Bad to the Bone

It began with a common paddock accident--crashing into a rail--and progressed into a bone infection. After several months of nursing and repeated surgeries, veterinary efforts failed to save the life of Alywow, a former Canadian Horse of the Year and million-dollar-plus Thoroughbred broodmare.

While bone infections don't automatically end with euthanasia, they can be difficult to treat. That's because most bone infections occur in the lower limbs, where blood flow under normal conditions is minimal, and it can be even less during active infection. As a result, the infection is harder to control.

Nicola C. Cribb, MA, VetMB, MRCVS, a resident in large animal surgery working with Ludovic Bouré, DVM, MSc, DES, Dipl. ACVS, ECVS, associate professor of large animal surgery at Ontario Veterinary College at the University of Guelph, details what happens when a bone in the lower limb becomes injured and infected:

"Lower limb infections occur when bacteria colonize bone or synovial structures, including joints, tendon sheaths, and bursa. An acute inflammation occurs following bacterial inoculation, which results in disruption of the blood supply to the affected tissue. Because joint, tendon, and bony structures in the lower limb of the normal horse are all minimally vascularized (supplied with blood vessels), this disruption of the already nominal blood supply results in protection of bacteria not only from the natural defense mechanisms of the horse, but from antibiotics that need to be carried to the site of infection in the bloodstream."

Additionally, Cribb says that as part of the inflammatory response, the body can produce a substance called fibrin (which forms an essential part of blood clots) and an accumulation of fluid in the tissues called a seroma. In fibrin and fluids, ¬microbes are protected from the horse's immune system and antimicrobials.

"It is very difficult for the infection- fighting blood cells to penetrate these infected areas, making them ideal places for the bacteria to hide from any attempts to eradicate them," explains Cribb. "As a result, treatment of lower limb infections is problematic, and the prognosis for return to athletic soundness is guarded."

Whys and Wherefores

Bone infections occur when bacteria is introduced through the bloodstream, traumatic injuries, or following surgery or joint injections.

"Adult horses usually develop bone infection subsequent to traumatic incidents involving fractures, lacerations, and puncture wounds," says Rustin M. Moore, DVM, PhD, Dipl. ACVS, professor of equine surgery and director of the Equine Health Studies Program, School of Veterinary Medicine, Louisiana State University. "Horses with long bone fractures, especially if they are open fractures (skin penetrated or broken during or after fracture), are predisposed to develop bone infection. Long bone fractures often require internal fixation with plates and screws or other materials, making the area prone to infection even if the primary incident did not lead to an open fracture. This is often associated with long surgical procedures, damaged/injured soft tissues, and the presence of implants."

The cannon, splint, pastern, and coffin bones are most commonly affected, says Moore, as the lower limbs are more prone to injury due to their location and sparse tissue coverage of the bones in these areas. Adult horses can experience either osteitis (infection of the outer cortex) or osteomyelitis (infection involving both the cortex and medullary cavity inside the bone shaft where the marrow is stored).

"Foals usually develop bone infection secondary to bacteremia or septicemia (systemic infections spread through the blood), which are relatively common in neonatal foals that have failure or partial failure of passive transfer from colostral antibodies," states Moore. "Bacteria can also enter the body shortly after birth through the umbilicus or by ingestion or inhalation. These environmental pathogens then circulate in the foal's bloodstream and localize in joints and bones."

The most common locations for bone infections in foals, Moore says, are in the ends of long bones, including the physis (growth plate), epiphysis (the rounded end of the long bone), and metaphysis (the area between the growth plate and the diaphysis or shaft of the bone), but any bone can be involved since the infectious agent reaches the bone via the bloodstream.

"However, the distal radius, distal third metacarpus/metatarsus, distal tibia, and distal femur are frequently affected," Moore notes.

Foals have a much greater tendency to develop osteomyelitis.

