Arthritis in horses encompasses a spectrum of inflammatory joint diseases that significantly impact equine health, performance, and welfare.
Overview and Clinical Importance
Arthritis in horses encompasses a spectrum of inflammatory joint diseases that significantly impact equine health, performance, and welfare. Understanding the distinction between septic (infectious) arthritis and non-septic arthritis (including osteoarthritis/degenerative joint disease) is critical for NAVLE success. Septic arthritis represents a true veterinary emergency requiring immediate intervention, while osteoarthritis (OA) accounts for approximately 60% of all equine lameness cases and is the leading cause of poor performance in athletic horses.
| Route |
Description |
Common Bacteria |
| Hematogenous |
Most common in FOALS; spread from umbilicus, lungs, or GI tract |
Enterobacteriaceae (E. coli most common), Salmonella, Actinobacillus |
| Traumatic/Wounds |
Penetrating injuries, lacerations near joints; most common in ADULTS |
Mixed flora: Gram-positive, Gram-negative, and anaerobes |
| Iatrogenic |
Post intra-articular injection or surgery; increased risk with corticosteroids |
Staphylococcus aureus, Streptococcus spp., coagulase-negative Staphylococci |
| Adult Horses |
Foals |
| Lameness: Acute onset, moderate to severe (AAEP 3-5/5), rapidly progressive
Joint effusion: Marked swelling, heat, pain on palpation
Systemic signs: Variable; fever, tachycardia in severe cases
Wound/drainage: May be present; draining joints may be LESS lame |
Fever: Often PRECEDES lameness
Lameness: May develop 8-24 hours after infection; sometimes subtle
Multiple joints: Common with hematogenous spread
Recumbency: Increased lying time; decubital ulcers
Septicemia signs: Depression, poor suckle, diarrhea |
Part 1: Septic Arthritis
Definition and Pathophysiology
Septic arthritis (SA) is a joint infection caused by bacterial invasion of the synovial space. It represents a potentially life-threatening or career-ending emergency requiring immediate veterinary attention. The synovial membrane is highly susceptible to infection because it lacks a basement membrane, allowing rapid bacterial colonization.
Pathophysiologic Cascade: Bacterial invasion triggers an inflammatory response with release of cytokines (IL-1, TNF-alpha) and degradative enzymes (matrix metalloproteinases). This leads to synovial membrane inflammation, fibrin accumulation, cartilage degradation, and potentially subchondral bone destruction. Without treatment, irreversible cartilage damage can occur within 24-48 hours.
High-YieldSeptic arthritis is a TIME-SENSITIVE emergency. Cartilage damage begins within hours of infection. The mantra is: 'The sun should never set on a septic joint without treatment.'
Etiology and Routes of Infection
NAVLE TipRemember the age-route association: FOALS = Hematogenous (often E. coli, Salmonella), ADULTS = Traumatic wounds or iatrogenic (often Staphylococcus, Streptococcus). Failure of passive transfer (FPT) is present in 50-88% of foals with septic joints!
Clinical Signs
Diagnosis
Synovial Fluid Analysis
Arthrocentesis with synovial fluid (SF) analysis is the cornerstone of diagnosis. The gold standard for confirming septic arthritis is a positive bacterial culture, although cultures are negative in 25-50% of cases.
High-YieldNAVLE Thresholds for Septic Arthritis: TNCC greater than 30 x 10^9/L, Neutrophils greater than 80%, Total Protein greater than 40 g/L. A TNCC greater than 100 x 10^9/L is considered PATHOGNOMONIC for sepsis. In FOALS, use lower thresholds: TNCC greater than 10 x 10^9/L and TP greater than 25 g/L.
Additional Diagnostic Tools
- Bacterial Culture and Sensitivity: Essential but only positive in 32-74% of cases; use enrichment media (blood culture bottles) to improve yield
- Serum Amyloid A (SAA): Acute phase protein; elevated in sepsis; useful for monitoring treatment response
- Radiography: Early: soft tissue swelling, effusion. Late (7-14 days): periosteal reaction, subchondral lysis, joint space narrowing
- Ultrasonography: Joint effusion, synovial thickening, fibrin, communication with wounds; helpful for guided arthrocentesis
- Contrast Radiography/Arthrography: Documents communication between wound and joint
Treatment of Septic Arthritis
Treatment goals: (1) Eliminate infection, (2) Remove inflammatory debris and fibrin, (3) Reduce pain and inflammation, (4) Preserve cartilage and joint function. Treatment should begin IMMEDIATELY upon suspicion of septic arthritis.
NAVLE TipFor NAVLE, remember the multimodal approach: Joint Lavage + Systemic Antibiotics + Local/Regional Antibiotics + Analgesia. Amikacin is the most commonly tested intra-articular antibiotic choice.
