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Synovial infections in horses refer to infections in the synovial structures, which include joints, bursae, and tendon sheaths. These infections can be severe and potentially career-ending for equine athletes.
Most horses develop synovial infections from wounds over top of a joint or tendon sheath. If there is damage to the synovium, bacteria can proliferate within the synovial structure, causing pain and tissue damage.
Symptoms of synovial infections in horses may include lameness, swelling of the joint, heat or oozing wounds. In severe cases, the horse may develop signs of systemic illness, such as fever, lethargy, and loss of appetite.
Any wound near a joint should be thoroughly examined by a veterinarian. Foals are also prone to developing synovial infections from septicemia (blood infection).
Synovial Infections
Synovial infections are infections of the synovium, the specialized tissue that lines joint capsules and tendon sheaths.
These structures contain synovial fluid, a lubricating fluid that ensures smooth joint and tendon movement. Synovial fluid is rich in nutrients, which predisposes it to bacterial infections.
Synovia are present throughout the body; however, they are most common in the limb joints. Common locations for synovial infections include: [1]
- Navicular bursas
- Coffin joints
- Pastern joints
- Digital flexor tendon sheaths
- Fetlock joints
- Hock joints
- Calcaneal bursa of the hocks
- Tendon sheaths of the hocks
- Carpal joints
Foals are more likely to have multiple synovial structures affected, as the bacteria usually spreads from a blood infection. [2][3] The hocks are most commonly affected in foals. [3]
Causes
Synovial infections can develop in several different ways, all of which involve bacteria or other pathogens infiltrating the synovial structures of a joint or tendon.
Traumatic Injuries
The most common cause of synovial infections are wounds that penetrate the joint capsule or tendon sheath, introducing bacteria. [1] Any wound damaging a synovial structure should be considered to have an infection after 24 hours. [1]
Bacteria commonly found in synovial infections from wounds include: [4]
- Staphylococcus
- Streptococcus
- Rhodococcus
- Corynebacterium
Septicemia
Foals also commonly develop synovial infections, however these infections do not usually arise from traumatic injuries. In foals, joint infections are a consequence of septicemia (blood infection) that spreads to the joints. [1]
Foals experiencing failure of passive transfer have the highest risk of synovial infections. [1] Studies show up to 78% of foals with failure of passive transfer may develop synovial infections. [5]
Common bacteria causing synovial infections in foals include: [2][3][4]
- Escherichia coli
- Streptococcus
- Staphylococcus
- Actinobacillus
- Salmonella
- Klebsiella
- Clostridium
- Rhodococcus
Around 0.5-1% of foals develop septic arthritis due to a joint infection. This is an inflammatory joint disease caused by the presence of pathogenic microorganisms, resulting in pain, swelling and difficulty in moving the affected joint. [2]
Joint Injections
Rarely, synovial infections can occur following the administration of joint injections, where a veterinarian injects therapeutic substances into the joint capsule.
