle Abraham Linton, BSc, BVMS, DACVIM (LAIM), of the University of Pennsylvania at New Bolton Center. PHF tends to be seasonal, occurring mainly in the hot summer months when aquatic species are most active.

The disease was named after the Potomac River Valley, where it was first recognized in 1979, but it can occur practically anywhere under the right conditions. “If we look at blood tests on horses all across the United States, many of them show evidence that they’ve been exposed,” Linton says. “Incidence and severity varies, possibly because some horses are more susceptible, and it may also depend on how much they ingest.”

•THE VECTOR: N. risticii infects a parasitic fluke (a type of flatworm). At different stages, infected flukes can be carried by freshwater snails and may be free-swimming in the water, where they can be consumed by larvae of aquatic insects like mayflies and caddisflies. When the adult flies later emerge, they carry N. risticii.

Although a horse can become infected with N. risticii by drinking contaminated water or picking up infected snails while grazing, the more common route is by ingesting mayflies, caddisflies and other aquatic insects. That’s why the risk of PHF is greater at barns that are located within a few miles of the natural waterways where N. risticii is present in the local fauna.

Nonetheless, PHF sometimes appears in unexpected places. “Caddisflies can be found in hay,” says William Gilsenan, VMD, DACVIM, of Rood and Riddle Equine Hospital in Lexington, Kentucky. “This is why we sometimes see cases of PHF in horses that are nowhere near a river. If the hay was grown near a river, it is possible that infected flies may be present in the hay. We don’t know how many flies a horse would have to ingest to induce disease. It may vary, with differing pathogenicity among the strains of this bacteria.”

• SIGNS: PHF can produce one or more of three distinct problems: a high fever, watery “pipestream” diarrhea and laminitis. “Scientists have not yet completely figured out how N. risticii causes terrible diarrhea and enterocolitis,” Linton says, “but once the horse ingests it, the pathogen infects a specific type of white blood cell [monocyte] and causes severe inflammation in the lining of the gastrointestinal tract. This leads to diarrhea and often laminitis.”

The first sign of PHF is high fever, which may reach 107 degrees Fahrenheit, accompanied by lack of appetite and depressed behavior. “There is often a two-stage fever,” says Nora Grenager, VMD, DACVIM, of Grenager Equine Consulting in northern Virginia. “The first fever spike might be brief and the owner may miss it because most people aren’t taking their horse’s temperature twice a day. Then up to a week later the horse has a second fever that is more severe.”

Within 24 to 48 hours after the severe fever begins, about 60 percent of horses develop loose stools or diarrhea that may be profuse and watery. At this stage, the horse may also appear colicky and become dehydrated.

However, the greater concern with PHF is laminitis: As many as 40 percent of affected horses develop potentially crippling inflammation of the connective tissues within the hooves. “Horses become dehydrated, lose protein through the inflamed intestinal wall, and develop endotoxemia—that is, when the intestine wall is damaged and inflamed, toxins that are usually contained within the gut can leak out into the bloodstream,” says Linton. The endotoxemia can trigger laminitis.

“Laminitis can be a very early sign, or it can occur even after you get the colitis and diarrhea under control,” says Linton. “The laminitis might occur a week later as the horse seems to be recovering. So even if you don’t see the laminitis early, the horse is not out of the woods yet. I always warn owners that even if their horse is improving, there is potential for laminitis, despite intensive treatment.”

• TREATMENT: The primary treatment for PHF is oxytetracycline, an inexpensive antibiotic that is effective against N. risticii, along with supportive care. “This includes intravenous (IV) fluids and medications to bind up the toxins in the intestine and decrease the diarrhea,” says Linton. “We also give IV medications to decrease the toxins’ effect on the bloodstream and reduce the potential impact of laminitis.”

In addition, icing the feet and lower limbs, along with other steps to prevent laminitis, is advisable. “We use anti-inflammatories and anti-endotoxic medications,” says Linton. “The treatment can become very involved so these horses are often hospitalized so we can keep up their fluids. Milder cases can be treated on the farm, but many horses with PHF need to be hospitalized.”

Horses with PHF usually respond quickly after treatment begins. “If the horse had a fever, it usually goes away within 24 hours after we’ve started him on oxytetracycline,” says Linton. “Often the response to treatment is enough to convince us that it is PHF.”

Now that veterinarians know what to look for and how to treat PHF, the survival rates are high—at least 70 percent of affected horses recover. Those that do not are often put down due to complications from laminitis. “The prognosis is pretty good,” says Grenager, “but the ones that get laminitis often have severe laminitis, which becomes a long-term management issue. This can even be a cause for euthanasia.”

