What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Feline Upper Respiratory Infections (Feline Herpesvirus, Calicivirus, and Bordetella): Etiology, Clinical Signs, Zoonotic Potential, and Therapeutics

Introduction

Feline upper respiratory infections (URIs) represent a complex of viral and bacterial diseases that are among the most common infectious conditions encountered in domestic cats worldwide [1, 2]. The primary etiological agents include felid alphaherpesvirus 1 (FeAHV-1, commonly termed feline herpesvirus type 1 or FHV-1), feline calicivirus (FCV), and the bacterium Bordetella bronchiseptica [3, 4]. These pathogens frequently co-infect feline patients, complicating clinical diagnosis and therapeutic management [3, 5]. Understanding the biophysical mechanisms of host cell interaction, the molecular epidemiology of circulating strains, and the zoonotic implications of these agents is essential for veterinary practitioners and diagnostic specialists.

Etiology and Biophysical Mechanisms

Feline Herpesvirus Type 1 (FHV-1)

FHV-1 is an enveloped, double-stranded DNA virus belonging to the genus Varicellovirus within the family Herpesviridae [6]. The viral particle is approximately 150-200 nm in diameter and possesses a lipid envelope derived from the host cell nuclear membrane [6]. FHV-1 exhibits a predilection for epithelial cells of the upper respiratory tract, conjunctiva, and cornea [7, 1]. Following primary infection, the virus establishes lifelong latency in sensory neurons of the trigeminal ganglia, with periodic reactivation triggered by stress, immunosuppression, or corticosteroid administration [6]. Transcriptomic analyses of Crandell-Rees feline kidney (CRFK) cells infected with FHV-1 field and vaccine strains have revealed differential expression of genes involved in innate immune signaling, apoptosis, and cellular stress responses [6].

Feline Calicivirus (FCV)

FCV is a non-enveloped, single-stranded positive-sense RNA virus belonging to the family Caliciviridae [8, 9]. The viral capsid is composed of the major structural protein VP1, which forms 90 dimers arranged in a T=3 icosahedral lattice [10, 9]. The leader of the capsid protein (LC) is post-translationally modified by palmitoylation and forms oligomers stabilized by disulfide bonds, a process essential for efficient viral replication [9]. FCV exhibits significant genetic diversity, with multiple genogroups and strains circulating globally [2, 11]. A novel genogroup has been identified through molecular evolution studies in group-housed cats in China [11]. The virus manipulates central carbon metabolism in infected cells, upregulating glycolysis and glutaminolysis to support viral replication [12]. Reverse genetics systems have been constructed for FCV strains such as FCV-BJ616, enabling proteomic analysis of host-virus interactions [8].

Bordetella bronchiseptica

Bordetella bronchiseptica is a Gram-negative, aerobic, motile coccobacillus that colonizes the ciliated respiratory epithelium of cats [13, 14]. The bacterium expresses multiple virulence factors, including filamentous hemagglutinin, pertactin, and a type III secretion system, which facilitate adherence to host cells and evasion of mucociliary clearance [13]. B. bronchiseptica is closely related to Bordetella pertussis and Bordetella parapertussis, the causative agents of whooping cough in humans, and shares several virulence determinants [13].

Epidemiology and Transmission

How Do Cats Get Respiratory Infections?

Transmission of FHV-1, FCV, and B. bronchiseptica occurs primarily through direct contact with infected cats via aerosolized respiratory secretions, fomites, and contaminated environments [3, 2]. FHV-1 is shed in ocular, nasal, and oral secretions during acute infection and during periods of reactivation [1]. FCV is shed in oral and nasal secretions and can persist in the environment for weeks due to its non-enveloped structure [15, 16]. B. bronchiseptica is transmitted through aerosol droplets and direct contact, with carrier cats serving as reservoirs [13].

Risk factors for infection include multi-cat environments such as shelters, catteries, and boarding facilities [3, 2]. Young kittens, geriatric cats, and immunocompromised individuals are at increased risk of severe disease [7, 17]. Co-infections with multiple respiratory pathogens are common, with studies reporting high rates of concurrent FHV-1 and FCV detection in clinical samples [3, 5]. The epidemiology of these pathogens was influenced by changes in cat population dynamics during the COVID-19 pandemic, with alterations in shelter intake and adoption rates affecting transmission patterns [3].

