Complicated Case of Osteomyelitis - The Horse Owner's Resource

Complicated Case of Osteomyelitis

When two potentially deadly infections strike a young filly simultaneously, her veterinarians rush to sort through her confusing signs to devise a treatment plan.
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A bone infection in the vertebrae of her neck caused Star such intense pain that she could barely move. Photo © EQUUS

A bone infection in the vertebrae of her neck caused Star such intense pain that she could barely move. Photo © EQUUS

On any sunny spring day, as late afternoon turned into early evening, you'd likely find Superstar waiting by the pasture gate to come in for dinner. The then-2-year-old warmblood filly was friendly with her two pasturemates on the private farm in Potomac, Maryland, where she lived, but she wasn't shy about claiming her place in line at mealtimes. So when barn assistant manager Stephanie Bennett didn't see Star by the gate one evening in May of 2012, she knew something was wrong.

"I immediately went looking and found her in the run-in shed," says Bennett. Star was standing in the far corner of the shelter, with her head?lowered and facing the wall. She?didn't react when Bennett called her name. Worried, Bennett carefully grasped the filly's halter and asked?her to turn away from the wall. Star wobbled slightly but didn't move. Bennett attached a lead and pulled?gently to attempt to turn her head. That's when Star fell over.

The filly tried to get to her feet, but couldn't. "She was struggling but?wasn't panicky," says Bennett. "She's never been the panicky type." Bennett ran to get help from the other barn manager, Jack Louth. By the time they returned, Star was on her feet. But something was seriously wrong.

"She wasn't standing on her left front leg," says Bennett. "She was resting the hoof on the toe but not bearing any weight on it." Star's left shoulder and elbow were "dropped," causing the limb to hang. In addition to her strange stance, the filly seemed lethargic, standing with her head lowered and a disinterested expression. Bennett stayed by the filly's side, comforting her, while an emergency call was made to the veterinarian.

Puzzling clues
The physical examination the veterinarian performed on the farm turned up some new clues that only added to the confusion. Star's left foreleg showed no signs of swelling or trauma, but she continued to rest it on the toe. When the veterinarian placed the hoof flat, however, Star was able to bear weight and walk normally for a single step. If he placed her foot flat for her again, she'd bear weight to take another step. The limb seemed to function perfectly well, but Star was unwilling or unable to use it. Suspecting an elbow fracture, the veterinarian took a series of radiographs of the area. All of the images were normal.

Star's neck also seemed to be extremely stiff, but there were no signs of trauma. She did have a fever of 102 degrees Fahrenheit, which undoubtedly contributed to her lethargy and sluggish demeanor. Interestingly, Star had also had a fever just 10 days earlier, but that one had passed with no other signs of illness.

The veterinarian suspected Star might have an abscess in her left hoof. He gave her an injection of antibiotics and prescribed a course of additional oral antibiotics for the coming days. Bennett also began a routine of soaking the hoof and paring away at the sole?to try to uncover a pocket of infection and pus. After two days of treatment,?no abscess had turned up, and Star?was no better.

"It was clear there was something really strange going on," says Bennett. "We were still having to literally?move her limb each step to get her?to walk. She was also still carrying
her head very low. I'd never seen?anything like it." Stumped, Star's?veterinarian referred her to the?Marion duPont Scott Equine Medical Center in Leesburg, Virginia, for a more in-depth evaluation.

"She definitely had some confusing signs," says Harold McKenzie, DVM, who examined Star when she arrived at the clinic. "I wasn't as worried about her fever---all her other vital signs were good---as I was about the way she was moving. A dropped elbow like she had is normally a very specific sign of neurological problems. But the other signs that typically accompany that weren't there. Usually, a neurological case can't advance or bear weight on an affected limb, but she could. The puzzle was that she just didn't want to."

A typical neurological exam wasn't an option for Star. "In order to perform a neurological exam, we normally back them up, trot them in a circle and spin them around in both directions to see how they move," says McKenzie. "We couldn't do any of that with her. She wasn't willing to move at all."

