I would also be very afraid of what they may be tracking into your yard from another yard. Parvo is my main worry, but I'm sure they could spread other things off their feet. Even my gardener isn't allowed to use his own mower because he won't pick poo up before mowing and I don't want it spread into my yard. I'm even worried about him walking in our yard from other yards, but not all his customers have dogs like a person that goes only to homes that do have dogs. He was also spreading dandelions in our yard before we stopped him from using his own mower. We don't have those anymore now.
He also left our gate open one time, but after we told him to make sure it was shut, it's never happen again. I still check and lock the gate before I let the girls out after he leaves. Can never be too careful.
