Hyperoestrogenism is caused by the excess production of oestrogen in the body and has different aetiologies depending on the sex of the animal.
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In the bitch it is usually associated with cystic ovaries or more rarely a functional ovarian tumour. These are mostly granulosa-theca cell in origin and 10-20% are malignant. It can also occur iatrogenically through the administration of oestrogens used to treat mismating and urinary incontinence.
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In the male dog the syndrome is associated with a Sertoli cell tumour of the testicles or a male feminising syndrome for which the cause in unknown.
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Hyperoestrogenism also occurs in the female ferret if she remains unbred for an extended period of time.