Causes cancer in chickens or humans, I doubt a chicken is going to live long enough in the first place to develop any disease from occasional applications of Vaseline.
I however don't buy into any of this Vaseline on the comb stuff, I can understand how it works by keeping water from settling in the skin, but I'm in northern Wisconsin, we can get some extended periods of extreme cold, -20 during the day and down to -35 or occasionally colder and I've never done the Vaseline thing, I only ever had 1 rooster get frostbite and that is odd to me because I've had others with bigger combs without frostbite. The biggest thing is ventilation not temperature, you need to get rid of moisture and you won't get frostbite. I don't have leghorns but I've got a Sicilian buttercup rooster a nh rooster and various single comb breeds. I wouldn't worry about it at all if your coop isn't cramped and you have good ventilation unless maybe you know your going to get down to severe extreme below zero temps, even then a dry coop will not allow frostbite
Edited by blucoondawg - 12/18/15 at 12:04am