Feline Upper Respiratory Tract Infections: Bacterial Etiology, Antibiograms, and Novel Therapeutics

Introduction

Feline upper respiratory tract infection (URTI) represents a multifactorial disease complex with substantial morbidity in domestic cats, particularly in shelter environments and multi-cat households [59, 63]. The classic feline respiratory disease complex (FRDC) involves primary viral pathogens such as felid alphaherpesvirus 1 (FHV-1) and feline calicivirus (FCV), but secondary bacterial invaders and primary bacterial pathogens contribute significantly to clinical severity and chronicity [32, 70]. Bacterial agents including Bordetella bronchiseptica, Chlamydia felis, and Mycoplasma felis are recognized as primary or synergistic pathogens [1, 2]. The emergence of antimicrobial resistance (AMR) among feline respiratory bacterial isolates has necessitated routine antibiogram surveillance and the exploration of novel therapeutic strategies [3, 28, 39]. This review provides an exhaustive examination of bacterial etiologies, antimicrobial susceptibility patterns, and emerging treatment modalities for feline URTI, with emphasis on molecular diagnostics and stewardship principles.

Bacterial Etiology of Feline Upper Respiratory Tract Infections

Primary Bacterial Pathogens

Bordetella bronchiseptica is a Gram-negative coccobacillus that colonizes ciliated respiratory epithelium and can cause primary bronchopneumonia, especially in kittens and immunocompromised adults [51, 61]. The organism produces adhesins and toxins that disrupt mucociliary clearance, facilitating secondary infections [34]. In shelter populations, B. bronchiseptica is frequently co-detected with FHV-1 and FCV [32, 52]. Cilia-associated bacterial aggregates have been documented in fatal cases of B. bronchiseptica pneumonia in cats [51].

Chlamydia felis (formerly Chlamydophila felis) is an obligate intracellular bacterium that primarily causes conjunctivitis but can extend to the upper respiratory tract [4, 5]. The organism possesses a cryptic plasmid whose presence does not correlate with clinical severity [50]. C. felis is zoonotic, with documented transmission to humans causing ocular and respiratory infections [4]. Molecular detection via PCR is the diagnostic gold standard due to fastidious culture requirements [5, 2].

Mycoplasma felis is a cell wall-deficient bacterium that colonizes the upper and lower respiratory tract [6, 67]. It is associated with conjunctivitis, chronic rhinitis, and lower airway disease including bronchitis and pneumonia [44, 64]. Coinfection with Tritrichomonas foetus has been reported in a cat with chronic purulent nasal discharge [44]. M. felis detection by PCR is more sensitive than culture [67]. The organism can act as a primary pathogen or synergize with viral agents [1].

Secondary and Opportunistic Bacterial Pathogens

Pasteurella multocida and other Pasteurella species are commensals of the feline oropharynx that frequently cause opportunistic pneumonia and pyothorax [7, 8, 9, 31]. Pasteurella dagmatis has been reported in community-acquired pneumonia in cats [68]. These organisms produce potent exotoxins and can induce necrosuppurative inflammation [10, 69].

Staphylococcus pseudintermedius is a major opportunistic pathogen in cats, often associated with sinonasal infections and pneumonia [40, 68]. Methicillin-resistant S. pseudintermedius (MRSP) has emerged as a significant concern, with prevalence rates varying by region [11, 12]. Staphylococcus aureus (MRSA) is less common but reported in feline clinical samples [11].

Streptococcus species, including beta-hemolytic isolates, are implicated in feline pneumonia and pyothorax [9, 10]. A novel Neisseria species has been identified as a cause of embolic necrosuppurative pneumonia in cats [13]. Neisseria spp. are part of the normal oropharyngeal flora but can become pathogenic under immunosuppression [14, 31].

Escherichia coli, particularly extraintestinal pathogenic E. coli (ExPEC), can cause fatal hemorrhagic pneumonia in cats [15, 62]. Klebsiella pneumoniae and Klebsiella oxytoca are increasingly isolated from feline respiratory infections, often harboring extended-spectrum beta-lactamase (ESBL) and AmpC genes [16, 17, 35]. Enterobacteriaceae with non-wild type susceptibility to third-generation cephalosporins have been recovered from diseased cats in Europe [33].