Prognosis
Survival rates: Adults 90-94%, Foals 78-89%. Return to athletic function: 50-81% depending on duration before treatment, joint affected, and presence of osteomyelitis.
| Parameter |
Normal |
Septic Arthritis |
Non-Septic/OA |
| Color/Turbidity |
Clear to light yellow; transparent |
Yellow-orange to green; turbid/cloudy |
Clear to slightly cloudy |
| Viscosity |
High (forms strand greater than 2 inches) |
Decreased (watery) |
Normal to mildly decreased |
| TNCC (cells/L) |
Less than 1 x 10^9/L |
Greater than 30 x 10^9/L (often greater than 50) |
Less than 5 x 10^9/L |
| Neutrophils (%) |
Less than 10% |
Greater than 80-90% |
Variable; predominantly mononuclear |
| Total Protein (g/L) |
Less than 20 g/L |
Greater than 40 g/L |
20-40 g/L |
| Treatment Modality |
Details and Recommendations |
| Joint Lavage |
Through-and-through needle lavage: Appropriate for early cases and foals
Arthroscopic lavage: Gold standard for adults; allows debridement of fibrin, pannus, and visualization of cartilage damage
Use sterile polyionic solution (LRS or saline); 10-20 L for thorough lavage |
| Systemic Antimicrobials |
Begin IV broad-spectrum antibiotics IMMEDIATELY pending culture results
First-line: Penicillin + Gentamicin (covers Gram-positive and Gram-negative)
Duration: Minimum 2-4 weeks; adjust based on culture and clinical response |
| Regional Limb Perfusion (RLP) |
Achieves antibiotic concentrations 10-100x higher than systemic administration
Drugs: Amikacin (1-2g), Gentamicin (1g), or Ceftiofur
Technique: Tourniquet proximal to joint, inject into peripheral vein, maintain 15-30 minutes
Frequency: Daily or every other day for 3-5 treatments |
| Intra-articular Antibiotics |
Amikacin: 250-500 mg; most commonly used due to broad spectrum and synovial fluid activity
Higher synovial concentrations achieved than RLP; often combined with lavage |
| Analgesia/Support |
NSAIDs: Phenylbutazone or flunixin; anti-inflammatory and analgesic
Limb support: Prevent contralateral limb laminitis; cryotherapy, supportive bandaging |
Part 2: Osteoarthritis (Non-Septic Arthritis)
Definition and Pathophysiology
Osteoarthritis (OA), also called degenerative joint disease (DJD), is a progressive, non-infectious inflammatory condition characterized by cartilage degradation, subchondral bone changes, and synovitis. It is the most common cause of lameness in horses, accounting for approximately 60% of all equine lameness cases.
Pathophysiologic Cascade: Mechanical stress or injury initiates an inflammatory response. IL-1 and TNF-alpha stimulate chondrocytes to release matrix metalloproteinases (MMPs) that degrade proteoglycans and collagen. PGE2 increases pain sensitivity and further promotes cartilage degradation. The result is progressive cartilage thinning, subchondral bone sclerosis, osteophyte formation, and decreased joint space.
Etiology and Risk Factors
- Post-traumatic OA (most common): Secondary to repetitive loading, fractures, OCD, soft tissue injuries
- Normal wear and tear: Age-related cartilage degeneration in older horses
- Conformation abnormalities: Cow hocks, sickle hocks, base-narrow/wide conformation
- Developmental orthopedic disease: OCD, subchondral bone cysts
- Previous septic arthritis: Secondary OA following resolved infection
Commonly Affected Joints
Focus: Bone Spavin (Distal Tarsal OA)
Bone spavin is OA of the distal tarsal joints (DIT, TMT, and occasionally PIT). These are low-motion joints where fusion (ankylosis) can actually improve soundness. It is responsible for up to 80% of chronic, low-grade hindlimb lameness in horses.
Clinical Signs of Bone Spavin
- Lameness: Gradual onset, low-grade hindlimb lameness; worse after rest, may 'warm out of it'
- Gait abnormality: Low foot flight, toe drag, hypermetric 'stabby' gait
- Positive flexion test: 'Spavin test' - upper limb/hock flexion for 60-90 seconds exacerbates lameness
- Churchill's test: Pelvic limb abduction with forced hock adduction
- Bony swelling: Firm enlargement on medial aspect of hock in chronic cases
- Responds to intra-articular anesthesia: DIT and/or TMT blocks improve lameness
High-YieldOccult spavin or blind spavin refers to clinical pain without radiographic changes. Nuclear scintigraphy may be more sensitive for detecting active inflammation in these cases.
