Septic Arthritis and Osteomyelitis

Joint and bone infection are a sequel to bacteremia in neonatal foals. Common names for this condition are: joint-ill, navel ill, infectious arthritis, septic polyarthritis, septic epiphysitis, and septic physitis.1 Rapid diagnosis and aggressive therapy are important in preventing irreversible cartilage or bone damage. It is most common in foals less than 30 days. Most commonly isolated organisms are gram-negative enteric organisms including E. coli, Actinobacillus suis like spp., Klebsiella pneumoniae, and less commonly Salmonella spp [2]. Several bacterial species may be recovered from the same foal or joint.

I. Sources of Infection

Previous or concurrent bacteremia ± failure of passive transfer.

1. Gastrointestinal tract

1. Probably most common portal of entry.
2. May or may not be associated with obvious signs of diarrhea or enteritis.
3. Diarrhea may have occurred 20 to 30 days previously - signs of a single joint infection then develop in a bright, alert foal.

2. Umbilical infection

1. Often exterior of navel appears normal, but can also be enlarged or swollen.
2. Infection may be associated with umbilical arteries, vein or urachus.
3. Ultrasound may assist with diagnosis.

3. Concurrent pneumonia

1. Concurrent pneumonia in approximately 35% cases.
2. Pneumonic lesion often focal.

4. Penetrating wounds - Rare in young foals (<60 day). More common in older foals.
5. Extension into joint from adjacent osteomyelitis.
6. Intra-muscular abscess - Uncommon.

II. Clinical Signs

1. Sudden onset of lameness with or without joint distension, pain and edema.
2. Swollen joints with or without periarticular edema in a recumbent foal.
3. Sudden onset of lameness with systemic signs of illness - i.e. fever, depression, diarrhea or anorexia.
4. Sudden onset of lameness without systemic signs of illness - i.e. bright, alert foal.
5. Onset of stiffness, with back or neck pain and a low grade fever - consider vertebral osteomeylitis.
6. Sudden onset of lameness with owner history of trauma to foal (mare stepped on foal). In foal less than 45 days, any lameness should be proven NOT to be related to an infectious etiology.
7. More than one joint may be involved in approximately 50% of cases.

III. Diagnosis

1. Evidence of septicemia and above clinical signs. Older foals can be bright and alert.
2. Fever, lameness (joint swelling in recumbent foal), inflammation, hemogram, leukocytosis ± joint swelling and periarticular edema.
3. Ultrasound of joint (See Section V, Musculoskeletal Imaging).
4. Joint aspiration

1. Aseptic technique; shave or clip, prep, gloves, etc.
2. If concerned about passing needle through periarticular edema, may instill 0.5 ml gentomicin following aspiration of synovial fluid.
3. Synovial fluid examination [2]

1. May be normal with bone infection prior to invasion into synovia or with an open draining wound.
2. Color - Cloudy, turbid, flocculent fluid and low viscosity.
3. Normal 800 WBC/μl - Greater than 1000 WBC/μl with 70% neutrophils suspect infection.
4. Normal total protein <1 g/dl - Greater than 2.5 g/dl may be infection or trauma.
5. Fibrin clots may clump and lower WBC count - May cause difficulty in aspirating fluid.
6. Gram stain may reveal intracellular and extracellular bacteria.

5. Culture synovial fluid [2]

1. Aseptic technique.
2. Aerobic and anaerobic cultures.
3. Media - Thioglycolate broth or brain, heart broth w/agar slant and SPS to prevent clotting.
4. Attempt culture even if foal is receiving antimicrobials - Recovery of bacteria only slightly less than foals not receiving antimicrobials.

6. Culture of blood

1. Culture any foal with systemic signs or those foals <14 days.
2. Greater probability of obtaining causative organism than joint culture - Do both joint and blood culture.
3. Excellent correlation between joint and culture of blood - If joint culture negative assume infected with organism(s) cultured from blood [2].
4. Significant number of foals have more than one bacterial species.

7. Ultrasonography (See Section V. Musculoskeletal Imaging)

1. Very sensitive at determining synovial effusion, proliferative synovial membrane and presence of intra-articular fibrin.
2. Useful in determining location for needle placement to obtain synovial fluid.
3. Assess cartilage damage.
4. Appearance: Normal - Small amount anechoic fluid; Synovial effusion without infection - Large quantity anechoic fluid; Synovial effusion with infection - Large quantity echoic fluid (note: echoic may be blood - Require synovial aspirate).
5. Very sensitive to detect osteomeylitis - Echoic fluid >2 mm between periosteum and bone cortex.

8. Radiography

1. May be normal on initial examination - Repeat in 5-7 days.
2. Examine metaphysis, physis or epiphysis for osteolysis, sclerosis, or reactive cortical bone.
3. Soft tissue swelling.

