A Vet's Guide to Cat Flu | Treatment, Recovery + Much More


Jul 22, 2023
A Vet's Guide to Cat Flu | Treatment, Recovery + Much More
Although this article has been written by a veterinarian and is based on scientific research and expert knowledge, it is intended for informational purposes only. It does not replace consulting your own veterinarian and establishing a patient-doctor relationship. To support our efforts, this site contains affiliate links to products we recommend and find helpful, which may earn a small commission at no extra cost to you. Read more.
Table of Contents


Welcome to the cat flu guide, where you will discover everything you need to know about cat flu, including the following: 

  • Symptoms: it's more than just a snotty nose and frequently affects the eyes too
  • Causes: it can be viral or bacterial, or both!
  • Diagnosis: do tests need to be performed?
  • Transmission: can humans and dogs also get it, and how long does it survive in the environment?
  • Prevention: are vaccinations 100% effective?
  • Treatment: medication from your veterinarian and nursing care at home
  • Recovery and prognosis: why does it sometimes keep coming back, and can it cause lifelong issues?
  • Differentials: what other illnesses can cause a snotty nose or watery eyes?

What Is Cat Flu?

A feline upper respiratory tract infection, sometimes referred to as "cat flu," is a viral or bacterial infection that affects a cat's nose, eyes, and throat.

Kitten with an upper respiratory tract infection
A kitten with an upper respiratory tract infection presents with conjunctivitis and squinting, and green discharge from the eyes and nose that's crusting around the eyelids and nostrils

Cat Flu Symptoms

Symptoms are usually mild to moderate. However, signs can be more severe in kittens due to their immature immune systems or in adults with weakened immune systems due to illness or medications. Typical symptoms include:

  • Conjunctivitis. Often referred to as 'pink eye' in humans, and is when the conjunctiva, a thin, transparent membrane that lines the inside of the eyelids, the third eyelid, and the sclera (white part of the eye), becomes red and inflamed. The conjunctiva can also become significantly swollen and puffy, known as chemosis. 
  • Blepharospasm. Refers to squinting of one or both of the eyes due to discomfort.  
  • Ocular and nasal discharge. Discharge from the eyes and nose that can be transparent or colored (typically green). If significant, crust may form around the nostrils and eyelids.  
  • Sneezing. It can be almost continuous or infrequent, and they will typically expel mucus throughout your home! 
  • Fever. Some cats will have a higher temperature than normal as they fight the infection. 
  • Inappetence. A reduced or complete loss of appetite.  
  • Lethargy. Duller and more subdued than usual.  
  • Voice changes. Their "meow" and "purr" may sound more hoarse, raspy, or quieter than normal.  
  • Difficulty swallowing. Due to a sore throat, swallowing may be exaggerated. 
  • Retching and gagging. May occur if food or nose discharge (dripping backward) touches their sensitive throat.

What Is the Conjunctiva?
What is the conjunctiva in a cat?

What Are Chemosis and Conjunctivitis?
What Is Conjunctivitis and Chemosis in a Cat?

Causes of Cat Flu

Cat flu is mainly caused by two viruses: feline herpesvirus and feline calicivirus. Less commonly, cat flu is caused by a bacterial infection from Chlamydia felisMycoplasma felis, or Bordetella bronchiseptica. It is also possible for cats to have more than one upper respiratory tract infection simultaneously and develop secondary bacterial infections.

Viral Causes of Cat Flu

The two viral causes of cat flu are feline herpesvirus (FHV) and feline calicivirus (FCV).

Most cats infected with herpesvirus remain lifelong carriers. Infections become latent (dormant), and shedding (release of virus from the body) is intermittently reactivated by stress. Cats infected with calicivirus continue to shed the virus after symptoms have resolved, with the amount gradually decreasing over weeks to months until it has completely cleared, although some remain lifelong carriers.

Herpesvirus can cause corneal (eye) ulcers, whereas calicivirus can cause ulcers on the tongue, lips, and hard palate.

