Feline herpesvirus type 1 (FHV-1) and feline calicivirus (FCV) are the two most common causes of cat flu, together responsible for the vast majority of feline upper respiratory tract infections. Because both pathogens spread readily between cats, cause overlapping clinical signs, and frequently co-infect the same animal, differentiating between them on clinical grounds alone can be genuinely challenging.

However, the two viruses have distinct biologies, produce subtly different clinical patterns, and — critically — require different long-term management strategies. This article explains how to distinguish FHV-1 from FCV, what diagnostic tests are available, and why the distinction matters in practice.

The Two Viruses: A Brief Overview

Feline Herpesvirus Type 1 (FHV-1)

FHV-1 is a double-stranded DNA virus belonging to the Alphaherpesvirinae subfamily. Like all herpesviruses, it establishes lifelong latency in the host — specifically in the trigeminal ganglia — after the primary infection resolves. Latent virus can reactivate periodically, particularly during physiological or psychological stress such as illness, boarding, introduction of new cats, or surgery, causing recurrent bouts of clinical disease.

FHV-1 is antigenically stable and does not mutate rapidly. This means vaccines remain reliably effective over time, and immunity from prior infection provides consistent (though not sterilising) protection against future disease episodes.

Feline Calicivirus (FCV)

FCV is a non-enveloped, positive-sense, single-stranded RNA virus. Unlike FHV-1, it does not establish latency. Instead, many cats remain persistently infected and continuously shed virus for extended periods after their initial infection — approximately 25% develop chronic infection, and up to 50% of infected cats shed virus even after apparent clinical recovery.

FCV mutates extremely rapidly due to the error-prone nature of RNA virus replication. Circulating strains differ considerably in their biological properties and antigenic profiles, meaning no single vaccine provides complete protection against all field strains.

Clinical Signs: Similarities and Differences

Both viruses produce sneezing, nasal discharge, conjunctivitis, fever, inappetence, and lethargy — the classic syndrome of feline upper respiratory disease, or cat flu. Distinguishing them requires attention to the specific character, severity, and anatomical distribution of signs. The following comparison covers the most diagnostically useful clinical features.

Clinical Feature	FHV-1	FCV
Nasal discharge	Prominent; often profuse and mucopurulent	Moderate; often more serous in character
Conjunctivitis	Very common; frequently severe	Present but generally milder
Corneal ulceration	Classic feature; dendritic (branching) ulcers on fluorescein stain	Not a feature of FCV infection
Oral ulceration	Rare	Very common; tongue, hard palate, nasal planum
Limping syndrome	Not associated with FHV-1	Caused by two specific FCV strains; kittens 8–12 weeks
Pneumonia	Rare in immunocompetent adults	Possible with pneumotropic FCV strains
Recurrence pattern	Stress-triggered reactivation from latency	Continuous shedding; no classical stress-triggered pattern
Chronic disease	Recurrent keratitis, corneal sequestrum	Chronic stomatitis (LPS), persistent shedding
Systemic virulent form	Not described in literature	FCV-VSD: high mortality, multi-organ disease

Distinctive Clinical Hallmarks

Corneal Disease: The Hallmark of FHV-1

One of the most diagnostically useful distinguishing features of FHV-1 is its capacity to cause corneal disease. FHV-1 corneal ulcers are typically dendritic (branching) in morphology, reflecting viral replication within corneal epithelial cells. Affected cats present with ocular pain, excessive blinking, photophobia, and a lesion visible on fluorescein stain examination. Corneal disease is not a feature of FCV infection and, when present, should immediately raise clinical suspicion for FHV-1.

Chronic or recurrent FHV-1 infection can also cause corneal stromal disease (interstitial keratitis) and, in severe cases, corneal sequestrum — a dark brown-to-black necrotic plaque embedded in the corneal stroma that typically requires surgical removal.

Oral Ulceration: The Hallmark of FCV

Oral ulceration — particularly affecting the tongue, hard palate, and nasal planum — is characteristic of FCV rather than FHV-1. FCV-associated vesicles rupture rapidly, leaving behind clean, painful erosions that cause drooling and reluctance to eat. FHV-1 rarely causes oral ulceration, and its presence should substantially raise clinical suspicion for FCV.

In cats with refractory chronic oral inflammation, FCV (alongside feline immunodeficiency virus) should be high on the differential list. Lymphocytic-plasmacytic stomatitis (LPS) — a severe, painful chronic gingivostomatitis — has been associated with persistent FCV infection.

The Limping Syndrome: Unique to FCV

Two specific FCV strains produce a distinctive limping syndrome characterised by sudden-onset fever, alternating leg lameness, and joint pain on palpation — without the oral ulceration or respiratory signs typical of other FCV strains. This syndrome occurs predominantly in kittens aged 8 to 12 weeks and can also appear following recent FCV vaccination. It typically resolves without treatment within 7 to 10 days. This presentation has no equivalent in FHV-1 infection.

Recurrence Patterns

The latent nature of FHV-1 produces a recognisable clinical pattern: a cat recovers from primary infection, remains well for weeks to months, and then develops signs again following an identifiable stressor such as rehoming, veterinary hospitalisation, introduction of a new pet, or a concurrent illness. This stress-triggered recurrence pattern is characteristic of herpesvirus latency and is not seen with FCV.

FCV does not produce this pattern. Cats that remain persistently infected shed virus relatively continuously, and clinical signs — when they occur — are not reliably preceded by identifiable stressors. Importantly, FCV-shedding cats often appear clinically well while remaining infectious to other cats.

