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. [1]

If left untreated, the infection can spread to adjacent muscles and bones, causing severe and debilitating lameness. [2]

Resolving cellulitis in horses can be challenging and may necessitate surgical drainage. [2] 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. [3]

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.

Clinical Signs

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.

Swelling usually begins in one spot, quickly spreading to the rest of the limb. Other symptoms that can accompany swelling include: [1][4]

  • Lameness
  • Heat in the limb
  • Sensitivity to touch
  • Pitting (swelling that produces indentation when pressed)
  • Fever (greater than 101.5oF or 38.6oC
  • Depression
  • 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.[4][5]

Secondary Complications

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: [4]

  • 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.

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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. [6]

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. [6]

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. [1]

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. [7]

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. [7]

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. [8]

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. [9]

Diagnosis

Early detection and timely diagnosis of cellulitis are critical factors for a favorable prognosis.

Cellulitis is typically diagnosed based on clinical signs in the affected horse. It is important to consult with your veterinarian to obtain an accurate diagnosis. [1][4]

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.

Diagnostic Imaging

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. [4]

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. [10]

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. [10]

Differential Diagnosis

Cellulitis is characterized by rapid onset of extensive swelling and lameness, making an accurate diagnosis crucial for effective treatment and optimal outcomes.

It is also important to consider and rule out other conditions that can present with similar signs, including: [4][11]

Cellulitis vs. Lymphangitis

Cellulitis is often confus