PREVENTIVE MEASURES:

• Vaccinate. The vaccine against PHF is recommended by the American Association of Equine Practitioners (AAEP) for horses likely to be exposed to the disease. “The vaccine is typically administered twice yearly, in spring and fall, but this schedule varies geographically and is dependent upon recommendations of the veterinarian,” says Gilsenan. “In regions with a long hot season, it might be appropriate to vaccinate more frequently.”

• Reduce exposure to species that carry N. risticii. Turn off barn and outdoor lights that may attract mayflies and caddisflies, which are known to fly for miles to swarm under streetlights and other light sources. Research shows that horses stabled at the ends of aisles, near open doors and night-lights, are at a much higher risk of contracting PHF.

“There was one report of a show barn that was having problems with PHF even though it was miles from a river,” says Grenager. “They had a lot of big safety lights on at night and a number of the horses got PHF. After they turned off the lights at night, the PHF cases dropped off significantly. Many people like to have a security light left on at the barn, but you have to weigh in the risks for PHF.” If security is a concern, consider installing motion detectors on your exterior barn lights, so they will come on only when someone is present.

• Store hay indoors or under a cover.

• Keep horses at some distance from water sources. If your pastures contain natural ponds or streams, consider fencing them off; horses grazing too close to the water’s edge may pick up infected insects, or they may drink contaminated water.

• Take special precautions when insect populations are at their highest. Finally, be aware of when populations of local aquatic insects are likely to peak. In some areas, insects that carry PHF may hatch and emerge by the thousands to mate and die within hours. Your local agricultural extension office may be able to help warn you of impending hatches. At those times, you’ll want to take extra precautions to protect your horse, perhaps by keeping him inside with the lights off.

LEPTOSPIROSIS

A bacterial infection caused by Leptospira spp. spirochetes0, leptospirosis is found in many species of animals worldwide. The spirochetes are shed in the urine of infected animals, and horses may pick up the infection through physical contact with contaminated water or moist soil. “We think of it entering through the mouth or nose, but any broken skin can be a route of entry, and the leptospires can penetrate mucous membranes,” Grenager says. “If the horse has nicks and scrapes on his feet and legs, or scratches, and wades through water contaminated by urine, there could be risk. There are reports of horses getting lepto from infected urine/water splashing into the eyes.”

• THE VECTOR: Leptospira spp. spirochetes are subdivided into multiple species with hundreds of serovars, which are variants within a species that produce different surface antigens. Six different serovars affect horses in North America, but the one that causes the most equine disease is Leptospira interrogans serovar Pomona, usually shortened to L. pomona.

“Certain types are host-adapted to a specific species of animal. They are ‘normal’ in that species; those animals can carry and shed it, but it doesn’t cause illness. If that type gets into another species, it does cause infection and illness,” says Grenager. “[L. pomona] can be carried in other animals, particularly skunks. The skunk is one of the most common maintenance hosts for L. pomona in the horse’s environment, and the horse gets it by accident.”

But skunks aren’t the only potential source of L. pomona, which can also be carried by pigs, cattle, dogs, rodents and other domestic and wild species. “The horse is always an accidental infection,” says Grenager. “Skunks, rodents and other wild animals urinating in feed or water or in a barn stall can spread lepto to horses. Infected cattle or other farm animals can spread it as well, in a shared environment.”

• SIGNS: When a horse is first infected with L. pomona, the bacteria multiply and circulate in the bloodstream. As he develops antibodies, he may have a fever, go off his feed and be depressed and lethargic. These signs can be subtle and easy to miss. After the initial infection, most horses never develop further signs of illness. However, if the horse’s immune system doesn’t control the bacteria, they may colonize specific organs—usually the eyes, the kidneys or the uterus—producing signs weeks or months after the exposure.

In the eyes, leptospirosis causes equine recurrent uveitis (ERU), also called “moon blindness,” an autoimmune disease that causes repeated inflammation of the uveal tract, the middle layer of the wall of the eyeball. “We often see recurrent uveitis and immune-mediated keratitis—eye diseases which are commonly associated with lepto,” says Linton. There seems to be a genetic component to susceptibility to ERU—the disease is more prevalent in Appaloosas than in other breeds.

