Key Insights
  • Hyaluronan injections (hyaluronic acid) restore joint lubrication and reduce inflammation in horses with osteoarthritis
  • HA is a natural component of synovial fluid that supports smooth movement and shock absorption
  • Low to medium weight HA preparations offer optimal anti-inflammatory effects
  • Intra-articular administration provides targeted relief but carries risk of flare or joint infection
  • Repeated HA treatments can improve synovial viscosity, pain management and cartilage integrity
  • Combining HA with corticosteroids may enhance pain relief and extend duration of therapeutic benefit
  • Proper sterile technique and veterinary supervision are essential for safe and effective use

Hyaluronan injections, commonly referred to as hyaluronic acid (HA) injections, are widely used in equine veterinary medicine to support joint health and manage joint disease.

Hyaluronan is a naturally occurring component of synovial fluid in joints, which acts as a lubricant and shock absorber. Synovial fluid promotes smooth joint movement and reduces friction between cartilage surfaces.

Whether your horse is dealing with osteoarthritis, recovering from an injury, or requires joint maintenance to support performance, HA injections may help reduce inflammation, improve joint function, and relieve pain.

This guide provides an evidence-based overview of hyaluronan injections, including their mechanism of action, potential benefits, risks, and considerations, to help you make informed decisions about your horse’s joint care.

Hyaluronan Injections for Horses

Horses and other mammals naturally produce hyaluronan, also known as hyaluronic acid (HA), within their joints. This molecule plays a role in joint lubrication and shock absorption, but its natural stores may become depleted due to joint injury or disease. [1]

To counteract this depletion, HA injections are commonly administered to horses with osteoarthritis or other joint dysfunctions. In sport horses, they are also used as a preventive measure to support joint health and mobility. [1]

Research suggests HA injections increase the viscosity of synovial fluid, improve joint lubrication, and provide an anti-inflammatory effect in horses with osteoarthritis. Additionally, HA injections may help alleviate pain in cases of traumatic arthritis. [2][3]

HA is also thought to promote connective tissue repair and provide protective benefits for cartilage, but more research is required to fully understand its effects and mechanisms in horses. [3][4]

Hyaluronic Acid

Hyaluronic Acid (HA) is a disaccharide (two-molecule) sugar consisting of d-glucuronic acid and N-acetyl-d-glucosamine. HA is a key component of synovial fluid and articular cartilage, contributing to smooth movement and shock absorption in the joints. [1]

HA is produced by builder cells called synovial fibroblasts and chondrocytes, found in joint and bone tissue, respectively. It is excreted into synovial fluid where it acts as a boundary lubricant and provides shock absorption between surfaces of articular cartilage. However, its effectiveness as a lubricant depends on the concentration and molecular structure of HA. [3]

HA binds with proteoglycan core proteins to form a large cluster of proteoglycans. These proteins are found in the extracellular matrix of articular cartilage. Because of this, HA also plays an important role in maintaining the stiffness of articular cartilage. [3]

Hyaluronan for Osteoarthritis in Horses

Osteoarthritis (OA) is a common disease in horses, reportedly causing up to 60% of all equine lameness. Given this, managing joint disease is a common requirement for older horses. [5][6]

Osteoarthritis is characterized by changes in the joint, which can include the following: [6]

  • Calcification and degeneration of articular cartilage
  • Osteophyte (bony growth near joint) formation
  • Synovitis (inflammation of joint lining)
  • Sclerosis (abnormal hardening of subchondral bone)

Additionally, ligaments, surrounding tissue, and the joint capsule can all be affected by OA. This further contributes to joint dysfunction and pain. Inflammation also leads to hypersensitization in affected joints. [5][6]

Inflammation in the joint causes the release of enzymes that can degrade proteoglycans, hyaluronan, and collagen. As a result, cartilage cannot retain as much water and is more easily damaged. [5] Once joint changes have occurred, OA cannot be cured; it can only be managed. [7]

Horses with osteoarthritis have a disruption in HA synthesis due to increased levels of inflammatory cytokines, free radicals, and proteinases in the joint. This results in HA with significantly reduced molecular weight, more molecular polysaccharides, and reduced synovial fluid viscosity and elasticity. [8]

Poor hyaluronan levels or function are often evident with joint inflammation or OA, especially if radiographic changes are present. HA supplementation may help restore joint function through improvement of synovial lubrication. Its anti-inflammatory properties may also promote pain relief, which also contributes to improved joint function and mobility in some cases. [5]

