Cause
The cause of autoimmune diseases is the production of autoantibodies against the animals own tissues. The underlying reasons for these disorders occurring are complex and not fully understood.

Infectious agents are thought to be the initiating factor in some of these disorders eg Systemic Lupus Erythematosus (SLE)

The common signs of autoimmune disease in animals depend upon the target organ(s) for the autoantibodies :

Adrenal gland

• Some cases of canine hypoadrenocorticism (Addisons disease) are due to autoantibody production against the adrenal glands.
• Signs include :
  • Muscle weakness - the dog collapses during exercise
  • Depression
  • Sudden collapse and shock
  • Kidney failure
  • Inappetance
  • Diarrhoea - sometimes contains blood
  • Abdominal pain
  • Increased thirst (polydipsia)
  • Increased urine production (polyuria)
  • Weight loss

Blood 

• Autoimmune haemolytic anaemia (dogs, cats (rare - often secondary to leukaemia), cattle (very rare), horses (rare - usually secondary eg abscesses). It can occur with autoimmune thrombocytopenia.
• Due to autoantibodies against the animals own red blood cells causing them to disrupt (called haemolysis)
• Coombs test is positive in these patients
• Anaemia may be present on routine haematological examination
• Free haemoglobin is present in blood and urine
• The animal may develop jaundice (accumulation of yellow pigment in tissues)
• Affected animals are lethargic
• Extremities (ears, feet, nose and tail) may become blue (cyanosed) or reddened, swollen, ulcerated and crusts may form.
• Enlargement of the spleen may occur
• Enlargement of lymph nodes may occur
• Sometimes autoimmune haemolytic anaemia has a primary cause eg it can be induced by drugs

• Autoimmune thrombocytopenia  (dogs, cats (rare), horses. It can be secondary to systemic lupus erythematosus - see below). It can occur with autoimmune haemolytic anaemia
  • Due to autoantibodies against platelets which are important in blood clotting.
  • Haemorrhages into the skin (small petechial haemorrhages, larger ecchymotic haemorrhages (patches) or sometimes massive haemorrhages - nose bleeds or internal bleeding into the abdomen or chest )
• Systemic lupus erythematosus (SLE) - see below

Musculoskeletal System

• Myasthenia gravis (dogs and cats)
  • The acquired form of this disease is associated with the production of autoantibodies against acetylcholine receptors on muscle. Signs include :
  • Poor exercise tolerance
  • Muscle weakness
  • Difficulty eating and swallowing
  • Regurgitation of food
  • Dilation of the oesophagus (megaoesophagus)
• Rheumatoid arthritis - Autoantibodies against IgG are associated with rheumatoid arthritis the signs of which are :
  • Lameness
  • Swollen joints - often affects the same joints in the left and right limbs.
  • Restricted range of movement in joints, and in advanced cases there may be no movement in the joint
  • In some cases the joint dislocates because local ligaments rupture, and so there is an excessive range of movement
  • Crepitus (a grating sensation and clicking or cracking sound) is present when the joints are manipulated
  • High body temperature
  • Inappetance
• Systemic lupus erythematosus (SLE) - see Multiple organ autoimmune diseases below.

Skin

• Bullous Autoimmune Skin Disease
  • Pemphigus vulgaris (dogs and cats)
    • Erosions or ulcers around the mucocutaneous junctions (mouth, nails)
    • The lesions discharge serum and crusts form over the ulcers
    • Depression
    • Inappetance
    • Death
  • Pemphigus foliaceus (most common canine and equine autoimmune skin disease, also reported in cats and goats)
    • Small swellings under the surface of the skin (bullae or pustules)
    • Crusting
    • Scale (scurf or dandruff) formation
    • Loss of hair (alopecia)
    • Black pigmentation of the skin (hyperpigmentation)
    • Sloughing of the foot pads
    • Itchiness (pruritus)
    • Usually affects the head and nose, but can be generalised
  • Pemphigus vegetans (rare- dog))
    • Pustules
    • Crusting
    • Papilloma formation
    • Can be generalised or in the groin area
  • Pemphigus erythematosus (dog and cat)
    • Discharging sores
    • Crusting
    • Itchiness (pruritus)
    • Affects mainly the skin around the eyes, the ears and on the bridge of the nose.
    • The disease gets worse if the skin is exposed to sunlight
  • Bullous pemphigoid (dog and horse)
    • Erosions or ulcers around the mucocutaneous junctions (eg lips and mouth), the ears or the groin
    • The lesions discharge serum and crusts form over the ulcers
    • Depression
    • Inappetance
    • High body temperature - due to secondary infection

