Page 1305 - Problem-Based Feline Medicine

63 – THE CAT WITH ABNORMAL IRIS APPEARANCE 1297

Pathogenesis Diagnosis

Infection occurs by: Diagnosis may be based initially on the clinical signs,
● Ingestion of tissue cysts (bradyzoites) from an but obtaining supportive laboratory data is essential in
intermediate host (most common) or making a definitive diagnosis.
● Ingestion of sporulated oocysts from soil or water
Serology.
(far less common) or
● Demonstration of a rise in IgM titers indicates
● Transplacental transmission (rare).
a recent active infection.
After ingestion a gut replication cycle occurs. The cat ● Comparison of levels of aqueous humor T. gondii
is the definitive host and is the only species in which antibody levels with serum levels (Goldman–
this occurs. This results in the production of unsporu- Witmer coefficient or C-value) has been advocated
lated (non-infective) oocysts which are passed in feces. to determine that anterior uveitis has been caused
The oocysts sporulate in the environment after 1–5 by T. gondii, although the use of C-values is still
days and become infective. Intestinal replication will controversial.
also result in the formation of tachyzoites (active
Definitive diagnosis requires demonstration of the
acute infection) or bradyzoites (inactive latent
organism in inflamed tissues or fluid samples by his-
infection).
tology, immunohistochemistry, or polymerase chain
Activation of bradyzoites, typically found in muscle, reaction (PCR) techniques.
brain and liver, may occur because of other disease
states such as FIV, which act as stressors or immuno-
suppressors. High-dose corticosteroid use has been
known to activate dormant T. gondii infections. Treatment
When bradyzoites are activated, they undergo rapid Clindamycin (Antirobe, Upjohn) at a dose rate of
replication causing destruction of tissue and inciting an 12.5 mg/kg twice daily for 3–4 weeks is usually effec-
inflammatory response in various tissues, notably the tive against the organism. If no response is evident
central nervous system, uveal tract of the eye, liver after 3 weeks of antibiotic therapy, reconsider the
and lungs. diagnosis.
Concurrent anti-inflammatory therapy decreases
the anterior uveitis, e.g. topical 0.5% prednisolone
Clinical signs
acetate drops applied bid–qid.
Signs may be acute, or chronic and intermittent. The
acute fatal form occurs mostly in kittens.
IRIS NEOPLASIA***
Fever, anorexia, depression, weight loss.
Muscle or joint pain may be evident. Classical signs

Dyspnea is common in kittens and cats with the acute ● Diffuse pigmentary change across the
form, and is associated with pneumonia. anterior iris surface.
● Mass lesions bulging forward from the
Multifocal neurologic signs.
anterior iris stroma or distorting normal
Signs of liver disease such as jaundice, enlarged liver, pupil shape.
ascites or elevated liver enzymes.
Ocular signs, especially anterior uveitis, chorioretinitis Pathogenesis
and optic neuritis. The organism may cause a lympho-
cytic-plasmacytic uveitis evidenced by gray to tan- Various tumors occur in the iris such as melanoma,
colored nodules on the iris surface. lymphosarcoma, sarcoma and ciliary body tumors.