Bacterial Respiratory Infections in Cats: Etiology, Diagnosis, and Treatment

Introduction

Bacterial respiratory infections in cats represent a significant component of feline respiratory disease complex (FRDC), a multifactorial syndrome involving viral and bacterial pathogens [1]. While primary viral agents such as felid alphaherpesvirus 1 (FHV-1) and feline calicivirus (FCV) are frequently incriminated, secondary bacterial invaders and primary bacterial pathogens contribute substantially to morbidity and disease progression [1, 2]. The clinical presentation of a [cat bacterial respiratory infection] ranges from mild serous nasal discharge and conjunctivitis to severe bronchopneumonia, pyothorax, and systemic sepsis [3, 4, 5]. Understanding the etiological agents, their pathogenic mechanisms, and the diagnostic and therapeutic approaches is essential for effective clinical management. This review provides a detailed examination of the primary bacterial pathogens involved in feline respiratory disease, their epidemiology, diagnostic methodologies, treatment protocols, and control strategies.

Etiology and Epidemiology

Bordetella bronchiseptica

Bordetella bronchiseptica is a Gram-negative, aerobic coccobacillus that colonizes the ciliated respiratory epithelium of cats, dogs, and other mammals [6, 7]. It is a primary pathogen capable of causing tracheobronchitis and bronchopneumonia, particularly in kittens and immunocompromised adults [7]. Transmission occurs via direct contact with infected respiratory secretions or fomites, and the bacterium can persist in the environment for extended periods [7]. Outbreaks in shelter environments are well documented, with multidrug-resistant strains posing significant therapeutic challenges [7]. The bacterium expresses adhesins such as filamentous hemagglutinin and pertactin, which facilitate attachment to ciliated epithelial cells, and produces toxins including tracheal cytotoxin and dermonecrotic toxin that impair mucociliary clearance [6]. An inactivated vaccine has been developed and shown to induce protective immune responses in cats [6]. Bacteriophage therapy has also been explored as a potential alternative for combating B. bronchiseptica infections [8].

Mycoplasma felis and Mycoplasma cynos

Mycoplasma felis and Mycoplasma cynos are cell wall-deficient bacteria belonging to the class Mollicutes [9, 10, 11]. M. felis is a common commensal of the feline upper respiratory tract but can act as an opportunistic pathogen, particularly in the lower respiratory tract [10]. M. cynos is more frequently associated with canine respiratory disease but has been identified in cats [9]. These organisms lack a peptidoglycan cell wall, rendering them intrinsically resistant to beta-lactam antimicrobials [9]. They possess adhesins that mediate attachment to respiratory epithelial cells and secrete metabolites such as hydrogen peroxide that cause ciliostasis and epithelial damage [9]. The fastidious nature of mycoplasmas complicates culture-based diagnosis, necessitating molecular methods for accurate detection [11]. Long-read sequencing workflows have been developed to improve genomic characterization of these organisms [11]. Antimicrobial susceptibility testing has revealed variable resistance profiles, including macrolide and fluoroquinolone resistance [9].

Chlamydia felis

Chlamydia felis is an obligate intracellular Gram-negative bacterium that primarily causes conjunctivitis in cats but is also implicated in upper respiratory tract disease [12, 13, 14, 15]. The organism exists in two morphological forms: the infectious elementary body (EB) and the replicative reticulate body (RB). EBs attach to and are internalized by conjunctival and respiratory epithelial cells, where they differentiate into RBs and replicate within a membrane-bound inclusion [13]. C. felis is zoonotic, with documented cases of human conjunctivitis and respiratory infection following exposure to infected cats [13]. Prevalence studies have demonstrated widespread distribution of C. felis in cat populations, with risk factors including multi-cat households, outdoor access, and young age [12, 14]. Co-infection with FHV-1 and mycoplasmas is common and can exacerbate clinical signs [15, 1].

Pasteurella multocida

Pasteurella multocida is a Gram-negative, facultatively anaerobic coccobacillus that is part of the normal oral and upper respiratory flora of cats [16, 17]. It is a common opportunistic pathogen that can cause pneumonia, particularly following aspiration or in the context of concurrent viral infection [17]. The bacterium produces a polysaccharide capsule that inhibits phagocytosis and a variety of virulence factors including dermonecrotic toxin and lipopolysaccharide [16]. Serotyping and molecular detection methods, including quadruplex real-time quantitative PCR, have been developed for identification and typing of P. multocida [16].