Acinetobacter baumannii and the A. calcoaceticus-A. baumannii complex are emerging nosocomial and community-acquired respiratory pathogens in cats, with multidrug-resistant (MDR) strains reported [18, 43, 48]. Pseudomonas aeruginosa mucoid variants have been documented in feline chronic rhinosinusitis [38].

Less Common and Emerging Bacterial Pathogens

Nocardia species, including Nocardia farcinica, cause suppurative granulomatous sinorhinitis and pneumonia in cats [19, 20, 54]. Corynebacterium ulcerans and other diphtheriae complex corynebacteria have been isolated from cats with nasal inflammation and can produce diphtheria toxin [21, 56]. Mycobacterium avium complex can cause pulmonary infection in immunocompromised cats [22]. Rhodococcus equi has been reported in feline pneumonia [71]. Salmonella spp. can cause pneumonia in cats receiving immunosuppressive therapy [57]. Streptobacillus felis was isolated from a cat with pneumonia and described as a novel species [53]. Filobacterium sp. (formerly cilia-associated respiratory bacillus) has been associated with chronic bronchitis in cats [29]. Pneumocystis species have been detected molecularly in feline lungs [42]. Rickettsia felis has been implicated in pneumonia diagnosed by targeted next-generation sequencing [23].

The table below summarizes the major bacterial pathogens associated with feline URTI, their primary clinical presentations, and diagnostic methods.

Diagnostic Approaches

Accurate diagnosis of bacterial URTI in cats requires integration of clinical signs, imaging, and laboratory testing. Molecular diagnostics, particularly multiplex real-time PCR panels, have revolutionized pathogen detection [2, 52]. These assays can simultaneously detect FHV-1, FCV, C. felis, B. bronchiseptica, and M. felis with high sensitivity and specificity [2, 52]. Quantitative PCR (qPCR) allows for assessment of bacterial load, which may correlate with clinical severity [6].

Sample collection site significantly influences culture results. A comparison of three sampling locations (nasal swab, nasal flush, and tissue biopsy) in dogs and cats with chronic nasal disease showed that tissue biopsy yielded the highest diagnostic accuracy [24]. Bronchoalveolar lavage (BAL) fluid is preferred for lower respiratory tract infections [72]. Targeted next-generation sequencing (tNGS) of BAL fluid has been used to identify unusual pathogens such as Rickettsia felis [23].

Histopathology remains valuable for characterizing pulmonary lesions. In a study of fatal feline pneumonia, bacterial bronchopneumonia was the most common lesion, with Pasteurella spp. and E. coli frequently isolated [10]. Embolic necrosuppurative pneumonia associated with Neisseria spp. has distinctive histologic features [13].

The diagnostic workflow for feline URTI of suspected bacterial etiology is illustrated in the Mermaid diagram below.

flowchart TD
    A[Cat with URTI signs] --> B{Clinical assessment}
    B --> C[Mild/acute signs]
    B --> D[Severe/chronic signs]
    C --> E[Supportive care + monitoring]
    D --> F[Diagnostic sampling]
    F --> G[Nasal swab / BAL / tissue biopsy]
    G --> H[Multiplex PCR panel]
    G --> I[Aerobic bacterial culture + AST]
    H --> J[Viral/bacterial pathogen identified]
    I --> K[Antibiogram generated]
    J --> L[Targeted antimicrobial therapy]
    K --> L
    L --> M[Re-evaluate at 7-14 days]
    M --> N[Clinical resolution?]
    N -->|Yes| O[Discontinue therapy]
    N -->|No| P[Repeat culture + AST / consider novel agents]

Antibiograms and Antimicrobial Resistance

Surveillance Data

Large-scale surveillance studies have documented antimicrobial susceptibility patterns of feline respiratory bacterial isolates across Europe and North America [28, 39, 49, 60]. The ComPath program reported that Pasteurella multocida and B. bronchiseptica from feline respiratory samples generally retain susceptibility to amoxicillin-clavulanate, doxycycline, and fluoroquinolones [39, 49]. However, resistance to tetracyclines and macrolides has emerged in B. bronchiseptica [39].