But even without that diagnostic tool, McKenzie suspected that there was something else at work: "The lowered head and neck in combination with the neck stiffness weren't typical of a neurological case. I was pretty sure something else was going on; we just had to figure out what it was."

McKenzie took a closer look at Star's neck and found that pressing on the left side seemed to cause her pain. To investigate further, he ordered a series of ultrasound and radiographic images of that area.

"With the ultrasound we could see an area of heat and inflammation in the soft tissue, but it was possible that these changes were simply associated with the intramuscular injections she had been given two days before, rather than being the primary cause of her pain," McKenzie says. The radiographs, however, were much more telling: "When we looked at the x-rays, we could see immediately that a significant amount of bone had been eaten away in one of the vertebrae in that area."

The changes in Star's sixth cervical vertebra weren't like those from a traumatic injury, nor did they look like those seen in chronic conditions such as arthritis. "What we saw was unusual: A young horse with dramatic bone remodeling and loss in a very central area of the vertebra," says McKenzie. "And it had obviously happened quickly; she developed her signs within a few days."

That information, combined with the inflammation seen in the ultrasound and her history of fever, added up to an unusual and very troubling diagnosis: Star had osteomyelitis, a serious and life-threatening bone infection.

Bad for the bone
"Our bodies are teeming with bacteria, but our immune system does a good job of keeping [pathogens] in check," explains McKenzie. "Sometimes, however, for whatever reason, bacteria can colonize a site they aren't supposed to and just wreak havoc."

In Star's case, bacteria had penetrated into her sixth cervical vertebra, toward the base of her neck, and the resulting infection was quickly eroding the bone. Her body's inflammatory response was causing swelling and pain that kept the filly from turning her neck and left her reluctant to move and bear weight on her left foreleg. "It was an unusual collection of signs, but once we found the spot in her vertebra, it all began to fit together," says McKenzie.

Osteomyelitis is more common in foals than in grown horses. "Foals seem to be more susceptible to blood-to-bone infections in general because their immune systems are still getting up and running," says McKenzie. "When it happens in an adult horse, it's usually from an infection in soft tissue next to the bone moving over, such as you might find after a traumatic injury, but many times you'll never know why the bacteria had an opportunity to invade."?Osteomyelitis is rare in mature?horses; a large clinic might see only?five to 10 cases a year, he says.

Osteomyelitis can develop in?any bone, but Star's case was especially dire because the infection was in a?vertebra. "The damage is done very rapidly, literally within days, and by the time we'd figured it out, the bacteria had a head start," says McKenzie. "There was a very real possibility that if we didn't get it under control quickly, the vertebra would be weakened so much [the bone] would crack, damaging the spinal cord contained within the bone, and she'd be paralyzed."

The veterinary team immediately began a very aggressive treatment regimen, with large doses of intravenous broad-spectrum antibiotics, including ceftiofur and gentamicin, as well as oral metronidazole. Star was also started on phenylbutazone to control pain and inflammation and omeprazole to guard against ulcers. "It is possible in some cases of osteomyelitis to do surgery to clean out the infection," says McKenzie. "If there is enough healthy bone around the infection, you can actually drill it out. But in this case, that wasn't an?option. It was too risky."

Star would have to remain at the clinic for some time. "We knew we faced a long, involved and uncertain treatment," says Bennett, "but fortunately her owners were dedicated to helping her."

The filly was monitored continu-ously, and the clinical team hoped to see a rapid improvement once the?antibiotic therapy began. But 48 hours later, Star's fever persisted, she was
still unwilling to turn her head or?walk, and she continued to keep her left foreleg propped up on its toe. "We weren't seeing the response we had hoped for," says McKenzie, "and I was very, very concerned."