Other Bacterial Pathogens

A diverse array of other bacterial species can cause respiratory disease in cats. Mycobacterium bovis and Mycobacterium avium are causes of feline tuberculosis, presenting with granulomatous pneumonia and lymphadenopathy [18, 19, 20]. Nocardia farcinica and other Nocardia species can cause sinonasal and pulmonary infections, particularly in immunocompromised cats [21, 22]. Rhodococcus equi and Klebsiella pneumoniae have been associated with severe bronchopneumonia [23]. Acinetobacter baumannii has been identified as an emerging pathogen in cats with respiratory illness, raising public health concerns due to its multidrug-resistant profile [24]. Neisseria species, including a novel species, have been implicated in embolic necrosuppurative pneumonia and sepsis [4, 25]. Rickettsia felis has been documented as a cause of pneumonia, diagnosed via targeted next-generation sequencing of bronchoalveolar lavage fluid [26]. Streptococcus species and anaerobic bacteria are frequently isolated from cases of pyothorax [3, 5].

Clinical Signs and Pathology

Upper Respiratory Tract Infection

Clinical signs of bacterial upper respiratory tract infection in cats include serous to mucopurulent nasal discharge, sneezing, conjunctivitis, ocular discharge, and submandibular lymphadenopathy [14, 27, 1]. C. felis typically induces severe conjunctival hyperemia, chemosis, and ocular discharge, often with minimal nasal involvement [12, 14]. B. bronchiseptica infection is characterized by paroxysmal coughing, gagging, and nasal discharge [7]. Co-infections with viral agents such as FHV-1 and FCV are common and result in more severe and prolonged clinical signs [1, 2].

Lower Respiratory Tract Infection

Lower respiratory tract involvement manifests as bronchopneumonia, lung abscessation, or pyothorax [3, 10, 4, 5]. Affected cats present with tachypnea, dyspnea, productive cough, fever, lethargy, and anorexia [10, 23]. Auscultation may reveal crackles, wheezes, and dull lung sounds over consolidated lung lobes [23]. M. felis has been associated with chronic bronchitis and bronchiectasis, with PCR detection correlating with disease severity [10, 28]. Embolic necrosuppurative pneumonia caused by Neisseria species results in multifocal pulmonary abscessation and systemic illness [4]. Pyothorax, characterized by purulent pleural effusion, is often associated with Pasteurella spp., Bacteroides spp., and other anaerobic bacteria [3, 5].

Pathological Findings

Gross pathological findings in bacterial pneumonia include cranioventral consolidation, multifocal to coalescing abscesses, and fibrinous pleuritis [4, 28]. Histologically, suppurative bronchopneumonia with neutrophilic infiltration, necrosis, and fibrin exudation is observed [4]. Granulomatous inflammation with caseous necrosis and acid-fast bacilli is characteristic of mycobacterial infections [18, 19, 20]. C. felis infection induces conjunctival epithelial hyperplasia, goblet cell loss, and lymphoplasmacytic infiltration with intracytoplasmic inclusions [13].

Diagnosis

Sample Collection

Appropriate sample collection is critical for accurate diagnosis. For upper respiratory tract disease, conjunctival swabs, nasal swabs, and oropharyngeal swabs are commonly collected [14, 29]. For lower respiratory tract disease, bronchoalveolar lavage (BAL) fluid, transtracheal wash, or fine-needle aspiration of consolidated lung tissue is preferred [26, 10, 29]. Pleural fluid should be collected in cases of pyothorax [3, 5]. Samples should be placed in appropriate transport media for bacterial culture and molecular testing [29].

Cytology

Cytological examination of respiratory specimens provides rapid preliminary information. Neutrophilic inflammation with intracellular and extracellular bacteria supports a diagnosis of bacterial infection [4]. Gram staining can differentiate Gram-positive and Gram-negative organisms. Mycoplasmas appear as small, pleomorphic, basophilic structures on Diff-Quik or Giemsa-stained preparations [11]. C. felis inclusions may be visible as basophilic intracytoplasmic inclusions in epithelial cells [13].

