Feline Calicivirus vs Herpesvirus: Telling Cat Flu Apart

Q: What's the difference between calicivirus and herpesvirus in cats?

Both cause upper-respiratory disease, but differently: herpesvirus (FHV-1) causes conjunctivitis, corneal ulcers and goes latent for life (reactivating under stress), while calicivirus (FCV) causes mouth ulcers, has many strains, and in rare cases a severe systemic form.

Q: How do I tell which virus a cat has?

Clinical signs point the way — corneal/eye involvement suggests herpes, oral ulcers suggest calicivirus — but the two frequently co-infect. Antigen or PCR testing on ocular, nasal or oral swabs confirms which virus is actively shedding.

Q: Why does herpes keep coming back?

Because FHV-1 goes latent in nerve tissue after the first infection and reactivates under stress — boarding, moving, illness. The same virus resurfaces, which is why flare-ups recur.

Q: Why isn't calicivirus vaccine protection complete?

Because FCV has many antigenically distinct strains, and antibodies to 1 vaccine strain may not neutralise the strain a cat meets in the field. It reduces severity but doesn't guarantee protection against every strain.

Q: Do I need to test both viruses?

In an outbreak or a cat with recurrent signs, yes — because they present similarly and co-infect, and management differs. In routine practice, the clinical picture plus testing when signs are active is usually sufficient.

TL;DR — “Cat flu” is almost always 1 of 2 viruses: FHV-1 or FCV. They look similar but behave differently: herpes causes eye problems and goes latent for life (flaring under stress); calicivirus causes mouth ulcers and exists in many strains that can dodge vaccine protection. Testing with 1 swab — antigen or PCR during active signs — names the virus, and the answer changes management.

When a cat sneezes and gets congested, the cause is usually “cat flu” — and “cat flu” is almost always one of two specific viruses. Telling them apart matters, because they don’t behave the same way, and the distinction changes what you expect and how you manage it.

The Two Viruses at a Glance

	Feline Herpesvirus (FHV-1)	Feline Calicivirus (FCV)
Main signs	Sneezing, conjunctivitis, corneal ulcers	Mouth ulcers, drooling, upper-respiratory signs
The defining trait	Goes latent for life, reactivates under stress	Many strains, some highly virulent
Recurrence pattern	The same virus flares again and again	Reinfection with a different strain possible
Vaccine	Reduces severity, doesn’t prevent latency	Reduces severity, doesn’t cover every strain

The overlap is real — both cause sneezing, nasal discharge and fever — which is exactly why clinical signs alone often can’t settle it, and why the two frequently co-infect.

Herpesvirus: The Virus That Hides

FHV-1 is the one that explains the recurring, eye-focused flare-ups.

After the first infection — usually as a kitten — the virus goes latent, hiding in the trigeminal nerve for the cat’s entire life. Then, under stress (boarding, moving, illness, a new pet), it reactivates, and the cat starts shedding again with the same sneezing and conjunctivitis as before.

The practical signature of herpes is recurrence without reinfection: the cat isn’t catching it again — the same virus is resurfacing. That’s also why a negative test between flare-ups means little (the cat isn’t shedding at that moment), and why eye involvement — conjunctivitis, corneal ulcers — points toward herpes rather than calicivirus.

For the full picture on herpes, see the dedicated FHV-1 guide.

Calicivirus: The Many-Strain Virus

FCV is the one that explains the mouth ulcers — and the vaccine’s imperfect track record.

The classic calicivirus signs are oral ulceration (sores on the tongue, gums and palate), drooling, and upper-respiratory signs. Some cats also limp, and a rare severe systemic form can be highly fatal.

Calicivirus’s defining problem is strain diversity. FCV exists as many antigenically distinct strains, and the vaccine strain doesn’t cover all of them — field studies have found the vaccine strains are poorly matched to the field strains in circulation. So vaccination reduces severity and shedding, but an antibody to one strain may not neutralise the strain your cat actually meets. That’s why FCV immunity is the unreliable one of the “cat flu” pair, and why mouth-ulcer cats sometimes test “protected” yet still get sick.

How Testing Tells Them Apart

Because the signs overlap and the two co-infect, testing is the reliable way to name the culprit:

Test	What it answers
Antigen / PCR on ocular, nasal or oral swab	“Which virus is shedding right now?”
Antibody (serum)	“Has the cat been exposed?” (not which is active)

The rules that matter:

Test during active signs. Both viruses shed intermittently, so a negative swab between flare-ups is a snapshot, not a clean bill.
Antigen/PCR tells you the active virus; antibody only tells you exposure. Given how common both viruses are, most adult cats are antibody-positive for both — which is why antibody panels don’t distinguish “sick now” from “met it before.”
The clinical picture still guides you. Eye and corneal involvement leans herpes; oral ulcers lean calicivirus. The test confirms what the signs suggest.

For the antibody-immunity angle across all three core feline viruses, see the feline triple test guide.

Related products: FHV Antigen Test (MG-CA-R007) · FCV Antigen Test (MG-CA-R008) — both quantitative, run on the FIA680/FIA880.

FAQ

What’s the difference between calicivirus and herpesvirus?

Two viruses, two behaviours: both cause upper-respiratory disease, but herpes causes eye problems and goes latent for life (reactivating under stress), while calicivirus causes mouth ulcers and has many strains — 1 vaccine strain can’t cover them all.

How do I tell which virus a cat has?

Two clues point the way — eye involvement suggests herpes, oral ulcers suggest calicivirus — but antigen/PCR on 1 swab during active signs confirms it.

Why does herpes keep coming back?

One virus, lifelong latency: herpes goes latent in the trigeminal nerve for life (in ~100% of recovered cats) and reactivates under stress, so the same virus resurfaces rather than a new infection. That’s why a negative test between flare-ups means little — the cat isn’t shedding at that moment.

Why isn’t calicivirus vaccine protection complete?

One strain mismatch breaks it: FCV has many antigenically distinct strains, and antibodies to 1 vaccine strain may not neutralise the strain a cat meets in the field. That’s why a vaccinated cat can still get a calicivirus flare.

Do I need to test both viruses?

Yes — test both in 2 scenarios: outbreaks and recurrent-signs cats, because the viruses present similarly, co-infect, and management differs.

Key Takeaways

“Cat flu” = 1 of 2 viruses — herpes or calicivirus, similar signs, different behaviour.
Herpes goes latent for life — 1 infection means recurring eye flare-ups triggered by stress.
Calicivirus has many strains — 1 vaccine cannot promise protection against every field strain.
Test during active signs — antigen/PCR names the 1 active virus; antibody only shows exposure.
The signs guide you — 2 patterns: eyes lean herpes, mouth ulcers lean calicivirus.

References

European Advisory Board on Cat Diseases (ABCD). “Guideline for Feline Calicivirus Infection.” — strain diversity and antibody-testing limits.
Gaskell R, et al. “Feline herpesvirus.” Veterinary Research. 2007;38(2):337–354.
Thiry E, et al. “Feline herpesvirus infection: ABCD guidelines.” J Feline Med Surg. 2009;11(7):547–555.

This content is for educational purposes only. It is not a substitute for veterinary diagnosis — a cat with respiratory disease should be seen by a veterinarian. Product specifications are as published by Migibio (Guangzhou Magic Biotech Co., Ltd.) and may change.