Diagnosis of Osteoarthritis
Radiographic Findings
NAVLE TipFor bone spavin, the dorsolateral-plantaromedial oblique (DLPMO) radiographic view is considered most useful for detecting dorsomedial osteophytes. Always obtain 4 standard hock views for complete evaluation.
Treatment of Osteoarthritis
There is NO CURE for OA. Treatment goals are: reduce pain, slow disease progression, and maintain function. A multimodal approach is typically used.
High-YieldFor NAVLE, know that corticosteroids are symptomatic treatment only and do NOT stop disease progression. Unlike most joints, the distal tarsal joints CAN be repeatedly injected with corticosteroids because eventual fusion improves soundness.
Supportive Management
- Corrective shoeing: Egg bar shoes, wedges to alter breakover; farrier consultation essential
- Exercise modification: Maintain low-level exercise; complete rest may worsen stiffness
- Weight management: Keep BCS at 4-5/9; reduce joint loading
- Nutraceuticals: Glucosamine, chondroitin sulfate, MSM; variable evidence but commonly used
| Favorable Prognosis |
Poor Prognosis |
| Early diagnosis and treatment (less than 24 hours)
Single joint involvement
Low virulence organism
Negative culture after treatment
No radiographic bone changes |
Delayed treatment (greater than 48-72 hours)
Multiple joint involvement (especially foals)
Concurrent osteomyelitis or physitis
Positive culture for resistant organisms
High-motion joints (fetlock, carpus) |
| Joint |
Common Name |
Predisposed Breeds/Use |
| Distal intertarsal (DIT) and Tarsometatarsal (TMT) |
Bone Spavin |
Icelandic horses, Standardbreds, Dressage, Western performance |
| Fetlock (MCP/MTP) |
Fetlock OA |
Racehorses (Thoroughbreds, Quarter Horses) |
| Carpus (middle carpal, antebrachiocarpal) |
Carpal OA |
Racehorses, jumpers |
| Proximal interphalangeal (PIP) |
High Ringbone |
Draft horses, Western performance |
| Distal interphalangeal (DIP) |
Low Ringbone |
All breeds; often secondary to navicular disease |
Summary: Septic vs Non-Septic Arthritis
| Radiographic Sign |
Significance |
| Joint space narrowing |
Cartilage loss |
| Osteophyte formation |
New bone at joint margins; appears as 'lipping' or 'spurs' |
| Subchondral sclerosis |
Increased bone density beneath cartilage |
| Subchondral lysis |
Bone resorption; radiolucent areas |
| Enthesiophytes |
New bone at ligament/tendon insertions |
| Joint fusion (ankylosis) |
End-stage; may improve soundness in low-motion joints |
| Treatment |
Details |
| Systemic NSAIDs |
Phenylbutazone: Most commonly used; anti-inflammatory and analgesic
Firocoxib: COX-2 selective; may have fewer GI side effects
Long-term use: monitor for GI ulceration, renal toxicity |
| Intra-articular Corticosteroids |
Triamcinolone acetonide: Most commonly used; potent anti-inflammatory
Methylprednisolone acetate: Longer duration; higher laminitis risk
Can repeat as needed in low-motion joints (DIT, TMT) |
| Hyaluronic Acid (HA) |
Restores synovial fluid viscosity; may be chondroprotective
Routes: Intra-articular or IV (Legend)
Often combined with corticosteroids for synergistic effect |
| PSGAG (Adequan) |
Polysulfated glycosaminoglycan; inhibits degradative enzymes
Protocol: 500 mg IM every 4 days for 7 treatments, then monthly |
| Biologic Therapies |
IRAP: Interleukin-1 receptor antagonist protein; anti-inflammatory
PRP: Platelet-rich plasma; growth factors promote healing
Pro-Stride (APS): Combines ACS and PRP technologies |
| Surgical Fusion (Arthrodesis) |
Appropriate for LOW-MOTION joints: DIT, TMT, pastern (PIP, DIP)
Methods: Chemical (alcohol, MIA), laser, drilling, or surgical
Goal: Accelerate natural ankylosis to achieve pain-free fusion |
| Feature |
Septic Arthritis |
Osteoarthritis (OA) |
| Onset |
Acute (hours to days) |
Gradual (weeks to months) |
| Lameness |
Moderate to severe; rapidly progressive |
Mild to moderate; 'warms out of it' |
| Effusion |
Marked; warm; painful |
Variable; minimal in low-motion joints |
| Synovial Fluid |
TNCC greater than 30 x 10^9/L; greater than 80% neutrophils; TP greater than 40 g/L |
TNCC less than 5 x 10^9/L; mononuclear predominance |
| Treatment Urgency |
EMERGENCY - immediate intervention |
Elective - long-term management |
| Primary Treatment |
Joint lavage + systemic/local antibiotics |
NSAIDs + IA corticosteroids/HA |