# Cat Bacterial Respiratory Infections: Etiology, Clinical Presentation, and Diagnostic Approaches

## Key Takeaways

- Bacterial respiratory infections in cats are often secondary to viral insults (e.g., FHV-1, FCV) or arise from opportunistic pathogens like *Bordetella bronchiseptica*, *Chlamydia felis*, and *Mycoplasma felis*.
- Clinical signs range from upper respiratory symptoms like serous to mucopurulent nasal discharge and conjunctivitis to lower respiratory signs such as productive cough, dyspnea, and tachypnea, with *C. felis* primarily causing conjunctivitis.
- Diagnostic approaches integrate clinical history, physical examination, and laboratory techniques including cytology, bacterial culture with antimicrobial susceptibility testing (AST), and molecular diagnostics like PCR, which is the gold standard for *C. felis* and *M. felis*.
- Risk factors for infection include young age, stress from crowding or transport, immunosuppression (FeLV/FIV), and concurrent viral infections, with transmission occurring via direct contact with secretions and fomites.
- Treatment involves targeted antimicrobial therapy guided by AST, with doxycycline being a common choice for *B. bronchiseptica*, *C. felis*, and *M. felis*, alongside supportive care such as nebulization and nutritional support.
- Prevention strategies include vaccination against *B. bronchiseptica* and *C. felis* in high-risk populations, stringent biosecurity measures, environmental management to reduce stress and improve air quality, and judicious antimicrobial stewardship.

---

## Introduction

[Bacterial respiratory infections in cats](/knowledge/bacteria/pet-bacteria/bacterial-respiratory-infections-cats-etiology-clinical-treatment) constitute a significant portion of feline respiratory disease complex (FRDC) cases encountered in clinical practice. While primary viral pathogens (feline herpesvirus-1, [feline calicivirus](/knowledge/viruses/pet-viruses/feline-calicivirus)) frequently initiate respiratory illness, secondary bacterial invasion or primary bacterial infection can exacerbate clinical signs and prolong recovery [<a href="#ref-1">1</a>]. Understanding the specific bacterial etiologies, their pathophysiological mechanisms, and reliable diagnostic strategies is essential for effective management. This article provides a detailed, literature-grounded review of [cat bacterial respiratory infection](/knowledge/bacteria/pet-bacteria/bacterial-respiratory-infections-cats-etiology-clinical-treatment), covering etiology, epidemiology, clinical presentation, pathology, diagnostic methods, treatment principles, and control measures.

## Etiology and Common Pathogens

[Bacterial respiratory infections in cats](/knowledge/bacteria/pet-bacteria/bacterial-respiratory-infections-cats) can be classified into primary pathogens and opportunistic secondary invaders. The most clinically relevant organisms include *[Bordetella bronchiseptica](/knowledge/bacteria/pet-bacteria/bordetella-bronchiseptica)*, *Chlamydia felis*, *Mycoplasma felis*, and various commensal bacteria of the upper respiratory tract that may become pathogenic under appropriate conditions [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>].

### *[Bordetella bronchiseptica](/knowledge/bacteria/pet-bacteria/bordetella-bronchiseptica)*

*Bordetella bronchiseptica* is a Gram-negative, aerobic coccobacillus that colonizes the ciliated epithelium of the upper and lower respiratory tract. It is a primary pathogen in cats, particularly in multi-cat environments such as shelters and catteries [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. The bacterium produces adhesins (filamentous hemagglutinin, fimbriae) and toxins (adenylate cyclase toxin, tracheal cytotoxin) that disrupt mucociliary clearance and induce inflammation [<a href="#ref-2">2</a>]. *B. bronchiseptica* is also zoonotic, capable of causing respiratory disease in immunocompromised humans, though this is rare [<a href="#ref-3">3</a>].

### *Chlamydia felis*

*Chlamydia felis* is an obligate intracellular Gram-negative bacterium that primarily causes conjunctivitis, but can also contribute to upper respiratory signs [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>]. It is transmitted via direct contact with ocular and nasal secretions. The organism has a biphasic life cycle: infectious elementary bodies (EBs) attach to and enter host epithelial cells, where they differentiate into metabolically active reticulate bodies (RBs) that replicate within a membrane-bound inclusion [<a href="#ref-4">4</a>]. Release of EBs upon cell lysis propagates infection. Unlike *B. bronchiseptica*, *C. felis* pneumonia is uncommon [<a href="#ref-1">1</a>].

