Mammary tumours are exceedingly rare in the horse, occurring much less frequently than in other mammalian species such as humans and dogs.
The factors that protect horses from such tumours are not yet understood, but are of considerable interest. If such factors could be identified, they may provide insight to help reduce the incidence of breast cancer in other species.
When mammary tumours do occur in equines, they are generally invasive and spread to other organs. Tumours can be present alongside mastitis, so further investigation should be undertaken if a horse with mastitis does not respond as expected to medical therapy.
Ultrasound can be useful to evaluate the structure of the affected gland and also to guide biopsy sample collection. Biopsy is required to confirm the diagnosis and to identify the type of tumour that is present.
In human breast cancer patients, the presence of oestrogen receptors (a group of proteins found inside and on the cells) in biopsied tissue is used as a prognostic marker — a biological characteristic that can be measured and evaluated to predict the course of the disease.
These receptors have also been identified in the biopsied tissue from some equine mammary tumours. This suggests that hormones may play a role in the cause of mammary tumours in the horse, as in other species.
Radical surgery may be performed to remove such tumours but the prognosis in these cases is frequently poor.
A more common mammary condition is mastitis, an infection of the mammary gland. This usually — but not always — occurs in lactating mares or following weaning of the foal.
Typically, the mare’s udder is enlarged and can be warm and painful to touch. Abnormal mammary secretions are usually present and milk may be lumpy or blood-tinged. An irregular gait may be the first abnormality noted, sometimes with oedema (swelling) in front of the mammary gland. In severe cases the mare may also be dull and off her feed.
Treatment of mastitis involves stripping out the affected quarter of the udder to remove all milk. Antibiotics and anti-inflammatory drugs are also helpful in more severely affected cases. Abscesses can occasionally form within the mammary gland, which may require more lengthy antimicrobial treatment or surgical drainage.
The prognosis is generally good for a horse with mastitis, although future milk production may sometimes be reduced.
Inappropriate lactation may be observed in both pregnant and non-pregnant mares. Premature lactation in the pregnant mare may be a sign of fetal distress or indicate other complications associated with pregnancy, such as placentitis, and should be investigated immediately.
Lactation in mares that are not in foal usually coincides with the mare coming in to oestrous (season). This is more commonly seen in overweight mares and sports horses. Treatment is not usually required in these cases and milk should not be stripped out as this will encourage further milk production.
Conditions affecting the mammary glands are much less common in horses than in other species, but they do still occur.
Any change in size or shape of the glands should therefore prompt veterinary evaluation.
Ref: Horse & Hound; 4 February 2016