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There is no age or sex predilection, but Malassezia dermatitis
or otitis are more often diagnosed in dogs
between 1 to 3 years of age. Many breeds seem to be
predisposed to Malassezia dermatitis: West Highland
White Terrier, Basset Hound, Dachshund, Cocker
Spaniel, Poodle, German Shepherd, Collies, Shetland,
Jack Russell Terrier, Silky Terrier, Australian Terrier,
Springer Spaniel, Newfoundland and Shar Pei.
Malassezia dermatitis is often recognized during warm
period at the time at which allergic dermatites are generally
diagnosed. Malassezia overgrowth or dermatitis
is not demonstrated to be contagious for companion
animals or man (Morris DO, 2005). In certain breeds,
for example Shar Pei and Newfoundland, clinical signs
may be particularly severe and epidermal dysplasia in
WHWT might be an inflammatory or hypersensitivity
reaction to the Malassezia infection or a result of
excessive self-trauma, rather than a congenital keratinization
disorder.
Pruritus is always present but its severity is related to
the importance of predisposing factors. Diffuse, regional
or localized alopecia, lichenification, hyperpigmentation,
erythema, erythematous macules, excessive scaling,
and oily skin and hair are the clinical features of
canine Malassezia dermatitis. The classical areas
involved are neck, axillae, abdomen, pinnae and external
ear canal, lips, and peri-anal area. Dogs with generalized
lesions have a rancid odour. In allergic cats multifocal
alopecia, erythema, crusting and greasy adherent
brownish scales are signs of Malassezia overgrowth
(Toma S et al., 2006). The distribution is depending
from the concurrent diseases, but face, ventral neck,
pinnae and ear canal, chin, interdigital and claw fold
skin are the most common site of infection. Especially
in Devon Rex cats, high number of yeasts in cytology is
associated with an abundant brown, greasy material in
this natural intertrigo area (Colombo S et al., 2007).
Malassezia dermatitis is suspected upon clinical evidence
and diagnostic methods used to identify overgrowth
of Malassezia organisms. The response to treatment
with specific antifungal therapy is considered the
best tool for a definitive diagnosis. Cytological examination
allows to rapidly observe yeasts and to “quantify”
their number. Many techniques are available to
obtain material from the skin: 1) direct impression
smear; 2) acetate tape (Scotch test); 3) scrape smear;
and 4) swab smear. Impression smear and Scotch test
seem to be the best techniques if the skin surface is flat
or greasy, whereas swab smears should be more useful
for cytological examination of the external ear canal.
Heat-fixing does not seem to increase numbers of
Malassezia on cytology of ear swab samples for cytologic
evaluation, and a 1-step dip in the blue reagent
alone as a rapid method of staining samples from
canine ear canals has been proposed (10). Slides or
acetate tape may be stained with Diff-Quick or other
rapid methods, May Grünwald-Giemsa, Giemsa or new
methylene blue. Microscopic examination with an oil
immersion lens (100X) reveals free or adhered to keratinocytes
yeasts appearing as oval or elongated cells of
3 to 5 µm in diameter, with a typical single polar bud-
S. Nardoni et al. -Veterinary malasseziosis
ding. The minimum number of yeasts that indicates the
possibility of a true Malassezia dermatitis is not really
known. Variations between breeds and body sites have
to be considered. Several criteria has been proposed to
establish Malassezia overgrowth; as a general guide, 1-
2 organisms per field (100X) in several fields in the
presence of typical clinical signs are suggestive of
Malassezia dermatitis. Methods to sample Malassezia
from the skin for culturing are cotton swabs, acetate
tapes, detergent scrubs and contact plates. Appropriate
media for the growth of all Malassezia species are
mDixon agar and modified Leeming and Notman agar,
while selective medium for dermatophytes allows the
growth of M.pachydermatis only. As the yeast is a normal
component of the cutaneous flora of the dog, by
itself a positive culturing has no or little value.
However, as for all opportunistic agents, the number of
colonies is perhaps an indication (this is comparable to
the number of yeasts demonstrated by cytological
examination). Dermohistopathology may sometimes
show the yeasts on the surface of the epidermis and
occasionally in the infundibula, particularly in PAS
stained sections (although they are occasionally visible
on HE stained sections). However, if they are not seen
on biopsy, this does not exclude their presence. False
negative results could be caused by the sampling of a
non-infected area, removal of the stratum corneum
during processing, etc. Cutaneous histopathology is a
less sensitive technique than cytology. In contrast, the
finding of Malassezia inside hair follicles could indicate
a real pathogenicity. The common findings in biopsies
from dogs with Malassezia dermatitis, include: orthokeratotic
hyperkeratosis with prominent foyers of
parakeratosis; spongiosis with irregular ridges; lymphocytic
exocytosis of the epidermis; focal accumulation
of neutrophils; subepidermal linear alignment of
mast cells and moderate superficial perivascular to
interstitial dermal inflammation with lymphocyte exocitosis.
Clinical signs of Malassezia dermatitis are variable
and may mimic many dermatoses, then differential
diagnosis includes many pruritic dermatoses characterized
by erythema, hyperpigmentation and seborrheoa
together with all underlying dermatological diseases of
the yeasts overgrowth.
References
Cafarchia C , Gallo S., Capelli G:, Otranto D., 2005 Occurrence
and population size of Malassezia spp. in the external ear canal
of dogs and cats both healthy and with otitis. Mycopathologia
Sep;160(2):143-9.
Chen T, Hill PB 2005 The biology of Malassezia organisms and
their ability to induce immune response and skin disease Vet.
Dermatol. 16, 4-26.
Colombo S, Nardoni S, Cornegliani L, , Mancianti F Prevalence of
Malassezia spp. yeasts in feline nail folds: a cytological and
mycological study. Vet Derm 2007, 18, 278-283
Nardoni S ,Mancianti F.,Corazza M., Rum A: 2004 Occurrence of
Malassezia species in healthy and dermatologically diseased
dogs. Mycopathologia May;157(4):383-8.
Nardoni S, Dini M, Taccini F., Mancianti F. 2007 Occurrence, dis-