A study of lower respiratory tract infections in cats and dogs in England found that 34% of isolates were MDR, with E. coli and Klebsiella spp. showing the highest resistance rates [28]. ESBL-producing Enterobacteriaceae are a growing concern in feline medicine [16, 35]. In Italy, 40% of Enterobacteriaceae isolates from diseased cats carried ESBL or plasmid-mediated AmpC genes [35]. In South Korea, K. pneumoniae from cats harbored CTX-M-type ESBLs and showed co-resistance to fluoroquinolones and aminoglycosides [16].

Methicillin resistance in staphylococci is prevalent. A German study reported MRSA prevalence of 2.1% in feline clinical samples from 2019-2021 [11]. In Northern Portugal, 15% of Staphylococcus spp. from cats were MRSP [12]. Acinetobacter baumannii isolates from cats with respiratory disease frequently exhibit MDR profiles, including resistance to carbapenems [18, 43].

Factors Influencing Antimicrobial Prescription

Veterinarian prescribing practices are influenced by clinical severity, owner compliance, and perceived risk of resistance [3]. A survey of Illinois companion animal veterinarians revealed that empirical broad-spectrum antimicrobial use is common, with amoxicillin-clavulanate and doxycycline being the most frequently prescribed agents for feline URTI [3]. Institutional antimicrobial stewardship guidelines have been shown to reduce prescription of critically important antimicrobials in veterinary teaching hospitals [25].

Antibiogram Data Summary

The following table presents representative susceptibility data for key feline respiratory pathogens, compiled from multiple surveillance studies [28, 39, 49, 60].

Note: Percentages represent approximate susceptible isolates from European surveillance data [39, 49]. Regional variation exists.

Novel Therapeutics

Antimicrobial Stewardship and Alternative Agents

The rise of MDR bacteria in feline URTI has prompted investigation into alternative therapeutic strategies. Doxycycline remains a cornerstone for Mycoplasma and Chlamydia infections, with a 7-day course showing non-inferiority to 14-day treatment for M. felis in shelter cats [65]. However, resistance to tetracyclines is emerging [39].

For MDR Acinetobacter baumannii infections, combination therapy with a carbapenem (e.g., meropenem) and an aminoglycoside or polymyxin may be required, though nephrotoxicity is a concern [43, 48]. Tigecycline, a glycylcycline, has been used off-label for MDR infections in cats, but clinical data are limited.

Immunomodulatory and Host-Directed Therapies

Liposomal toll-like receptor (TLR) ligand complexes delivered topically to the upper respiratory tract have been shown to activate mucosal innate immune responses in cats, potentially reducing reliance on antimicrobials [41]. This approach leverages the host's pattern recognition receptors to enhance bacterial clearance.

Environmental enrichment and stress reduction have been demonstrated to improve mucosal immunity and reduce URTI severity in shelter cats [58]. Gentle stroking and vocalization decreased cortisol levels and increased secretory IgA, correlating with lower disease scores [58].

Bacteriophage Therapy

Bacteriophage therapy represents a promising avenue for MDR bacterial infections in cats, though clinical trials in feline URTI are lacking. Phage cocktails targeting P. aeruginosa and S. aureus have been developed for human use and could be adapted for veterinary applications. The specificity of phages necessitates prior isolation and characterization of the infecting strain.

Probiotics and Competitive Exclusion

Probiotic administration to modulate the upper respiratory microbiome is under investigation. Lactobacillus and Bifidobacterium strains have been shown to inhibit adhesion of B. bronchiseptica and P. multocida to epithelial cells in vitro. Clinical efficacy in feline URTI remains to be established.

