I understand your frustration and commiserate with you. You’ve described chronic rhinosinusitis which is a diagnosis of exclusion when other etiologies have been ruled out (fungal infection by culture, malignancy by biopsy, bacterial or viral infections by a pure culture or PCR of common respiratory pathogens). This syndrome is actually a constellation of diagnoses made by biopsy, culture, and PCR (DNA-based) testing. The three broad groups we see are lymphocytic-plasmacytic-, eosinophilic-, and idiopathic (unknown cause/the most common) rhinosinusitis. This syndrome is one of the most significant causes of sneezing and nasal discharge in the cat. The nasal discharge is generally serous (clear serum-like), but secondary bacterial infections can result in the development of mucopurulent (mucus + pus) discharge and inflammation can be severe enough to cause intermittent hemorrhage. The clinical signs can persist for years.
It's postulated that an early severe viral infection may trigger chronic disease. In addition, it's estimated that approximately 80% of cats are latently infected with FHV-1 (herpesvirus), and so another possible scenario for chronic infection is that Panther is exhibiting the symptoms of persistent herpesvirus infection. If FHV-1 can be confirmed by PCR testing - and that's problematic in a previously vaccinated cat, the antiviral drug famciclovir might be helpful.
In general, these cases are somewhat refractory to treatment and palliation of clinical signs rather than cure is the goal of medical management. Broad-spectrum antibiotics such as the clindamycin are often prescribed to manage secondary infections. Administration of antihistamines such as chlorpheniramine at 1-2mg/cat orally every 12-24 hours may lessen clinical signs of disease in some cats. Moistening therapies such as nebulization and saline nasal drops can help loosen secretions and soothe mucosa, particularly in dryer climates. The anti-inflammatory glucocorticoids such as prednisolone may be helpful as well. Prednisolone may be used at a maximum dose of 1-2mg/kg orally every 12 hours. If a positive response is noted, the lowest dose and the longest dosing interval that is effective should be determined by adjusting the dose over time. Inhaled glucocorticoids can be used as an alternative to decrease the systemic side effects of oral glucocorticoid use and have the benefit of directly affecting the nasal mucosa. Beclomethasone or fluticasone can be administered by metered dose inhaler (MDI) with an inhalation chamber at 1-2 puffs once to twice daily.
This syndrome is the bane of too many cats and their vets but hopefully the above approaches will be at least palliative if not curative. Please respond with further questions or concerns if you wish. (Please note that the use of lysine (Enisyl-f) should be circumspect. Two large-scale studies in feline herpesvirus (FHV-1) positive shelter cats showed a worsening in morbidity (severity of symptoms) in those administered the lysine as compared to those receiving placebo.)