Although the skin surface is thoroughly cleaned before the procedure and sterile needles are used, there is still a risk of introducing bacteria into the joint capsule with injectable medications. [1]
Studies show that around 1 out of 1000 joint injections administered to horses result in a synovial infection. [6] There is no difference in the risk of joint infection for injections performed in a hospital setting versus an ambulatory setting. [6]
Known factors that increase the risk of synovial infections after joint injections include: [6]
- Clipping the hair over the joint
- Using non-sterile needles or gloves
- Limited practitioner experience
- Using multi-dose vials of medications
- Larger needle sizes
Symptoms
Most horses with joint infections have an obvious wound over the joint, indicating the cause of the infection. Some wounds may ooze the yellowish, clear, sticky fluid found within the joint capsule. [1]
Horses without an obvious wound may show symptoms such as: [1][4]
- Severe lameness
- Swelling of the joint area
- Heat when touching the affected area
- Fever
Foals tend to show minimal lameness, however, will still develop swelling and heat over the affected joints. [1]
Diagnosis
For horses with wounds over or near a synovial structure, the first step in diagnosing an infection is determining whether the joint capsule or tendon sheath is compromised. [1]
Your veterinarian may perform the following procedures to assess this: [1]
- Probing the wound with a metal instrument or finger to identify depth
- Injecting sterile saline into the synovial space to see if fluid exits the wound
- Injecting X-ray contrast fluid into the synovial space to see if there are any leaks visible on an X-ray
- Identifying air within the synovial space on X-rays or ultrasound
If there is no obvious wound, other diagnostic techniques include: [1][7]
- Ultrasound of the synovial structure to identify swelling and thickening of the synovium
- Measuring white blood cell levels in the synovial fluid
- Bacterial culture of the synovial fluid
- Biopsy of the synovial membrane
- Bloodwork showing signs of infection
Treatment
Treatment of synovial infections focuses on controlling the bacterial infection with antibiotics and flushing the synovium. [8]
Systemic Antibiotics
Systemic antibiotics are medications that are administered orally, intramuscularly, or intravenously to ensure that the antibiotic therapy reaches all parts of the horse’s body. This type of antibiotic treatment is particularly useful for joint infections in foals, who often have multiple affected joints. [1]
The veterinarian will select which antibiotics to used based on a culture and sensitivity test. Several antibiotics are tested on bacteria grown from the affected synovial structure to determine the most effective antibiotic for the specific bacteria present. [1]
Common antibiotics used to treat synovial infections include: [8][9]
- Penicillin and gentamicin in combination
- Trimethoprim-sulfa
- Doxycycline
- Enrofloxacin
- Chloramphenicol
Appropriate antibiotic selection is very important in foals, who can develop toxicity from certain antibiotics. [10] In particular, foals can develop kidney failure from antibiotics in the aminoglycoside class, and cartilage damage from enrofloxacin. [10]
Intrasynovial Antibiotics
Veterinarians can also inject antibiotics directly into the synovial structure, allowing for a high concentration of antibiotics in the infected area. [8]
Amikacin is the most common antibiotic used for this purpose. [8] Usually, this treatment is administered daily until the infection resolves. [8]
Regional Limb Perfusions
Regional limb perfusion (RLP) is a veterinary technique employed in the treatment of lower limb infections in horses. This procedure involves administering antibiotics directly into the blood vessels of a specific limb to achieve high local concentrations of the drug in the target area.
The primary aim of RLP is to achieve a higher concentration of antibiotics in the infected area while minimizing systemic exposure and potential side effects. [8]
To perform an RLP, the veterinarian places a tourniquet higher up on the limb than the affected joint, to prevent blood flow back to the body. Following this, antibiotics are injected into a blood vessel, resulting in an elevated concentration of antibiotics in the area below the tourniquet. [8]
The horse stands with the tourniquet in place for 15-30 minutes after antibiotic administration to allow the medication time to work. [8]
Similar to intrasynovial injections, amikacin is the most common antibiotic used for RLPs. [8]
Antibiotic-Impregnated Materials
Antibiotic-impregnated materials are a novel and increasingly popular approach in the treatment of joint infections in horses.
These materials include small discs or beads that deliver high concentrations of antibiotics directly to the site of infection. The medication releases slowly after the material is placed in an infected synovial space. [8]
The most common material is polymethylmethacrylate (PMMA) beads, which are made of a hard acrylic. [9] Due to their firm texture, most surgeons only place PMMA beads in low-motion joints or the soft tissues around an affected synovial structure to avoid damage to the synovium. [9] PMMA beads are usually removed after the antibiotic depletes. [8][9]
European veterinarians may have access to gentamicin-collagen sponges, which are also effective in treating synovial infections. [9] The collagen in these sponges degrades over time without causing further damage to the synovium. [9] Currently these products are not available in North America. [9]
Needle Lavage
Needle lavage involves placing several large needles into the synovial structure, then injecting a large amount of sterile saline through one of the needles. [8] The saline drains out of the other needles, allowing for flushing of the entire synovial membrane surface. [8]
This flushing allows inflammatory debris, such as pus, and bacteria to leave the joint, making antibiotic treatment more effective. [8]
Arthroscopic Lavage
Some severely affected synovial structures accumulate so much inflammatory debris that needle lavage is not possible, due to the viscosity of the synovial fluid.