Hyaluronan Supplementation for Joint Disease

Sodium hyaluronate is the sodium salt of hyaluronic acid and the drug product approved for treatment of osteoarthritis and synovitis in horses. [4]

HA preparations can be administered to horses in several different ways, including:

  • Intra-articular injection (in the joint)
  • Intra-muscular injection (in the muscle)
  • Intravenous injection (in the vein)
  • Oral supplement
  • Topical product

Intra-articular injections are the most common method used by veterinarians for treating lameness caused by joint disease. [7]

HA trials have been performed in horses for over 50 years. Though study results have varied, some trials showed that HA injections can lead to significant improvements in lameness, weight-bearing scores, and quality of synovial fluid. [2]

In humans, intra-articular (joint) injections of HA have been shown to alleviate pain and improve joint mobility. Human medical research shows that HA is more successful than NSAIDs and corticosteroids in managing osteoarthritis. Research suggests similar effects in horses, but more study is required to determine the full effects of HA injections in horses. [2][7]

Mechanisms of Action

The molecular weight of HA varies depending on how polymerized it is, meaning preparations with higher numbers of repeating disaccharide units have a higher molecular weight. HA preparations used in the treatment of joint disease are categorized as either low, medium, or high according to their molecular weight. [3]

When HA is injected into a joint, the molecular weight of the preparation may help equalize the concentration and distribution of molecular weight of existing HA within the joint. Supplementing with hyaluronic acid may also increase natural production of HA within the joint. [3]

Further, HA injections may help modulate pain by interacting with nerve cells within the synovium. This results in a decreased firing of nociceptors and reduced pain perception. [3]

Effects of HA Injections

The efficacy of HA drug products may be related to how quickly HA reaches the target cells. In humans, low-molecular weight HA products have enhanced penetration into the joint space, allowing it to bind more effectively to cellular receptors. [3]

In animal models, medium-molecular weight preparations appear to be more effective in reducing synovial inflammation and restoring lubricating properties compared to higher-molecular-weight preparations. [3]

Studies of intra-articular injection of HA in humans show that 60-75% of patients receiving repeated treatments experienced significant benefits that lasted longer than those gained from corticosteroid injections. [7]

However, for intra-articular injections to be successful, veterinarians must identify the correct joint for intervention. Your veterinarian will identify the correct joint with a combination of flexion tests, imaging, and joint blocks.

It should also be noted that though HA helps to maintain lubrication and pain relief of the joint, its effect is temporary. Therefore, continued treatment is often necessary. [1]

Osteoarthritis Treatment

A number of studies have been performed to assess the efficacy of HA in management of equine osteoarthritis. One study showed that 40% of horses with middle carpal joint OA were free from lameness six weeks post treatment with sodium hyaluronate. [6]

Another study found that the effect of HA treatment can vary in duration of effectiveness. Initially, horses may respond well and lameness scores may improve, but some horses may have a return of lameness while others remain sound for months post-treatment. [6]

Historically, researchers have questioned whether HA has disease-modifying effects or just symptom-modifying effects. Research in humans and horses has since shown that multiple HA treatments can result in decreased histologic articular cartilage fibrillation (condition in which cartilage splits) and lead to improvements in synovial membrane parameters. These findings suggest that HA can improve overall disease markers of OA. [1]

Clinical Use

In a survey of equine practitioners, mid-molecular weight HA joint injections were the most preferred method of administering HA to horses. [1] For intra-articular HA injections, the standard dose is approximately 20 mg per site. Doses are usually repeated within 2-3 weeks. [5]

There are multiple HA injections approved for use in horses, including:

  • Legend®
  • Hyalovet®
  • Hylartin® V
  • Hyvisc®

Legend® is arguably the most well-known brand of HA injection in the equestrian community. In addition to symptomatic treatment, this product is often used as a preventive drug in performance horses. One study showed that horses treated prophylactically with Legend® raced longer before requiring further joint injections and earned more money than horses treated with a placebo. [1]

Corticosteroids & Hyaluronan Combination Therapy

Another popular and potentially beneficial practice is to inject HA in conjunction with a corticosteroid. This method has proven effective in human patients with osteoarthritis and the effect may be similar in horses. This approach is appealing because it allows for a reduced dose of corticosteroid and while leveraging the cartilage-repair potential of HA. [7]

Research on human OA concluded that combination therapy provided a more rapid improvement in pain, had beneficial effects during one-year post-treatment, and showed no negative side effects on joint structure. [1]

Another study performed with racing Thoroughbreds and Standardbreds showed that a combination of intra-articular HA and the corticosteroid methylprednisolone had better and longer lasting effects than methylprednisolone alone. [5]

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Use in Competition

Horses that perform high-intensity exercise are prone to joint and soft tissue injuries that can affect their ability to compete. Performance horses are especially susceptible to OA due to damage to the articular cartilage in joints. Among other changes within the joint, decreased synovial fluid viscosity is associated with loss of high molecular weight HA. [9]

Because of this, HA injections are commonly given to competitive horses, both for preventive and treatment purposes. HA preparations are allowed in competition, but intra-articular injections do come with some safety concerns due to risk of joint infection.

