My cat keeps getting horrible looking insects stuck on his head?

My cat likes lazing around just about anywhere, but mostly in the flower beds. For several weeks now he has had these horrible looking blood sucking insects stuck to his head. He gets about 4 or 5 a week. My husband has to pull them out - they have wiggly legs, their body is grey and the more blood they suck the bigger their body gets - gross eh! We dab infected area with disinfectant, but am a bit worried. Could they be causing damage? They don't seem to annoy him at all. Has anyone else encountered this problem with their cat picking up parasites from outdoors? Thanks