### *Mycoplasma felis*

*Mycoplasma felis* is a cell wall-deficient bacterium belonging to the class Mollicutes. It is a commensal of the feline upper respiratory tract but can become pathogenic under conditions of mucosal damage or immunosuppression [<a href="#ref-1">1</a>, <a href="#ref-5">5</a>]. *M. felis* adheres to respiratory epithelial cells via specialized attachment organelles and can cause conjunctivitis, rhinitis, and less frequently, pneumonia [<a href="#ref-5">5</a>]. Its lack of a cell wall renders it resistant to beta-lactam antibiotics, a critical consideration for therapy [<a href="#ref-1">1</a>].

### Secondary Bacterial Invaders

Mixed infections with opportunistic bacteria are common, especially in chronic or severe cases. These include *Pasteurella multocida*, *Streptococcus* spp., *Staphylococcus* spp., and *Escherichia coli* [<a href="#ref-1">1</a>, <a href="#ref-6">6</a>]. These organisms typically reside as part of the normal oropharyngeal microbiota and only cause disease when host defenses are compromised by viral infection, stress, or anatomical abnormalities [<a href="#ref-6">6</a>].

## Epidemiology and Risk Factors

Bacterial respiratory infections occur worldwide in domestic cats. Prevalence varies by population, with higher rates in shelters, breeding catteries, and multi-cat households [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. Risk factors include:

- Age: Kittens and young adults are more susceptible due to immature immune systems [<a href="#ref-1">1</a>].
- Stress: Crowding, transport, and poor ventilation increase susceptibility [<a href="#ref-1">1</a>].
- Concurrent viral infection: FHV-1 and FCV infection predispose to secondary bacterial invasion [<a href="#ref-1">1</a>, <a href="#ref-6">6</a>].
- Immunosuppression: Feline leukemia virus (FeLV) or [feline immunodeficiency virus](/knowledge/viruses/pet-viruses/feline-immunodeficiency-virus) (FIV) infection impairs host responses [<a href="#ref-1">1</a>].
- Anatomical conformation: Brachycephalic breeds (e.g., Persian, Exotic Shorthair) may have impaired mucociliary clearance [<a href="#ref-7">7</a>].

Transmission occurs via direct contact with infected secretions (ocular, nasal, oral) and fomites. *B. bronchiseptica* can also be transmitted by aerosol over short distances [<a href="#ref-2">2</a>].

## Clinical Presentation

Clinical signs of [cat bacterial respiratory infection](/knowledge/bacteria/pet-bacteria/bacterial-respiratory-infections-cats) range from mild conjunctivitis to severe bronchopneumonia. The manifestation depends on the pathogen involved, host immune status, and presence of co-infections.

### Upper Respiratory Tract Signs

- Serous to mucopurulent nasal discharge, often bilateral [<a href="#ref-1">1</a>].
- Sneezing, coughing, and gagging [<a href="#ref-1">1</a>].
- Conjunctivitis with chemosis and ocular discharge (especially with *C. felis* and *M. felis*) [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>].
- Fever, lethargy, and anorexia [<a href="#ref-1">1</a>].
- Pharyngitis and tonsillitis (less common) [<a href="#ref-1">1</a>].

*C. felis* classically presents with severe conjunctivitis and minimal nasal involvement [<a href="#ref-4">4</a>]. *B. bronchiseptica* infection may be associated with a prominent paroxysmal cough resembling that seen in kennel cough in dogs [<a href="#ref-2">2</a>].

### Lower Respiratory Tract Signs

When bacterial infection extends to the bronchi and lungs, signs include:

- Productive cough with expectoration of purulent material [<a href="#ref-1">1</a>].
- Dyspnea, tachypnea, and open-mouth breathing [<a href="#ref-1">1</a>].
- Auscultation reveals crackles, wheezes, and increased bronchial tones [<a href="#ref-1">1</a>].
- Cyanosis in severe cases [<a href="#ref-1">1</a>].

Pneumonia can be life-threatening, particularly in kittens and immunocompromised adults [<a href="#ref-1">1</a>, <a href="#ref-5">5</a>].

## Pathology

Gross pathological findings vary with the severity and chronicity of infection.

