Bretaigne Jones, DVM
Scientific Communications
Royal Canin, USA

It never seems to fail, you are within weeks of three major shows, and your bitch blows her coat. Talk about rotten timing, and it's going to take months for her to grow that hair back! Why now? What went wrong?

Well, for all the frustration it can cause, it is totally natural for dogs, especially bitches, to blow coat periodically. All dogs have the potential for that sudden, heavy shedding that leaves them looking like a grooming experiment gone terribly wrong. It is most noticeable in those dogs that have more undercoat, commonly referred to as double-coated.

Dogs have three types of hair. Primary hairs are also called guard hairs, and these are the outer coat hairs that are typically longer and stiffer. Secondary hairs refer to the short fluffy hairs that make up the undercoat, and serve as insulation. Puppies have all secondary hairs until about 6 months of age, which explains their soft coats. The third hair type is the tactile hairs which are commonly recognized as the whiskers found on the face. 

The coat differences seen between breeds results from the ratio of primary to secondary hairs, as well as the texture of the individual hairs. Some breeds have an equal amount of guard hairs and undercoat, providing the water proofing and insulator effects needed for their original geographic location. Other breeds may have little undercoat, and their guard hairs will differ in length, such as Irish setters and beagles. Obviously, even within a breed there are coat variations, as seen in dachshunds which have smooth, wire-haired, and long-haired genes. Furthermore, in the long-haired lines, there are differences with the amount of undercoat present.

In order to fully understand how the sudden blown coat can occur, it is necessary to understand normal hair growth. The season and changing day lengths can influence this normal process, though with indoor dogs that influence is minimized. Outdoor dogs will usually have two fairly distinct sheds each year as they respond to environmental stimuli. Indoor dogs tend to shed at a continuous rate for the most part, unless some other factor triggers a more pronounced effect.

The life of a hair begins with the anagen stage, where the hair follicle is actively producing hair cells. This activity begins and continues in response to hormonal signals. Of the entire "life span" (hairs are dead material) of a hair, 80 to 90% of that time is in the anagen stage, actively growing from the hair follicle.

The next stage is called catagen, and occurs when new hair production ceases. This stage, too, is in response to signals, although in this case it is the sudden absence of the stimulatory signals that triggered the anagen, or growth, phase. This is a dormant stage for the hair follicle. The hair is not actively attached at the base of the follicle any more, but stays in the follicle due to friction. In this stage it is easily pulled out. These are called club hairs.

The last phase is a transitional stage called telogen. The follicle is reactivated out of the dormancy of catagen, and prepares for active production of hair in the anagen phase.

All of the hair follicles on a dog's body will be in any of these three stages concurrently. The amount of hair at any one time in the anagen phase (remember that 80 to 90% of the lifespan of a hair is spent in anagen phase), compared to catagen and telogen determines when shedding occurs, the length of the hair coat, and whether or not there is an undercoat present.

Poodles have the majority of their hair follicles in anagen stage consistently, meaning that their hair grows in length almost constantly. Therefore, their hair needs to be clipped routinely to control matting and remain aesthetically pleasing.

Alternatively, hairless breeds such as the Chinese Crested have the majority of their hair follicles in the telogen stage.

Other breeds fall in-between these two extremes.

There are many things that influence hair growth and shedding. These include genetics, nutrition, age, sex, the health status, the season of the year, and if it is an intact animal - where it is in the normal hormonal ebb and flow. Radiation and some drugs will affect rapidly dividing cells and usually stop hair growth. This is commonly seen in people experiencing hair loss during chemotherapy and radiation treatments for cancer. Normal radiation from taking diagnostic radiographs is not a cause of hair loss.

Some diseases can mimic a blown coat, such as endocrine imbalance involving the adrenal, pancreas or thyroid glands. Parasitic infestations, both internal and external, can have a profound affect on coat quality, but won't necessarily mimic a normal blown coat. An important consideration is the health of the skin in areas of hair loss. When a dog blows coat, the underlying skin is normal, healthy skin. With external parasites such as fleas, there will generally be evidence of skin reaction to the bites, and itchiness.

The sex of the animal definitely impacts the coat. Spayed and castrated dogs may have more pronounced undercoats, giving a cottony appearance. Excessive or absent sex hormones can cause changes in a normal coat. These are not considered to be "blown coats".

When a dog does blow coat, there is extensive hair loss, but the underlying skin is normal. Other names for this are telogen effluvium and telogen defluxion, referring to the resetting of the follicles to the telogen stage. The most common influence causing a dog to blow coat is reproductive cycle activity. Usually, it results from a pregnancy and birth, but it can occasionally occur after an estrous cycle. This can be confusing, because the hair loss doesn't happen at the same time as the causative event, but instead follows 1 to 3 months after. The hair follicles get simultaneous signals to reset to a telogen stage, so the hairs shed all at once, over a short period of time. The animal doesn't generally go entirely bald, and the areas with the most pronounced hair loss will be in the lumbar area, on the sides over the ribcage and on the flanks. The hair loss pattern is more or less symmetrical from side to side. The most important thing to remember is that the coat will regrow fully; it will just take 2 to 4 months to regain its full coat.

Sometimes extreme stress will trigger this sudden excessive shedding. Events such as surgery, or just general anesthesia, will stimulate the synchronization of the hair follicles to the telogen stage. This of course results in a blown coat. Just like the reproductive causes, it usually happens 1 to 3 months after the event, and a new coat will grow in within 2 to 4 months after shedding. Sudden severe illness and fever, or shock, can have the same affect.

A different situation that is sometimes confused with blowing coats is "after clipping alopecia". When this happens, the hair does not grow back immediately after having been clipped. The clipping may have been cosmetic grooming, or it may have been part of surgical preparation. In any event, there is no way to predict which animals may be affected. It is more common with breeds that have thick undercoats (Huskies, Chows, Shelties). The coat will eventually regrow, but it may take up to 24 months.

As you can imagine, there are a myriad of things that can cause hair loss, which may or may not have skin lesions associated. Veterinary consultation and diagnostics are necessary to fully identify the primary cause in most cases.

As we now know, with typical blown coats, all we need is time.