3 out of 4 of my chickens have red swollen vents and messy butts. How to treat?

[bobathesexyclown]

So I noticed my chickens butts have been messy with poop so I decided to bathe them and I noticed their butts were red and swollen and kinda inflated looking. What ailment is this and what can I do to treat it? Thank you

 

[nuthatched]

What's their diet?
Have your treated for mites or worms?
Has it been especially hot?

 

[bobathesexyclown]

Their diet is I free range them part of the day when I’m off work and hanging out outside and then when their in their run I feed them layer pellets, mealworms and fresh fruit, and homegrown vegetables. It has been in the 70s and low 80s where I live. I think I need to treat them for worms but I’m not sure what to get for that. Any advice on deworming them would be appreciated