Merck Manual

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Professional Version

Mastitis in Small Animals


Clare M. Scully

, MA, DVM, MS, DACT, Louisiana State University, Department of Veterinary Clinical Sciences

Reviewed/Revised Jul 2023

Mastitis is most commonly seen in postpartum lactating bitches. It can, however, occur in nonpregnant dogs with galactorrhea. It may involve only one or multiple glands and it can be chronic or acute.

Etiology of Mastitis in Small Animals

Mastitis is caused by the introduction of bacteria into a mammary gland via nursing, trauma, or hematogenous spread. The bacteria associated with mastitis include Escherichia coli, Staphylococcus spp, and Streptococcus spp, which are all found in the normal skin flora of the bitch. Staphylococcus aureus is the most common bacteria isolated.

The incidence of chronic or subclinical mastitis is unknown and is usually suspected when puppies fail to thrive. The weight of puppies should be monitored daily to be certain they are gaining appropriately.

Clinical Signs of Mastitis in Small Animals

Clinical signs of acute mastitis include:

  • anorexia

  • firm, painful, and warm mammary glands

  • fever

  • decreased maternal interest in puppies

Milk can appear normal, blood-tinged, or purulent.

Diagnosis of Mastitis in Small Animals

Microscopic examination of the milk will show increased degenerative neutrophils and can be diagnostic. The milk from the affected gland should be submitted for microbial culture and antimicrobial susceptibility testing, even if it looks normal.

Differential diagnoses include inflammatory mammary carcinoma in dogs and feline mammary hyperplasia in cats.

Treatment of Mastitis in Small Animals

Treatment of acute or chronic mastitis should include broad-spectrum antimicrobial treatment, which cover both gram-negative and gram-positive organisms. When choosing an antimicrobial, it is important to consider the effects that antimicrobials may have on the offspring. Common empirical choices are cephalexin (15–30 mg/kg, PO, every 8 hours) and amoxicillin-clavulanate (14 mg/kg, PO, every 8–12 hours).

Additional treatments that should be considered are warm or cold compresses on the gland and stripping the teat. Pain medication needs special consideration due to the transfer of those medications via the milk to the pups. The puppies can nurse the nonaffected teats but the bitch may not allow this and milk supplementation may be necessary.

If acute mastitis is not treated immediately it will spread from gland to gland and can lead to rupture. Abscess or gangrene of the glands can develop rapidly in severe cases and lead to sepsis. If there is evidence of teat rupture, surgical debridement of abscessed or gangrenous tissue may be necessary.

Prevention of Mastitis in Small Animals

Whelping areas and any bedding used should be kept very clean. Claw nails of puppies should be kept trimmed short to avoid trauma to the gland and create an opening for opportunistic bacteria.

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