To mitigate the risk of infection, many owners and trainers opt for oral or intravenous HA products as an alternative to intra-articular injections. [9]

Side Effects

Horse owners should be aware that HA injections, specifically those given in the joint, can have complications. Repeated intra-articular injections are associated with an increased risk of flare reactions, also known as “joint flares”. Joint flares are reactive synovitis caused by inflammation soon after a joint infection.

A joint flare can result in grade 3/5 lameness, severe joint effusions, and marked changes in synovial fluid protein content. [5][10] While “joint flares” can mimic symptoms of a septic joint, prognosis and treatment are quite different. “Joint flares” typically resolve in 1-3 days with only rest and anti-inflammatories.

Though rare, one of the most concerning complications of injection of HA in any joint is sepsis. Symptoms of sepsis include pronounced synovial effusion (joint swelling) and severe lameness. [7]

It’s important to note that adverse reactions can occur with any type of intra-articular injection, which is why they should only be administered by a qualified veterinarian.

Qualified equine practitioners strictly adhere to aseptically preparing the injection site, using sterile gloves, single-use sterile syringes and needles, and avoid multiple-dose bottles or vials. [7][11]

Frequently Asked Questions

Here are some frequently asked questions about hyaluronic acid injections for horses:

Summary

Hyaluronan (hyaluronic acid) injections are widely used in horses to improve joint health and manage arthritis, as they help increase lubrication and reduce pain.

  • HA injections come in several different delivery methods: intravenous, intra-articular, and oral
  • HA injections are typically needed in repeated doses for the most beneficial effect
  • Medium-weight molecular preparations appear to be the most successful for use in horses
  • Combination HA and corticosteroid injections may provide more rapid pain relief than HA injections alone
  • HA injections are allowed in competition, but there are some safety concerns with intra-articular injections such as joint flares
  • The most serious side effect of intra-articular HA injections is sepsis, a serious injection of the joint
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References

  1. Frisbie. D. D., Joint Disease in the Horse. Elsevier. 2016.
  2. Richardson, D.W. and Loinaz, R., An Evidence-Based Approach to Selected Joint Therapies in Horses. Vet Clinics of N. America: Equine Practice. 2007.
  3. Williams, V.S., Intra-articular Hyaluronic Acid Supplementation in the Horse: The Role of Molecular Weight. Journal of Equine Vet Sci. 2007.
  4. Popot, M.A. et al., Hyaluronan in Horses: Physiological Production Rate, Plasma and Synovial Fluid Concentrations in Control Conditions and Following Sodium Hyaluronate Administration. Equine Vet J. 2004. View Summary
  5. Malone, E.D., Managing Chronic Arthritis. Vet Clinics: Equine Practice. 2002. View Summary
  6. Nedergaard, A. et al., Evidence of the Clinical Effect of Commonly Used Intra-Articular Treatments of Equine Osteoarthritis. Equine Vet Education. 2024.
  7. Caron, J.P., Intra-Articular Injections for Joint Disease in Horses. Vet Clin Equine. 2005. View Summary
  8. Niemelä, T.M. et al., A Randomised, Double-Blinded, Placebo-Controlled Clinical Study on Intra-Articular Hyaluronan Treatment in Equine Lameness Originating from the Metacarpophalangeal Joint. BMC Vet Research. 2016.
  9. Fernando de Souza, A. et al., What is the Evidence of Hyaluronic Acid and Polyacrylamide Hydrogel in Intra-Articular Therapy in Equines? Systematic Literature Review. J Equine Vet Sci. 2020. View Summary
  10. Frisbie, D.D., Evaluation of Polysulfated Glycosaminoglycan or Sodium Hyaluronan Administered Intra-Articularly for Treatment of Horses with Experimentally Induced Osteoarthritis. Am J Vet Res. 2009. View Summary
  11. Zanotto, G.M. and Frisbie, D.D., Current Joint Therapy Usage in Equine Practice: Changes in the Last 10 Years. Equine Vet J. 2021. View Summary