Septic arthritis and osteomyelitis in the neonatal foal (Proceedings)

Septic arthritis and osteomyelitis are recognized sequela to bacteremia/sepsis in the neonatal foal. It can present in the newborn foal that shows the progression of signs we saw in the last presentation of septicemia in the foal or it can develop in the absence of signs of septic shock and occur when the foal is between 2-4 weeks of age.  The mean age of presentation for this problem is around 8 days. In one study approximately 25% of foals presenting to a referral hospital with sepsis score >11, presented with or developed septic orthopedic disease.  Cohen et al stated that septic arthritis was the cause of death is 12.5% of foals (8-31 day of age) on farms.

 Blood supply to the joint of the young foal is supplied through a main arteriole that branches into the synovial membrane and the epiphysis. In the newborn foal the blood supply of the epiphysis also supplies the metaphysis through transphyseal vessels.   Hematogenous bacteria can settle in any of these three areas creating septic foci. Sluggish blood flow and decreased oxygen tension may encourage the proliferation of bacterial growth at the chondo-osseous junctions. The types of lesions in the foal have been described by Firth according to their location in joint or bone. 

Synovial lesions (S-type) are located within the joint and involve the synovial structures only, no bone involvement.   Epiphyseal lesions (E-type) occur in the subchondral bone adjacent to the articular cartilage.  Physeal infection (P-type) involves bone on the metaphyseal side of the growth plate.  A fourth type of osteomyelitis in young foals involves the small tarsal bones (T-type). 

There is usually a history of complete or partial failure of passive transfer of maternal antibodies (FPT).  Foals with FTP were 1.7-1.9 times more likely to develop septic arthritis than foals that received adequate colostrum. Owners will often report that the foal must have been stepped on by the mare, assuming that trauma was the most likely cause of “sudden” lameness.  They may also report that the foal seems to be laying down more than other foals.

Septic orthopedic disease (SOD) should be assumed in any lame foal until proven otherwise. The future soundness of the foal depends on the early recognition and treatment of the septic process. Besides lameness, clinical signs may consist of swollen joints or edema over the region of the growth plates.  Careful palpation of the joints and growth plates help to identify the affect area or areas.  More than one joint may be involved. In our clinic, the stifle – is the most common, followed by hock, carpus, fetlock, shoulder and hip.

Diagnostic tests

A complete blood count (CBC) will reflect the state of sepsis the foal.  The newborn foal with clinical signs of septicemia as well as SOD may have a neutropenia with a left shift.  Foals that are somewhat older and less systemically involved may have a normal to elevated white blood cell count. Fibrinogen levels are often elevated.

The Gold standard diagnostic in defining septic arthritis is the arthrocentesis.  Arthrocentesis of a septic joint is usually straight forward because of the joint distension.  The joint should be surgically prepared and a needle directed into the joint. Stall side evaluation of the synovial fluid can be done.  Normal synovial fluid should be clear yellow in color with a high level of viscosity. Viscosity can be demonstrated by placing a drop of fluid between two fingers and pulling them apart.  Normal synovial fluid should be “stringy” between the two fingers.

The total protein of the normal joint should be less than 2 mg/ml. Fluid from a septic joint may be cloudy in appearance, total protein may exceed 3 mg/dl and there is little to no viscosity. Hyaluronan in the synovial fluid is responsible for the viscosity of the fluid.  In a septic joint the hyaluronan is depolymerized by enzymes released by the destruction of white blood cells in infected joints.  The normal cell count in synovial fluid is < 200 WBC/mm3 with a less than 25% neutrophil count.  The septic synovial fluid may have a WBC count from a few to a hundred thousand cell that are >90% neutrophils.

Cultures of blood and synovial fluid are helpful in the identification of the offending bacteria.  It is important to do both because different bacteria may grow from each culture and synovial fluid cultures are often negative.  The clinician can increase the yield of bacteria from synovial fluid by placing the sample to be cultured into blood culture media and incubation for 24 hours before plating out for growth.

Diagnostic imaging is needed to determine whether osteomyelitis is present.  Radiographic examination has been the primary imaging modality used to determine if osteomyelitis is a component of the septic process. Unfortunately radiographic imaging is hampered by several factors. Bone lysis which is indicative of osteomyelitis often lags behind the actual bone infection.

Lytic changes don't appear until 30-50% of bone mineral is removed. This may take several days to weeks.  Thus serial radiographs may need to be done every 4-5 days to insured that the bone is not involved. A second disadvantage to radiographs is the overlap of structures which can decrease the ability to see more subtle lesions.  The advantage of the radiograph is that most clinicians have the ability to take them.  With the new DR machines, foals can be screened on the farm.  If a lytic area is identified it can be treated immediately.