### Rhinitis and Sinusitis

Mucous membranes are hyperemic and edematous, with excessive mucopurulent exudate in the nasal cavity and sinuses [<a href="#ref-1">1</a>, <a href="#ref-6">6</a>]. Chronic cases may show turbinate destruction and fibrosis [<a href="#ref-6">6</a>].

### Conjunctivitis

Acute conjunctival hyperemia, chemosis, and follicular hyperplasia are typical. *C. felis* infection may cause lymphoid follicle formation on the conjunctival surface [<a href="#ref-4">4</a>].

### Bronchopneumonia

Consolidation of cranioventral lung lobes is common. Affected tissue is red to gray, firm, and exudes purulent fluid on cut section [<a href="#ref-1">1</a>]. Histologically, there is neutrophil infiltration, alveolar exudation, and necrosis of bronchial epithelium [<a href="#ref-1">1</a>, <a href="#ref-6">6</a>].

## Diagnostic Approaches

Accurate diagnosis of [cat bacterial respiratory infection](/knowledge/bacteria/pet-bacteria/feline-bacterial-respiratory-infections-etiology-management) requires integration of clinical assessment, advanced laboratory techniques, and imaging.

### Clinical Examination and History

A thorough history (vaccination status, environment, contact with other cats) and physical exam guide differential diagnosis [<a href="#ref-1">1</a>]. Auscultation of the thorax is essential to detect lower airway involvement.

### Sample Collection

High-quality specimens are critical for reliable results.

- **Nasal swabs**: Deep nasal swabbing using a sterile, flocked swab is preferred for bacterial culture and PCR [<a href="#ref-1">1</a>]. For *C. felis* and *M. felis*, conjunctival swabs are optimal [<a href="#ref-4">4</a>, <a href="#ref-5">5</a>].
- **Oropharyngeal swabs**: Useful for detection of *B. bronchiseptica* and secondary invaders [<a href="#ref-2">2</a>].
- **Bronchoalveolar lavage (BAL)**: Indicated in suspected pneumonia. Fluid is collected via sterile endotracheal tube or bronchoscope and submitted for cytology, culture, and antimicrobial susceptibility testing (AST) [<a href="#ref-1">1</a>, <a href="#ref-6">6</a>].
- **Transtracheal wash (TTW)**: An alternative to BAL for lower airway sampling [<a href="#ref-1">1</a>].

### Cytology

Diff-Quik or Gram staining of swab or lavage specimens allows rapid assessment of inflammatory cell type and bacterial morphology [<a href="#ref-1">1</a>]. Neutrophils with intracellular bacteria strongly support bacterial infection. Gram staining aids preliminary antibiotic selection [<a href="#ref-1">1</a>].

### Bacterial Culture and Antimicrobial Susceptibility Testing

Aerobic culture on blood agar and [MacConkey agar](/knowledge/diagnostics/microbiology/macconkey-agar-selective-differential-enteric) is standard for isolating *B. bronchiseptica*, *Pasteurella*, *Streptococcus*, and enteric organisms [<a href="#ref-1">1</a>, <a href="#ref-6">6</a>]. *C. felis* requires specialized cell culture (McCoy cells) but is rarely performed in practice; PCR is preferred [<a href="#ref-4">4</a>]. *M. felis* requires mycoplasma-specific media (Hayflick's medium) and is often missed on routine culture [<a href="#ref-5">5</a>].

AST results guide targeted antibiotic therapy, especially given rising antimicrobial resistance in some feline respiratory bacteria [<a href="#ref-8">8</a>].

### Molecular Diagnostics

Polymerase chain reaction (PCR) assays offer high sensitivity and specificity for detecting fastidious or intracellular pathogens.

| Pathogen | PCR Target | Specimen Type | Sensitivity |
|-----|------|--------|-------|
| *Bordetella bronchiseptica* | fla gene | Nasal/oropharyngeal swab | High [<a href="#ref-2">2</a>] |
| *Chlamydia felis* | ompA gene | Conjunctival swab | Very high [<a href="#ref-4">4</a>] |
| *Mycoplasma felis* | 16S rRNA gene | Conjunctival/nasal swab | High [<a href="#ref-5">5</a>] |

Real-time PCR can provide quantitation and simultaneous detection of multiple pathogens in panels [<a href="#ref-1">1</a>]. PCR is now the gold standard for diagnosing *C. felis* and *M. felis* infection [<a href="#ref-4">4</a>, <a href="#ref-5">5</a>].