Bacterial Culture and Antimicrobial Susceptibility Testing

Aerobic and anaerobic bacterial culture should be performed on respiratory specimens [29, 5]. B. bronchiseptica grows on MacConkey agar and Bordet-Gengou agar, producing characteristic colonies [7]. Pasteurella multocida is readily cultured on blood agar [17]. Mycoplasmas require specialized media such as Hayflick's or Friis medium and incubation in a 5% CO2 atmosphere for 3-10 days [9, 11]. C. felis cannot be cultured on routine media and requires cell culture techniques [13]. Antimicrobial susceptibility testing should be performed on bacterial isolates to guide therapy, particularly given the emergence of multidrug-resistant strains [30, 9, 7].

Molecular Diagnostics

Polymerase chain reaction (PCR) assays are the gold standard for detection of fastidious organisms such as Mycoplasma spp. and C. felis [14, 10, 31]. Multiplex real-time PCR panels that simultaneously detect viral and bacterial pathogens associated with FRDC have been developed and validated [31]. These assays offer high sensitivity and specificity and can detect co-infections [31]. Quantitative PCR (qPCR) can provide information on bacterial load, which may correlate with disease severity [10]. Targeted next-generation sequencing (tNGS) of BAL fluid has been used to identify unusual pathogens such as Rickettsia felis [26]. Long-read sequencing workflows have been developed for improved genomic characterization of fastidious mycoplasmas [11].

Serology and Biomarkers

Serological assays for C. felis and B. bronchiseptica are available but have limited diagnostic utility in individual cases due to background seroprevalence [12]. Measurement of serum procalcitonin (PCT) levels and PCT mRNA expression has been investigated as a biomarker to differentiate bacterial from viral infections in cats, with elevated levels observed in bacterial infections [32].

Diagnostic Imaging

Thoracic radiography is essential for evaluating lower respiratory tract disease. Findings include alveolar, interstitial, or bronchial patterns, lung consolidation, abscessation, and pleural effusion [10, 4, 23]. Computed tomography (CT) provides superior detail for characterizing pulmonary lesions and guiding sample collection [20, 28].

Diagnostic Algorithm

flowchart TD
    A[Cat with respiratory signs], > B{Upper or lower respiratory tract?}
    B, >|Upper| C[Collect conjunctival/nasal/oropharyngeal swabs]
    B, >|Lower| D[Collect BAL fluid/transtracheal wash/pleural fluid]
    C, > E[Cytology + Gram stain]
    D, > E
    E, > F{Intracellular bacteria seen?}
    F, >|Yes| G[Aerobic + anaerobic culture + AST]
    F, >|No| H[PCR for Mycoplasma, Chlamydia, Bordetella, viruses]
    G, > I[Identify pathogen + susceptibility profile]
    H, > J[Multiplex PCR panel]
    I, > K[Targeted antimicrobial therapy]
    J, > K
    K, > L[Re-evaluate at 48-72 hours]
    L, > M{Clinical improvement?}
    M, >|Yes| N[Complete course]
    M, >|No| O[Repeat diagnostics + consider tNGS]
    O, > P[Adjust therapy based on results]

Treatment

Antimicrobial Therapy

Selection of antimicrobial therapy should be guided by culture and susceptibility testing whenever possible [30]. Empirical therapy is often initiated pending results, particularly in severe cases [30]. Doxycycline is a first-line agent for suspected Mycoplasma spp., C. felis, and B. bronchiseptica infections due to its efficacy and tissue penetration [27, 7]. A prospective, randomized, masked, placebo-controlled trial demonstrated the efficacy of doxycycline in treating infectious ophthalmic and respiratory disease in kittens [27]. Fluoroquinolones such as pradofloxacin or marbofloxacin are effective against many Gram-negative pathogens and mycoplasmas but should be reserved for cases with confirmed susceptibility due to resistance concerns [9, 7]. Amoxicillin-clavulanate is useful for Pasteurella multocida and other Gram-positive and anaerobic organisms [17]. Macrolides such as azithromycin have activity against B. bronchiseptica and mycoplasmas but resistance has been reported [9]. A systematic review and meta-analysis comparing shorter versus longer durations of antibiotic treatment for pneumonia in dogs and cats found no significant difference in outcomes, supporting the use of shorter courses to reduce antimicrobial selection pressure [30].

