Nasty smell in my Samsung window unit

My brother and I own several Samsung window units which we have professionally cleaned each spring. However, 3-4 weeks into the summer, the window units begin smelling like a dead animal. The window units have water which pools in them and it attracts mosquitos which attract flies. What can we do to get rid of that nasty smell without having to remove the entire unit every 3-4 weeks? Your advice will be greatly appreciated. Thanks, Dee!