Alopecia areata in the dog

By Irina Matricoti, Veterinary Surgeon, European Specialist in Veterinary Dermatology, Dipl. ECVD

Alopecia areata is an infrequent autoimmune disease of the dog, rarely reported in the cat. It arises as a result of an immune dysregulation in the hair follicle, leading to an attack by T lymphocytes at the root (bulbitis), resulting in hair loss. In particular, there is a loss of immune privilege, present in the lower portion of the hair follicle, as well as other tissues, e.g. the anterior chamber of the eye, the testis and the central nervous system. These tissues are exempt from strict immune control: T-lymphocyte surveillance of antigens is inhibited at these sites by overexpression of immune-suppressive molecules such as TGF-beta. Antigen-presenting cells, such as Langerhans cells, are also very scarce at these sites. As a result of the loss of this immune privilege, there is an infiltration of Natural Killer lymphocytes into the follicle, together with an increased expression of major histocompatibility complex (MHC) molecules, which are responsible for presenting antigens to the lymphocytes. These factors lead to lymphocyte reactivity towards follicular autoantigens.

The autoantigens that are presumably recognised and attacked by the immune system are some melanocyte antigens and trichoialin. In patients with alopecia areata, high levels of antibodies to melanocyte proteins, such as melanoma-associated antigen 3 (MAGE-3), and trichoialin have been found. Melanogenesis occurs only during the anagen phase of the follicular cycle, so that the hair follicles involved in the process are exclusively in the growth phase. An attack of the follicle by lymphocytes induces a hair cycle arrest, resulting in follicular dystrophy, early transition to the catagenic phase and subsequent hair loss. The cause of this loss of immune privilege is unknown: microtrauma, bacterial antigens, viral infections or stress are hypothesised. Genetics could also play a role, in human medicine for example alopecia areata is sometimes observed in related patients, and is frequently associated with other immune disorders, notably Hashimoto's thyroiditis, vitiligo and atopic dermatitis (1,2)

Clinically, dogs with alopecia areata are frequently adult males. German shepherd dogs, dachshunds and beagles according to some studies appear more represented, in others the pinscher is mentioned, however the disease can be seen in any breed and also in mongrel dogs. The main clinical sign is focal or multifocal alopecia, sometimes accompanied by leucotrichia.

In most cases the area involved is on the muzzle, and in particular the periocular or perilabial skin, but lesions are also often observed on the limbs. Alopecia may become more diffuse, and in only one case has it been described as generalised, involving the whole body (2). Rarely, traconichia and onychodystrophy have also been described, which are more frequently reported in humans (1,2).

The diagnosis is obtained by histopathological examination, to be performed after excluding, by means of basic dermatological examinations, other diseases that more frequently cause multifocal alopecia, and in particular all infectious follicular diseases (dermatophytosis, bacterial folliculitis and demodicosis). Histological examination shows a lymphocytic bulbitis, i.e. the marked presence of peri- and intra-bulbar lymphocytes. The appearance of the bulb is often described as that of a swarm of bees. The biopsy should be performed from the peripheral portions of the lesion, where affected anagen follicles and dystrophic follicles can still be seen. In chronic cases, only peribulbar fibrosis with profound atrophy or absence of follicles is usually observed.

The dermoscopic examination, used routinely in human medicine, shows in humans an appearance of hair similar to exclamation marks. In veterinary medicine, the most frequently described dermoscopic feature is the presence of yellow dots, which correspond to keratin accumulating at the level of the follicular infundibula (3).?

Lesions may regress spontaneously and hair may regrow within 6-24 months. When the lesions do not regress, cyclosporine is generally used, due to its specific immunomodulatory action on T lymphocytes (4). The hair frequently grows back white. In human medicine, numerous therapies are used, including topical steroids, tacrolimus, phototherapy and topical minoxidil. The latter has been tested for topical use in Mexican nude dogs and proved effective in partially regrowing hair, however there is no data on its safety and possible adverse effects related to possible oral intake through licking. It is important to remember that oral administration of minoxidil in dogs is highly toxic to the myocardium, and this therapy is currently not recommended due to the possibility of serious adverse effects. Anecdotal data on the efficacy of oclacitinib in alopecia areata in dogs have also been reported recently.

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Bibliographic references

1.???Tobin JD. Alopecia areata in: Hair loss disorders in domestic animals eds: Mecklenburg L Linek M, Tobin DJ; Wiley-Blackwell, 2009

2.???Outerbridge CA, White SD, Affolter VK,Alopecia universalis in a dog with testicular neoplasia. Veterinary Dermatology 2016 27:513-e139

3.???Scarampella F, Roccabianca P Alopecia areata in a dog: clinical, dermoscopic and histological features. Skin Appendage Disorders 2018, 4: 112-117

4.???Noli C, Toma S. Three cases of immune-mediated adnexal skin disease treated with cyclosporin. Veterinary Dermatology 2006 17:85-92?