I get a lot of phone calls and questions from worried mothers who have noticed that their baby's tongue has a bit of white coating and is this thrush? Thrush is fungal infection of the mouth that is seen in babies (about 2-5% of babies), but thrush typically affects the sides of the inside of the baby's mouth or under the lips and along the gum line. A white tongue alone is most likely residual milk.
There are many cases of thrush that are mild enough that they may resolve on their own. On the other hand, a severe case of thrush may be painful and may make it difficult for a baby to feed, which then leads to a fussy, irritable baby.
Thrush is caused by the fungus candida and despite everyone's best efforts at cleanliness, candida like bread mold, can just happen. Candida may be acquired at the time of delivery as the baby passes through the birth canal that is colonized with candida, or during nursing from the skin of the breast, or from a pacifier or the nipple of a bottle.
Thrush is typically treated by wiping the inside of the mouth with a soft washcloth followed by an antifungal medication given as drops in the baby's mouth after the baby has eaten. In a breast fed infant I treat the mother's breast with a topical antifungal cream as well.
Best way to look for thrush may be when you baby yawns and you get a good look at the inside of their mouths (bucal mucosa). You don't need to be a detective to find thrush, it is usually fairly evident and the biggest clue that it is not milk as it will not wipe off with a soft washcloth.