Why Do Cat Or Dog Bites On Skin Take Longer To Heal?

hello...i would like to ask about my skin. i go directly, when a lice of a cat or dog bites me it get into a very red and so itchy, and the wounds/scars will get a very long time(1 year or more) to heal/disappear and it get a black color into my skin. when mosquito bites me, as if nothing happen after or an hours goes. and when i get wounded by a knife or any sharp things it will heal for a weeks or a shorter time... what i want to ask is that...why this cat/dog bites get a very long time to heal? and i am worrying about this. can you suggest me?