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How to treat external horse parasites
Look for the horse biting or rubbing itself excessively.
A horse that has external parasites likely is irritated by them. In order to relieve any itching or pain, your horse will rub, bite, or scratch any areas that it can get to.

-It may also bite or rub at other external areas, like the inner ears, neck and temples. This can also be caused by external parasites such as inside ear mites, ringworm, etc.
-Rubbing and biting areas of discomfort can lead to areas of hair loss, as well as open sores. If your horse has these issues, then there may be an underlying parasitic infection causing it.
-Internal parasites are usually harmless and natural, and should be kept at bay. Aggressively licking any metal devices might cause an internal parasite to make it's way through your body, or, worse, they could cause a parasite called plasmosis, which can turn bad bacteria inside the body into a dangerous bacteria.
-Look for any rash, redness, or swelling. For example, your horse may rub itself instead of biting.
Answer:
Rubbing and biting areas of discomfort can lead to areas of hair loss, as well as open sores. If your horse has these issues, then there may be an underlying parasitic infection causing it.