Cellulitis refers to bacterial infection and inflammation that affects tissues under the skin anywhere in the body. In horses, this severe condition typically occurs in the hindlimbs. It is characterized by a sudden onset with significant swelling and intense pain. 
If left untreated, the infection can spread to adjacent muscles and bones, causing severe and debilitating lameness. 
Resolving cellulitis in horses can be challenging and may necessitate surgical drainage.  Some horses experience recurrent flare-ups but implementing proper management practices can help reduce the likelihood of cellulitis flare-ups in your horse.
If you observe signs of cellulitis in your horse, seek immediate veterinary care. Cellulitis can rapidly spread up the leg and lead to sepsis, a severe and life-threatening condition. 
Cellulitis in Horses
Cellulitis is an inflammatory condition characterized by infection and acute inflammation of subcutaneous tissue.
It is commonly localized in the distal limbs, particularly the hindlimbs. It typically occurs due to pathogenic bacteria entering through skin wounds or breaks.
The infection leads to the infiltration of inflammatory cells, such as neutrophils and macrophages, which can lead to swelling (edema) and formation of abscesses.
Signs of cellulitis include severe swelling, pain, heat, and compromised limb function. Affected horses often exhibit a rapid onset of clinical signs, with severity ranging from mild to severe.
- Heat in the limb
- Sensitivity to touch
- Pitting (swelling that produces indentation when pressed)
- Fever (greater than 101.5oF or 38.6oC
- Anorexia (loss of appetite)
- Tachycardia (fast heartbeat)
- Draining tracts on the skin’s surface and fluid discharge
Blood tests may show high white blood cell, neutrophil and fibrinogen counts, indicating infection and inflammation. Discharge may start to leak from the affected area within 24 to 48 hours after onset. If the infection goes untreated, it can potentially lead to systemic illness.
Many affected horses develop supporting-limb laminitis in the contralateral leg (on the opposite side of the body). The limb with cellulitis is unable to bear weight, causing uneven and excessive weight bearing on the opposing limb. Unfortunately, affected horses that become laminitic often have a poor prognosis.
In addition to laminitis, several other secondary complications can develop in horses with cellulitis. These include: 
- Large areas of skin necrosis (dead tissue)
- Bacteremia (bacteria in the blood)
- Osteomyelitis (inflammation in the bones)
- Septic arthritis
- Endocarditis (inflammation of the heart)
If you observe signs of cellulitis, it is crucial to contact your veterinarian immediately. They can accurately diagnose the condition and initiate appropriate treatment.
Causes of Cellulitis in Horses
The precise underlying cause of cellulitis in horses remains unknown. Approximately 50% of cases develop without any apparent cause or specific trigger. 
Cellulitis is categorized into two types: primary and secondary. Primary cellulitis occurs without any identifiable underlying cause. While it is more commonly observed in Thoroughbred racehorses, any horse can develop primary cellulitis. 
Secondary cellulitis develops following trauma to the horse’s limb, resulting in an infection under the skin. Potential traumatic triggers for cellulitis include surgery, penetrating wounds, and joint injections. 
The bacterial organisms commonly associated with secondary cellulitis are Staphylococcus aureus and Streptococcus. These bacteria produce potent toxins that damage blood cells, skin and nerve cells in the affected area. 
These bacterial toxins also trigger a massive immune response involving several types of immune cells, including macrophages and white blood cells. The cytokines produced by these cells stimulate blood flow to the affected area, leading to increased fluid accumulation and subsequent swelling. 
Limb edema may be a risk factor for recurrent cellulitis in horses. Each bout of cellulitis is thought to impair the lymphatic system, resulting in persistent edema even after treatment and increasing the risk of future cellulitis flare-ups. However, more research is needed to understand this relationship. 
Bathing Practices & Skin Health
Another potential cause of cellulitis in racehorses is improper bathing practices and unhygienic equipment (i.e. hoses, scrapers, etc.). Cleaning equipment that comes in contact with multiple horses could transmit bacteria that can cause cellulitis.
It is important to maintain proper hygiene and cleanliness to minimize the risk of infection. Following good biosecurity protocols can also help to limit the transmission of disease.
Skin conditions that lead to dryness or cracking of the skin can create opportunities for bacteria to penetrate the subcutaneous tissue. Several different factors can contribute to skin conditions, including exposure to mud, dry climates, or improper bathing and drying practices. 
