Patch test responses to Malassezia pachydermatis in healthy dogs

The effects of the patch test application of Malassezia pachydermatis extracts to normal canine skin were evaluated in eight healthy beagle dogs. Antigens (4 and 0.4 mg/ml) and saline controls were applied for 48 h using filter paper discs in Finn chambers. At the first test, two dogs showed patch test reactivity 20 min and 24 h after patch removal. Four out of six dogs that did not react to the first patch test showed reactivity when re-tested on day 8. Two remaining dogs were patch tested for a third time on day 15, after 7 days of cutaneous challenge with suspensions of M. pachydermatis cells, but failed to display reactivity. Positive patch test reactions were characterized histologically by mild epidermal hyperplasia, superficial dermal oedema and mild to moderate perivascular, periadnexal and interstitial infiltrates of neutrophils and CD3+ lymphocytes. Four dogs showed delayed intradermal test reactivity to M. pachydermatis antigens but intradermal and patch test reactivity did not correlate. This study indicates that patch test reactivity to M. pachydermatis antigen occurs in some healthy dogs exposed to the yeast, or may develop after a short period of antigen exposure. Further studies of patch test reactivity are warranted in dogs with disease associated with this cutaneous yeast.