COULD RAT POISON CAUSE THESE SYMPTOMS?

I have a flock of 12 chickens of many various breeds, one rooster and the rest are hens. They live in a large pen, weekly going out to forage. They have a nicely sized coop, are well watered and fed, have a balanced diet, and all are very content. No problems have been encountered so far with lodgings. No stress is being induced to my knowledge. I went out today and one hen (she is a 1 1\2 yr old Buff Orpington hen) was sitting there listlessly so I looked her over. Normally she is very perky and has actually never been sick or had any diseases before. She had a discolored, dark purple comb and wattles, whereas normally they're bright red. Her feet were swollen, and she does not have the desire too eat or drink. She has no respiratory symptoms of any kind, and her vent is not obstructed. She looks healthy in every other way. No other chickens have these symptoms yet.