Yes she can be part of the problem. Just like kids in a daycare swapping lice around, they all need to be treated at once. Permethrycin spray will kill lice too, I think, it is usually in a blue pump spray bottle. I have never used it. Ask the farm supply store what they recommend. Also, you can buy oregano food supplement made for chickens. It is sort of powdered, I have heard of gringing up green oregano leaves and feeding it to them. In humans, oil of oregano and garlic help make you more immune to these kinds of vectors as well as mosquitoes and are said to fight infections. I have seen the oregano for chicken products for sale but have not used them. They also have a natural antibiotic quality.