Are Cat Respiratory Infections Dangerous?

Feline URIs can range from mild, self-limiting disease to severe, life-threatening illness [17, 1]. Virulent systemic FCV (VS-FCV) infections are associated with high morbidity and mortality, characterized by severe pneumonia, footpad edema, ulcerative skin lesions, and multi-organ failure [17]. Chronic sequelae of FHV-1 infection include corneal sequestra, symblepharon, and chronic rhinosinusitis [7, 13]. Computed tomographic evidence has demonstrated concurrent middle ear, upper airway, and lower airway disease in cats with chronic respiratory signs, supporting the concept of united airway disease [18]. Secondary bacterial infections, including those caused by Mycoplasma species and B. bronchiseptica, can exacerbate clinical signs and complicate treatment [19, 14].

Clinical Signs and Pathology

FHV-1 Clinical Signs

Acute FHV-1 infection typically presents with serous to mucopurulent ocular discharge, conjunctivitis, chemosis, sneezing, and nasal discharge [7, 1]. Ophthalmic manifestations are particularly prominent in kittens and may include corneal ulceration, keratitis, and eosinophilic keratitis [7]. Chronic FHV-1 infection is associated with stromal keratitis, corneal sequestra, and nasolacrimal duct obstruction [7, 13]. Slaviero et al. documented the occurrence and pathology of FHV-1 and FCV in cats with respiratory disease, noting that FHV-1 was associated with necrotizing rhinitis and conjunctivitis [1].

FCV Clinical Signs

FCV infection typically causes acute oral ulceration, salivation, pyrexia, and mild upper respiratory signs [15, 16]. Oral ulcers are most commonly observed on the tongue, hard palate, and lips [20]. Some FCV strains cause a chronic lymphoplasmacytic stomatitis, which is difficult to manage therapeutically [20]. VS-FCV infection presents with severe systemic signs including pyrexia, facial and limb edema, ulcerative skin lesions, icterus, and fatal pneumonia [17]. Postoperative outbreaks of FCV infection have been reported following routine ovariohysterectomy, suggesting that surgical stress may trigger viral shedding and transmission in veterinary hospital settings [20].

Bordetella bronchiseptica Clinical Signs

B. bronchiseptica infection in cats typically causes a mild to moderate tracheobronchitis characterized by coughing, sneezing, and nasal discharge [13]. In kittens and immunocompromised cats, infection can progress to bronchopneumonia [13]. Chronic bronchitis and bronchiolitis with prominent globule leukocyte infiltration has been described in a cat co-infected with Filobacterium felis and B. bronchiseptica [14].

Pathology

Gross pathological findings in FHV-1 infection include conjunctival hyperemia, corneal edema, and turbinate necrosis [1]. Histologically, FHV-1 causes intranuclear eosinophilic inclusion bodies in epithelial cells, with associated necrosis and neutrophilic inflammation [1]. FCV infection is characterized by epithelial necrosis and vesicle formation in the oral mucosa, with lymphoplasmacytic infiltration [17, 1]. In VS-FCV cases, pulmonary lesions include severe interstitial pneumonia with alveolar edema and hyaline membrane formation [17]. B. bronchiseptica infection results in suppurative bronchopneumonia with peribronchial lymphoid hyperplasia [13, 14].

Zoonotic Potential

Is Cat Respiratory Infection Contagious to Humans?

The zoonotic potential of FHV-1 and FCV is considered negligible. FHV-1 is highly species-specific and does not replicate in human cells [1]. FCV is similarly restricted to felids, with no documented cases of human infection [10, 9]. However, B. bronchiseptica is a zoonotic pathogen capable of causing respiratory disease in immunocompromised humans, particularly those with underlying pulmonary conditions or immunosuppression [13]. Human infection with B. bronchiseptica typically presents as a pertussis-like illness with paroxysmal coughing [13]. Veterinary personnel and immunocompromised individuals should exercise appropriate infection control measures when handling cats with suspected B. bronchiseptica infection [13]. For a broader discussion of zoonotic considerations, see the article on Feline Upper Respiratory Infections: Zoonotic Potential and Public Health.

