Dynamics and aetiology of ovarian follicular cysts in post-partum dairy cattle

Ovarian follicular cysts affect approximately 7-13% of dairy cattle during the post-partum period, prolonging the interval from calving to conception. This amounts to significant losses to the dairy industry. Cysts are commonly accompanied by the expression of behavioural oestrus, resulting in additional costs because of unsuccessful artificial insemination. Although follicular cysts have been the subject of extensive study, particularly in relation to their diagnosis and veterinary treatment, relatively little is known about the mechanisms underlying their development. Some reports have shown that cows with cysts are associated with higher milk production than non-cystic herdmates. A number of experimental approaches have been utilised to induce follicular cysts, including giving erogenous steroids and gonadotrophins and/or altering endogenous gonadotrophin secretion. Utilising these approaches, coupled with rectal ultrasound, it has been observed that follicular cysts are dynamic structures. Follicular cysts can inhibit the growth of subordinate follicles, although the loss of this dominance is not necessarily accompanied by a decline in their diameter. However, their ability to increase in size is not always associated with dominance. It appears as though follicular cysts continue their growth because they are still responsive to the stimulatory effects of gonadotrophins. Evaluation of the success of treatment for cysts is complex because some cysts spontaneously regress and misdiagnosis of cysts occurs. Natural resolution occurs by luteinization of the cyst or by ovulation of the follicular cyst. Some treatments incorporate the induction of luteinization followed by luteolytic treatments. As the improved understanding of the reasons for cyst formation and the mechanisms involved should lead to the development of improved management and preventative strategies to reduce their occurrence.