These horses require arthroscopic lavage, where a surgeon introduces a small camera and forceps into the synovial cavity to remove inflammatory debris. [8] Arthroscopic lavage is also the treatment of choice for any joint infections older than 24 hours. [8]
Some surgeons may place a catheter into the synovial structure during the surgery. [8][9] Placing a catheter allows lavage or antibiotic administration to occur up to 3-4 times a day.
Additionally, the catheter can be attached to a continuous infusion system, which allows constant delivery of small amounts of antibiotic. [9] Typically these catheters are left in place for around 5 days and have a low complication rate. [9]
Hyaluronic Acid Injection
Some studies show that administering hyaluronic acid (HA) into an infected joint can reduce cartilage damage and inflammation. [3][7] Hyaluronic acid is a major component of synovial fluid, and functions as a lubricant for smooth joint function. [3]
Pain Control
Synovial infections are very painful, making pain control an important part of the treatment plan.
Pain management options for horses include: [11]
- NSAIDs: Non-steroidal anti-inflammatory drug such as phenylbutazone
- Epidural analgesia: Administering pain control into the spinal canal to numb the entire limb
- Perineural analgesia: Administering pain control around a nerve close to the affected structure
There are also reports of injecting morphine or local anesthetic, such as mepivacaine, into the affected synovial structure to provide strong local pain control. [10]
Wound Management
Horses that have wounds or surgical sites around their infected joint require careful management to prevent introduction of more bacteria. [7]
Sterile bandages, changed daily, are the best method for protecting the wound. [7] Bandaging can also provide pressure that reduces swelling of the tissues surrounding the joint. [7]
Physical Therapy
Initially, horses with joint infections require prolonged stall rest while their infection resolves. Inflammation can reduce the amount of proteoglycans, the component responsible for shock absorption, within the joint cartilage. [3] Allowing for 3-4 weeks of stall rest enabled the cartilage to replenish fully before resuming normal activity levels. [3]
After the infection resolves and the cartilage begins to recover, hand-walking and gentle range-of-motion exercises can also help restore normal function of the joint. [3][4] This is particularly important in cases with accumulation of large amounts of inflammatory debris, as the debris can cause adhesions within the joint capsule. [4]
As recovery continues, active range of motion exercises and stretching exercises can be added to the physical therapy protocol. [10] This can progress to controlled exercise over a variety of non-slippery surfaces, then paddock turnout, and eventually a return to normal activity and pasture turnout. [10]
For foals, it is particularly important to note that reduced use of a limb can cause tendon laxity, tendon contraction, or angular limb deformity in the opposing weight-bearing limb. [3][10]
Prognosis
Overall, synovial infections have a good prognosis, with between 84-90% of adult horses recovering with appropriate treatment. [8]
Around 50% of horses return to their previous athletic performance, with some studies showing as high as 89% returning to some level of athletic function. [8][11]
Foals
For foals, the prognosis is less favorable, with around 42-45% of foals surviving the infection. [3] However, many foals are euthanized due to concerns about their future athletic performance, which may artificially lower the survival rate. [3]
Foals diagnosed with infections before they reach 30 days of age typically have a worse prognosis. [3][12] However, foals have an improved prognosis if they are treated within 24 hours of developing symptoms or receive multiple types of antibiotic treatment. [13]
Athletic Performance
Research on athletic performance after septic arthritis shows variable results, with studies reporting a range between 30-70% of foals having an athletic career. [3]
Studies in Thoroughbred foals with septic arthritis report a 48-67% chance of going on to a racing career. [3][14][15] In sport horses, one study showed only 28% of affected foals had a successful sporting career. [10]
Some studies also report on competition performance in affected foals. One study showed no difference in racing performance between foals affected by synovial infections and their maternal siblings. [12]
However, another study found that affected foals had reduced total winnings compared to their maternal siblings. [15] Therefore, the exact effect of synovial infections on athletic performance is not fully understood.
