Cat Flu: Herpesvirus vs Calicivirus, and Why It Matters

Key takeaway: Most cat flu comes from one of two viruses, Feline Herpesvirus (FHV-1) or Feline Calicivirus (FCV), and both cause sneezing, runny nose, and watery eyes. The quickest clue is where the trouble sits: FHV-1 usually hits the eyes hardest (squinting, conjunctivitis, corneal ulcers), while FCV usually hits the mouth hardest (ulcers, drooling, sore gums). The answer matters because the two viruses respond to different antiviral pathways, so your veterinarian needs to know which one is driving the illness before choosing treatment.

If your cat has been sneezing with a runny nose, you are not alone, and there is a clear path forward. Let us walk through it together.

A cat resting comfortably at home.

What are the most common cat upper respiratory infection symptoms?

The most common cat upper respiratory infection symptoms are sneezing, a runny or blocked nose, watery or discharging eyes, mild fever, low appetite, and tiredness. These signs are often grouped under the everyday name "cat flu", and in most cases the cause is a virus rather than bacteria.

Two viruses cause the large majority of cat flu cases: Feline Herpesvirus (FHV-1) and Feline Calicivirus (FCV). Both spread easily between cats through sneeze droplets, shared bowls, grooming, and human hands.

Because the early symptoms overlap so much, you often cannot tell the two apart by sneezing alone. That is exactly why a proper look from your veterinarian is so helpful.

Kittens, senior cats, and cats in multi-cat homes or shelters are the most likely to catch and spread these viruses. Stress and crowding make flare-ups more common.

How is Feline Herpesvirus (FHV-1) different from Feline Calicivirus (FCV)?

Feline Herpesvirus (FHV-1) and Feline Calicivirus (FCV) are two separate viruses that both cause cat flu, but they tend to target different areas. FHV-1 is more of an "eyes and nose" virus, while FCV is more of a "mouth and gums" virus.

Here is a simple side by side to help you spot the pattern.

Feature	Feline Herpesvirus (FHV-1)	Feline Calicivirus (FCV)
Main trouble spot	Eyes and nose	Mouth and gums
Classic signs	Sneezing, nasal discharge, squinting, conjunctivitis, corneal ulcers	Mouth ulcers, drooling, gingivitis, stomatitis, sometimes limping
Nature of infection	Lifelong, with flare-ups often triggered by stress	Often clears, but some cats carry and shed it
Antiviral a vet may prescribe	HerpX (famciclovir)	CaliciX or CaliciX Max (EIDD-1931)
Role of antibiotics	Only for secondary bacterial infection, not the virus	Only for secondary bacterial infection, not the virus

Neither virus is treated with antibiotics on its own, because antibiotics do not act on viruses. Your veterinarian decides whether antivirals, supportive care, or both are right for your cat.

What are the signs of Feline Herpesvirus (FHV-1) in cats?

Feline Herpesvirus (FHV-1) most often shows up as sneezing, nasal discharge, and clear eye trouble such as squinting, red conjunctiva, and eye discharge. In some cats FHV-1 also causes painful corneal ulcers, which is why eye symptoms are such an important clue.

Feline herpesvirus, also called feline viral rhinotracheitis, is a lifelong infection. Once a cat catches it, the virus stays in the body and can flare up again later, usually when the cat is stressed by travel, a new pet, boarding, or illness.

Because FHV-1 is lifelong, the goal of care is never to eliminate the virus. Instead, care focuses on calming flare-ups, keeping the eyes and nose comfortable, and reducing how often symptoms return.

Watch closely if your cat is pawing at an eye, holding an eye shut, or has thick eye discharge. Eye ulcers can worsen quickly, so these signs deserve a same-week visit to your veterinarian.

What are the signs of Feline Calicivirus (FCV) in cats?

Feline Calicivirus (FCV) classically causes painful mouth ulcers along with sneezing and nasal discharge. Many cats with FCV drool, paw at the mouth, refuse hard food, or develop red, inflamed gums (gingivitis) and stomatitis.

Some cats with FCV also show a short-lived limping or stiff, sore joints, sometimes called "limping syndrome". This mouth-and-joint pattern is a helpful hint that you may be dealing with FCV rather than FHV-1.

In more severe cases, FCV drives long-lasting mouth inflammation such as feline chronic gingivostomatitis, which can make eating genuinely painful. If your cat has stopped eating because the mouth hurts, treat that as urgent.

You can read one family's account of oral ulcers improving with veterinary-guided antiviral care in this story about how one cat recovered from FCV mouth ulcers. Every cat is different, so let your veterinarian tailor the plan.

Why does the answer change the cat flu treatment?

The answer changes the cat flu treatment because Feline Herpesvirus (FHV-1) and Feline Calicivirus (FCV) respond to different antiviral medicines. A medicine chosen for one virus is not automatically the right choice for the other, so identifying the virus helps your veterinarian pick the correct pathway.

For Feline Herpesvirus (FHV-1), the antiviral a veterinarian may prescribe is famciclovir, the active ingredient in HerpX. For Feline Calicivirus (FCV), the veterinarian-prescribed antiviral option is EIDD-1931, found in CaliciX and CaliciX Max.

These are not interchangeable. Famciclovir and EIDD-1931 are different molecules aimed at different viruses, which is the whole reason diagnosis comes first.

