Photo: Kenneth L. Marcella
Horses have a higher incidence of ovarian tumors than do any other domestic animal.
While the percentage of ovarian tumors is reported to be 2.5 percent of all equine cancers, this particular tumor
Photo: Kenneth L. Marcella
Horses have a higher incidence of ovarian tumors than do any other domestic animal.
While the percentage of ovarian tumors is reported to be 2.5 percent of all equine cancers, this particular tumor location
is significant for two reasons.
Mares with ovarian tumors, specifically granulosa-theca cell tumors, which are by far the most common type of equine ovarian
cancer, tend to have reproductive dysfunction, and they tend to be aggressive. Both of these problems generally cause clients
to seek veterinary help.
Behavior changesThe granulosa-theca cell tumor in the horse causes an increase in various hormone levels and an imbalance in these hormones
or the changes from normal levels can profoundly affect a mare's reproductive behavior and her personality.
Not all granulosa-theca cell tumors behave similarly so mares can have elevated levels of testosterone, estrogen, both or
neither. Certain mares will become anestral and show no cyclical behavior. Some mares will exhibit continual estrous behavior
or will have extended cycles with short periods of non-responsiveness that do not follow any pattern. Other mares will become
aggressive and exhibit almost stallion-like behavior. These mares may herd and "drive" other mares as a stallion would, and
they may show overt aggression and bite or kick herd-mates that they