Foals that develop infections in multiple synovial structures, develop bone infections, or have a prolonged treatment time have a worse chance of future athletic performance. [3]
Summary
- Synovial infections are bacterial infections within a joint capsule or tendon sheath
- The most common cause of joint infections in adult horses is a wound over the synovial structure
- Foals can develop synovial infections from septicemia due to bacteria circulating in the blood
- Treatment involves extensive antibiotic therapy, often through multiple methods including joint injections
- The overall prognosis with prompt treatment is good with 50% of adult horses returning to athletic performance
References
- Baxter. G. M., Ed., Manual of Equine Lameness. Wiley Blackwell, 2011.
- Rinnovati. R. et al., Diagnosis, Treatment, Surgical Management, and Outcome of Septic Arthritis of Tarsocrural Joint in 16 Foals. Journal of Equine Veterinary Science. 2018. doi: 10.1016/j.jevs.2018.04.003.
- Annear. M. J. et al., Septic Arthritis in Foals. Equine Veterinary Education. 2011. doi: 10.1111/j.2042-3292.2011.00240.x.
- Tremaine. H., Infection of Equine Joints and Tendon Sheaths. In Practice. 2000. doi: 10.1136/inpract.22.5.262.
- Hardy. J., Etiology, Diagnosis, and Treatment of Septic Arthritis, Osteitis, and Osteomyelitis in Foals. Clinical Techniques in Equine Practice. 2006. doi: 10.1053/j.ctep.2006.09.005.
- Krause. D. M. et al., Comparison of Equine Synovial Sepsis Rate Following Intrasynovial Injection in Ambulatory versus Hospital Settings. Equine Veterinary Journal. 2022. doi: 10.1111/evj.13485. View Summary
- Morton. A. J., Diagnosis and Treatment of Septic Arthritis. Veterinary Clinics of North America: Equine Practice. 2005. doi: 10.1016/j.cveq.2005.08.001. View Summary
- Baxter. G. M., Ed., Adams and Stashak’s lameness in horses, Seventh edition. Hoboken, NJ: Wiley-Blackwell, 2020.
- Orsini. J. A., Update on Managing Serious Wound Infections in Horses: Wounds Involving Joints and Other Synovial Structures. Journal of Equine Veterinary Science. 2017. doi: 10.1016/j.jevs.2017.01.016.
- Lindegaard. C. et al., Haematogenous Septic Arthritis, Physitis and Osteomyelitis in Foals: A Tutorial Review on Pathogenesis, Diagnosis, Treatment and Prognosis. Part 2. Equine Veterinary Education. 2022. doi: 10.1111/eve.13393.
- Crosby. C. E. et al., Current Treatment and Prevention of Orthopaedic Infections in the Horse. Equine Veterinary Education. 2023. doi: 10.1111/eve.13770.
- O’Brien. T. J. et al., Factors Associated with Survival and Racing Performance of 114 Thoroughbred Foals with Septic Arthritis Compared with Maternal Siblings (2009-2015). Equine Veterinary Journal. 2021. doi: 10.1111/evj.13387.
- Vos. N. J. and Ducharme. N. G., Analysis of Factors Influencing Prognosis in Foals with Septic Arthritis. Ir Vet J. 2008. doi: 10.1186/2046-0481-61-2-102. View Summary
- Neil. K. et al., Retrospective Study of 108 Foals with Septic Osteomyelitis. Australian Veterinary Journal. 2010. doi: 10.1111/j.1751-0813.2009.00539.x. View Summary
- Whisenant. K. D. et al., Prognosis for Survival to Discharge and Racing Performance in Thoroughbred Foals Treated for Single Joint Septic Arthritis (2009–2016). Equine Veterinary Journal. 2023. doi: 10.1111/evj.13892.View Summary
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