Subclinical mastitis is a form of the disease in which there is no detectable change in the udder and no observable abnormalities in milk. However, the presence of microorganisms in milk usually can be demonstrated by microbiological culture, and inflammatory changes in the milk can be detected by special tests, such as conducting a somatic cell count.

Subacute clinical mastitis is a condition in which abnormalities of the udder and secretion are readily observable. This form of mastitis can vary in severity, depending in part, on the microorganism causing the infection. Changes in the milk, such as flakes, clots, and a watery appearance are the most obvious abnormalities. Heat, swelling, and udder sensitivity are slight or absent.

Acute mastitis is a condition characterized by sudden onset, redness, swelling, hardness, pain, grossly abnormal milk, and reduced milk yield. Systemic symptoms may also be present and include fever, loss of appetite, reduced rumen function, rapid pulse, dehydration, weakness, and depression. When the disease onset is very rapid and the signs are very severe, the disease is termed peracute mastitis.

Chronic mastitis is an udder infection that is of long duration. Chronic mastitis may remain in a subclinical phase indefinitely, or the infection may alternate between subclinical and clinical phases; at times, clinical signs may persist for long periods.

Nonbacterial mastitis is a mammary inflammation that occurs when microorganisms cannot be isolated from milk samples. Such cases may be either clinical or subclinical.