When it affects the uterus of a pregnant mare, leptospirosis can cause abortion late in gestation. “And since lepto abortions are likely underreported—meaning a definitive diagnosis was not made—we probably underestimate how significant a problem this is,” says Grenager. In some cases, the foal may be born alive but very sick with leptospirosis. “The fetal fluids and placental fluids are infective,” says Linton. “If another horse comes into contact with these fluids this is another way the disease can be transmitted.”

Kidney disease from leptospirosis isn’t common in horses, but some do develop acute renal failure. A kidney infection may result in shedding of the pathogen via urine. “Even after the infection is under control, the horse may continue to shed bacteria for several weeks,” Linton says. “Since the bacteria can be passed in the urine, it’s important to isolate any horse with an active kidney infection.”

• TREATMENT: If the leptospirosis is still in the acute stage of infection, your veterinarian will treat it with the appropriate antibiotic. “There are different antibiotic recommendations based on the body system that the bacteria happens to be affecting,” says Gilsenan. “Your veterinarian will recommend the appropriate treatment.”

Likewise, other treatments will depend on the body system affected. For example, says Grenager, “a horse with kidney failure will also need supportive care and lots of fluids.” Treatment cannot help a mare who has already aborted, but affected foals born alive may respond to antibiotics. In both cases, the horses may need to be kept isolated for up to 16 weeks with periodic tests to make sure they are not shedding bacteria in their urine.

ERU is difficult to treat effectively because it is more than just a simple bacterial infection—the disease, which can appear two to eight months after the initial leptospirosis infection, is an autoimmune disorder. Although how Leptospira organisms cause ERU isn’t fully understood, it appears that the proteins on L. pomona are similar enough to proteins found in the tissues within the eye to cross-react with the antibodies. In other words, the anti-bodies created to fight the pathogen end up attacking the body’s own tissues as well.

“One of the things that complicates therapy is that often it’s not simply the organism that causes a problem but rather the animal’s immune response to the organism, such as when an eye is damaged,” says Gilsenan. “Part of the treatment involves quieting down the inflammation in the eye with some kind of corticosteroid, and another component is using an antibiotic that will penetrate into the eye.”

PREVENTIVE MEASURES:

• Vaccinate. In late 2015, Zoetis introduced the first vaccine against leptospirosis approved for use in horses. “The new vaccine has only been out for a short time, but Zoetis did rigorous safety trials and an efficacy trial that showed it works,” says Grenager. “It is labeled as effective against L. pomona.”

In controlled trials, the vaccine prevented leptospiremia—the presence of the bacteria in the bloodstream—after initial exposure. It is believed that reducing the infection in the bloodstream reduces the risk of the complications associated with leptospirosis—ERU, kidney disease or abortion.

• Adjust management practices to minimize exposure. “It is better to not feed hay on the ground. It also helps if you can minimize areas of standing water, which is helpful in preventing many other problems as well,” says Grenager. Keep water troughs and buckets clean, and fence off access to ponds, marshy areas and other natural water sources. “If a mare aborts, you need to test for lepto and if the test is positive, treat and isolate her until proven that she is no longer shedding bacteria,” Grenager says. “All the aborted tissues should be removed and the area disinfected appropriately.”

SALMONELLOSIS

A common cause of diarrhea in mature horses, salmonellosis is caused by Salmonella enterica bacteria. The most frequent method of transmission for salmonella is fecal-to-oral, meaning that the bacteria are shed in manure, then consumed with contaminated feed or water. Strictly speaking, salmonellosis isn’t solely a water-borne disease, but it is passed easily through contaminated water—both in unclean buckets and troughs as well as natural ponds and streams.

“Salmonella can be readily transmitted via water or human hands and boots, tools, hay carts, etc.,” says Grenager. “The other challenging factor is that horses can look healthy and still be shedding the pathogen in their manure, especially when they are stressed. This is probably also true of the other animals that periodically shed it—and then it gets into the water supply.”

• THE VECTOR: S. enterica is divided into six subspecies and more than 2,600 serovars, some of which are normally found in warm-blooded animals and some in cold-blooded animals. But all are zoonotic—meaning that they can cause disease in people as well as animals. In horses, the two most common forms that cause diseases are S. enterica serovar Typhimurium and S. enterica serovar Agona.

“There are thousands of kinds of salmonella and different kinds infect different species of animals,” Grenager says. “Almost any animal can get salmonella, and it can be a serious disease in horses—foals and adults.” Salmonella is common around large livestock operations, and the bacteria can survive in wet environments for months. Many forms of salmonella are becoming resistant to antibiotics.

