There are many reasons why a dog's coat will thin and become bald in some spots, however we are going to discuss the most common reasons seen with the Pug dog breed.
1) Skin Problems -
Fur consists of 3 main elements: the follicle, bulb and the shaft. The follicle of fur is what is underneath the skin. The shaft is the strands of fur that you visibly see. The bulb is at the base of the follicle, deep within the skin. If a Pug is having issues that affect the skin, this will have a direct impact on the health of the hair shaft's bulb. In turn, the strength and integrity of hairs can weaken to the point of falling out.
2) Parasites -
The most common parasite that causes hair loss with Pug dogs is demodectic mange. There are different types of this mite infestation that may cause hair loss around the eye
area, paws or other parts of the body. In some cases, the balding areas will begin to crust over.
Ringworm (which is a fungus) is also a possibility. The telltale signs of this are circular bald spots that appear on the head, paws, ears or limbs. They may have a red spot in the middle and the area may or may not crust over.
3) Thyroid Issues -
Both low level of hormones (hypothyroidism) or high levels (hyperthyroidism) can cause hair loss. With this issue, problems are often seen over larger areas of the Pug's coat and are not concentrated in one area. If this is the cause, hair is often dry and brittle.
4) Stress -
This is an emotional breed and for that reason the Pug is highly susceptible to stress related conditions. Major changes such as a loss of a family member (human or pet), moving to a new home, severe levels of separation anxiety
can really throw the body off kilter.
When a Pug is highly stressed, a condition called telogen effluvium may develop, in which large number of coat follicles are forced into a prolonged resting phase. One to two months afterward those hairs may then fall out, especially when the dog is bathed or brushed. Since this is a delayed reaction, a Pug that suffered through a bout of stress may only show this symptom of hair loss a couple of months afterward once things have settled down.
When all test results are normal, this is often the cause. If the triggers have been resolved, the coat often grows back to its normal dense and shiny state of health.