Virulent Systemic Disease: An FCV-Specific Concern

Virulent systemic FCV disease (FCV-VSD) is a severe, potentially fatal syndrome caused by specific hypervirulent FCV strains. It has no FHV-1 equivalent. FCV-VSD presents with high fever, facial and limb oedema, ulcerative dermatitis, respiratory distress, jaundice, and multi-organ failure, with reported case fatality rates of 40 to 67% in outbreak settings. Affected cats include vaccinated adults, which distinguishes it from typical cat flu presentations.

Any cluster of severe, rapidly progressive systemic illness in cats — particularly in a cattery or clinic setting — should prompt consideration of FCV-VSD and immediate isolation measures.

Diagnostic Testing

In mild, uncomplicated cat flu in a previously vaccinated cat, laboratory testing may not be necessary and empirical supportive care is reasonable. However, PCR testing is recommended in the following situations:

• Severe or protracted disease not responding to initial supportive treatment
• Cattery or shelter outbreak settings where identifying the causative agent guides control measures
• Chronic stomatitis, conjunctivitis, or recurrent respiratory signs
• Suspected FCV-VSD or unusually severe systemic illness
• Immunocompromised cats or young kittens with severe disease
• Distinguishing vaccine-strain virus from wild-type field virus in recently vaccinated cats

Multiplex respiratory PCR panels simultaneously testing for FCV, FHV-1, Chlamydia felis, Bordetella bronchiseptica, and Mycoplasma felis are commercially available worldwide and provide the most comprehensive diagnostic yield from a single conjunctival or nasopharyngeal swab.

Management Differences

Supportive care — including broad-spectrum antibiotics for secondary bacterial infections, nutritional support, mucolytic agents, and adequate hydration — is appropriate for both infections. However, the distinct biologies of FHV-1 and FCV drive important differences in targeted and long-term management.

Managing FHV-1

• L-Lysine supplementation: Lysine competes with arginine, which is essential for FHV-1 replication. Supplementation has been used to reduce recurrence frequency and severity. While clinical trial evidence for efficacy is mixed, it remains widely used and is well tolerated. L-Lysine has no effect on FCV.
• Famciclovir (oral antiviral): The most evidence-supported antiviral for FHV-1 in cats. Used for severe acute disease, chronic recurrent keratitis, and herpetic skin disease. Requires metabolism by feline-specific enzymes; acyclovir (the human equivalent) is poorly converted in cats and should not be substituted.
• Topical ophthalmic antivirals: Cidofovir and trifluridine are used topically for herpetic corneal ulceration. Cidofovir 0.5% applied twice daily is preferred for its less frequent dosing schedule and good corneal penetration.
• Stress reduction: A cornerstone of long-term FHV-1 management given the stress-triggered reactivation pattern. Environmental enrichment, pheromone diffusers (Feliway), and minimising unnecessary household disruption can meaningfully reduce relapse frequency.

Managing FCV

• L-Lysine: Not effective against FCV. Do not use as a substitute for FCV-specific management.
• Famciclovir: Not effective against FCV. Antiviral management options for FCV are currently limited.
• Vaccination: More complex than for FHV-1 due to FCV’s extensive genetic diversity. Newer multivalent vaccines incorporating a broader range of field strains are available and preferred in high-risk environments.
• Isolation and shedding management: FCV-shedding cats in multi-cat environments should be identified by PCR and managed carefully to reduce onward transmission. Chronically infected cats that are shedding high-titre virus are a persistent infection risk.
• Management of chronic stomatitis: Cats with severe LPS may benefit from dental extractions, immunomodulatory therapy, and close long-term monitoring. Some cats require full-mouth extraction to achieve remission.

Co-Infection with Both Viruses

Co-infection with FHV-1 and FCV is common in clinical practice and results in more severe disease than either pathogen alone. Cats presenting with both corneal ulceration and oral ulceration should be considered likely co-infected, and multiplex PCR testing is especially valuable in these cases. Concurrent infection with Chlamydia felis or Mycoplasma felis further complicates the clinical picture and reinforces the value of comprehensive panel testing over single-pathogen testing.

Key Takeaways

• FHV-1 and FCV both cause cat flu but differ fundamentally in biology, clinical pattern, and long-term management
• Corneal ulceration (especially dendritic) strongly points to FHV-1; oral ulceration strongly points to FCV
• Stress-triggered recurrence is a hallmark of FHV-1 latency; continuous shedding without reactivation pattern is more typical of FCV
• The limping syndrome and virulent systemic disease are FCV-specific — they have no FHV-1 equivalent
• Multiplex PCR on conjunctival or nasopharyngeal swabs reliably differentiates the two viruses
• L-Lysine, famciclovir, and topical antivirals are FHV-1 specific tools with no efficacy against FCV
• Co-infection with both viruses is common and typically produces more severe disease

References

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2. Sykes JE (2014). Canine and Feline Infectious Diseases. Elsevier Saunders.

3. Maggs DJ (2010). Antiviral therapy for feline herpesvirus infections. Vet Clin North Am Small Anim Pract 40(6):1055–62.

4. Radford AD et al. (2009). Feline calicivirus. Vet Res 40(2):29.

5. Hurley KF, Sykes JE (2003). Update on feline calicivirus: new trends. Vet Clin North Am Small Anim Pract 33(4):759–72.

6. Lommer MJ, Verstraete FJ (2003). Concurrent oral shedding of feline calicivirus and FHV-1 in cats with chronic gingivostomatitis. Oral Microbiol Immunol 18(2):131–4.