Luteal cystic ovary disease is characterized by enlarged ovaries with one or more cysts, the walls of which are thicker than those of follicular cysts because of a lining of luteal tissue. Incidence ratios of follicular versus luteal cysts vary greatly because of diagnostic tendencies of individual veterinarians. Classically, luteal COD is defined as the presence of a fluid-filled ovarian structure >25 mm diameter persisting >7 days in the absence of a CL and with a wall diameter >3 mm, usually associated with abnormal reproductive signs. Normal lacunae formation in CL may be incorrectly classified as luteal COD.
The basic causes of true luteal cysts are believed to be the same as for follicular cysts. The release of luteinizing hormone (LH) may be somewhat greater than that occurring when follicular cysts develop, and sufficient to initiate luteinization of follicles but inadequate to cause ovulation. Luteal cysts may be an extension of follicular cysts such that the nonovulatory follicle is partially luteinized spontaneously or in response to hormonal therapy.
Luteal cysts are accompanied by normal conformation and anestrous behavior. Rectal palpation reveals a quiescent uterus characteristic of the luteal phase of the estrous cycle. Luteal cysts are recognized as smooth, fluctuant domes protruding above the surface of the ovary. Usually, they are single structures.
Luteal cysts are differentiated from follicular cysts on the basis of palpable characteristics of both the structure and the uterus and, to some extent, on the cow’s behavior. Progesterone assay and ultrasonography can help differentiate between follicular and luteal cysts, although with either method a final diagnostic decision remains somewhat subjective. On attempts to manually rupture the cystic structure, follicular cysts burst or rupture under minimal pressure whereas luteal cysts cannot be ruptured with reasonable force. Both types of cysts respond to LH or GnRH therapy, but PGF2α will lyse some luteal cysts and generally all diestrual CL structures.
The treatment of choice is luteolytic doses of PGF2α if a correct diagnosis can be ascertained. A normal estrus is expected in 3–5 days. The major limitation of this treatment is the difficulty in accurately estimating the amount of luteal tissue present. If the structure being diagnosed as a luteal cyst is really a developing CL (as discussed above, sometimes called a cystic CL), it may not respond because dairy cows do not become highly responsive to the luteolytic action of PGF2α until day 6 after estrus. Ultrasound examination is increasingly common and facilitates diagnosis of ovarian structures. Luteal cysts also respond to human chorionic gonadotropin and GnRH therapy that is effective in the treatment of follicular cysts, but the next estrus could occur 5–21 days after treatment. Manual rupture of luteal cysts is not recommended because of the risk of trauma and hemorrhage. Because of poor estrus detection practices on many dairy farms, the treatment of choice for both follicular and luteal cysts is intravaginal progesterone/prostaglandin (a fixed timed artificial insemination protocol) (see Treatment of Follicular Cystic Ovary Disease in Large Animals). Application of this protocol in affected cows promotes timely breeding after treatment.