Diagnostics

Sample Collection and Laboratory Testing

Diagnostic confirmation of feline URI pathogens relies on molecular detection methods, virus isolation, and serological assays [21, 4]. Polymerase chain reaction (PCR) panels targeting FHV-1, FCV, and B. bronchiseptica are widely used in veterinary diagnostic laboratories [4, 5]. A retrospective analysis of respiratory PCR panels performed at a veterinary diagnostic center demonstrated that FHV-1 and FCV were the most frequently detected pathogens in feline samples [4].

Point-of-Care and Molecular Diagnostics

Rapid and sensitive detection of FHV-1 has been achieved using fluorescent microspheres as labels for immunochromatographic test strips, enabling point-of-care diagnosis [22]. An automated portable loop-mediated isothermal amplification (LAMP) based centrifugal microfluidic system has been developed for nucleic acid detection of multiple feline URI pathogens, offering rapid, multiplexed detection in field settings [21]. For a detailed review of point-of-care molecular diagnostics, see the article on Point-of-Care Molecular Diagnostics for Feline Upper Respiratory Pathogens: FHV-1, FCV, and Bordetella.

Serological and Biomarker Assays

Serum amyloid A and haptoglobin concentrations have been evaluated as biomarkers of inflammation in cats with respiratory diseases, with elevated levels correlating with disease severity [23]. Commercial ELISA kits are available for detection of FHV-1 and FCV antibodies, although serology is of limited utility in distinguishing acute infection from prior exposure or vaccination [23].

Diagnostic Prediction Models

Noninvasive diagnostic prediction models have been constructed for feline nasal and nasopharyngeal diseases using clinical examination findings and imaging data [24]. Computed tomography is increasingly used to evaluate the extent of upper and lower airway disease in cats with chronic respiratory signs [18].

Differential Diagnosis

Differential diagnoses for feline URI include other viral pathogens such as feline gammaherpesvirus, which has been investigated in cats with and without upper respiratory tract disease [25]. Mycoplasma species, including Mycoplasma felis, are also important differentials and can cause respiratory disease in cats [19]. For a comprehensive overview of bacterial etiologies, see the article on Feline Upper Respiratory Tract Infections: Bacterial Etiology, Antibiograms, and Novel Therapeutics.

Treatment and Therapeutics

Antiviral Therapy

Antiviral therapy for FHV-1 includes topical and systemic administration of nucleoside analogues such as famciclovir, which is metabolized to penciclovir and inhibits viral DNA polymerase [7]. Cidofovir, a nucleotide analogue, is used topically for ophthalmic FHV-1 infections [7]. No specific antiviral therapy is approved for FCV infection, although ribavirin and interferon-omega have demonstrated in vitro activity [10].

Antimicrobial Therapy

Secondary bacterial infections, including those caused by B. bronchiseptica, require appropriate antimicrobial therapy [13]. Doxycycline is the antimicrobial of choice for B. bronchiseptica infection, with fluoroquinolones and amoxicillin-clavulanate as alternative options [13]. Antimicrobial susceptibility testing is recommended to guide therapy, particularly in cases of recurrent or refractory infection [13].

Supportive Care

Supportive care is the mainstay of treatment for feline URI and includes nutritional support, fluid therapy, and airway humidification [7, 13]. Ophthalmic manifestations require topical lubrication, antibiotic ointments, and in severe cases, surgical intervention such as conjunctival grafting or corneal sequestrectomy [7].

Vaccination

Vaccination against FHV-1 and FCV is a cornerstone of preventive medicine in cats [10, 15]. Modified-live and inactivated vaccines are available, with the modified-live intranasal vaccine providing more rapid onset of mucosal immunity [10]. An engineered VP1 mRNA vaccine has demonstrated induction of immunity and complete protection against FCV challenge in cats, representing a promising next-generation vaccine platform [10]. Vaccine strains of FHV-1 and FCV have been characterized at the transcriptomic level, revealing differences in host gene expression profiles compared to field strains [26, 6].

