Upper respiratory infection (URI) is one of the most common infectious syndromes in cats and is widely known as “cat flu”. In 80-90% of cases, Feline Herpesvirus-1 (FHV-1) and Feline Calicivirus (FCV) are the primary etiologic agents. Prevalence is especially high in shelter cats, unvaccinated kittens, and multi-cat households. FHV-1 infection can remain latent and later reactivate under stress, causing recurrent episodes. This article reviews the etiology, clinical presentation, diagnostic approach, treatment options, vaccination, and nutritional immune support strategies for feline URI.
Situations That Require Urgent Veterinary Attention
- Kittens younger than 8 weeks — URI can deteriorate rapidly
- Open-mouth breathing — always an emergency sign in cats
- More than 24 hours without eating (risk of hepatic lipidosis)
- Corneal ulceration, corneal opacity, or eyelid adhesion
- Fever (>39.5°C) together with lethargy and dehydration
- Purulent nasal discharge (green/yellow) lasting more than 5 days or not responding to antibiotics
1. Etiology — What Causes “Cat Flu”?
| Agent | Frequency | Typical Features | Latent Infection? |
|---|---|---|---|
| Feline Herpesvirus-1 (FHV-1) | 40-50% | Severe conjunctivitis, corneal ulceration, nasal ulceration | Yes — remains latent in the trigeminal ganglion and may reactivate during stress |
| Feline Calicivirus (FCV) | 30-40% | Oral ulceration (tongue, palate), lameness (FCV-associated arthritis), pneumonia | Chronic carriage possible (up to 50% — oropharyngeal shedding) |
| Chlamydia felis | 5-10% | Initially unilateral conjunctivitis that often becomes bilateral, mild nasal discharge | Chronic carriage may occur |
| Bordetella bronchiseptica | 5-10% | Coughing (less common in cats), pneumonia in kittens | Chronic carriage possible |
| Mycoplasma felis | Secondary | Usually a secondary pathogen; conjunctivitis | — |
Why Does It Recur? — FHV-1 Latent Infection
In approximately 80% of cats infected with FHV-1, the virus remains dormant in the trigeminal ganglion. Stressors such as moving, boarding, introduction of a new cat, lactation, or corticosteroid treatment may reactivate the virus and trigger recurrent sneezing, nasal discharge, and ocular signs. These cats may also shed virus and infect other cats (Gaskell et al., 2007).
2. Routes of Transmission and Risk Factors
Routes of Transmission
- Direct contact: nose-to-nose contact, sneeze droplets (within 1-2 meters)
- Fomites: food bowls, litter trays, hands, clothing
- Aerosol droplets: spread during sneezing
- Queen-to-kitten: during parturition and nursing period
- Carrier cats: asymptomatic viral shedders
Risk Factors
- Unvaccinated kittens (maternal antibodies wane around 6-8 weeks)
- Shelter or multi-cat housing — high population density
- Stress: moving, boarding, introduction of a new cat
- Immunosuppression: FIV/FeLV-positive cats
- Brachycephalic breeds: Persian, Exotic (narrow nasal passages)
- Poor ventilation and inadequate hygiene
3. Clinical Signs
3.1 FHV-1 vs. FCV — Clinical Comparison
| Clinical Feature | FHV-1 | FCV |
|---|---|---|
| Sneezing | Prominent, often severe | Mild to moderate |
| Nasal discharge | Initially serous, later mucopurulent | Mild serous discharge |
| Ocular involvement | Marked — conjunctivitis, corneal ulceration, keratitis | Mild conjunctivitis |
| Oral ulceration | Uncommon (nasal or lip ulceration may occur) | Prominent — tongue, palate, lips |