IV. Treatment

1. Treat early - within hours. Consider as a medical emergency. If suspect infection due to clinical signs and synovial fluid appearance - treat aggressively before obtaining results of synovial aspirate [3].
2. Broad spectrum bacteriocidal antimicrobials given systemically with activity against gram negative and gram positive bacteria. Commence immediately - prior to results of culture and sensitivity. Ideal choice amikacin and ampicillin or first generation cephalosporin - effective against 93% isolates [2]. Other less broad spectrum: gentocin with ampicillin/penicillin; chloramphicol, 3rd generation cephalosporins. Procaine penicillin is least appropriate. Cases of resistant Salmonella strains may be treated with Enrofloxacin or Imipenem (See Guidelines for Drug Use in Equine Neonates).
3. Intra-articular antimicrobials [4]

1. Can combine with systemic antimicrobial therapy.
2. Inject 0.5 ml of 50 mg/ml gentocin (25 mg) or 0.5 ml 250 mg/ml amikacin (125 mg) into joint.
3. Do not buffer solution.
4. Use along with drainage and lavage and systemic antimicrobials and anti-inflammatory drugs.
5. Injecting multiple joints could potentially raise gentocin or amikacin levels to toxic levels.

4. Regional Limb Perfusion [5-7]

1. Advantages

1. High and persistent concentrations in joints (up to 100 x concentration after IV administration).
2. MIC of many bacteria exceeded for 24 hours.
3. Infusate reaches peri-articular tissues, bone, synovium.
4. Infusate reaches normal, inflamed, necrotic tissue.
5. Outcome of treatment of orthopedic infection better than with conventional IV administration.

2. Method

1. Anesthesia or deep sedation
2. Catheter - 20 gauge x 2.5 inch
3. Tourniquet placed proximal to the joint involved - 30 minute duration
4. Dosages used in foals:

• Amikacin - 50 mg in 10 to 12 ml of saline
• Gentamicin - 50 mg in 10 to 12 ml of saline
• Imipenem may be used in very valuable animals with resistant infections, 200 mg in 10 ml of saline

5. Assure adequate serum immunoglobulin concentration
6. Drainage - mechanical lavage and removal of debris.

1. Removal of degenerative neutrophils, fibrin, high WBC, proteolytic enzymes, will benefit from aspiration and flush.
2. Flush

1. Distension - irrigation with one puncture and 3-way stopcock and 2 syringes with saline or preferably lactated Ringer’s. Use 500 ml to 1 liter total volume.
2. Through and through - two needles in joint and continuous lavage of 1-2 liters sterile fluid.

3. Dilute Betadine® no advantage over saline
4. Arthroscopy - allows look at cartilage, removal of fibrin and synovectomy (removal of bacteria sequestered in synovial membrane) in addition to flushing joint. Submit synovial biopsy for culture.
5. Flushing generally repeated until WBC <30,000 cell/μl (clinical impression).
6. Arthrotomy

1. Previously indicated for advanced cases which were refractory to flushing, but is effective in acute cases.
2. Arthrotomy in distal portion of joint ± penrose drain (I do not use due to risk of ascending infection) and sterile wraps (changable stent bandage over more proximal joints) - healing by second intention or delayed primary healing if joint infection resolved.

7. Curettage of physeal lesions
8. Use of antimicrobial impregnated beads placed adjacent to bone - under sterile conditions mix antimicrobial with poly methyl methacrolate, string together beads and place surgically adjacent to lesion. Gives increased concentrations locally.
9. Interosseous antimicrobial perfusion [6]. Useful to increase concentration of antimicrobial agent to infected joint and bone in distal limb. Place tourniquet proximal to lesion - place canulated screw distal to tourniquet into medullary cavity - infuse systemic dose of antimicrobial into medullary cavity. Care to avoid toxicity if systemic antimicrobials are being administered.
10. Immobilization of joint - splints and support wraps
11. Pain control

1. Low dose NSAID over short time - weigh foal to prevent overdose and measure dose carefully.
2. Banamine™ (flunixin meglumine) IM or IV for five days did not produce ulcers in foals. Phenylbutazone more ulcerogenic [9].

12. Post infection treatment

1. Synovial fluid transfer or intra-articular hyaluronic acid to decrease pain and persistent inflammation.
   Editor's Comments – Consider avoiding intra-articular Adequan™ following therapy-some joints have re-inflamed - unknown if potentiates infection if organisms remain in joint?
2. Systemic hyalauronic acid or Adequan™ may improve joint environment.

13. Correction or treatment of underlying nidus of infection or systemic disorders.

Prognosis - General comments

Good with early treatment (1st day), broad spectrum systemically administered antimicrobials, aggressive flushing/drainage of joint, regional limb perfusion where indicated and better prognosis with the absence of bone involvement.
Poor with a delay in treatment, use of procaine penicillin only and radiographic evidence of bone involvement, and failure to treat underlying septicemia.