Early detection and timely diagnosis of cellulitis are critical factors for a favorable prognosis.
Your veterinarian will perform a thorough physical examination, evaluate the horse’s medical history, and may conduct additional diagnostic tests if necessary to confirm the diagnosis and determine the cause of cellulitis.
Diagnostic tests may include a complete blood count (CBC) to assess any abnormal changes, a serum chemistry profile to evaluate organ function and electrolyte levels, bacterial culture analysis to identify the specific bacteria causing the infection, and diagnostic imaging tests such as ultrasound or radiography to assess the extent of tissue involvement and rule out other underlying conditions.
These tests help the veterinarian to gather important information and guide appropriate treatment decisions for the affected horse.
Invasive procedures such as sampling of the synovial fluid from joints should be avoided. These procedures can potentially introduce pathogens into healthy tissue, leading to the spread of infection.
Ultrasonography is a valuable diagnostic tool for evaluating soft tissue structures and blood flow in cases of cellulitis. Ultrasound can show the presence of necrotic tissue, gas, or deep-seated fluid accumulation (edema) within the limb. This allows clinicians to determine the areas that may need to be drained. 
If the lameness and inflammation associated with cellulitis are severe, your veterinarian may opt to perform an X-ray examination to determine whether the infection has spread to joints or bones.
Flow-phase scintigraphy is a physiological imaging method that can be used to examine the entirety of the horse, instead of a restricted area. It can assess blood flow in the body and identify excessive soft-tissue inflammation and lesions. 
Many horses with cellulitis tolerate scintigraphy better than ultrasound or radiography as it does not involve putting external pressure on the affected leg.
Unlike ultrasound or radiography, scintigraphy does not involve applying external pressure to the affected leg, making it a well-tolerated option for many horses with cellulitis. This imaging method offers valuable insights monitoring the condition. 
Cellulitis is characterized by rapid onset of extensive swelling and lameness, making an accurate diagnosis crucial for effective treatment and optimal outcomes.
- Bone fracture
- Mud fever (dermatophilosis)
- Necrotizing fasciitis or flesh-eating bacteria
- Purpura hemorrhagica (PH)
- Tendon or tendon sheath infection
- Bowed tendon
- Suspensory ligament desmitis (SLD)
- Equine viral arteritis (EVA)
Cellulitis vs. Lymphangitis
Cellulitis is often confused with lymphangitis, otherwise known as big leg disease or stovepipe leg. Both conditions involve bacterial infection leading to excessive swelling and pain in the lower legs. 
While cellulitis involves infection and inflammation of subcutaneous tissue, lymphangitis affects the lymphatic vessels in the horse’s body. These vessels run alongside blood vessels to collect fluid from around cells and return it back to general circulation.
Lymphangitis involves bacteria not commonly seen in cellulitis, namely Corynebacterium pseudotuberculosis. Bacterial culture can identify which organism is involved and which tissues are most likely affected.
Horses with lymphangitis may have small pus-filled nodules on the legs accompanying the swelling, which can extend further up the leg to the chest and abdomen. The necrotizing tissue sometimes seen with cellulitis is not common in lymphangitis.
If your horse is diagnosed with cellulitis, your veterinarian will determine the most appropriate treatment and advise you on providing necessary supportive care for your horse.
A cellulitis infection must be treated promptly to prevent the bacteria from spreading and reaching the bloodstream. If left untreated, a systemic inflammatory response (known as blood poisoning or sepsis) may occur, which can be fatal. 
Treatment with antibiotics will prevent the spread of the infection and decrease the risk of chronic lameness and disfigurement. 
Veterinarians typically treat cellulitis infections with broad-spectrum antibiotics, including high doses of penicillin or trimethoprim-sulfonamide. Broad-spectrum antibiotics are used until bacterial culture results are received to identify which bacteria to target.
Antibiotics such as ceftiofur, ampicillin, or gentamicin can be used as alternatives if penicillin proves ineffective.  The antibiotic metronidazole is often combined with penicillin for the treatment of deep fascia cellulitis. 
Polymicrobial infections, or infections caused by several microorganisms are also possible and may require specific antibiotic combinations. 
