Septic synovitis refers to infection that occurs within a synovial structure such as a joints, bursa, or tendon sheath.
There are three main ways bacteria can gain access to a synovial structure and cause infection:
- Wounds/lacerations into synovial structures allow bacterial contamination to enter the synovial structure and lead to infection (Figure 1). This is the most common cause of synovial infections in adult horses.
- Hematogenous spread, or spread through the blood, occurs in in sick animals. Synovial infections in these cases are often secondary to a primary problem elsewhere, such as an umbilical infection, pneumonia, or diarrhea. This is the most common cause of synovial infections in foals.
- Iatrogenic spread can occur following joint injections or following surgical procedures involving joints, such as arthroscopy.
Contamination and infection of synovial structures is considered an emergency. The infection results in a significant inflammatory response that can cause severe lameness and damage to articular cartilage, bone, and/or soft tissue structures within that affected synovial structure. Over time, this intense inflammatory response and secondary tissue damage can lead to the development of osteoarthritis and degenerative joint disease, not only limiting the future athletic activity of the animal but also the normal day to day activity.
Horses that are suspected of having a contaminated or infected synovial structure should receive prompt veterinary attention to institute appropriate treatment and decrease the likelihood of the devastating effects of the inflammatory and infectious process.
- Lameness – generally severe but can be mild-moderate in some cases
- If synovial fluid drains through an open laceration the horse may not initially exhibit severe lameness
- Swelling and heat of the affected limb, especially centered around the affected synovial structure
- Fever- most commonly identified in foals
- Wounds or lacerations that occur over joints, especially those on the lower limb, should be treated cautiously and evaluated by a veterinarian to ensure synovial structure involvement hasn’t occurred.
- In foals, multiple synovial structures can be affected- the most common signs include heat and swelling around the structure in addition to sudden lameness. Foals at risk of developing joint infections should be monitored daily for the above signs. If you notice your foal has increased lameness, heat, or swelling surrounding a joint, contact your veterinarian for further evaluation.
- A combination of multiple diagnostics are often used to diagnose infection of a synovial structure
- Obtaining a synovial fluid sample is one of the most important diagnostics to perform in cases with suspected infection. The character of the fluid (color, viscosity), the amount of protein in the fluid, and the number and types of white blood cells in the fluid will be determined. A sample may also be submitted for bacterial culture and antimicrobial sensitivity. In horses with open wounds, it may not always be possible to obtain a joint fluid sample if enough of the fluid has drained out of the wound.
- In cases with wounds, sterile saline can also be injected into the synovial structure to see if it exits the wound. If sterile saline comes out of the wound while the veterinarian injects the synovial structure from a distant site, that structure is considered contaminated.
- Other diagnostics that can may be performed include radiographs (x-rays) with and without contrast and also ultrasound
- Outcome is dependent on many factors including the chronicity of infection, if additional structures (bone, soft tissue) are affected, and type of bacteria involved in the infection. Even with appropriate initial treatment, some cases may require multiple joint lavage procedures or surgeries.
- Luckily, as our treatments have improved over time, patient outcome has also improved. Studies have found survival to hospital discharge to be good with return to athletic use being fair-good. These injuries can still be life threatening and come with life threatening complications (i.e. laminitis).
- Following discharge from the hospital, horses may continue to receive antibiotics and may require continued bandaging and wound care- this will depend on the extent and severity of wounds/lacerations.
- Some form of stall or small pen rest may also be required, again depending on the extent and severity of disease and any wounds/lacerations.
- It is important to consider the time and financial commitment required in these cases. Decisions for treatment should be made in collaboration with your primary care veterinarian and/or board certified veterinary surgeon and should consider the overall health status and future expectations for the horse.