There are many reasons the skin of dogs can become alopecic, but there are two main categories to consider. When a dog has an alopecic area, either the hair is not re-growing due to a hair cycle issue (lesions are symmetrical, large areas, and not itchy), or the hair is falling out due to folliculitis: staphylococcus, demodicosis, dermatophytosis, atopic dermatitis, or other causes. Nearly every inflammatory condition of the skin can have some aspect of alopecia.
First, get a complete history of the lesions, as well as previous skin issues the patient has had, the dog’s parasite prevention program, if the owner has tried any therapies, and what medications the patient is on. If the alopecia is symmetrical, determine if the dog is having signs of hypothyroidism (young- to middle-aged dogs) or hyperadrenocorticism (old dogs). Be specific in your questions. Do not ask, “Are there any other symptoms?” Instead, for example, you should ask, “Is he drinking a lot? Is he waking at night to urinate or having accidents inside?”
Determine if the dog is itchy or not. The itch question is very important to have the owner answer correctly as it will potentially change your rule outs and also your therapy. If the dog is itchy, that means the skin is inflamed. Sometimes owners assume if the lesion looks bad, it must be itchy, so they say yes. Some owners think itching is only scratching; they do not realize behaviors like licking or rolling are ways a dog itches, and say no, the dog is not itchy. Be very specific and ask, “Does he scoot his butt, shake his head, roll on his back, or lick his feet?” Ask about itchiness on the rest of the body, not only the alopecic area.
Next, examine the entire dog, do not just focus on the alopecic area. You might find other areas are affected as well as gain clues as to the underlying cause of the alopecia.
If lesions are at all inflamed (crusted, red, scaly, or itchy) perform a skin cytology (tape or slide impression) to look for yeast or bacteria and deep skin scrapings to look for mites, and if those are not helpful, perform a fungal culture for dermatophytes (Figure 2). For strongly pruritic patients, even when scrapings are negative, external parasites need to be ruled out with a trial of isoxazoline antiparasitic medications for one to two months. Biopsies should be performed when other diagnostics or treatments are not rewarding, or if the lesions are dramatic or unusual.
Sometimes dogs can have multiple conditions all at once contributing to hair loss and/or recurrent infections like atopic dermatitis (Figures 3 and 4). Sometimes therapy is not working, or issues are recurrent despite therapy. These cases are best referred to a dermatologist before infections become resistant or the owner gets frustrated.
Hair cycling abnormalities
Hair cycling abnormalities are caused by endocrine, iatrogenic, and idiopathic conditions. Alopecia or thin hair is noticed when many hairs have been shed and not regrown for many months, the remaining unshed hair (which is not cycling normally) may be faded, dull, or matted. The owner may not notice the problem until the pet is groomed or clipped, and they will blame the groomer for the hair condition because “there was no problem until he was groomed.”
It is important to educate owners that the grooming or clipping did not cause the underlying issue with hair cycling. Giving a diagnosis of “Post clipping alopecia” is not recommended as it implies the act of clipping was somehow involved in the disease. “Hair cycle arrest” is more appropriate.
On a dog with hair cycle arrest, bloodwork should be performed, including a thyroid PANEL with total T4, Free T4, and TSH, along with a urinalysis. If a total T4 alone is normal, that can usually rule out hypothyroidism, however, you should not diagnose true hypothyroidism from a total T4 level alone. Total T4 can be low due to many other conditions and random fluctuations. Always perform a thyroid panel if your T4 level is low or if you have a strong suspicion of hypothyroidism prior to starting thyroid supplementation.
I do not recommend a “trial” with thyroid meds to see if it helps, as most “trials” become inappropriate and potentially harmful long-term therapy because the prescriber forgets it was just a trial. Hypothyroid dogs are not energetic happy dogs (Figure 1 A and B). When thyroid medications are started in a hypothyroid patient, you should see dramatic improvement in energy and hair regrowth within a month. If not, reassess the patient.
Hyperadrenocorticism can be tested for in older dogs, but I don’t perform hyperadrenocorticism testing if the patient has normal lab work and no other signs of disease other than hair loss. Because therapy for hypoadrenocorticism is potentially dangerous, I only perform testing for hyperadrenocorticism if I would have a strong need to treat (for example if the hyperadrenocorticism is causing recurrent infections, polyuria, calcinosis cutis (Figure 5), or if diabetes is difficult to regulate).
Iatrogenic Cushing’s syndrome is common. Oral steroid use for over a couple months can lead to hair thinning even without any other symptoms. Confirmation of the steroids causing the hair loss would be tapering and stopping the steroids; there is no test to diagnose whether the steroids are causing hair loss other than withdrawal of the steroids and seeing hair regrowth. Any systemic steroids, even budesonide, can cause hair cycle arrest. Topical steroids can be absorbed systemically and cause the same issues as oral steroids in sensitive patients. Be sure to question the owner about ear medications, eye medications, or the owner’s topical steroid or hormone replacement creams that the dog may come in contact with.
Seasonal flank alopecia can cause dramatic or mild, pigmented, symmetrical lesions on the flanks of short-haired dogs such as boxers and bulldogs (Figure 6). Sometimes the condition is not seasonal and not only on the flank areas. Flank alopecia can resolve spontaneously or will sometimes respond to a trial of melatonin.
Sometimes no abnormalities are found as to the underlying cause of the hair cycle arrest and the patient is happy and healthy. In these cases, Alopecia X is the typical diagnosis. Alopecia X is a condition of plush haired dogs like Pomeranians, keeshonds, and huskies and causes slowly progressive hair loss of mainly the trunk, sparing the legs and head (Figure 7). It can arise in young adults or older dogs and can be very disturbing for the owner who strongly desires a fluffy beautiful coat.
All other causes of hair cycle arrest should be ruled out so there is no concern about internal disease. Other than melatonin and/or doggie clothing, it is rare that I would recommend therapy for alopecia X in cases where the dog is otherwise healthy and happy. This condition can resolve spontaneously.
In summary, if you are presented with a dog with alopecia, determine if the condition is inflammatory or hair cycle arrest (or both). Then perform basic skin diagnostics and lab work to determine the cause of the lesions and the underlying reason. Biopsies can sometimes give clues to underlying conditions. Treat the infections and underlying reason and always recheck and follow up to make sure your treatments have resolved and controlled the condition long term.
A CLOSER LOOK
|Itchy and/or inflamed and losing hair:
Not itchy, not inflamed and losing hair:
Keep in mind that any of the conditions on this list can have a secondary infection present that may be inflamed.
● Tobin, D.J., Linek, M. (2009). Hair Loss Disorders in Domestic Animals. Germany: Wiley.
Melissa Eisenschenk, DVM, DACVD, is a board-certified veterinary dermatologist. She attended veterinary school at the University of Minnesota. After veterinary school, she was in private practice for four years, and then returned to the University of Minnesota for a dermatology residency and Master’s degree. She passed dermatology board exams in 2009. Dr. Eisenschenk opened Pet Dermatology Clinic in Maple Grove, Minn., in 2013.