Mastitis in Dogs

Causes, Treatment, and Prevention

Mastitis in Dogs

The Spruce / Madelyn Goodnight

Mastitis is a condition in which the mammary glands become inflamed, usually due to milk accumulation or bacterial infection. Three common causes of mastitis are trauma to the teat, excess milk accumulation, and dirty environmental conditions. While medication can be provided in most cases, untreated mastitis may lead to serious infections that can be fatal, so veterinary help should be sought as soon as possible.

Although this condition is often thought of as a problem associated with pregnant dogs or those that are nursing, it can affect the mammary glands of any dog, including males. Knowing how to prevent and treat this condition is important for your dog's health and well-being.

What Is Mastitis?

Mastitis is a condition in which the mammary glands located in the dog's breast become inflamed, typically due to a bacterial infection, milk accumulation, or fungal infection. Bacteria enter the gland through the opening in the teat and cause infection, inflammation, and other problematic symptoms of mastitis. Other times, bacteria are not involved and mastitis simply occurs as a result of excess milk accumulation in the mammary gland.

Symptoms of Mastitis in Dogs

The most common sign of mastitis in a female or male dog is a swollen breast. This condition typically starts out with inflammation, redness, and pain. Nursing dogs may also be reluctant to feed their puppies due to discomfort, which can prevent puppies from gaining weight at a healthy rate. As the infection in the mammary gland spreads, a dog may become septic if mastitis is not treated. Septic infections can be life-threatening. Vomiting, diarrhea, and even anorexia are typically seen in dogs with more severe cases.


  • Swollen or discolored breasts
  • Inflamed, swollen, or red teats
  • Ulcerated breasts
  • Mammary glands that feel hot to the touch
  • Blood or pus in milk or seeping from teats
  • Painful breasts
  • Lethargy
  • Loss of appetite
  • Vomiting
  • Diarrhea
  • Reluctance to allow nursing
  • Nursing puppies aren't gaining weight

Painful, Swollen, or Discolored Breasts and Teats

One or multiple breasts can be infected and swollen, and this swelling is not limited to just nursing or pregnant dogs. As breasts swell, they may also become inflamed or discolored like a bruise, and even develop ulcers or sores on them. The teats may also become very inflamed, red in color, and larger than usual.

Mammary Glands Feel Hot to the Touch

If you touch your dog's mammary glands, they may feel hot due to inflammation and infection. Belly rubs are not wanted by most dogs with mastitis due to the sensitivity and pain it causes their mammary glands.

Blood and Pus in Milk or Seeping From Teats

Blood and pus may seep from the teats, and in nursing dogs, these substances can even be present in the milk. This is usually accompanied by redness, swelling, and irritation.


When dogs are in pain, they may become lethargic and unwilling to move around unless necessary. The dog's energy should return to normal after treatment.

Loss of Appetite

Often in addition to lethargy, dogs can lose their appetite when they don't feel well. Depending on how long treatment takes, they may also lose weight during prolonged periods of limited eating.

Vomiting or Diarrhea

Vomiting and diarrhea can be caused by gastrointestinal issues and a variety of other health conditions, but both are also associated with mastitis in more severe cases.

Puppies Aren't Gaining Weight

In early cases of mastitis in a nursing dog, you may notice that the puppies aren't gaining any weight if they are attempting to nurse from an affected breast that is not producing enough milk. Your dog may be reluctant to nurse its puppies as the mastitis worsens. Painful mammary glands only hurt more during nursing, so dogs with mastitis may try to get away from their puppies and not allow them to nurse.

Mastitis in one mammary gland of a dog with dark fur close-up.
Ellen11 / Getty Images

Causes of Mastitis

The most common cause of mastitis is trauma to the teat, which can allow bacteria to enter the mammary gland. This condition can also occur simply from too much milk in the gland. Nursing dogs are more likely to develop mastitis because of repeated trauma while feeding puppies.

  • Trauma: The most common reason for a dog to develop mastitis begins with trauma to the teat. Trauma from a nursing puppy or another type of injury allows bacteria, such as E. coli, to enter the mammary gland through the teat canal. This results in a bacterial infection.
  • Excess milk accumulation: Occasionally, too much milk is produced, and excess milk accumulates in the mammary gland. This pressure can cause mastitis without a bacterial component.
  • Dirty environments: Trauma isn't the only way bacteria can enter the teat canal. Simply being in a dirty environment can allow bacteria to cause an infection in the mammary gland.

Diagnosing Mastitis in Dogs

After a medical history is obtained and a complete physical examination is performed, your veterinarian will most likely diagnose your dog with mastitis based on the physical findings, especially if it is a female dog that is pregnant or nursing. Occasionally, a sample of any discharge from the teat will be examined under a microscope to see if there is evidence of blood, bacteria, or fungus, or a sample may be sent out for culture and sensitivity to identify the infection. This can help your veterinarian determine whether the dog is experiencing mastitis or galactostatis, which also causes excess milk production but does not involve infection. If you have a male dog with mastitis or a female dog that is not pregnant or nursing, further testing may be performed to rule out other problems such as mammary cancer.

Treatment and Prevention

If your dog is diagnosed with mastitis, oral antibiotics and anti-inflammatory medications will most likely be prescribed. Mastitis does not typically require hospitalization unless your dog has become septic or requires surgical removal of severely affected glands.

If the mastitis is due to an over-accumulation of milk, the breast may need to be gently milked by hand to express the excess. A cabbage leaf compress may also be recommended to help with pain and inflammation.

Finally, keeping the mammary glands clean is important not only in helping treat mastitis but also in preventing it from occurring in the first place. Regular bathing and wiping of a lactating dog's mammary glands are important prevention methods to keep them clean and healthy. Owners of nursing dogs should also ensure the home is kept clean (especially within puppy enclosures).

Prognosis for Dogs With Mastitis

Most dogs that develop mastitis can be treated with medications and return to health within two to three weeks. However, severe cases involving major infections, sepsis, or removal of the affected breast may have longer recovery periods. Affected breasts may be permanently unable to produce milk, and blood infections can be fatal despite treatment. If you're concerned that your dog may have mastitis, seek veterinary help quickly to begin treatment for any possible infections before the condition progresses. This will also help reduce the amount of time that your dog experiences pain or discomfort from swelling and irritation.

Article Sources
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  1. Lunchick, Paisley. Mastitis in Nursing Mother Dogs. American Kennel Club, 2022