Supportive Care

Supportive care is critical in managing bacterial respiratory infections. This includes fluid therapy for dehydrated patients, nutritional support, and oxygen therapy for hypoxemic animals [3]. Nebulization with saline or bronchodilators may aid in loosening respiratory secretions [28]. In cases of pyothorax, thoracostomy tube placement for drainage and lavage is essential [3, 5].

Surgical Intervention

Surgical intervention may be necessary for cases of pyothorax that do not respond to medical management, for lung abscessation, or for removal of foreign bodies [3, 5]. Thoracotomy with debridement and drainage may be required in severe cases [5].

Control and Prevention

Vaccination

Vaccines are available for B. bronchiseptica in cats. An inactivated B. bronchiseptica vaccine has been shown to induce protective immune responses and reduce clinical signs following challenge [6]. Bacterium-like particle vaccines displaying protective FHV-1 antigens have been developed and can induce immune responses in mice and cats, representing a novel approach to controlling FRDC [33]. No commercial vaccines are currently available for M. felis or C. felis in cats.

Biosecurity

In multi-cat environments such as shelters and catteries, strict biosecurity protocols are essential to prevent the spread of bacterial respiratory pathogens [7]. This includes isolation of affected cats, disinfection of contaminated surfaces, and use of personal protective equipment by handlers [7]. B. bronchiseptica can survive in the environment for up to two weeks, necessitating thorough cleaning with disinfectants effective against Gram-negative bacteria [7].

Antimicrobial Stewardship

Judicious use of antimicrobials is critical to mitigate the emergence of multidrug-resistant bacteria [30, 7]. Culture and susceptibility testing should be performed whenever possible, and narrow-spectrum agents should be selected based on results [30]. Shorter treatment durations should be considered when clinically appropriate [30].

Conclusion

Bacterial respiratory infections in cats are caused by a diverse array of pathogens, including Bordetella bronchiseptica, Mycoplasma felis, Chlamydia felis, Pasteurella multocida, and emerging organisms such as Neisseria spp. and Acinetobacter baumannii. Accurate diagnosis requires a combination of cytology, culture, molecular methods, and imaging. Treatment should be guided by antimicrobial susceptibility testing, with doxycycline serving as a common empirical choice. Control strategies include vaccination for B. bronchiseptica, biosecurity measures, and antimicrobial stewardship. Continued research into novel diagnostics, therapeutics, and vaccines is needed to improve outcomes and combat antimicrobial resistance.

References

[1] Maboni G, Che S, Tallmadge R, et al. Feline respiratory disease complex: insights into the role of viral and bacterial co-infections. Front Microbiol. 2024. https://pubmed.ncbi.nlm.nih.gov/39290513/

[2] Slaviero M, Ehlers LP, de Almeida BA, et al. Generalized and fatal felid alphaherpesvirus-1 natural infection with liver involvement in a feline leukaemia virus-positive adult cat: a case report. Vet Res Commun. 2022. https://pubmed.ncbi.nlm.nih.gov/35854050/ *** 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.

[3] Marolf V, Fillonneau C, Diserens KA, et al. Pyothorax was rare and had favorable outcomes in cats: clinical findings from 31 cases in a Swiss private practice (2018-2021). J Am Vet Med Assoc. 2025. https://pubmed.ncbi.nlm.nih.gov/40882678/

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

[5] 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. https://pubmed.ncbi.nlm.nih.gov/37098692/

[6] Li Y, Dong X, Yang A, et al. Development and efficacy evaluation of an inactivated Bordetella bronchiseptica vaccine in cats. Vaccine. 2026. https://pubmed.ncbi.nlm.nih.gov/42217448/

[7] Rodriguez L, Berliner EA. Outbreak management of multidrug-resistant Bordetella bronchiseptica in 16 shelter-housed cats. J Feline Med Surg. 2023. https://pubmed.ncbi.nlm.nih.gov/36763462/

[8] Huang X, Hou Y, Zhao M, et al. Identification of a broad-spectrum lytic Bordetella phage and assessments of its potential for combating Bordetella infections. Virology. 2025. https://pubmed.ncbi.nlm.nih.gov/40306109/

[9] Framst I, Beeton ML, Peterson SW, et al. Antimicrobial susceptibility and genomic determinants of resistance and virulence in Mycoplasma cynos and Mycoplasma felis. Vet Microbiol. 2026. https://pubmed.ncbi.nlm.nih.gov/41274178/

