Cause
The primary cause of SLE is unknown in animals and humans although several factors have been suggested, including :

• Genetic inheritance
• Exposure to environmental factors - eg ultraviolet light - which definitely induces a photosensitivity reaction in affected animals
• Exposure to viral infections
• Endocrine factors
• Drug administration (anticonvulsants, contraceptives, hydralazine, isoniazid, penicillamine, procainamide and vaccinations (dogs))
• Suppressor T cell deficiency and other immunological disorders

Whatever the primary cause the disease is characterised by the production of non-specific antibodies (called antinuclear antibodies - ANAs)  by the animal which may attack specific cells or tissues, for example :

• Red blood cells - erythrocytes
• White blood cells - leukocytes
• Platelets

These ANAs combine with free DNA to form DNA-antiDNA complexes and can become deposited in a variety of tissues causing a Type III hypersensitivity reaction,  including :

• The walls of arterioles causing necrosis and fibrosis
• The synovial membrane
• The glomeruli - resulting in glomerulonephritis

It is interesting to note that both dogs and humans with SLE have low concentrations of thymic factors in their blood.

The clinical signs associated with SLE in dogs include :

• Anaemia - due to haemolysis of red blood cells *
• Coagulopathy due to thrombocytopenia
• Fever *
• Recurrent shifting lameness associated with a polyarthritis - seen in 75% of cases *
• Recurrent shifting lameness associated with a polymyositis 
• Lymphadenopathy
• Myocarditis and pericarditis
• Neurological signs including seizures, psychoses and polyneuropathies
• Oral ulcers *
• Pneumonitis or pleuritis
• Polydipsia and polyuria with proteinuria due to glomerulonephritis *
• Skin lesions including alopecia, cellulitis, crusting, erythema, furunculosis, panniculitis, seborrhoea, ulcers (mucocutaneous junctions and footpads), scar formation -50% of cases * The lesions can affect the face, ears , limbs,  and body
• Splenomegaly
• Thyroiditis

(* Most common signs)

Signs in the cat include :

• Anaemia
• Coagulopathies due to thrombocytopenia
• Conjunctivitis
• Fever
• Lung disease
• Lymphadenopathy
• Myopathy
• Neurological disorders (including behavioural changes)
• Oral ulcers
• Polyarthritis
• Skin lesions - in about 20% of cats with SLE
• Renal failure

Complications
Multiple organ system involvement makes diagnosis complicated.

Diagnosis
Patients should also have at least 2 manifestations of the disease aznd diagnosis is made by identification of ANAs by an indirect immunofluorescent test, and by histopathological examination of skin biopsie for .lymphohistiocytic interface dermatitis, thickened basement membrane, vasculitis, subepidermal vesicles, basal cell degeneration 

Anaemia may be present and a direct Coombs' test may or not be positive. Other haematological changes may include, thrombocytopaenia, leucopaenia or leucocytosis, proteinuria and hypergammaglobulinaemia.

Unfortunately the ANA test can give false  positive results in up to 20% of dogs with infectious diseases (eg leishmaniasis). The  Lupus Erythematosus (LE) test is not as useful as it lacks specificity and sensitivity and can vary daily.

Pathology findings in the skin include: .