| Fever | Moderate to high (39.5-40.5°C) | Mild to moderate |
| Lameness | Absent | May occur (FCV-associated arthritis, especially in kittens) |
| Pneumonia | Rare (usually secondary bacterial) | Possible in virulent systemic strains (VS-FCV) |
| Incubation period | 2-6 days | 2-10 days |
| Disease duration | 10-21 days (uncomplicated cases) | 7-14 days |
3.2 Severe Disease in Kittens
URI in Neonatal and Young Kittens
URI may be considerably more severe in kittens:
- Bilateral eyelid adhesion and severe ocular discharge — risk of corneal perforation
- Pneumonia — especially with Bordetella or secondary bacterial infection
- Dehydration + anorexia — kittens cannot smell properly when the nose is obstructed
- Chronic rhinitis / sinusitis — permanent turbinate damage may develop
- Mortality: can reach 50% in unvaccinated neonatal kittens
4. Diagnostic Approach
| Method | Test | Note |
|---|---|---|
| Clinical diagnosis | History + physical examination | In most cases, clinical presentation is sufficient; routine pathogen differentiation is not always required |
| PCR | Ocular, nasal, or oropharyngeal swab | Useful to distinguish FHV-1 from FCV; false negatives may occur with latent infection |
| Culture | Bacterial culture + susceptibility testing | Used when secondary bacterial infection is suspected, especially in chronic or refractory cases |
| Fluorescein staining | Detection of corneal ulceration | Important for FHV-1 keratitis — dendritic ulcers are highly suggestive |
| Thoracic radiography | When pneumonia is suspected | Indicated in severe cases or when dyspnea is present |
5. Treatment Approach
Medical Treatment
- Antibiotics: for secondary bacterial infection (doxycycline — also covers Chlamydia; amoxicillin-clavulanate)
- Antiviral therapy: famciclovir for FHV-1; oral and generally well tolerated in cats
- Ocular therapy: cidofovir eye drops or idoxuridine in FHV-1 keratitis when indicated
- NSAIDs: fever and pain control when appropriate (cat-safe dosing required)
- Nasal care: saline drops; topical decongestants are not routinely used
Supportive Care at Home
- Steam therapy: allow the cat to breathe humid air in a steamy bathroom for about 15 minutes
- Nasal and ocular cleaning: saline cleansing 3-4 times daily
- Warm environment: ideally 22-25°C, protected from cold stress
- Stress reduction: quiet environment, predictable routine, isolation if needed
- Encouraging appetite: wet food, warming food, enhancing aroma
6. Protection Through Vaccination
WSAVA Core Vaccination — FHV-1 and FCV
| Age | Recommendation |
|---|---|
| 6-8 weeks | First dose (FVRCP — FHV-1 + FCV + FPV) |
| 10-12 weeks | Second dose |
| 14-16 weeks | Third dose (final kitten dose) |
| 1 year later | First booster |
| Thereafter | Booster every 3 years (WSAVA 2024) |
Note: Vaccination does not completely prevent URI, but it significantly reduces disease severity and duration. Vaccinated cats usually experience a milder course.
7. Nutritional Management — The VetKriter Approach
VetKriter Nutrition Principle
Nutritional support is vital in cats with URI. Nasal obstruction reduces the ability to smell, which leads to anorexia, and more than 48 hours without food can trigger hepatic lipidosis, especially in overweight cats. Preserving appetite and supporting immune function directly affect treatment success.