Novel Therapeutics

Engineered VP1 mRNA vaccines represent a novel therapeutic approach for FCV, inducing robust humoral and cellular immune responses [10]. Characterization and immunogenic evaluation of epidemic FCV strains is ongoing to inform vaccine strain selection [15]. For a discussion of novel therapeutics in bacterial respiratory infections, see the article on Bacterial Respiratory Infections in Cats: Etiology, Clinical Signs, and Treatment.

Control and Prevention

Control of feline URI in multi-cat environments requires a comprehensive approach including vaccination, isolation of infected animals, environmental disinfection, and stress reduction [3, 2]. FHV-1 is inactivated by lipid solvents and common disinfectants, while FCV is more resistant to disinfection due to its non-enveloped structure [1]. B. bronchiseptica is susceptible to quaternary ammonium compounds and bleach solutions [13].

Diagnostic and Therapeutic Decision Tree

flowchart TD
    A[Cat presenting with URI signs], > B{Clinical examination}
    B, > C[Ocular signs predominant?]
    C, >|Yes| D[Suspect FHV-1]
    C, >|No| E[Oral ulcers present?]
    E, >|Yes| F[Suspect FCV]
    E, >|No| G[Coughing predominant?]
    G, >|Yes| H[Suspect Bordetella]
    G, >|No| I[Perform PCR panel]
    D, > I
    F, > I
    H, > I
    I, > J{Pathogen detected?}
    J, >|FHV-1| K[Antiviral therapy + supportive care]
    J, >|FCV| L[Supportive care + isolation]
    J, >|Bordetella| M[Antimicrobial therapy]
    J, >|Multiple pathogens| N[Combination therapy]
    J, >|None detected| O[Consider other etiologies]
    K, > P[Monitor for chronic sequelae]
    L, > P
    M, > P
    N, > P
    O, > Q[Advanced diagnostics: CT, biopsy]

References

[1] Slaviero M, de Almeida BA, de Castro LT, et al. Feline herpesvirus and calicivirus: Occurrence and pathology in cats with respiratory disease. Top Companion Anim Med. 2025. https://pubmed.ncbi.nlm.nih.gov/41015115/

[2] Acar G, Bilge-Dagalp S, Fedai T, et al. Molecular epidemiology of feline calicivirus (FCV) and felid alphaherpesvirus 1 (FeAHV-1) in cats with clinical signs and clinically healthy cats. Comp Immunol Microbiol Infect Dis. 2025. https://pubmed.ncbi.nlm.nih.gov/40644818/

[3] de Oliveira Santana W, Maciel JG, Flores RS, et al. Epidemiology and co-infection of URTD pathogens in domestic cats during COVID-19. Braz J Microbiol. 2025. https://pubmed.ncbi.nlm.nih.gov/41405820/

[4] Snedden K, Frye E, Conklin R, et al. A retrospective analysis of canine, feline, and equine respiratory polymerase chain reaction panels performed at the New York State Animal Health Diagnostic Center (January-December 2023). J Am Vet Med Assoc. 2025. https://pubmed.ncbi.nlm.nih.gov/40139158/

[5] Neely D, Mosley YC, Morris S, et al. Feline ocular and respiratory infections: a retrospective analysis of clinical cases submitted to Georgia Veterinary Diagnostic Laboratories (2012-2022). JFMS Open Rep. 2025. https://pubmed.ncbi.nlm.nih.gov/40078714/

[6] Kwan E, Legione AR, Hartley CA, et al. Transcriptomic profiles of Crandell-Rees feline kidney cells infected with Varicellovirus felidalpha-1 (FHV-1) field and vaccine strains. Virol J. 2025. https://pubmed.ncbi.nlm.nih.gov/40241206/

[7] Charnock L, Vernau K, Burkitt-Creedon J, et al. Clinical Management of Ophthalmic Manifestations of Upper Respiratory Tract Infections in Kittens. Vet Clin North Am Small Anim Pract. 2025. https://pubmed.ncbi.nlm.nih.gov/41484008/

[8] Xu C, Zhao J, Liu H, et al. Construction of Reverse Genetics System for Feline Calicivirus FCV-BJ616 and Proteomic Analysis. Microbiologyopen. 2026. https://pubmed.ncbi.nlm.nih.gov/41603492/

