Uterine Cysts - Horses

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Also Known As: Endometrial cysts — Glandular cysts — Lymphatic lacunar cysts


Uterine cysts are a common clinical and incidental finding in mares.

There are two types of uterine cyst recognised in the horse:

  1. Endometrial glandular cysts – 5-10mm in size and the result of fibrosis around endometrial glands.
  1. Lymphatic lacunar cysts – >1cm in diameter, can be several cm, caused by lymphangiectasia.

Endometrial cysts can be located centrally and luminally at the bifurcation or may be within the uterine body where they can be luminal or extraluminal. Smaller cysts are often luminal while large ones often form extraluminally from blocked lymphatic channels.[1]

Uterine cysts can complicate pregnancy diagnosis, particularly when scanning for twin pregnancies.


Mare older than 11 years of age are most likely to have cysts.[2]

Clinical Signs

Significance of uterine cysts is often difficult to assess due to many contributing factors on fertility. Presence of cysts is very common and small endometrial cysts are generally of no clinical significance[1]

Large cysts or many in a single area may prevent mobility of the embryo and thus result in failure of maternal recognition of pregnancy, thus the mare will return to oestrus after ~21 days.

Numerous cysts can also result in early embryonic death, usually between 22 and 44 days of gestation,[2] possibly due to interference with placentation.

Multiple small cysts have also been observed in mares with chronic, infiltrative endometritis, which carry a hopeless reproductive progonosis.[1]


Diagnosis is usually made by rectal palpation or trans-rectal ultrasound scanning. Ultrasonographically, cysts appear as fluid filled cavities that may be spherical or long and oval in shape and may be septated into cavities.[3]

Hysteroscopy can also be used and is particularly useful for very small cysts.

Cysts must be differentiated from twin pregnancies when ultrasound scanning pregnant mares. If any doubt is present, the mare should be rescanned after 2-3 days, after which time a conceptus would have undergone a detectable increase in size and likely also changed location if during the motile phase. Mapping cyst locations prior to breeding is very useful.


Multiple treatment modalities are described for uterine cysts including mechanical curettage, manual rupture by hand or via endometrial biopsy forceps, hysteroscopic rupture using forceps, electrocoagulation and laser ablation. All have been used with success and availability of equipment, personnel and cyst size should be used to determine which modality is most appropriate.


As no risk factors other than age have been identified, prevention is impossible.

The key factor is preventing interference with pregnancy diagnosis, thus pre-breeding trans-rectal ultrasound examination is advised for all mares, particularly those over 10 years of age, to allow mapping of existing cysts. Prudent scanning at pregnancy diagnosis is essential if these records are not available.

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  1. 1.0 1.1 1.2 England, G. C. W (2005) Fertility & Obstetrics in the Horse 3rd ed. Blackwell Publishing Ltd, Oxford
  2. 2.0 2.1 Samper, J. C., Pycock, J. F., McKinnon, A. O (2007) Current Therapy in Equine Reproduction, Elsevier Health, pp 121-125.
  3. Kähn, W., Volkmann, D (200) Veterinary Reproductive Ultrasonography:Horse, Cattle, Sheep, Goat, Pig, Dog, Cat. Schlűterche, Hannover, p79

Lavoie, J-P., Hinchcliff, K. W (2008) Blackwell’s Five-Minute Veterinary Consult: Equine 2nd ed. Wiley-Blackwell, Oxford

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