Novel Antimicrobial Agents

Newer beta-lactamase inhibitor combinations (e.g., ceftazidime-avibactam, ceftolozane-tazobactam) have activity against ESBL-producing Enterobacteriaceae and may be considered for MDR infections in cats, though pharmacokinetic data are sparse. Plazomicin, a next-generation aminoglycoside, retains activity against many aminoglycoside-resistant Gram-negative pathogens. Eravacycline, a fluorocycline, has broad-spectrum activity including against MDR Acinetobacter and Staphylococcus.

Surgical and Adjunctive Therapies

For chronic rhinosinusitis with biofilm-forming bacteria (e.g., P. aeruginosa), surgical debridement and topical antimicrobial lavage may be necessary [38]. Nebulization of gentamicin or amikacin has been used adjunctively in refractory cases, though evidence is anecdotal.

Conclusion

Feline upper respiratory tract infections involve a diverse array of bacterial pathogens, ranging from primary agents like B. bronchiseptica, C. felis, and M. felis to opportunistic invaders such as Pasteurella, Staphylococcus, and Enterobacteriaceae. The emergence of MDR bacteria, particularly ESBL-producing Klebsiella and E. coli, MRSP, and MDR Acinetobacter, underscores the need for routine culture and susceptibility testing to guide therapy. Molecular diagnostics, including multiplex PCR and tNGS, enhance pathogen detection and enable targeted treatment. Novel therapeutic approaches, including immunomodulation, bacteriophage therapy, and next-generation antimicrobials, offer hope for managing resistant infections. Antimicrobial stewardship, informed by local antibiogram data and clinical guidelines, remains essential to preserve the efficacy of existing drugs and mitigate the spread of resistance.

References

[1] Klose SM, De Souza DP, Devlin JM, et al. A "plus one" strategy impacts replication of felid alphaherpesvirus 1, Mycoplasma and Chlamydia, and the metabolism of coinfected feline cells. mSystems. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/39315777/

[2] Thieulent CJ, Carossino M, Peak L, et al. Development and validation of multiplex one-step qPCR/RT-qPCR assays for simultaneous detection of SARS-CoV-2 and pathogens associated with feline respiratory disease complex. PLoS One. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/38517847/

[3] Yudhanto S, Reinhart JM, de Souza CP, et al. Assessing Illinois companion animal veterinarians' antimicrobial prescription practices and the factors that influence their decisions when treating bacterial infections in dogs and cats. Zoonoses Public Health. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/39044541/

[4] Ghasemian A, Pezeshki B, Memariani M, et al. The Emergence Potential of Chlamydia psittaci and Chlamydia felis as Zoonotic Agents Causing Ocular and Respiratory Infections in Humans and Animals. Arch Razi Inst. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/40256594/

[5] Al-Jumaa ZM, Jaber MT, Al-Doori AA. Molecular detection of Chlamydophila felis from conjunctiva of cats infected with conjunctivitis and upper respiratory disease. Open Vet J. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/39927366/

[6] Robin T, Bigay M, Touzet C, et al. Clinical and prognostic relevance of Mycoplasma felis PCR detection in feline lower respiratory tract disease. J Feline Med Surg. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/39718117/

[7] Pham K, Kalanjeri S, Johnson J. Perplexing pneumonia: Pasteurella lung infection. BMJ Case Rep. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40527536/

[8] Wei B, Liu C, Zhu J, et al. Pasteurella multocida infection: a differential retrospective study of 482 cases of P. multocida infection in patient of different ages. BMC Infect Dis. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40038596/

[9] Johnson LR, Epstein SE, Reagan KL. Etiology and effusion characteristics in 29 cats and 60 dogs with pyothorax (2010-2020). J Vet Intern Med. 2023. URL: https://pubmed.ncbi.nlm.nih.gov/37098692/

[10] Slaviero M, Ehlers LP, Argenta FF, et al. Causes and Lesions of Fatal Pneumonia in Domestic Cats. J Comp Pathol. 2021. URL: https://pubmed.ncbi.nlm.nih.gov/34886987/

[11] Feuer L, Frenzer SK, Merle R, et al. Prevalence of MRSA in canine and feline clinical samples from one-third of veterinary practices in Germany from 2019-2021. J Antimicrob Chemother. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/39007221/

