Bit & Bridle

Equine Panniculitis

Other Names

  • Equine Steatitis
  • Nodular Panniculitis
  • Fat Necrosis (Subcutaneous)
  • Weber-Christian Disease (horses)

What is Equine Panniculitis?

Equine Panniculitis is an inflammatory condition affecting the subcutaneous adipose tissue — the layer of fat that sits just beneath the skin. Inflammation within this fat layer causes the formation of painful nodules, plaques, or diffuse swelling under the skin, and in some cases, necrosis (death) of fat tissue that can subsequently drain through the skin surface.

The condition is relatively uncommon in horses but can be challenging to manage. It can affect localised areas or present as a widespread multifocal disorder involving large areas of the body. When associated with systemic signs — fever, depression, weight loss — it is referred to as systemic panniculitis, which represents a more serious presentation than isolated cutaneous involvement.

Panniculitis in horses can occur at any age and in any breed, though some cases appear linked to underlying systemic diseases. Accurate diagnosis is important because treatment approaches vary significantly depending on the underlying cause.

Causes

Panniculitis in horses can be triggered by a variety of underlying conditions or agents:

  • Infectious causes: Bacterial infections — particularly Streptococcus equi, Staphylococcus species, Corynebacterium pseudotuberculosis (causing pigeon fever/dryland distemper), and mycobacterial infections — can directly infect subcutaneous fat. Fungal organisms and parasitic larvae have also been implicated.
  • Immune-mediated disease: Immune complex deposition within the subcutaneous tissue, analogous to lupus panniculitis or nodular panniculitis in other species, can drive sterile inflammation.
  • Trauma: Direct physical injury causing fat necrosis and subsequent sterile or septic inflammation
  • Injections: Some injectable medications, particularly oil-based preparations, can cause fat necrosis at injection sites — a form of localised panniculitis
  • Nutritional: Vitamin E deficiency has been associated with steatitis (fat necrosis) in various species and may play a role in some equine cases, particularly those involving fat deposits in the muscle and subcutis
  • Idiopathic: In some cases, no underlying cause is identified despite thorough investigation

Symptoms and Signs

Clinical signs depend on the extent and cause of the panniculitis:

  • Single or multiple firm to fluctuant nodules under the skin, typically ranging from 1 cm to several centimetres in diameter
  • Swelling, heat, and pain on palpation of affected areas
  • Skin discolouration over affected nodules — the overlying skin may become thinned, discoloured, or alopecic
  • Draining tracts — nodules may rupture and discharge oily, yellowish to brown fluid or purulent material
  • Fever and systemic illness in cases with widespread involvement
  • Lethargy, inappetence, and weight loss in systemic cases
  • Pain and reluctance to move if extensive lesions affect weight-bearing areas or are compressive

Lesions most commonly appear on the trunk, neck, and ventral abdomen. The number and distribution of nodules vary widely from a single isolated lesion to dozens of nodules scattered across the body.

Diagnosis

Diagnosis requires confirmation of panniculitis and investigation for an underlying cause:

  • Fine needle aspirate (FNA) and cytology: A needle is used to aspirate material from a nodule for microscopic examination. This can identify inflammatory cell types, bacteria, fungi, or fat necrosis, guiding further investigation.
  • Biopsy and histopathology: Full-thickness biopsy of an affected nodule is the definitive diagnostic procedure. Histopathology characterises the nature of the inflammation (neutrophilic, granulomatous, lymphocytic) and may identify infectious organisms.
  • Bacterial and fungal culture: From biopsy or aspirate material, to identify infectious pathogens and guide antibiotic or antifungal selection.
  • Haematology and biochemistry: Blood work assesses systemic involvement and inflammatory markers.
  • Vitamin E and selenium levels: To investigate nutritional deficiency as a contributing factor.
  • Immunological testing: In cases where immune-mediated disease is suspected, additional tests for antinuclear antibodies and other immune markers may be considered at referral level.

Treatment

Treatment is directed at the underlying cause where one is identified:

  • Infectious panniculitis: Targeted antibiotics based on culture and sensitivity, administered for extended courses (weeks to months). Surgical drainage of fluctuant abscess-like lesions may be required.
  • Immune-mediated panniculitis: Immunosuppressive doses of corticosteroids (dexamethasone or prednisolone) are used to suppress the abnormal immune response. Treatment courses are typically prolonged, with gradual dose reduction as lesions resolve.
  • Nutritional support: Vitamin E supplementation where deficiency is identified.
  • Wound management: Draining lesions require regular cleaning and protection. In some cases, surgical debridement of necrotic tissue accelerates healing.
  • NSAIDs: Anti-inflammatory pain relief with phenylbutazone or flunixin meglumine helps manage pain and systemic inflammation.
  • Idiopathic panniculitis: In cases with no identifiable cause, corticosteroids often produce improvement, though the mechanism is not entirely clear.

Prevention

Prevention is largely targeted at the known risk factors:

  • Maintain adequate Vitamin E intake — ensure access to fresh pasture or supplement appropriately for horses in dry lot or on stored forage
  • Practice careful injection technique and site rotation to minimise injection-site reactions
  • Manage wounds promptly to prevent secondary bacterial infection of subcutaneous tissues
  • Control insect vectors that may transmit organisms causing pigeon fever in regions where it is endemic

Prognosis

Prognosis varies considerably. Localised infectious panniculitis treated promptly with appropriate antibiotics and drainage typically resolves well. Immune-mediated panniculitis often requires long-term corticosteroid therapy and may relapse when medication is reduced. Idiopathic and multisystemic panniculitis cases are more variable — some respond well to treatment whilst others prove frustratingly refractory. The overall condition and immune status of the horse significantly influence the outlook.

When to Call the Vet

Contact your vet if you notice:

  • Lumps or nodules developing under the skin of your horse, particularly if they are painful or appear to be growing
  • Skin lesions discharging fluid or pus
  • Your horse develops skin changes alongside systemic signs such as fever, weight loss, or lethargy
  • Existing nodules appear to be increasing in number or size

Frequently Asked Questions

Is equine panniculitis contagious?

Most forms of equine panniculitis are not directly contagious between horses. However, infectious forms caused by Corynebacterium pseudotuberculosis (pigeon fever) can spread between horses in endemic areas via insect vectors. Standard biosecurity — isolation of affected animals, fly control, and hygiene — is advisable when an infectious cause is suspected.

How long does treatment take?

Treatment duration varies significantly. Localised cases may resolve within four to eight weeks with appropriate treatment. Immune-mediated panniculitis often requires months of corticosteroid therapy with gradual dose reduction. Owners should be prepared for extended treatment periods and the possibility of relapse.

Can panniculitis cause permanent scarring?

Yes. Draining nodules that require surgical drainage or that rupture spontaneously can leave areas of fibrosis and scarring under the skin and at the skin surface. In most horses this is cosmetic rather than functionally significant, though extensive scarring over pressure points or under tack may cause discomfort.

Is panniculitis related to skin tumours?

Panniculitis produces benign inflammatory nodules rather than neoplastic growths. However, some skin tumours — particularly mast cell tumours and lymphoma involving the subcutis — can mimic panniculitis clinically. Biopsy and histopathology are essential for accurate differentiation. See also: Equine Squamous Cell Carcinoma for an important neoplastic skin condition and subcutaneous tissue for anatomical background.