Rats often are so fastidious about their fur that you may never notice that they actually are shedding old hairs. At certain points in their life, however, the shedding may become so intense that they can't keep up a normal sleek groomed appearance. In either case, losing old fur as a result of growing new fur is normal.
A rat's first moult usually happens when he is young, at just six to eight weeks old. At this age, he sheds his downy baby coat and grows a sleek adult coat. He will moult again at around three months old. These are the most noticeable molting periods, because the rat's color may change or the fur may appear to be streaked with various colors. Other moults may not be as apparent, but your rat may have patches of soft new fur that is slightly lighter than the surrounding, coarser fur. Bald patches and irritated skin are not part of a normal moult.
Most healthy rats don't require any special grooming during a moult. Your rat normally will groom himself, which keeps him clean and removes any loose or dead hairs. If your rat isn't keeping up with the shedding hair, or he is old, ill or has an injury, he may need brushing to remove excess moulting fur. Use a soft-bristled brush and work in the direction of the fur growth.
External parasites, like fleas, mites or ticks, can cause fur loss in rats. This type of fur loss is not part of a normal moult and often occurs in a random pattern as a result of scratching and skin irritation. If not treated, an infestation could lead to a skin infection, anemia or diseases transmitted by fleas, such as bartonella, which is the cause of cat-scratch fever, but also can be spread to and by rats. If your rat has fleas or other parasites, he should be treated promptly with a rat-safe anti-flea shampoo or spray.
Unusual shedding of fur also may be caused by allergies, health problems, stress and even other rats who chew on the fur of their cage mates. This fur usually will fall out in patches, leaving behind bare or nearly bare skin, or may appear as if the rat has been given a close shave. Your rat may be scratching himself so intently that he pulls out fur, which eventually could lead to a skin infection. If you suspect your rat is shedding hair due to a health problem or stress, contact your veterinarian. If the hair loss is due to chewing by cage mates, it may be necessary to separate the rats.