Why Are My Dog’s Nipples Enlarged?

Your female dog’s mammary glands are a source of nutritious milk for her puppies. During pregnancy, which happens after successful mating when the female dog is in heat, the breasts and nipples will become enlarged in readiness for breastfeeding. However, pregnancy isn’t the only reason why a dog’s nipples enlarge.

Why are my dog’s nipples enlarged?

Your dog’s nipples become enlarged when she’s pregnant, has a mammary gland tumor, has mastitis, experiences an allergic reaction, or has an underlying pathological condition. However, it’s vital to differentiate between true and false pregnancy in dogs because frequent false pregnancies are linked to pyometra, womb infections, and breast cancers.

Below we highlight 5 possible reasons why your dog’s nipples are enlarged and what you need to look out for to identify the actual cause.

1. Pregnancy

A female dog’s nipples enlarge during the early stages of pregnancy. If the cause is pregnancy, the areolas will become rounded and the nipples will turn slightly dark red in color. This happens due to an increase in blood flow to the nipples. You should, however, notice other symptoms of pregnancy in dogs such as:

  • Decreased activity
  • Changes in appetite
  • Weight gain
  • Enlarged abdomen
  • Nesting behavior
  • Isolation behavior
  • Seeking more of your attention

2. False pregnancy

Pseudopregnancy or false pregnancy is a common issue among fertile female dogs. It often occurs when a female dog in heat mates with a male dog but the fertilization was incomplete. Your fluffy friend’s breasts will be swollen, nipples will be enlarged, her belly will distend, her appetite will increase, she will be moody or withdrawn, and might display nesting behavior & treat toys as if they were her puppies.

You should visit your vet in case your dog experiences prolonged pseudopregnancy symptoms. The best prevention for false pregnancy is to have your female dog spayed. If your dog is displaying signs of pregnancy yet she’s spayed, it could be due to ovarian remnant syndrome, where the dog displays behavior and hormone levels consistent with a female dog in heat (proestrus or estrus).

3. Allergic Reactions

An allergic reaction could be another reason your female dog’s nipples are enlarged. The reactions could be caused by eating certain foods, coming in contact with certain substances, inhaling an allergen, or consuming products she’s is allergic to. The most common food allergens for dogs include beef, dairy, wheat, egg, chicken, lamb, soy, pork, rabbit, and fish.

Contact dermatitis, for example, is a common allergy that causes swelling of the breasts and nipple enlargement. Worryingly, severe allergic reactions in dogs can spread to the central nervous system or the respiratory system, which can be fatal. It’s important to keep track of any food you feed your dog or hygiene product you use on them just in case of severe allergic reactions and your veterinarian asks about them.

4. Mastitis

Mastitis is a breast infection that affects the mammary glands. It causes breasts to swell and nipples to enlarge. A dog suffering from mastitis will also experience heightened sensitivity to pain and the breasts can release pus.

Mastitis in dogs often occurs in the first 2-3 weeks of the lactation period, and milk from the affected nipples can be toxic to puppies. Mastitis can also occur if a false pregnancy is not treated correctly. The infection can advance rapidly putting the dog’s life at risk.

Mastitis is caused by different strains of bacteria, including Staphylococcus bacteria and Prototheca zopfii, which is the most virulent bacterial strain causing mastitis. Prototheca zopfii is more prevalent in cows than dogs and cats.

The most common signs of mastitis in dogs include:

  • Lumpy, painful nipples
  • Discolored breast milk
  • Blood-tinged breast milk
  • Refusing to nurse puppies
  • Painful vocalizing (“crying”)
  • Dehydration and lethargy

5. Neoplasms and cysts

Other causes of enlarged nipples in dogs are neoplasms and cysts. If you notice your dog’s nipples are large than usual and you can feel lumps or hard masses underneath her breasts, you should rush her to a veterinarian for screening. Hard lumps underneath breasts can be mammary tumors or cysts, which are often a symptom of breast cancer.

According to a recent study, some dog breeds are more likely to develop malignant tumors of the mammary glands than others. They include Poodles, English Cocker Spaniels, and Dachshunds. Dogs over the age of 8 are also more likely to develop breast cancer.

Final thoughts

If you notice your female dog’s nipples or breasts are enlarged and they are not pregnant, it would be advisable to take her to your veterinarian. Enlarged nipples and breasts in dogs could be a sign of malignant tumors of the mammary glands, mastitis, an allergic reaction, and also false pregnancy. 

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