Antibiotics have a place, but a narrow one. They do not treat either virus itself, and are used only when a secondary bacterial infection sets in on top of the viral illness.

How is Feline Herpesvirus (FHV-1) treated and managed?

Feline Herpesvirus (FHV-1) is managed, not eliminated, because the virus stays in the body for life. Care aims to shorten and soften flare-ups, protect the eyes, and lower stress so symptoms return less often.

Famciclovir is an antiviral that a veterinarian may prescribe for FHV-1 flare-ups, and it is the active ingredient in HerpX. Because the right dose depends on your cat's weight and condition, all dosing decisions belong with your veterinarian rather than a fixed home recipe.

Supportive care matters just as much as any medicine. Gentle eye cleaning, a calm environment, humidity to loosen congestion, and tempting, warm, soft food all help a cat through a flare.

Reducing stress is a core part of FHV-1 management. Predictable routines, safe hiding spots, and avoiding sudden changes can genuinely lower how often flare-ups happen.

When should you see a vet for Feline Herpesvirus (FHV-1)?

See your veterinarian for Feline Herpesvirus (FHV-1) if your cat has eye pain, a held-shut eye, thick discharge, stops eating, or if symptoms drag on beyond a few days. Eye ulcers linked to FHV-1 can worsen fast, so early attention protects your cat's sight and comfort.

How is Feline Calicivirus (FCV) treated?

Feline Calicivirus (FCV) is treated with a combination of veterinary-guided antiviral therapy for the virus itself and supportive care for pain, hydration, and nutrition. The veterinarian-prescribed antiviral option for FCV is EIDD-1931, the ingredient in CaliciX and CaliciX Max.

CaliciX is aimed at mild to moderate FCV, such as gingivitis and mouth ulcers, while CaliciX Max is aimed at more severe or stubborn mouth disease. Which one fits your cat, and at what dose, is a decision for your veterinarian based on the severity and your cat's weight.

Here is the point many families miss: antibiotics do not treat the calicivirus itself. Antibiotics only help when a secondary bacterial infection appears on top of the viral illness, so leaning on antibiotics alone can leave the real driver untouched.

One safety note that is not optional. EIDD-1931 must never be given to pregnant, nursing, or breeding cats, so always tell your veterinarian if your cat is or may be pregnant.

How can you care for a sneezing cat at home?

You can support a sneezing cat at home by keeping them warm, fed, hydrated, and calm while your veterinarian guides the medical plan. Home care does not replace a diagnosis, but it makes a real difference to comfort and recovery.

1. Wipe away eye and nose discharge gently with a soft cloth and warm water, several times a day.

2. Warm the food slightly and offer soft, smelly options, since a blocked nose dulls appetite and mouth ulcers make chewing hurt.

3. Create steam by sitting with your cat in a bathroom after a hot shower, which helps loosen congestion.

4. Keep fresh water within easy reach and watch closely for dehydration.

5. Lower stress with quiet, warm resting spots and a steady daily routine.

6. Separate a sick cat from other cats where possible, and wash your hands between them, because both viruses spread easily.

Call your veterinarian promptly if your cat stops eating for more than a day, struggles to breathe, has a painful eye, or seems to be getting worse rather than better. These are signs that home care alone is not enough.

FAQ

Is cat flu caused by Feline Herpesvirus (FHV-1) or Feline Calicivirus (FCV)?

Cat flu is most often caused by one of these two viruses, and sometimes both at once. Feline Herpesvirus (FHV-1) tends to affect the eyes and nose more, while Feline Calicivirus (FCV) tends to affect the mouth and gums more, so your veterinarian looks at the pattern to decide.

Can antibiotics treat my cat's sneezing and cat flu?

Antibiotics do not treat either Feline Herpesvirus (FHV-1) or Feline Calicivirus (FCV), because antibiotics act on bacteria and not viruses. Your veterinarian may still prescribe an antibiotic if a secondary bacterial infection develops, but the viral cause needs a different approach.

Does Feline Herpesvirus (FHV-1) ever go away completely?

No, Feline Herpesvirus (FHV-1) is a lifelong infection that stays in the body even after symptoms settle. Care focuses on managing and reducing flare-ups, often triggered by stress, rather than removing the virus.

Which antiviral is used for Feline Calicivirus (FCV)?

For Feline Calicivirus (FCV), the veterinarian-prescribed antiviral option is EIDD-1931, found in CaliciX and CaliciX Max. The right product and dose depend on how severe the mouth disease is and on your cat's weight, so your veterinarian makes that call, and EIDD-1931 is never used in pregnant, nursing, or breeding cats.

How can I tell FHV-1 and FCV apart at home?

You cannot confirm it at home, but the strongest clue is location. Heavy eye involvement, like squinting and eye ulcers, points more toward Feline Herpesvirus (FHV-1), while painful mouth ulcers, drooling, and sometimes limping point more toward Feline Calicivirus (FCV). Only your veterinarian can confirm the diagnosis and match the treatment.

If your cat is sneezing and you are not sure which virus is behind it, the kindest next step is a proper diagnosis so the treatment fits. To learn more about the care options and to speak with people who understand what you are going through, reach out to the BasmiFIP India team and work alongside your own veterinarian. You are not facing this alone, and there is a clear path forward.