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Fur loss can be caused by a number of things so it's hard to guess really.
Female hams can sometimes lose fur due to hormonal problems, you could try giving a drop of evening primrose oil a day & see it that helps, it's generally good for skin/fur anyway so worth a try.
There is also the possibility of Cushings disease, they usually tend to drink more with this condition too, if it is that it can be treated, Coco's ham Mocho has had Cushing's for some time now & is being treated successfully but I'm not sure how common it is for vets to treat it.
Flax seeds (linseed) & nutritional yeast are beneficial for skin & fur so you could supplement with that too.
At around 18 months you might notice some early signs of ageing & slight fur loss but I wouldn't expect it too be as much as this so I doubt it's age related.