Pine is not good to keep rats on from what I've read. I choose not to use it, but many do. This is what I've read so far about using pine shavings.
http://ratguide.com/care/environment...ing_litter.php
There is a longer article I've read before, but I dont have it bookmarked. It's more indepth than that snippet.Cedar or Pine
Cedar and pine are choices that you want to avoid. Unfortunately they are usually sold in pet stores right next to the rodent supplies. Tests have shown that the phenols (aromatic hydrocarbons) that are found in these softwoods are toxic to rats and other small animals. This toxic substance is what makes the litter have the strong smell, which covers up the odors of urine but at a hazardous cost to your rat. It is the oils containing these phenols that you find in many household cleaning products like pine-fresh and pine-sol.
If you use pine or cedar you are jeopardizing your rat�s health in several ways. Long term inhalation of these chemicals can compromise the rat�s immune system making it more prone to respiratory problems such as mycoplasma. It has also been scientifically documented that the phenols in these softwoods alter the levels of the liver enzymes, which can lead to liver disease and even liver failure.
How old are your rats? You do communal nursing correct?
Try giving them a bunch of aspen shavings to use. Equine fresh can kill litters of pinks by stuffing up their noses.