Thyroid

• Autoantibodies against thyroglobulin (and occasionally against thyroid hormone T3)  are associated with lymphocytic thyroiditis  - the main cause of hypothyroidism in dogs. Signs of hypothyroidism include :
  • Bilateral hair loss (alopecia)
  • Thinning of the skin
  • Lethargy
  • Black pigmentation of the skin
  • Scurf (dandruff)
  • Lethargy
  • Obesity
  • Cold intolerance - seeks warm places to lie down
  • Reproductive problems.
  • Fat deposits in the cornea of the eye (lipidosis)
  • Dry eye (keratitis sicca)
  • Diarrheoa
  • Vomiting
  • Constipation
  • Slow heart rate (bradycardia)
  • Muscle weakness

Multiple Organ Autoimmune Diseases

• Systemic lupus erythematosus (SLE) - (dogs and cats)
  • Arthritis (rheumatoid) in several joints - see above - most common presenting sign
  • Skin disease - second most common presenting sign
    • Bilaterally symmetrical loss of hair and production of scurf (scale or dandruff) 
    • Ulcerations and crusting form in severe cases - often affecting the ears, feet and head
  • High body temperature - does not respond to antibiotics, does respond to corticosteroids (present in over 50% of cases)
  • Inappetance
  • Anaemia - causes lethargy. Positive Coombs test.
  • Thrombocytopenia (low platelet count - less than 50,000 per cubic mm - present in about 30% of cases) - may lead to haemorrhages
  • Neutropenia - low white cell count (neutrophils) (present in up to 50% of cases)
  • Kidney disease - glomerulonephritis (in about 25% of cases) - protein leaks into urine.
  • Weight loss 
  • High total serum protein

Complications
Multiple organ system involvement is common and this can make diagnosis difficult.

• Adrenal insufficiency (Addisons disease)
  • Indirect immunofluorescence tests
  • Low blood cortisol concentrations (below 35 nmol/l)
  • Low plasma sodium concentration (less than 135 mmol/l)
  • High plasma potassium concentration (over 5.5 mmol/l)
  • High blood calcium concentrations (in about 50% of cases)
  • High blood urea and creatinine (about 75% of cases)
  • Poor ACTH stimulation test
  • Histopathological examination of adrenal biopsy

Blood

• Autoimmune haemolytic anaemia
  • Coombes test positive
• Autoimmune thrombocytopenia
  • low platelet count (less than 40,000/cubic mm)
  • Sometimes prolonged ACT (activated coagulation time)
  • Immunofluorescence tests
  • ELISA test

• Myasthenia gravis
  • Immediate response to therapy with 0.1-1.0 mg edrophonium chloride (an anti-cholinesterase drug)
• Rheumatoid arthritis - diagnosis is difficult
  • Rheumatoid Arthirtis (RA) factors may be present on serology - titre needs to be 1:16 or higher
  • Synovial fluid examination - white blood cell counts exceed 3,000/ cubic mm and can be very high.
  • Histopathological examination of synovial membrane biopsies.
  • Radiological findings :
    • Loss of joint space (due to loss of articular cartilage)
    • Soft tissue swelling 
    • Radiolucent areas in subchondral bone - typical findings
    • Other secondary changes eg DJD, ankylosis, luxation

• Systemic lupus erythematosus (SLE)
  • Positive ANA assay test - low titres may be present in NORMAL dog serum - so care is needed in interpretation.The titre should be higher than 1:100.
  • Positive LE cell preparation

• Bullous autoimmune skin diseases
  • Drugs to suppress the immune response. Combinations of these are often given together :
    • Corticosteroids - eg prednisone or prednisolone 1-2mg/kg body weight by mouth twice daily
    • Cytotoxic drugs
      • azathioprine 2mg/kg body weight daily or on alternate days OR 
      • cyclophosphamide 2 mg/kg body weight daily for 4 days per week
    • Gold salt therapy - 1mg.kg body weight of sodium aurothiomalate every week
    • Megestrol acetate (cats only)
  • Avoid exposure to sunlight (cases of pemphigus erythematosus)

Thyroid

• Lymphocytic thyroiditis
  • Synthetic thyroid hormone (T4) replacement therapy