Clinical Signs and Diagnosis

"The hallmark sign of bone infection is pain or lameness, ranging from mild to non-weight bearing," Moore reports. "If the infection is severe or is located in strategic areas, then usually the associated pain is often quite marked. There can also be localized swelling, heat, and soreness to pressure. The animal may have a fever and may be depressed, lethargic, and/or inappetent (loss of appetite). Some horses, especially foals, have increased white blood cell (neutrophil) counts and increased fibrinogen levels in their blood. But often, bone infections in foals may not become clinically apparent for a few to several weeks, particularly if it is a well-defined abscess."

Diagnosis is based on visual exam and various imaging techniques--radiography, digital radiography, nuclear scintigraphy, ultrasonography, computed tomography, and magnetic resonance imaging. States Moore, "Radiographically, bony infection appears as lucency/lysis (decreased mineral content), which often takes 10 to 14 days after becoming infected to become apparent. Greater detailed radiographs such as those obtained with digital radiography and/or MRI or CT may be able to detect subtle differences sooner. Nuclear scintigraphy (bone scan) is more sensitive at detecting bony turnover, so this may be useful to detect areas of possible infection if a definitive diagnosis cannot be made with radiography."

Treatment

For a successful outcome, bone infection needs to be treated early and aggressively with bactericidal antibiotics and, in some cases, surgery.

Antibiotics Moore sug gests using a broad- spectrum bactericidal antibiotic based on the expected offending bacteria, then maintaining or adjusting the drug(s) according to the horse's response and the results of culture and susceptibility testing. He notes, "Most bony infections involve either aerobic or anaerobic bacteria, and without the selection and use of appropriate antibiotics, most established bony infections cannot be completely resolved."

However, antibiotic therapy can be expensive, carries the potential of leading to antibiotic resistance by the bacteria and--depending on the antibiotic--can cause toxic side effects such as kidney damage or diarrhea.

Often, a combination of systemic and locally administered antibiotics is required to achieve success. Systemically administrated drugs are delivered intravenously, intramuscularly, or orally, traveling through the bloodstream to the site of infection.

Cribb explains, "In the normal horse a very high dosage of antibiotics needs to be given systemically to achieve therapeutic concentrations in the structures of the lower limb. The dose required is often so high it could be toxic to the horse and produce unwanted side effects. When an infection is present, the disrupted blood supply makes it even less likely that appropriate levels of antibiotics will be achieved when safe levels of antibiotics are administered."

To overcome these obstacles, antibiotics are placed at the infection site. "With localized antibiotics, systemic side effects and toxic effects are minimal," reports Cribb. "Delivery is achieved by either placing a tourniquet on the leg and perfusing the tissues of the lower leg or the medullary cavity of the bone with antibiotics, or by placing antibiotic-releasing implants next to the infected tissues."

Moore says local antibiotic perfusion of the limb while using a tourniquet placed above the affected area allows more time for the antibiotics to remain in the vascular bed in the area of the bone infection, thereby providing a longer time for its diffusion into the affected tissues.

Perfusion is usually performed daily until the infection has resolved. It can be time-consuming, technically challenging, traumatic to the limb, and costly, Cribb notes.

Delivering increased concentrations of antibiotics via implanted materials allows for a slow, continued release of antibiotics in the affected area. "However, the implants readily available and in clinical use in North America are non-absorbable, meaning that often a second surgery is required for their removal," Cribb says. "Complications can occur any time a horse is operated on, so the requirement for a second surgery is an obvious disadvantage."

Surgical options "Surgery of lower limb infections differs depending on the structure involved," says Cribb. "Regardless, the principles of treating the infection remain the same and include debridement and lavage to remove unhealthy tissue, bacteria, and harmful inflammatory mediators, and antibiotherapy to eliminate all remaining deep-seated bacterial infection."

Additional Help

Other options are employed to help defeat an infection. Draining the affected area to remove inflammatory exudates (pus) is often helpful and necessary, says Moore. "This can either be achieved by lavage (flushing) using needles or the arthroscope, by open drainage, or by the use of closed-suction drainage systems. Non- steroidal anti-inflammatory medications for pain and inflammation, physical therapy, bandaging, and splinting/casting are also utilized to control pain and inflammation. Although probably not used that often in horses or very often necessary, hyperbaric oxygen therapy may be useful for chronic, resistant infections, particularly those involving anaerobic bacteria."