7.1 Strategies to Encourage Appetite
Food Presentation Strategies
- Prefer wet food: stronger aroma and higher moisture content
- Warm the food: approximately to body temperature (around 38°C) to improve smell
- Small, frequent meals: offer fresh food 4-6 times per day
- Texture and flavor variety: pate, shredded meat, fish-based options
- Hand-feeding assistance: placing a small amount on the palate or lips may stimulate eating
- Aroma enhancers: a small amount of chicken broth or tuna water may help
Critical Rules
- 48 hours without eating = veterinary attention (risk of hepatic lipidosis)
- Assisted feeding: only with veterinary guidance (syringe or tube feeding when indicated)
- Cats with oral ulceration: offer soft food at room temperature; very hot or cold food may irritate lesions
- Water intake: dehydration risk is high; wet food and water fountains can help
- Do not restrict protein: adequate protein is essential for immune response and recovery
7.2 Nutritional Components That Support Immunity
| Nutritional Component | Immune Function | Source |
|---|---|---|
| L-Lysine | Historically proposed to suppress FHV-1 replication, but current evidence is conflicting and modern reviews generally find no consistent benefit | Supplement form; low levels may also appear in foods |
| Omega-3 (EPA/DHA) | Anti-inflammatory support, mucosal recovery, immune modulation | Fish oil, salmon oil |
| Vitamin E + Selenium | Antioxidant defense and T-cell support | Complete food formulas, mixed tocopherols |
| Vitamin A (Retinol) | Mucosal integrity and epithelial barrier support | Liver, egg, complete food formulations |
| Zinc (Zn) | T-cell maturation and antioxidant support | Prefer chelated sources such as zinc methionine or zinc proteinate |
| Beta-glucan | Immunomodulation; macrophage and NK-cell activation | Yeast cell wall, hydrolyzed yeast |
| Probiotics | Support GALT activity and mucosal IgA response | Enterococcus faecium, Bacillus coagulans |
The L-Lysine Controversy
L-lysine supplementation was recommended for many years against FHV-1. However, a comprehensive meta-analysis by Bol and Bunnik (2015) found that lysine supplementation did not reduce FHV-1 infection and may even have had negative effects in some studies. Current ABCD guidance does not recommend routine lysine supplementation. If lysine is considered, the decision should be individualized by the attending veterinarian.
8. Management in Multi-Cat Households
Infection-Control Protocol
Isolation:
- The affected cat should be isolated in a separate room
- Use separate food and water bowls and a separate litter tray
- The caregiver should wash hands and ideally change clothing between cats
- Isolation should continue for at least 1 week after clinical signs resolve
Disinfection:
- FHV-1: an enveloped virus, susceptible to most disinfectants (for example diluted bleach at 1:32)
- FCV: a non-enveloped virus, more resistant; potassium peroxymonosulfate-based products are useful
- Surfaces, food bowls, and litter trays should be disinfected regularly
- Ventilation is important
9. Management of Chronic / Recurrent URI
Because FHV-1 can persist latently, some cats experience recurrent URI. Long-term management includes:
- Stress minimization: pheromone support, stable routines, quiet environment
- Immune support: high-quality nutrition, omega-3 fatty acids, antioxidants
- Consistent vaccination: keep boosters up to date
- Antiviral prophylaxis: famciclovir may be considered during major stress periods under veterinary guidance
- Chronic rhinitis / sinusitis management: saline flushing, culture-guided prolonged antibiotics in selected cases
- FIV/FeLV testing: indicated when recurrent URI suggests underlying immunosuppression
10. References
- Gaskell RM, Dawson S, Radford AD. Feline Respiratory Disease. In: Greene CE, ed. Infectious Diseases of the Dog and Cat. 4th Ed. Elsevier, 2012:151-162.
- Bol S, Bunnik EM. Lysine supplementation is not effective for the prevention or treatment of feline herpesvirus 1 infection in cats: a systematic review. BMC Vet Res. 2015;11:284.
- ABCD (European Advisory Board on Cat Diseases). Feline Upper Respiratory Tract Disease Guidelines. 2023.
- Thiry E, et al. Feline Herpesvirus Infection — ABCD Guidelines. JFMS. 2009;11(7):547-555.
- Radford AD, et al. Feline Calicivirus Infection — ABCD Guidelines. JFMS. 2009;11(7):556-564.
- Day MJ, et al. WSAVA Guidelines for the Vaccination of Dogs and Cats. JFMS. 2024;26(4):249-317.
- WSAVA Global Nutrition Committee. Nutritional Assessment Guidelines. 2024.