The term FUO is used to define any febrile animal patient for which there is no obvious cause after “routine” diagnostic tests have been done. What constitutes “routine” will vary amongst different vets. The main diagnostic categories for FUO include infections (local, systemic), neoplasms, and collagen-vascular (immune-mediated) diseases. Other less common causes include granulomatous diseases; noninfectious inflammatory diseases such as pancreatitis, steatitis, tissue necrosis. It is only after a thorough search for the above causes that we are left with the diagnosis of FUO. Generally, the etiology of FUO is not a rare disease but is rather a common disease presenting in an atypical fashion.
Infections should always be considered first in the search for the etiology of FUO because of their frequency and potential response to treatment. It is helpful to categorize infections as either localized or systemic in order to allow for an eventual well-focused approach to the patient's problem. In the cat, viral agents such as parvovirus, herpesvirus, or calicivirus usually cause fairly classical clinical signs and rarely persist long enough to fit the definition of true FUO. Feline infectious peritonitis coronavirus infection is a common cause of FUO because it produces insidious, chronic illness and frequently defies easy diagnosis. Fever is often associated with retroviral infections (FeLV, FIV) as a direct result of the virus infection but more often due to an opportunistic infection occurring secondary to the immunosuppressive effects of the viral disease.
Your new cat is unlikely to have contracted whatever caused your male cat's illness. Even if there had been contact, the incubation would be less than a year.
However, if your cat is out doors alot she could run into a source of infection, such as other cats, rodents, birds, etc. that infected your boy.
If she is indoor, she is probably just slowly recovering (dare I suggest she's basking in your attention?)I would not be concerned since she is responding appropriately to each treatment.
Take each symptom seriously though and don't be afraid to seek the advice of your vet!
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