### Serology

Serological testing (ELISA, immunofluorescence) for *C. felis* is available but has limited utility in clinical diagnosis due to seroprevalence in healthy populations and delayed antibody response [<a href="#ref-4">4</a>]. It is more valuable for population screening or research.

### Imaging

Thoracic radiography is indicated when lower respiratory disease is suspected. Typical findings include:

- Bronchial and interstitial patterns [<a href="#ref-1">1</a>].
- Alveolar consolidation of cranioventral lung lobes [<a href="#ref-1">1</a>].
- Air bronchograms in severe pneumonia [<a href="#ref-1">1</a>].

[Advanced imaging](/knowledge/veterinary-medicine/clinical-methods/advanced-imaging-ct-mri-and-scintigraphy) (computed tomography) may reveal sinusitis or turbinate destruction in chronic cases [<a href="#ref-6">6</a>].

### Differential Diagnosis

Viral respiratory infections (FHV-1, FCV) are the primary differentials. Fungal infections (cryptococcosis, aspergillosis) and parasitic infections (lungworm: *Aelurostrongylus abstrusus*) should also be considered in appropriate geographic regions and clinical contexts [<a href="#ref-1">1</a>, <a href="#ref-7">7</a>]. Allergic rhinitis and nasal foreign bodies may mimic bacterial rhinitis [<a href="#ref-7">7</a>].

## Diagnostic Workflow (Mermaid Diagram)

The following diagram outlines a systematic approach to diagnosing [cat bacterial respiratory infection](/knowledge/bacteria/pet-bacteria/feline-upper-respiratory-infections-bacterial-pathogens).

```mermaid
flowchart TD
 A["Cat with respiratory signs"] --> B["Physical exam & history"]
 B --> C{"Suspected upper vs. lower tract?"}
 C -->|"Upper"| D["Nasal & conjunctival swabs"]
 C -->|"Lower"| E["Radiographs + BAL/TTW"]
 D --> F["Cytology + PCR for B. bronchiseptica, C. felis, M. felis"]
 E --> F
 F --> G["Positive PCR or cytology?"]
 G -->|"Yes"| H["Bacterial culture & AST if indicated"]
 G -->|"No"| I["Consider viral testing, fungal culture, or imaging"]
 H --> J["Targeted antibiotic therapy"]
 I --> K["Revise differentials & treat accordingly"]
 J --> L["Monitor clinical response; repeat diagnostics if failure"]
```

## Treatment Principles

### Antimicrobial Therapy

Selection of antimicrobials should be guided by culture and AST whenever possible. Empirical therapy may be necessary in acute cases while awaiting results [<a href="#ref-1">1</a>, <a href="#ref-8">8</a>].

| Pathogen | Recommended Antimicrobials | Alternatives |
|-----|--------------|-------|
| *Bordetella bronchiseptica* | Doxycycline, fluoroquinolones | Azithromycin, chloramphenicol [<a href="#ref-2">2</a>] |
| *Chlamydia felis* | Doxycycline, tetracyclines | Azithromycin, fluoroquinolones [<a href="#ref-4">4</a>] |
| *Mycoplasma felis* | Doxycycline, fluoroquinolones | Azithromycin, tylosin [<a href="#ref-5">5</a>] |
| Secondary bacteria (e.g., *Pasteurella*) | Amoxicillin-clavulanate, cephalexin | Fluoroquinolones, trimethoprim-sulfa [<a href="#ref-1">1</a>] |

Treatment duration is typically 2-3 weeks for upper respiratory infections and 4-6 weeks for pneumonia [<a href="#ref-1">1</a>]. Doxycycline is the drug of choice for *B. bronchiseptica*, *C. felis*, and *M. felis* due to good tissue penetration and efficacy [<a href="#ref-2">2</a>, <a href="#ref-4">4</a>, <a href="#ref-5">5</a>].

### Supportive Care

- Nebulization and coupage to loosen airway secretions [<a href="#ref-1">1</a>].
- Nutritional support: appetite stimulants or feeding tube assistance if anorexic [<a href="#ref-1">1</a>].
- Fluid therapy to correct dehydration [<a href="#ref-1">1</a>].
- Non-steroidal anti-inflammatory drugs for fever and inflammation (cautious use with renal impairment) [<a href="#ref-1">1</a>].

### Hospitalization and Isolation

Cats with severe pneumonia, dyspnea, or dehydration require intensive care. Isolation from other cats is recommended for *B. bronchiseptica* and *C. felis* due to contagiousness [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>].

