Continue writing the next sentence.

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.
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.