A Lesson in Anatomy
The skin is the largest organ of the body and its primary role is to protect the body from dangerous microbes and the elements including UV lights, extreme temperatures, chemicals and various forms of mechanical trauma which can take place when Rover is romping through the woods, engaging in boisterous play with other pals at the park, or getting punctured by pesky bugs.
A dog's skin is also known for housing an abundance of hair follicles which provide most dogs with a thick coat of fur. Unlike humans, who have single follicles, which means one hair shaft sprouts from one single pore, dogs are blessed with compound hair follicles, meaning that each pore contains a central stiff hair surrounded by as many as 20 finer secondary hairs.
All in all, the fine, secondary hairs make up the dog's undercoat, while the stiff central hairs make up the dog's top, guard coat. Variations in size and quantity of the central stiff hairs along with the finer secondary hairs sprouting from compound follicles is ultimately what determine the type of coats dogs end up having, explains Linda P. Case, in the book: "The Dog: Its Behavior, Nutrition, and Health."
Stages of Hair Growth in Dogs
In order to better understand why dogs shed when stressed or nervous, it helps to take a glimpse into the stages of hair growth in dogs.
Just as it happens in people, a dog's hairs tends to grow in stages. More specifically, there are four stages of hair growth in dogs.
The anagen stage takes place when your dog's fur is actively growing. The catagen stage signals the end of the active growth stage, meaning that each hair has reached its genetically determined length. The telogen stage is the resting, dormant stage of the hair cycle, and finally there's the exogen stage which takes place when a dog starts actively shedding.
Interestingly, a dog's coat moves rather rapidly to the dying and falling off stage to give place to new growth, which means that they are shedding quite often-to their owner's surprise.
On top of dogs shedding profusely in the early spring and early fall when they "blow their coats" to give space to their cold-weather coats and hot-weather coats, dogs also tend to shed in response to changes in temperature or amount of sunlight making shedding an almost year-round ordeal.
Generally, unless you notice obvious hair loss leading to bald spots, what you are likely witnessing is the natural replacement of your dog's hair coat, however, in the case of acute or chronic stress, this can play a number on Fido's shedding too.
Did you know? In humans, a single hair may grow for up to 6 years before being shed and replaced by a new hair, while in dogs the growing cycle is much shorter and averages about 130 days. The only exceptions to this general rule are non-shedding dog breeds like the poodle whose hair grows for several years before being replaced, explains Amy Shojai in the book: "Dog Facts: The Pet Parent's A-to-Z Home Care Encyclopedia: Puppy to Adult."
The Role of Stress in Dogs
All living beings are subjected to several challenges in their lives including exposure to stressful stimuli and events. These stressful events may cause general, widespread or localized biological responses which may lead to a variety of effects that impact the body.
A dog's skin, which is richly innervated by sensory nerves, reacts to the effect of hormones released during stressful events. It's therefore not a coincidence that, in humans, a variety of skin disorders such as psoriasis, atopic dermatitis, itchiness, and urticaria have been shown to be exacerbated by psychological stress.
There is accumulating evidence that hormones, neurotransmitters, and cytokines released during a stress response can negatively impact hair growth cycles-and this applies to dogs too.
According to a study conducted on mice, stress has been found to alter the actual hair follicle (HF) cycle prematurely, terminating the normal duration of active hair growth ( the anagen phase) in mice.
Why Do Dogs Shed When at the Vet's Office?
You are not imagining things if you notice significant hair loss when your dog is stressed as it may happen when in your veterinarian's examination room. Many dogs get stressed at the vet due to a variety of reasons.
For instance, dogs may dislike being restrained, touched in various places and poked by needles. It also doesn't help the fact that other stressed dogs may emit alarm pheromones that put your dog on high alert.
So how does the shedding happen? More specifically, what you are witnessing is the effect of your dog's fight or flight response.
To sum it up, your dog's body goes on high-alert mode, triggering physiological changes in an effort to create a boost of energy sufficient to get himself out of trouble and survive. Your dog's heart rate and breathing will therefore increase. His blood pressure will rise and his pupils will dilate so to see with more clarity.
On top of this, blood flows to your dog's muscles so that he can sprint into action and his his hair follicles dilate so to accomodate the blood flow to those escape muscles.
Hairs that are in the telogen phase (the resting phase) are therefore more likely to fall out as the arrector pili muscles (small muscles attached to hair follicles) contract as it happens in stressful situations such as being at the vet, explains board-certified veterinary dermatologist Karen L. Campbell in the book "The Pet Lover's Guide to Cat and Dog Skin Diseases."
- Stress inhibits hair growth in mice by induction of premature catagen development and deleterious perifollicular inflammatory events via neuropeptide substance P-dependent pathways. Arck PC, Handjiski B, Peters EM, Peter AS, Hagen E, Fischer A, Klapp BF, Paus RAm J Pathol. 2003 Mar; 162(3):803-14.
- Dog Facts: The Pet Parent's A-to-Z Home Care Encyclopedia: Puppy to Adult by Amy Shojai
- The Dog: Its Behavior, Nutrition, and Health by Linda P. Case
- The Pet Lover's Guide to Cat and Dog Skin Diseases by Karen L. Campbell