In the more recent studies, computed tomography (CT) has been used to diagnose osteomyelitis in horses.  It is felt that it is a more sensitive modality and may pick up lesions earlier in the disease than radiographs.  CT eliminates the overlapping structures and more clearly defines the boney lesion. Another advantage of the CT is that you can investigate more joints quickly and see structure in areas that are difficult to radiograph such as hip joints. Disadvantages of the CT are that it is located at a tertiary referral clinic and requires anesthesia.  Cost of the CT may also be a drawback but should be weighed against the need to heavily sedate or anesthetize the foal anyway for therapy.

Treatment

Treatment goals for the foal with SOD are similar to that of the septic foal – immunologic support in the form of colostrum or plasma transfusion, broad spectrum antibiotic coverage, cardiovascular support with intravenous fluids and nutritional support if the foal is not eating.  In addition to these goals one needs to add the goal of joint health.

As previously discussed septic joints have an increase in joint effusion and an influx of WBC into the synovial fluid.  The extent of the joint fluid can alter the blood flow to that particular joint.  The WBC brought into destroy the bacteria are destroyed themselves releasing inflammatory mediators and proteolytic enzymes into the joint. Hyaluronate is depolymerized and connective tissue such as cartilage is degraded.  Joint lavage is an important intervention to decrease the joint distension and to dilute the amount of destructive substances (bacteria and proteolytic enzymes) in the affected joint. The technique of joint lavage involves placing a needle into the affected joint distending it further with a sterile crystalloid fluid and inserting another needle for egress of the fluid.

For the larger joints a liter of fluid can be flush through and through this system.  Smaller joints will take less volume. This procedure is best done under a short acting anesthetic.  Arthroscopic evaluation and large volume flushing may be indicated in joints where fibrin has accumulated or where a synovectomy would be useful in returning function to the affected joint. In chronic nonresponsive septic arthritis, arthrotomy with open drainage has been advocated. The open drainage technique requires daily sterile bandaging.  Arthroscopy with closed suction drainage has also been successful in treatment of septic arthritis.

Systemic antibiotics alone are not sufficient in the treatment of SOD. Local antibiotics administered either intra-articular (instilled after joint lavage) or through regional limb perfusion (RLP) are needed to eliminate the bacteria in the affected bone and synovia.  These methods of drug deliver will increase the articular/bone antibiotic concentration up to 1000 times. RLP is limited to the lower joints in the limb distal to and including the carpus and hock. It is accomplished with the foal anesthetized and placed in lateral recumbency. A venous or intraosseous catheter is placed above the affected joint. An Esmarch bandage is wrapped from the distal limb upward to above the joint of focus and a tourniquet is placed above the catheter.  The Esmarch bandage is released around the joint and amikacin (500 mg) diluted in 20-30 mls of sterile saline is injected into the catheter and left for 20-30 minutes.  This method is useful when trying to treat osteomyelitis.

An adjunct therapy that may be helpful for the foal with SOD includes the use of chondroprotective agents such as chondroitin sulfate and glucosamine. Intra-articular injection of hyaluronic acid post through and through lavage has been shown to decrease inflammation.  Intravenous use of hyaluronate sodium has also been advocated  in affected foals. None of these products have been tested for efficacy in the foal.

Many foals that experience orthopedic pain will develop pressure sores secondary to recumbency.  Skin over bony processes will begin to feel thickened and leathery.  This usually progresses to large decubital ulcerations. Deep bedding is not always sufficient to stop this process. Keeping the wound clean and well padded helps the healing but it often takes several weeks before bandaging is unnecessary.  Bandaging decubita of the hip area can be difficult but can be accomplished by encircling the wound with preplacing stent sutures through which umbilical tape can be threaded.  The wound can be covered with gauze sponges and then umbilical tape can be laced over them to hold the bandage in place.

Prognosis

Improvements in treatments have increased the short term survival of foals with septic arthritis. From 1983-1989 - 8/15 (53.3%) of foals with SOD presented to Tufts survived to discharge. During the period of 1990-1998 – 25/36 (71%) survived to discharge.  This was equal to the general sick foal population presented to the hospital. Foals with bony complications (osteomyelitis) had an overall lower survival rate of 67%.

An important question that needs to be answered is not the short term survivability of affected foals but long term athletic ability. Residual dysfunction in children post septic arthritis ranges from 10-25%. Schneider et all reviewed the records of 52 adult horses that survived septic arthritis – 62.8% returned to racing and made at least 1 start. Steele reviewed the outcomes of 93 foals with septic arthritis – 1/3 of the animals went on to race at least once. Unfavorable factors included isolation of Salmonella and the presence of multi-systemic disease.  Smith et al found 50% of foals with a diagnosis of septic arthritis went on to start at least 1 race.  When compared with unaffected siblings, septic arthritis foals were significantly less likely to race and those that raced took a significantly longer time to start.

Key to success is early recognition and aggressive therapy but long hospitalization, multiple anesthesias, long term antibiotics and possibly surgery without knowledge of long term outcome for future soundness creates a financial and emotional dilemma for owners. Often financial considerations play a role in the decision to treat.