Page 1309 - Problem-Based Feline Medicine

63 – THE CAT WITH ABNORMAL IRIS APPEARANCE 1301

Clinical signs
Pathogenesis
Typically, there is anorexia, depression, fever and
In general, all deep fungal infections enter the body by
weight loss combined with other systemic signs which
the inhalation of aerosolized spores leading to either
can involve many other organs, especially the lungs,
fungal rhinitis (e.g. Cryptococcus) or pneumonia
brain, nasal cavity but also the orbital tissues,
(e.g. blastomycosis, histoplasmosis) with subsequent
lymph nodes, bones, toenails and skin.
dissemination to other parts of the body, including the
eye, by hematogenous or lymphatic spread. Most intra-ocular fungal infections cause either:
● Choroidal granulomas which are typically evident
The initial ocular site for establishment of infection is
on fundoscopy as a raised area of tapetal hypore-
usually the choroid, while the anterior uveal tract is
flectivity, or small swollen brownish discolored
often involved later in the course of the infection.
areas in the tapetal and non-tapetal retina.
The different intra-ocular fungi vary in their geograph- ● Generalized chorioretinitis and secondary retinal
ical distribution. detachments. This may progress to anterior uveitis
● Blastomycosis is caused by a dimorphic fungus, and endophthalmitis.
which grows as a yeast in mammalian tissue and as ● Optic neuritis, which is seen as a red, swollen
a mycelial form in the environment. Found in the optic nerve that is hyperemic.
USA (in the midwestern and southeastern states ● Anterior uveitis, which appears as anterior cham-
along the Mississipi, Ohio and Missouri Rivers and ber flare, miosis and an inflamed discolored iris.
occasionally mid Atlantic states), Central America, ● Vision loss and reduced pupil light reflexes in
parts of Asia, Africa, Europe and Israel. Close proxi- affected eyes.
mity to water may be a factor in its geographical
distribution. Diagnosis
● Cryptococcosis is caused by a budding capsulated
Initial diagnosis is based on the clinical presentation
yeast-like fungus with more worldwide distribu-
of a cat with a rapidly progressive, usually bilateral
tion. C. neoformans is shed in pigeon feces and
chorioretinitis progressing to anterior uveitis, with signs
can be found in soil or in avian excreta. C. neofor-
of other systemic disease, and occurring in a geographic
mans var. gatti is found in the developing flower
area where such fungal disease is known to occur.
of some Australian Eucalypt trees (red river gums
and forest red gums). High concentrations of Imaging techniques can be used to obtain more sup-
organisms are found in the bark and the accumu- portive evidence of a deep fungal infection:
lated debris surrounding the base of the tree. ● Thoracic radiography for pulmonary granulomas.
These trees have been widely exported around the ● Nasal cavity radiography.
world from Australia. ● Ocular ultrasound for signs of retinal detachment.
● Histoplasmosis is caused by a dimorphic fungus
Confirmation of the diagnosis is based on the
which forms a free-living mycelial form and yeast-
demonstration of the organism in:
like organisms in mammalian tissues. Found in
● Cerebrospinal fluid.
central USA especially in Ohio, Mississippi and
● Samples taken by centesis of vitreous or subretinal
Missouri River basins and in Central and South
exudate.
America. There is association with moist humid
● Histopathology of enucleated globes.
conditions, nitrogen-rich soils, and bat and bird
● Bone marrow biopsy samples (histoplasmosis).
excrement.
● Lung or lymph node aspirates or draining tracts.
● Coccidioidomycosis is caused by a soil mycelial
organism with a natural resevoir in desert soils and Serological tests can be performed, looking for
around animal burrows. Found in Southwestern elevated:
USA, Mexico, Central and South America. Animal ● Antibodies (blastomycosis, histoplasmosis, coccidio-
infection occurs by inhalation of arthrospores, domycosis).
which can transform into spherules and then ● Antigen – Cryptococcal capsular antigen in blood,
endospores in lung tissue. CSF, or urine is sensitive and specific.