Other Names
- Equine Systemic Granulomatous Disease
- Generalised Granulomatous Disease
- Idiopathic Systemic Granulomatosis
- Multisystemic Eosinophilic Epitheliotropic Disease (MEED) — a related but distinct condition
What is Equine Sarcoidosis?
Equine Sarcoidosis — also referred to as Equine Systemic Granulomatous Disease — is a rare, poorly understood inflammatory condition characterised by the formation of non-caseating granulomas (organised collections of inflammatory cells, primarily macrophages) in multiple organs throughout the body. The condition was originally named by analogy to human sarcoidosis, which shares the same histopathological pattern, though the two conditions are not necessarily equivalent in cause or behaviour.
In horses, granulomas form most commonly in the skin, lymph nodes, liver, lungs, and bone marrow, though virtually any organ can be involved. The condition is uncommon and its exact prevalence is uncertain, but it is recognised globally and can cause significant morbidity and weight loss in affected horses.
Equine sarcoidosis should not be confused with equine sarcoids, which are a distinct and far more common condition — locally invasive skin tumours of fibroblastic origin associated with bovine papillomavirus.
Causes of Equine Sarcoidosis
The cause of equine sarcoidosis is not established. As in human sarcoidosis, it is believed to represent an abnormal immune response — likely a dysregulated granulomatous reaction to an unidentified antigen (a foreign substance or altered self-antigen). Proposed causes include:
- Infectious agents: Mycobacteria have been investigated as potential triggers (by analogy with human sarcoidosis, where mycobacterial antigens have been implicated), though no consistent infectious cause has been proven in horses.
- Environmental antigens: Inhalation of or exposure to environmental particles triggering an aberrant immune response has been hypothesised.
- Immune dysregulation: An underlying defect in immune regulation may predispose certain individuals to form granulomas in response to stimuli that would not affect normal horses.
- Drug reactions: Rarely, granulomatous reactions in horses have been linked to specific drug or vaccine exposures, though this does not account for most cases.
Symptoms and Signs
Clinical presentation depends on which organs are predominantly affected and the extent of disease:
- Skin lesions: Scaling, crusting, and thickening of the skin, often with hair loss — particularly affecting the face, limbs, and perineum. The skin may appear crusty, scurfy, or depigmented in patches. Pruritus (itching) is variable.
- Weight loss and poor condition: Often one of the most prominent features — significant, progressive weight loss despite apparently adequate nutrition, due to systemic inflammation and malabsorption.
- Generalised lymph node enlargement
- Respiratory signs: Coughing, exercise intolerance, and laboured breathing when the lungs are significantly involved
- Peripheral oedema: Ventral oedema and leg swelling due to hypoproteinaemia from liver involvement or protein-losing enteropathy
- Exfoliative dermatitis: In severe cutaneous cases, large sheets of skin may scale and slough
- Lethargy and depression
- Fever: Intermittent pyrexia in some cases
Diagnosis
Equine sarcoidosis is a diagnosis of exclusion — other more common causes of systemic illness, granulomatous disease, and skin disease must be ruled out before this diagnosis is made. The diagnostic process typically includes:
- Biopsy: Tissue biopsy (skin, lymph node, or liver if indicated) showing non-caseating granulomas is the hallmark finding. The granulomas in sarcoidosis do not contain central necrosis (hence “non-caseating”), distinguishing them from infectious granulomas caused by mycobacteria or fungi.
- Exclusion of infectious causes: Culture and sensitivity testing, fungal culture, and PCR testing for mycobacteria should be performed on biopsy material to exclude infectious granulomatous conditions.
- Haematology and biochemistry: Often reveals anaemia of chronic disease, elevated globulins (hyperglobulinaemia), hypoalbuminaemia, and elevated liver enzymes. A marked increase in total protein with a reversed albumin:globulin ratio is a common pattern.
- Rectal biopsy: Granulomas may be identifiable in rectal mucosa biopsies in cases with gastrointestinal involvement, providing a relatively accessible diagnostic sample.
- Chest radiography and abdominal ultrasound: To assess pulmonary involvement and intra-abdominal lymph node and organ changes.
Treatment
Treatment is challenging and outcomes are variable:
- Corticosteroids: Immunosuppressive doses of dexamethasone or prednisolone are the primary treatment, aimed at suppressing the aberrant granulomatous response. Some horses respond initially but relapse when treatment is reduced or withdrawn.
- Long-term immunosuppression: Cases that respond to steroids may need prolonged or lifelong low-dose immunosuppressive therapy to maintain remission — this carries risks including susceptibility to infection and laminitis (from steroid use).
- Nutritional support: High-quality, easily digestible nutrition is important to counter the cachexia (muscle wasting and weight loss) associated with the condition. Feeds with high fat content and good-quality protein support muscle maintenance.
- Topical treatment: Skin lesions may benefit from topical emollients, medicated shampoos, or topical steroids to manage discomfort and crusting.
- Euthanasia: In horses that do not respond to treatment and whose quality of life deteriorates significantly, euthanasia is the most humane option.
Prevention
There is no established means of prevention for equine sarcoidosis, as the underlying cause is unknown. Routine veterinary examinations and attention to unexplained weight loss or skin changes allow for earlier diagnosis — earlier intervention offers the best chance of sustained response to treatment.
Prognosis
Prognosis is guarded to poor for most cases of equine sarcoidosis. Some horses achieve partial or temporary improvement with corticosteroid treatment, but complete and sustained remission is uncommon. The condition tends to be progressive, and many affected horses are eventually euthanised due to deteriorating quality of life. Horses with predominantly cutaneous disease appear to have a somewhat better prognosis than those with widespread systemic involvement, though even skin-predominant disease can progress.
When to Call the Vet
Consult your vet if your horse shows:
- Progressive, unexplained weight loss
- Generalised skin scaling, crusting, or hair loss — particularly if involving the face and multiple body regions
- Persistent respiratory signs without a clear infectious cause
- Generalised lymph node enlargement
- Ventral oedema combined with weight loss and lethargy
- Any combination of the above signs that does not respond to standard treatments for more common conditions
Frequently Asked Questions
Is equine sarcoidosis the same as an equine sarcoid?
No — these are entirely different conditions despite the similar names. An equine sarcoid is a common, locally invasive skin tumour caused by bovine papillomavirus, affecting the skin only and not causing systemic illness. Equine sarcoidosis is a rare systemic inflammatory condition causing granuloma formation in multiple organs. The naming overlap is unfortunately confusing for horse owners encountering one or both conditions.
Can equine sarcoidosis be cured?
Current evidence suggests that true cure is uncommon. Many horses achieve periods of improvement with corticosteroid treatment, but the condition tends to relapse. Research into this condition in horses is limited compared to the human disease, partly due to its rarity. Referral to an equine internal medicine specialist is advisable for any confirmed case.
How is equine sarcoidosis diagnosed conclusively?
Definitive diagnosis requires tissue biopsy showing the characteristic non-caseating granuloma pattern, combined with exclusion of infectious causes (particularly mycobacterial and fungal disease) that produce similar histological findings. Blood tests and imaging are supportive but not sufficient alone. The diagnosis is, in practice, often presumptive based on the clinical picture, blood work, and response to treatment in horses where biopsy is not performed.
Is equine sarcoidosis contagious?
No. Equine sarcoidosis is an immune-mediated condition and is not contagious. There is no evidence of transmission between horses or from horses to humans. However, since the condition may be triggered by an as-yet-unidentified environmental antigen, it is worth considering whether other horses in the same environment are showing any similar signs, and reporting this to your vet.