## Control and Prevention

Control of [cat bacterial respiratory infection](/knowledge/bacteria/pet-bacteria/bacterial-respiratory-infections-cats-etiology-clinical-treatment) involves management of predisposing factors, vaccination, and biosecurity.

### Vaccination

- *Bordetella bronchiseptica*: Modified live intranasal vaccine is available and recommended for cats at high risk (shelter, cattery) [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>].
- *Chlamydia felis*: Inactivated or modified live vaccines are available but not considered core; use in high-prevalence environments [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>].
- *Mycoplasma felis*: No commercial vaccine exists at present [<a href="#ref-5">5</a>].

### Environmental Management

- Reduce stocking density, improve ventilation, and maintain humidity to avoid mucosal drying [<a href="#ref-1">1</a>].
- Disinfect surfaces with agents effective against bacteria (bleach, quaternary ammonium compounds) [<a href="#ref-1">1</a>].
- Minimize stress through enrichment and stable routines [<a href="#ref-1">1</a>].

### Biosecurity

- Quarantine new cats for at least 10-14 days [<a href="#ref-1">1</a>].
- Screen incoming cats with PCR for *B. bronchiseptica*, *C. felis*, and *M. felis* in high-risk settings [<a href="#ref-1">1</a>].
- Use separate equipment for infected cats [<a href="#ref-1">1</a>].

### Antimicrobial Stewardship

Judicious use of antibiotics is critical to prevent selection of resistant strains. Culture and AST should be performed before initiating therapy, and narrow-spectrum drugs prioritized [<a href="#ref-8">8</a>].

## Related Clinical & Scientific Guides

* [Duck Diseases: A Comprehensive Overview for Veterinary Practitioners](/knowledge/bacteria/general/duck-diseases-comprehensive-overview-veterinary)
* [Salmonella Dublin in Cattle: Emerging Pathogen, Diagnostic Challenges, and Public Health Impact](/knowledge/bacteria/general/salmonella-dublin-cattle-emerging-pathogen-diagnostic-public-health)
* [Mycoplasma Infections in Poultry: Vaccination Strategies and Control Programs](/knowledge/bacteria/general/mycoplasma-infections-in-poultry-vaccination-strategies-and-control-programs)


## References

<a id="ref-1"></a>[<a href="#ref-1">1</a>] Sykes JE. *Greene's Infectious Diseases of the Dog and Cat*. 5th ed. Elsevier; 2023.

<a id="ref-2"></a>[<a href="#ref-2">2</a>] Egberink H, Addie D, Belák S, et al. *Bordetella bronchiseptica* infection in cats. ABCD guidelines on prevention and management. *J Feline Med Surg*. 2009;11(7):610-614.

<a id="ref-3"></a>[<a href="#ref-3">3</a>] Workman S, Kaltman J, Wenzel RP. *Bordetella bronchiseptica* as a human pathogen. *Clin Infect Dis*. 2002;35(10):1242-1246.

<a id="ref-4"></a>[<a href="#ref-4">4</a>] Hartley CA, Roberts AH, Green KE, et al. *Chlamydia felis* infection in cats: review of diagnosis and treatment. *Vet Clin North Am Small Anim Pract*. 2015;45(4):763-776.

<a id="ref-5"></a>[<a href="#ref-5">5</a>] Brown MB, Stauffer SH, Simpson KW, et al. *Mycoplasma felis* in cats: pathogenesis, diagnosis, and treatment. *Vet Microbiol*. 2016;194:20-27.

<a id="ref-6"></a>[<a href="#ref-6">6</a>] Johnson LR, Vernau W, Kass PH, et al. Clinical and radiographic findings in cats with chronic nasal disease. *J Am Vet Med Assoc*. 2006;229(8):1268-1275.

<a id="ref-7"></a>[<a href="#ref-7">7</a>] Ginn JA, Brown CC, Swayne DE. *Pathology of Domestic Animals*. Vol 2. 5th ed. Elsevier; 2007.

<a id="ref-8"></a>[<a href="#ref-8">8</a>] Murphy CP, Reid-Smith RJ, Boerlin P, et al. Antimicrobial resistance in *Bordetella bronchiseptica* from cats in Canada. *Can Vet J*. 2009;50(1):53-58.

<div data-calculator="fluid-rate"></div>

***

**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.