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Jon Mouncey Westpoint Veterinary Group
DEFINITION
The definition of an ovarian cyst has previously been described as a large, hard, or fluid filled structure, with a diameter greater than 25mm, persisting on the ovary for more than 10 days. However, in recent years the definition of a follicular cyst has been amended to include the absence of any luteal (progesterone producing) tissue and clear interference with normal ovarian cyclicity. Other definitions include follicles greater than 20mm on one or both ovaries that fail to regress or ovulate, yet maintain growth and the production of steroid hormones, thereby interfering with normal ovarian cyclicity in the absence of a corpus luteum.
Ovarian cysts are dynamic structures, they can luteinise (become corpus luteum and produce progesterone), regress and can also be replaced by new cysts, but veterinary understanding of which will occur is currently lacking. Non-steroid producing or ‘non-active’ cysts, which are hormonally inactive, do not influence normal ovarian cyclicity and so can occur together with a corpus luteum –
and indeed a pregnancy. Traditionally the presence of an ovarian cyst was associated with nymphomania and bull- like appearance in cows, however in more recent times, cysts clinically present as anoestrus (not expressing heat); irregular heats, negative pregnancy diagnosis and can also be seen in clinically ‘normal’ animals at a routine pre-breeding examination. From a practical point of view, an ovarian impairment may be suspected when apparently cystic structures are detected no earlier than seven weeks after calving, a time when reproductive functions are thought to be restored. Confirmation of an ovarian cyst is best achieved though transrectal ultrasound examination by your vet. Ultrasonography also assists in distinguishing between follicular and luteinised cysts and reliable identification of a corpus luteum. A luteal cyst is characterised by enlarged ovaries with one or more cysts with thicker walls than those of thin walled follicular cysts. Luteal cysts are associated with high concentrations of
progesterone (due to their luteal tissue lining), while follicular cysts are associated with relatively low concentrations of progesterone. Progesterone concentration measurement in the milk can also help to determine the degree of luteinisation of an ovarian cyst.
DEVELOPMENT OF CYSTS
Ovarian cysts can occur during any point of lactation and even during pregnancy, although they most often occur during the post calving period in high yielding cows when they are transitioning from the non-cycling state during pregnancy to returning to regular cyclicity. The disturbance of this transition that causes the formation of a cyst is multifactorial, in which both genetic and environmental factors are involved. The exact cause of cysts is unknown, however, it is believed there is a disruption in the normal pathway that triggers the ovulation of a follicle. This means the follicle continues to grow and produces a ‘cystic’ structure.
Tonia Simms Westpoint Veterinary Group
eproductive efficiency is a key driver for profitability on dairy farms. Many conditions associated with calving and the onset of lactation can impair reproductive performance of dairy cattle. One of the most common ovarian dysfunctions during the post-partum period is ovulation failure, resulting in the formation of ovarian cysts. Economic losses on dairy farms relating to ovarian cysts are incurred through an extension of calving to pregnancy interval and an increase in the culling rate.
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