Regional Limb Perfusion (RLP)
Antimicrobials can be administered intravenously (IV) or locally via regional limb perfusion (RLP).
A tourniquet is applied to the upper leg above the infection to put pressure on the veins and prevent the normal flow of blood out of the leg.  The antibiotic solution is then injected directly into the affected area. This procedure is usually performed while the horse is standing and sedated. 
RLP is beneficial as it delivers a high concentration of antimicrobials to the affected limb, maximizing efficacy and minimizing treatment costs. RLP not only targets and treats the affected area directly, but it also reduces the risks of toxicity, development of antibiotic resistance, and potential systemic effects that could compromise the horse’s overall health. 
This treatment should only be performed by your veterinarian, as there is a risk of tissue necrosis (tissue death) with IV perfusion. If the infection has spread above the leg, this treatment will likely be ineffective.
Corticosteroids may be utilized in the treatment of severe cellulitis that is localized to the leg. While these drugs can effectively reduce inflammation and edema in the affected area, they also suppress the immune system, potentially impacting the horse’s ability to combat infection. 
Dimethyl sulfoxide (DMSO) is a chemical solvent that has anti-inflammatory and analgesic properties. It acts as a diuretic in horses, pulling fluids from tissues. 
DMSO can be administered topically or intravenously to reduce soft tissue swelling, edema and inflammation that forms secondary to trauma. 
Anecdotally, it has been used in conjunction with other treatments to reduce wound-associated limb edema and endotoxin-induced tissue damage. DMSO may have bacteriostatic properties, meaning it can inhibit the reproduction of bacteria to decrease the spread of cellulitis. 
Although there is a lack of information about doses and proper handling, the beneficial properties of DMSO suggest it could be used alongside an antibiotic to help treat cellulitis.
While your horse is receiving treatment for cellulitis, your veterinarian may suggest further measures to help reduce swelling and promote healing.
It is crucial to keep your horse comfortable and to promote weight-bearing on the affected leg. This helps prevent laminitis in the hoof of the opposite limb. 
Applying a bandage to the affected leg can help reduce swelling, maintain cleanliness, and promote dryness. It is important to ensure correct bandaging techniques to avoid compromising circulation, which may worsen the condition.
The use of hydrotherapy, such as regularly rinsing the affected leg with cold water, can be beneficial in reducing inflammation and alleviating pain. Cooling the leg helps to decrease blood flow and prevent further damage caused by swelling. 
In severe cases, surgical debridement may be necessary to remove damaged tissues. This procedure aims to prevent the spread of infection, reduce bacterial load, and facilitate healing. 
Preventing Secondary Laminitis
Your veterinarian may also recommend supportive care measures for the contralateral leg to prevent laminitis from developing. Preventative measures may include the use of frog-support pads and supportive boots.
Non-steroidal anti-inflammatory drugs (NSAIDs), such as phenylbutazone (Bute) and flunixin meglumine, are frequently used in horses to alleviate pain and inflammation. For horses in severe pain, additional relief can be provided by xylazine, epidural morphine and/or transdermal opioid (i.e. fentanyl patches). 
The prognosis for horses with cellulitis varies greatly between individual horses. With early detection of the infection and prompt medical intervention, a favorable prognosis is highly probable for the horse.
The survival rate for horses with limb cellulitis is between 77 – 89%, with a return to athletic function or performance of approximately 69%.
The best way to prevent cellulitis is to be vigilant of injuries or skin damage that could enable pathogens to enter the wound and trigger cellulitis. Ensure fences, paddocks and stalls are well maintained to reduce the risk of injury and infections leading to cellulitis.
Additional measures to reduce the risk of cellulitis in your horse include:
- Keeping shelters dry and clean will help to minimize the risk of infection, should your horse develop a skin wound.
- Practice proper grooming techniques, especially on the legs and feet, to keep the skin and coat clean.
- Ensure good hygiene practices in your barn, including regularly cleaning stalls and avoiding sharing of grooming tools.
- Contacting your veterinarian immediately if your horse has a penetrating wound to begin prophylactic antibiotic treatment.
Sebum, a natural oily substance produced by the sebaceous glands, plays a crucial role in maintaining a protective barrier on the skin’s surface. It helps to prevent bacteria from entering through damaged areas in the skin, reducing the risk of infections such as cellulitis. 
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