Runny noses

Hi all. I need some advice. I have 3 litters currently, two 6 week old litters and one 12 week old litter. Several of the kits have what looks like dirt on the fur under their noses. They are raised in dirt runs, but they must have some sort of runny nose for the dirt to stick there I'm assuming. I spend at least 1 to 2 hours in the rabbit area daily feeding and watering an only hear an occasional sneeze. This has been going on for at least 3 weeks and none of them have any other symptoms and have not gotten any worse. They don't have matted fur on their paws, they are eating well, gaining weight, and active. I have never experienced this in kits before. None of my adult rabbits are affected. I haven't brought in any new stock and none of my rabbits have ever left my property. We did have two of our dogs come down with kennel cough from the neighbors dog. I suppose I could have passed it to the rabbits, if that's even possible. If that is the case will they recover? Is it safe to sell them as pets? I had the 12 week old litter for sale and sold 4 before they were showing the runny noses, but have stopped selling them until I can figure this out. Any insight would be greatly appreciated.

If they have matted fur on the inside of their front paws, or are sneezing out mucus, ...it is of concern. However.. if it is just a "dirty face" rabbits dig and play in the dirt.