Follow-up care depends on the location, nature, and severity of the infection. Says Moore, "If the infection is in or near a joint, then passive range of motion exercise or light, controlled exercise once the acute inflammation subsides and the lameness improves is important to help maintain range of motion and prevent scarring and fibrosis of the joint capsule and other tissues that may limit future movement and soundness. If the infection involves a bone, then exercise should be restricted until the infection is under control and the mineral (calcium) is replenished to prevent a pathologic fracture. If the infection involves a bone that is fractured, then exercise should be completely curtailed until the fracture is healed. Fractures can heal in the presence of infection, but only if rigid fixation can be maintained. It is often a race against time to get the fracture to heal before the implants--screws or plates--become loose secondary to lysis/softening of the bone subsequent to the bacterial infection."

Prognosis

Moore explains, "The prognosis varies, depending upon the location, severity, and duration of the infection. Adult horses with joint infections not involving the bone reportedly have an 85-92% chance for discharge from the hospital and a 57-65% chance of returning to athletic function. Adult horses with infected orthopedic implants (fractures repaired with plates and screws) reportedly have a 53-63% prognosis for hospital discharge. Although there is no specific data, generally if horses survive to hospital discharge, they should be able to resume their career or at least an alternate useful purpose.

"In adult horses, the prognosis often depends on the location, severity, structures involved, and in the case of a fracture repair, whether the infection can be controlled while the fracture is healing and whether this occurs before the implants loosen, since a fracture will not heal in the presence of infection if there is instability," Moore adds.

The prognosis for foals is generally substantially worse (about 45%) than for adult horses. Moore says in foals, prognosis depends on the number of bones or joints involved, the severity of infection at the time of diagnosis and treatment, the presence of systemic illness with a specific area of infection, whether or not the infection involves the joint and/or physis, and the susceptibility or resistance of the causative pathogen to antibiotics.

Tomorrow's Treatment?

Because of the problems associated to antibiotic therapies, Bouré has been investigating a bioabsorbable implant that is placed at the site of infection and releases antibiotic slowly over time. Bouré says he and his colleagues combined amikacin (an antibiotic effective against most common equine pathogens involved in orthopedic infection) with hyaluronan (a natural constituent of the joint containing analgesic, anti-inflammatory, and cartilage- protecting effects) to form a gel. That is dried until a 1cm disc is formed. When arthroscopic lavage is used to remove inflammatory mediators and bacteria in the joint fluid and remove any fibrin in which the bacteria are sequestered, this implant could be placed in the joint. The disc rehydrates and breaks down, releasing the amikacin. The hyaluronan is naturally degraded, eliminating the need for disc removal.

Bouré plans to primarily use the implant for joint infections. "However, the implant could be appropriate for use in any lower limb infections, including infected tendon sheaths, bursae, and bone infections," he says.

Although a study has been completed, Bouré says the implant is not ready for clinical use. Further research and modification is needed to provide better results.

Prevention

Good horsekeeping practices can reduce the risk of bone infection by eliminating sources of injury and contamination. "Remove all sharp objects that horses could either step on or become entangled with," suggests Moore. "A veterinarian should examine any horse that has sustained trauma to make sure there is no fracture or penetrating wound to a bone or joint.

For foals, "Provide the mare with a clean, non-contaminated environment in which to foal," he says. "Make sure the foal stands and nurses early and often during the first few hours to maximize the amount of colostrum (containing protective antibodies) that is ingested. Measure the foal's blood immunoglobulin levels within the first 24 hours to confirm adequate passive transfer; if levels are insufficient, administer immunoglobulin-rich plasma.

"Careful observation and monitoring of foals for any signs of infection (depression, malaise, poor appetite, lethargy, fever, etc.) and any evidence of swelling of joints or umbilicus, coughing, diarrhea, or lameness is important and should be brought to the attention of your veterinarian immediately," he adds.

Take-Home Message

Bone infection can be a grave, sometimes life-ending condition that might not be overcome despite treatment efforts. Take the diagnosis of bone or joint infection seriously and initiate aggressive veterinary action to achieve a successful outcome.

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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