I've just discovered that my cat has a huge patch of missing hair on her neck. Is this something serious?
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Loss of hair is called "alopecia", and in cats it can have many causes.
The only way to know what is causing your cat's hair loss is to have her examined by a veternarian. They will do tests on a sample of her blood to make sure that she's normal, and look at some skin cells under a microscope to see if they can find any parasites or evidence of disease.
Once the cause of the hair loss is determined, your vet will give you instructions on how to treat it. For allergies, this can mean a change in diet to eliminate the allergens. Treatment may also include medication and/or bathing your cat in medicated shampoo.
It looks like the edges of the spot are "wet", so either some fluid is leaking from somewhere or your cat is grooming the area severely from discomfort. A vet visit is important to diagnose the problem!
My cat had similar hair loss too. Often, this is a sign that fungus may be present on her skin. It can be something very serious; I think you should go to the vet for some advanced tests, find out what kind of fungus are there and get the appropriate treatment. Keep in mind that fungus is easily contagious to humans.
Also, this could be a sign of a bad diet, if no fungus will be found then maybe you should control her diet better.