Heard this from a friend so I did not experience it first hand, but thought I would pass it on anyway. Be sure to shake up the fly spray as you use it. Apparently these people heavily dosed their horses with spray on a mosquito infested trail ride. The horses all came up sick and covered with bumps. The vet said it was due to the concentration at the bottom of the non shaken bottle. Maybe they mixed their own and did not mix it correctly as well.