Thrush in chickens is very similar to thrush that babies get.
You’ll notice a white oozy substance inside their crop (which is a space between their neck and body.) They will have a larger than normal appetite. The chicken will appear lethargic and have a crusty vent area. And their feathers will look ruffled.
It is important to mention that thrush is a fungal disease. This means it can be contracted if you allow your chickens to eat molded feed or other molded food. And they can also contract the disease from contaminated water or surfaces.
Even though there is no vaccine, it can be treated by an anti-fungal medicine that you can get from your local vet. Be sure to remove the bad food and clean their water container as well.