One possibility was that Star had a second area of infection the team hadn't discovered. To look for it, they performed a bone scan, called nuclear scintigraphy. For this procedure, a horse is injected with a radioactive tracer that is attracted to, and drawn into, metabolically active bone cells. When the radiation is detected and recorded with a specialized camera, any regions of increased metabolic activity resulting from injury or disease will show up as a distinct "hot spot" on the image. The good news was that Star's scans confirmed that there was only the one site of infection, but it was still a mystery why she wasn't improving.

McKenzie decided to perform a?spinal tap---to extract a sample of the protective fluid from the space surrounding her spinal cord---in order to test for white blood cells and bacteria within her central nervous system. "While we were not able to culture any bacteria from the sample, we did find dramatically elevated white blood cells, which is indicative of inflammation?and likely infection," he says. So, in?addition to osteomyelitis, Star had meningitis, a potentially deadly infection?of the nervous system.

Which came first---the osteomyelitis or the meningitis---wasn't clear. "Seeing as we had infected bone sitting right next to the spinal cord, we suspect [the osteomyelitis] developed first," says McKenzie. But it also didn't really matter at this point; Star was getting sicker by the hour.

A change in tactics
With this new information, McKenzie reconsidered Star's treatment plan: "We needed to change the antibiotics we were using. We had been focused on drugs that penetrated bone well, but we needed ones that could also reach the central nervous system." Star was switched to oral chloramphenicol and rifampin, two less common but powerful antibiotics that are very effective at reaching the central nervous system as well as penetrating into bone.

Star began to improve the very next day. "Once we changed the antibiotics, she really turned a corner and steadily improved," says McKenzie. "Her fever broke and didn't return, and she began moving around more on her own. She was still a sick filly with a very sore neck, but each day she looked a bit more comfortable. We were still very cautious in our prognosis but getting more hopeful."

After six days in the clinic on the new antibiotics, Star had improved enough to go home. Bennett and Louth would have to keep up an intensive medication schedule, however, with doses coming as frequently as every six hours. "We also told them to watch the filly closely, take her temperature a few times a day, and call us immediately if she showed any signs of relapsing," says McKenzie. "It was possible that there was a pocket of bacteria we weren't reaching that could flourish if given the chance."

The aftercare took a team effort, and Bennett and Louth spent many hours monitoring the filly until her health?returned. Bennett devised an elaborate record-keeping system to track Star's medication, temperature and vital signs. "In the beginning we were?checking her every four hours, then eventually we felt OK with every six," she says. The filly was kept on stall rest with hand-walking twice a day for the first two weeks, then turned out in a small paddock as her pain subsided,?her energy levels rose and her movement became more normal. Her fever never returned.

Star was returned to the clinic for recheck examinations on a weekly basis at first, with the intervals increasing with improvement in her condition. After one month she was moving her neck and left leg normally, with no signs of pain or fever. "We did see a bit of weakness and asymmetry in her movement, which may have been a result of the amount of inflammation she'd had around her spinal cord," says McKenzie. Another series of radiographs showed that the inflammation was completely resolved and the bone in her vertebra was filling back in quickly, restoring strength in the affected area.

After another month of antibiotic treatments at home, Star was brought back to the clinic for a final check. McKenzie found her even more improved and nearly back to normal. "One of the pleasures of internal medicine is seeing the body heal itself," he says. "We don't get to cut and sew things back together in a single day, like surgeons do, but we can find the right medications to give the body the running start it needs to take care of things on its own, and that's very cool."

There's no way to know exactly how Star contracted osteomyelitis and meningitis, says McKenzie: "It's possible her immune system was somewhat compromised at the time she got sick. Maybe that fever she had the week before was somehow involved, but it's impossible to say. Ultimately she was able to clear the infections, and her final blood work showed a normal immune system, so there's no reason to think this will ever happen to her again."

Star hasn't suffered any long-term effects from her ordeal and has recently been started under saddle. "I'm still in the habit of watching her very closely," says Bennett, "but she really hasn't looked back. She's a beautiful mover with a very sensible mind. I think she's going to grow up to be a really nice addition to our current string."