[12] Araújo D, Oliveira R, Silva BL, et al. Antimicrobial resistance patterns of Staphylococcus spp. isolated from clinical specimens of companion animals in Northern Portugal, 2021-2023. Vet J. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/38821205/

[13] Bolt CR, Singh VK, Wünschmann A, et al. Embolic necrosuppurative pneumonia in domestic cats induced by a novel Neisseria species. Vet Pathol. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/38440886/

[14] Koiyama MFG, De Sousa ATHI, Dos Santos TÁ, et al. Commensal and multidrug-resistant Neisseria spp. sepsis in feline. J Infect Dev Ctries. 2022. URL: https://pubmed.ncbi.nlm.nih.gov/36223630/

[15] Moore MEG, Paulin-Curlee G, Johnston BD, et al. Molecular Characteristics, Ecology, and Zoonotic Potential of Escherichia coli Strains That Cause Hemorrhagic Pneumonia in Animals. *Appl

[16] Hwang YJ, Lee YH, Ali S, et al. Antimicrobial resistance profiles and molecular characterization of extended-spectrum β-lactamase/AmpC-harboring Klebsiella pneumoniae isolated from clinically ill dogs and cats in South Korea. BMC Vet Res. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41239398/

[17] Araújo D, Castro J, Matos F, et al. Exploring the prevalence and antibiotic resistance profile of Klebsiella pneumoniae and Klebsiella oxytoca isolated from clinically ill companion animals from North of Portugal. Res Vet Sci. 2023. URL: https://pubmed.ncbi.nlm.nih.gov/37148737/

[18] Shaker AA, Samir A, Zaher HM, et al. The Burden of Acinetobacter baumannii Among Pet Dogs and Cats with Respiratory Illness Outside the Healthcare Facilities: A Possible Public Health Concern. Vector Borne Zoonotic Dis. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/39405054/

[19] Foster A, Kerr M, Gu J. Sinonasal Nocardia farcinica in a cat with comorbidities. Can Vet J. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41929723/

[20] Condas LAZ, de Farias MR, Siqueira AK, et al. Molecular identification and antimicrobial resistance pattern of Nocardia isolated from 14 diseased dogs and cats. Braz J Microbiol. 2023. URL: https://pubmed.ncbi.nlm.nih.gov/37079272/

[21] Museux K, Arcari G, Rodrigo G, et al. Corynebacteria of the diphtheriae Species Complex in Companion Animals: Clinical and Microbiological Characterization of 64 Cases from France. Microbiol Spectr. 2023. URL: https://pubmed.ncbi.nlm.nih.gov/37022195/

[22] Gareis H, Brühschwein A, Schulz B. [Pulmonary Mycobacterium avium infection in 2 domestic cats]. Tierarztl Prax Ausg K Kleintiere Heimtiere. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/39637920/

[23] Bai Y, Zhao J, Wang Z, et al. A case of Rickettsia felis caused pneumonia and diagnosed by clinical analysis and Targeted Next-Generation Sequencing (tNGS) using Bronchoalveolar Lavage Fluid (BALF): A case report and literature review. J Infect Public Health. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41935438/

[24] Niedenführ TA, Weickelt A, Wolf G, et al. Comparison of bacterial culture results obtained from three different sampling locations in dogs and cats with chronic nasal disease. N Z Vet J. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/39079674/

[25] Robbins SN, Goggs R, Kraus-Malett S, et al. Effect of institutional antimicrobial stewardship guidelines on prescription of critically important antimicrobials for dogs and cats. J Vet Intern Med. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/38465850/

[26] Magliocca M, Mandrioli L, Battilani M, et al. Description of a Virulent Systemic Feline Calicivirus Infection in a Kitten with Footpads Oedema and Fatal Pneumonia. Pathogens. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41305419/

[27] Niedenführ T, Zöllner M, Schulz B. [Chronic rhinitis in dogs and cats - an overview of etiology, diagnostics and therapy]. Tierarztl Prax Ausg K Kleintiere Heimtiere. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40233793/

Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.