• SIGNS: Once ingested, salmonella bacteria penetrate the cells in the wall of the intestine and multiply, causing damage that leads to diarrhea. In some cases, the bacteria may get into the bloodstream (septicemia), and infections may develop in other parts of the body. “Foals can become bacteremic or septicemic with infection throughout the body and then localizing in joints and other organs, which can be devastating,” Grenager says. Salmonellosis generally takes three forms in adult horses:

• A subclinical carrier shows no signs of illness but may shed the bacteria in his manure, which can then spread to others. “There are some other species of animals that can harbor salmonella for long periods of time, but in most cases horses don’t become long-term carriers,” says Grenager. “They generally have the disease and then get over it, rather than being infected for life, but they can be silent shedders for some time, which are a big risk for other horses.”

• A horse with mild illness might have a fever and a low white cell count, and his manure may become soft but not watery. “They may never actually break with diarrhea,” Grenager says.

• A horse with acute disease starts with fever, depression, loss of appetite and abdominal pain. Severe, watery diarrhea develops within a day. Fluid and electrolyte losses and other sudden physiological changes can rapidly progress to shock. If not treated rapidly and aggressively, the disease can be fatal.Stress plays a large role in the development of salmonellosis—horses who are sick with other diseases or who have recently traveled, undergone surgery or been subject to other stressors are more likely to become seriously ill. For that reason, university hospitals and large veterinary clinics are always on guard against outbreaks. “A horse may come in for some other reason and doesn’t really look sick or isn’t showing any signs but is shedding it and spreads the bacteria,” Grenager says. “Now many hospitals have a protocol where you have to demonstrate that a horse with suspicious clinical signs does not have salmonella before it can enter the main hospital.”

• TREATMENT: Horses with severe diarrhea need supportive care—those with the acute form of the disease will require large volumes of intravenous fluids, possibly with dissolved or oral electrolytes, along with anti-inflammatory treatments. “They also need treatment aimed at laminitis prevention, with cold therapy for their feet and deep, soft bedding,” Grenager says.Foals with septic salmonella infections are often treated with antibiotics, but due to resistance issues, using these drugs in adult horses is controversial. “We want to be careful about giving adult horses antibiotics for salmonella because there are strains of the bacteria that are becoming resistant to multiple types of these drugs,” says Grenager. “Some veterinarians use antibiotics for every case because they don’t want those horses to become sicker, and others never prescribe antibiotics for salmonella in adult horses. I try to assess it case by case. If the horse is really sick with a very low white blood cell count, and it is going to be difficult for him to fight it, we might use antibiotics to save the horse, but it’s not something I do routinely.”

PREVENTIVE MEASURES:

• Practice good hygiene. Because there’s no salmonella vaccine for horses, biosecurity measures are the first line of defense against salmonellosis. That means scrubbing feed tubs and water buckets periodically and taking other steps to make sure your horse’s feed and water don’t get contaminated. 

Good hygiene is especially important if a horse on your property contracts salmonellosis: Either wear a protective coverall or change out of your barn clothes immediately after caring for the sick horse, and wash your hands with soap and water or use a skin sanitizer as soon as you leave the stall. Once he recovers, replace all the bedding and disinfect the walls and floor. “Salmonella can be killed with disinfectants; there are many good resources on how to disinfect stalls. You can use detergents to clean all surfaces and then let them completely dry,” says Grenager. “Diluted bleach and some other disinfectant compounds can be used.” You’ll also need to disinfect manure buckets, grooming brushes and other tools used to care for the horse while he was sick.

• Keep rodents out of your barn. “Mice can be a problem, spreading salmonella around a facility,” says Grenager. “I remember a situation where people had gotten rid of the barn cats because they were worried the cats were spreading salmonella, but then the mouse population exploded and the situation was even worse. Cats are very clean, compared with mice! They had to bring the cats back to keep mice from spreading this disease.”

• Isolate ill horses and potential shedders. If you have a horse who is sick with salmonella or who is a known shedder, take steps to keep him isolated from other horses until the danger has passed. “Use separate boots or boot covers when you are in that stall, wear gloves, etc.,” says Grenager. “Make sure no other people or animals go in and out of that stall.”

Splashing in cold water is a great way to cool off on a hot summer day, for people and horses alike. And there’s no reason not to enjoy your local rivers and lakes—as long as you take the right precautions, you’ll be able to have your summer fun and keep your horse safe and healthy, too.

This article first appeared in EQUUS issue #466, July 2016.