06/08/2026
Spayed Cats Can Still Develop Mammary Tumors—This Alone Does Not Indicate Incomplete Ovarian Removal
Ovarian hormones, particularly estrogen and progesterone, are recognized risk modifiers for feline mammary tumor development. However, ovarian hormones are not the sole cause. Feline mammary tumorigenesis is multifactorial and may involve age, breed predisposition, genetic susceptibility, previous hormonal exposure, exogenous progestins, and accumulated cellular alterations.
Ovariohysterectomy (OHE/OVH) reduces the risk of feline mammary carcinoma, particularly when performed early in life, but it does not provide complete protection.
A case-control study reported that cats spayed before six months of age had an approximately 91% reduction in risk, while those spayed before one year had an approximately 86% reduction compared with sexually intact cats. These figures represent risk reduction—not complete risk elimination. The available evidence is also primarily observational and should be interpreted with its methodological limitations.
Therefore, the development of a mammary tumor in a previously spayed cat is not, by itself, diagnostic evidence of ovarian remnant syndrome (ORS) and does not justify the conclusion that the ovaries were incompletely removed.
ORS should be considered when there is evidence of functional ovarian tissue, particularly recurrent or cyclical estrous signs such as:
• Increased vocalization or restlessness
• Urine marking
• Lordosis and tail deviation
• Attraction of male cats
• Vulvar changes or other hormone-responsive signs
Diagnostic evaluation may include vaginal cytology and measurement of serum anti-Müllerian hormone (AMH), luteinizing hormone, and progesterone, interpreted according to reproductive status, sampling time, and laboratory limitations. A positive AMH result supports the presence of ovarian tissue; however, a negative result may not exclude ORS in every clinical circumstance. Progesterone measurement and appropriate stimulation testing may provide additional evidence. Histopathological examination of surgically removed tissue can confirm ovarian tissue.
Clinical conclusion
A mammary tumor in a spayed cat
≠ confirmation of ovarian remnant syndrome.
Mammary neoplasia and ORS are separate clinical conditions. The completeness of a previous ovariohysterectomy should be assessed using evidence of functional or identifiable ovarian tissue—not inferred solely from the subsequent development of a mammary tumor.
References
Overley B, Shofer FS, Goldschmidt MH, Sherer D, Sorenmo KU. Association between ovariohysterectomy and feline mammary carcinoma. J Vet Intern Med. 2005;19(4):560–563. doi:10.1892/0891-6640(2005)19[560]2.0.CO;2
Gogua E. Should we recommend spaying cats for mammary tumour prevention, and if so, at what age? Veterinary Evidence. 2025;10(3). doi:10.18849/ve.v10i3.711
Flock U, Fischer S, Weeger J, Reese S, Walter B. Anti-Müllerian hormone as a diagnostic tool to identify queens with ovarian remnant syndrome. J Feline Med Surg. 2022;24(8)–e174. doi:10.1177/1098612X221099195
Merck Veterinary Manual: Ovarian Remnant Syndrome in Small Animals
Cornell University: Feline Reproductive Function Tests
