Nonpruritic hair loss
Monika Welle (Switzerland) opened the workshop by explaining that because many forms of alopecia in dogs are associated with hair cycle arrest, it is important to understand the hair cycle. Recently, two new stages have been characterized in dogs – exogen (shedding) and kenogen (hairless resting stage). The hair cycle consists of the phases anagen, catagen, telogen and exogen (shedding) with a resting kenogen (hairless telogen) phase.
In order to progress to anagen, stem cells from the bulge (an anatomical feature in mice but not in dogs or humans) grow down to form the inner root sheath and hair shaft. The precise stimulus is unknown. Later in anagen, the hair grows upwards. It then regresses in catagen and the lower part undergoes apoptosis producing a telogen follicle. The telogen resting phase hair is firmly attached to hair follicle and it is not pushed out. Exogen, or shedding, is an enzymatic process between the hair and root sheath and can occur at any time during the following cycle.
Kenogen follicles are hairless resting follicles (called hairless telogen by Dunstan) that need stimulation to enter anagen. Normally, 15–20% of follicles are in this phase and when activated enable dogs to grow more hair at colder times of year. The numbers of hair follicles in kenogen may be increased three to four times in clinical alopecia when stimuli to make hair downgrowth are missing.
The hair cycle is governed by epithelial and dermal tissues with systemic modifiers. Various mediators are involved in the hair cycle.
Canine recurrent flank alopecia (CRFA) (S. Vandenabeele)
Sophie Vandenabeele (Belgium) discussed CRFA in dogs. The typical presentation is a bilaterally symmetrical flank alopecia. However, not all cases are on the flanks or seasonal or symmetrical.
1 Typical: bulldogs and boxers have a clear breed predilection with symptoms developing from 8 months to 11 years. The lesions are classical bilaterally symmetrical areas of alopecia. Some also have a little alopecia on the dorsal muzzle. There is hyperpigmentation in some breeds but not the Dalmatian. Some dogs also have the dorsum affected, and other breeds, such as the great dane, show only hypotrichosis.
2 Atypical presentations include alopecia that is predominantly facial: on the bridge of the nose and periocular regions. Breeds developing this form include golden and Labrador retrievers. The hair may regrow and the dogs do not have concurrent hypothyroidism. Some have multifocal lesions and involvement of dorsal facial and pinnal folds.
3 Atypical presentations also occur in some bulldogs and boxers. In these dogs areas of alopecia develop inflammation with crusts and pigmentation. An interface dermatitis is found on histopathology.
4 An atypical presentation is also found in Rhodesian ridgeback dogs. These dogs exhibit a change in coat colour and not alopecia. The change occurs annually. Histopathology shows hyperkeratosis and malformed follicles.
The pathogenesis involves the photoperiod and the disease is usually seen at latitudes from 35 degrees upwards. The photoperiod may act directly or indirectly via prolactin secretion. There may be decreased endogenous melatonin production in genetically predisposed animals.
Shoulder (normal-haired) and flank (hairless) biopsies were transplanted to nude (athymic) mice from two classically affected 3-year-old dogs, a boxer and a mixed breed. After 30 days there was hair regrowth on both the affected and nonaffected skin biopsies. Both donor dogs stayed alopecic for a further 50 days. These results indicate that the pathogenesis of CRFA is mediated by systemic factors. A systemically induced alopecia can explain the diversity of clinical presentations of this disease.
Stefanie Kobrich (Germany) commented that Rhodesian ridgeback dogs have hair that looks different and asked if this is the same syndrome.
Sophie Vandenabeele replied that it is.
Monika Welle added that affected dogue de Bordeaux and cane corso dogs appear mucinotic.
Sophie Vandenabeele stated that she has not biopsied these breeds.
Jan Declercq (Belgium) added that he saw a Yorkshire terrier with texture and colour change that progressed to alopecia later in the season.
Ann Hargis (USA) stated that histologically there is a mix of growing and nongrowing hair types.
Sophie Vandenabeele agreed that both types are seen.
Sherry Myers (Canada) asked if the growth is in one implanted site.
Sophie Vandenabeele clarified that the transplanted biopsies taken from the shoulder area of the dogs regrew hair but not those taken from the flank.
Valerie Fadok (USA) commented that the disease is common in pitbulls in the USA and that melatonin is used regularly as a treatment. She asked if melatonin affects hair regrowth.
Sophie Vandenabeele answered that melatonin has only recently become available in Belgium. One group had reported that their experience is that regrowth is faster.
Valerie Fadok added that Ed Rosser (USA) believes that if the timing of the hair loss cycle is established, starting treatment with melatonin before the hair falls out results in less alopecia.