Pathophysiology Feline immunodeficiency virus (FIV) infection leads to a progressive immunologic dysregulation due to a constant loss of lymphocytes (particularly CD4+ T lymphocytes), changes in cytokine patterns, and an increase of circulating immunoglobulin G (IgG). The progressive immunopathology predisposes to severe secondary diseases such as opportunistic infections, neoplasia, neurologic disease, and wasting syndrome. Clinical Signs The initial acute phase of infection is characterized by nonspecific clinical signs such as anorexia, depression, fever and generalized lymphadenomegaly that may go unnoticed, evolving in a clinically latent stage. In the terminal phase of infection, infected cats may develop clinical signs related to secondary diseases, including opportunistic infections, myelosuppression, neurologic dysfunction, and neoplasia. Diagnosis Clinical suspicion is based on the association of known risk factors (sex, age and lifestyle) with clinical signs and biochemical abnormalities. A complete blood cell count may reveal mild neutropenia in the initial acute phase and pancytopenia (lymphopenia, anemia and neutropenia) in the terminal phase of infection. Hypergammaglobulinemia is normally reported. Other hematological and biochemical abnormalities may be due to secondary diseases. The detection of FIV-specific antibodies in blood confirms the diagnosis of infection. Therapy Proper management of infected cats is essential to reduce secondary infections and avoid viral transmission. If secondary diseases are diagnosed, appropriate treatment is recommended. Antivirals and immunomodulators can be considered for FIV-infected cats. Zidovudine (5-10 mg/kg PO q12h) can be effective in the treatment of FIV-associated stomatitis or neurological disorders. Human interferon-alpha (IFN-α) administered at high (104-106 U/kg SC q24h) or low (1-50 U/kg PO q24h) doses and feline interferon-omega (IFN-ω) administered subcutaneously (106 IU/kg q24h on 5 consecutive days) or orally (105 IU/cat q24h on 90 consecutive days) can be employed. Prognosis In most cases, FIV-infected cats live many years with a high quality of life and die at an old age from causes secondary to FIV infection.