Mastitis is almost always associated with lactating nursing animals. Trauma, poor sanitary conditions, and underling illnesses are potential predisposing factors. Most of the time, mastitis is not an emergency situation, and animals affected are not significantly ill. The only sign might be a bitch refusing to let the puppies nurse. This may be taken as a sign of the mother just being immature or uncaring. The mammary glands should always be observed closely if this is occurring.

Mastitis is usually an acute (sudden) condition. If it is left unnoticed, an animal is immunocompromised or a particularly pathogenic (causing significant disease) bacteria is present, the infection can spread to other glands. Occasionally it may cause septicemia (bacterial blood infection). These animals are usually quite ill and require much more intensive care.

Mammary glands may become so inflamed that they may be abscessed or even gangrenous. These conditions require surgical intervention. As apposed to abscessed glands, gangrenous glands have lost their blood supply and are cool, darker and sometimes ulcerated.

Nursing puppies may be in poor nutritional condition due to a lack of nursing allowed (from pain experienced by the dam during nursing) or poor nutritional content of the infected milk. The nursing on infected glands itself generally is not detrimental to the animal.

Disorders that can cause clinical signs similar to mastitis include:

Mammary gland enlargement caused by advanced pregnancy, lactation or pseudopregnancy. Sometimes there is an excessive accumulation (galactostasis) of milk in the glands, and they may become warm and somewhat painful.

Mammary fibroepithelial hypertrophy is a benign growth of the mammary tissue causing a firm swelling. The swellings may become extremely large.

Mammary gland tumors are fairly common and usually occur in older animals. They may occasionally be confused with mastitis especially if they are ulcerated.