[10] 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. https://pubmed.ncbi.nlm.nih.gov/39718117/

[11] Framst I, D Andrea C, Baquero M, et al. Development of a long-read next generation sequencing workflow for improved characterization of fastidious respiratory mycoplasmas. Microbiology (Reading). 2022. https://pubmed.ncbi.nlm.nih.gov/36374163/

[12] Szymańska-Czerwińska M, Zaręba-Marchewka K, Woś M, et al. Prevalence, risk factors and genetic diversity of Chlamydia felis in cats. J Vet Res. 2025. https://pubmed.ncbi.nlm.nih.gov/41497455/

[13] 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. https://pubmed.ncbi.nlm.nih.gov/40256594/

[14] 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. https://pubmed.ncbi.nlm.nih.gov/39927366/

[15] 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. https://pubmed.ncbi.nlm.nih.gov/39315777/

[16] Zhao M, Wu W, Song W, et al. A quadruplex real-time fluorescent quantitative PCR detection method for identification and serotyping of Pasteurella multocida. Microb Pathog. 2025. https://pubmed.ncbi.nlm.nih.gov/40639696/

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

[18] Peters H, Kaspers L, Brangsch H, et al. Outbreak of feline tuberculosis caused by Mycobacterium bovis in a German household, a possible domestic zoonosis. J Vet Diagn Invest. 2026. https://pubmed.ncbi.nlm.nih.gov/42136114/

[19] Borrás P, Piras I, Dominguez L, et al. Feline tuberculosis due to Mycobacterium bovis: case series with clinical course, genotyping and therapeutic outcomes from Buenos Aires, Argentina. JFMS Open Rep. 2026. https://pubmed.ncbi.nlm.nih.gov/41884498/

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

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

[22] 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. https://pubmed.ncbi.nlm.nih.gov/37079272/

[23] Aslam MW, Lau SF, Radzi R, et al. Clinicopathological and Radiological Features of Cats Presented with Infectious Respiratory Disease Signs: A Focus on Rhodococcus equi and Klebsiella pneumoniae. Microorganisms. 2023. https://pubmed.ncbi.nlm.nih.gov/36985312/

[24] 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. https://pubmed.ncbi.nlm.nih.gov/39405054/

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

[26] 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. https://pubmed.ncbi.nlm.nih.gov/41935438/

[27] Vernau KM, Kim S, Thomasy SM, et al. Doxycycline with or without famciclovir for infectious ophthalmic and respiratory disease: a prospective, randomized, masked, placebo-controlled trial in 373 kittens. J Feline Med Surg. 2024. https://pubmed.ncbi.nlm.nih.gov/39485362/

[28] Kang D, Womble M, Cullen JM, et al. Severe bronchiectasis resulting from chronic bacterial bronchitis and bronchopneumonia in a jungle cat. J Vet Diagn Invest. 2024. https://pubmed.ncbi.nlm.nih.gov/38014741/

[29] 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. https://pubmed.ncbi.nlm.nih.gov/39079674/

[30] Emdin F, Emdin A, Ong SWX, et al. Shorter versus longer durations of antibiotic treatment for pneumonia in dogs and cats: a systematic review and meta-analysis. J Am Vet Med Assoc. 2026. https://pubmed.ncbi.nlm.nih.gov/41547037/

[31] 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. https://pubmed.ncbi.nlm.nih.gov/38517847/

[32] Matur E, Özcan M, Ergül Ekiz E, et al. Use of serum procalcitonin (PCT) level and PCT mRNA expression as a potential clinical biomarker in cats with bacterial and viral infections. J Feline Med Surg. 2022. https://pubmed.ncbi.nlm.nih.gov/36350675/

[33] Jiao C, Jin H, Zhang M, et al. A bacterium-like particle vaccine displaying protective feline herpesvirus 1 antigens can induce an immune response in mice and cats. Vet Microbiol. 2023. https://pubmed.ncbi.nlm.nih.gov/37931577/

[34] 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. https://pubmed.ncbi.nlm.nih.gov/41305419/

[35] Martinavičienė G, Paulauskas L, Schnyder M, et al. Severe respiratory distress in a stray cat caused by the lungworm Aelurostrongylus abstrusus: A first case report in Lithuania. Vet Parasitol Reg Stud Reports. 2025. https://pubmed.ncbi.nlm.nih.gov/40121036/