Diagnosis
The diagnosis of dermatomyositis is based on five criteria, which are also used to differentially diagnose with respect to polymyositis:
Muscle weakness in both thighs or both upper arms
Using a blood test, finding higher levels of enzymes found in skeletal muscle, including creatinine kinase, aldolase, and glutamate oxaloacetate, pyruvate transaminases and lactate dehydrogenase
Using testing of electric signalling in muscles, finding all three of: erratic, repetitive, high-frequency signals; short, low-energy signals between skeletal muscles and motor neurons that have multiple phases; and sharp activity when a needle is inserted into the muscle
Examining a muscle biopsy under a microscope and finding mononuclear white blood cells between the muscle cells, and finding abnormal muscle cell degeneration and regeneration, dying muscle cells, and muscle cells being consumed by other cells (phagocytosis)
Rashes typical of dermatomyositis, which include heliotrope rash, Gottron sign, and Gottron papulesThe fifth criterion is what differentiates dermatomyositis from polymyositis; the diagnosis is considered definite for dermatomyositis if three of items 1 through 4 are present in addition to 5, probable with any two in addition to 5, and possible if just one is present in addition to 5.Dermatomyositis is associated with autoantibodies, especially antinuclear antibodies (ANA).
It is of tremendous economic importance for the dairy industry, and it is also of concern for public health. The same considerations apply to mastitis in sheep and goats and other milk-producing females. It is also of economic importance in the sow, but, in this species, it is not related to public health. In other domestic females (queen, mare,  etc. ), it is more an individual illness dealt with by veterinary practitioners. References
External links

Mastitis on mayoclinic.com
Australian Breastfeeding Association