[9] Peñaflor-Téllez Y, Gómez de la Madrid J, Monge-Celestino EI, et al. The leader of the capsid protein from Feline calicivirus must be palmitoylated and form oligomers through disulfide bonds for efficient viral replication. J Virol. 2025. https://pubmed.ncbi.nlm.nih.gov/40827917/

[10] Zhang MD, Xie ZF, Li XH, et al. Engineered VP1 mRNA Vaccine Induces Immunity and Complete Protection Against Feline Calicivirus in Cats. Transbound Emerg Dis. 2026. https://pubmed.ncbi.nlm.nih.gov/41658352/

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[12] Zhao G, Zhu H, Xue X, et al. Feline Calicivirus Infection Manipulates Central Carbon Metabolism. Vet Sci. 2025. https://pubmed.ncbi.nlm.nih.gov/40005898/ *** 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.

[13] 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. https://pubmed.ncbi.nlm.nih.gov/40233793/

[14] Shiga T, Kakinuma Y, Takada M, et al. Chronic bronchitis and bronchiolitis with prominent globule leukocyte infiltration in a cat with Filobacterium felis. Vet Pathol. 2025. https://pubmed.ncbi.nlm.nih.gov/40088090/

[15] Xue X, Zhao G, Fang C, et al. Characterization and immunogenic evaluation of feline calicivirus epidemic strains. J Vet Sci. 2025. https://pubmed.ncbi.nlm.nih.gov/40765225/

[16] Luo D, Xie W, Li N, et al. Identification and Pathogenicity Analysis of Feline Calicivirus in Shanghai and Guangdong, China. Transbound Emerg Dis. 2025. https://pubmed.ncbi.nlm.nih.gov/40503218/

[17] 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/

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[20] Karakaya-Bilen E, Akgül G, Yılmaz-Koc O. Suspected Feline Calicivirus Infection Triggering Ulcerative Oral and Skin Lesions in Cats Following Routine Ovariohysterectomy: A Postoperative Risk Assessment. Vet Med Sci. 2025. https://pubmed.ncbi.nlm.nih.gov/40728089/

[21] Bi W, Wen F, Cai S, et al. An automated portable LAMP-based centrifugal microfluidic system for nucleic acid detection of multiple pathogens in feline upper respiratory disease. Mikrochim Acta. 2025. https://pubmed.ncbi.nlm.nih.gov/40993306/

[22] Shao P, Lian Y, Liu X, et al. Rapid and sensitive detection of feline herpesvirus-1 using fluorescent microspheres as labels for immunochromatographic test strips. Vet Res Commun. 2026. https://pubmed.ncbi.nlm.nih.gov/41779066/

[23] Gareis H, Schulz B. Evaluation of amyloid A and haptoglobin in the serum of cats with respiratory diseases. Front Vet Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/42109870/

[24] Fujiwara-Igarashi A, Nakazawa Y, Ohshima T, et al. Construction of a Diagnostic Prediction Model for Feline Nasal and Nasopharyngeal Diseases in Japan Using Noninvasive Examinations. Vet Med Sci. 2025. https://pubmed.ncbi.nlm.nih.gov/40104847/

[25] Doğan F, Acar G, Fedai T, et al. Investigation of the presence of gammaherpesvirus infections in cats with and without upper respiratory tract disease. Comp Immunol Microbiol Infect Dis. 2026. https://pubmed.ncbi.nlm.nih.gov/42035571/

[26] Kwan E, Legione AR, Hartley CA, et al. Transcriptomic analysis of Crandell-Rees feline kidney cell infections with field and vaccine feline calicivirus strains. Virus Res. 2026. https://pubmed.ncbi.nlm.nih.gov/41435987/

[27] Zhang R, Zhu H, Zhao G, et al. Transcriptome analysis of Crandell Rees Feline Kidney (CRFK) cells infected with Feline calicivirus strain 023 (FCV 023). Virology. 2026. https://pubmed.ncbi.nlm.nih.gov/41110383/

[28] Acar G, Bilge-Dagalp S. Investigation of the epidemiology of calicivirus infection of cats using molecular and virus isolation techniques. Comp Immunol Microbiol Infect Dis. 2025. https://pubmed.ncbi.nlm.nih.gov/40081119/