Feline Herpesvirus

  • Over 90% of cats have been exposed to feline herpesvirus from their mother or other cats and kittens they encounter.
  • Upper respiratory symptoms can be mild to severe.
  • Drooling can be one of the first signs of infection.
  • Corneal ulcers can occur, which are open sores on the transparent front part of the eye.
  • Corneal ulcers can deepen due to an infection with secondary bacteria, leading to significant inflammation and even perforation of the eyeball.
  • If treatment is delayed, symblepharons can form, which are adhesions between the inside of the eyelid, third eyelid, sclera (white part of the eye), and cornea (transparent part of the eye). 
  • Rarely, ulcerated, crusty skin lesions form around their eyes and nose, and also the forelimbs if they have been rubbing their face. 
  • More than 80% of exposed cats become carriers of the virus for the rest of their lives. They develop a latent infection, meaning the virus hides in certain parts of their nervous system. During latency, they do not show any signs of cat flu. However, it can be intermittently reactivated by stress, causing shedding (release of virus from their body), which means they will be infectious to other cats and may experience a flare-up of symptoms. 
  • During a flare-up of herpesvirus in an adult cat, often only their eyes are affected, and usually just one. Often you'll see squinting and tearing caused by conjunctivitis, inflammation of the cornea, or corneal ulcers. In addition, they may have mild sneezing but often have no other cat flu symptoms.

 Corneal diseases in cats associated with feline herpesvirus
The different types of corneal diseases associated with feline herpesvirus

Feline Herpesvirus Dermatitis
Rarely, feline herpesvirus causes ulcerative skin disease, predominantly affecting the face

Feline Calicivirus

  • Flu symptoms can vary from mild to moderate and are typically less severe than with feline herpesvirus infection.
  • Ulcers (open sores) on the tongue, lips, and hard palate (roof of the mouth) sometimes occur, which are painful and can cause drooling. 
  • Rarely, pneumonia (lung infections) or limping is reported. 
  • Once symptoms have resolved, cats continue to shed the virus and remain a source of infection to other cats. Shedding gradually decreases over several weeks to months. Most cats eventually clear the virus, but some will remain long-term carriers and continue to shed the virus for the rest of their lives. 
  • Long-term carriers usually don't have a relapse of cat flu symptoms.
  • Carriers may be at an increased risk of developing feline chronic gingivostomatitis, which is challenging to manage and characterized by severe inflammation in the mouth. 
  • After an infection, a cat will develop natural immunity, but since calicivirus readily mutates, reinfection is possible with different strains.   
  • There have been rare outbreaks of virulent systemic feline calicivirus, a mutated virus that is highly contagious and often fatal. Symptoms are severe and include facial and limb edema (swelling), sloughing of the skin, pneumonia, and multi-organ failure.

Tongue ulcers due to feline calicivirus
Painful tongue ulcers caused by an infection with feline calicivirus

Feline Chronic Gingivostomatitis
There is a link between calicivirus and chronic gingivostomatitis, a condition characterized by inflammation throughout the mouth due to an exaggerated immune response to plaque

Bacterial Causes of Cat Flu

The bacterial causes of cat flu are mycoplasma, chlamydia, and bordetella.

Mycoplasma and chlamydia mainly cause conjunctivitis. Bordetella may cause mild flu symptoms, including a cough in some cases.

Other types of bacteria, such as staph and strep, can also invade once a primary infection has caused damage.

Chlamydia Felis

  • Mainly the eyes are affected, causing conjunctivitis (inflammation of the conjunctiva) and chemosis (swelling of the conjunctiva). 
  • There may be discharge from the nose, sneezing, fever, and lethargy, but these symptoms are not common and, if they do occur, are mild. 
  • After symptoms have resolved, some cats can remain carriers for several months and are a potential source of infection.
  • In some cases, conjunctivitis, affecting one or both eyes, will improve or completely resolve with antibiotic eye drops, only to reoccur later. A long course of oral antibiotics (typically doxycycline for 3-4 weeks) is required to clear the infection completely. Some vets will also treat all in-contact cats, as they may be asymptomatic carriers (carrying and shedding the bacteria but not showing symptoms). 

Bordetella Bronchiseptica

  • In adult cats, it typically causes mild or no symptoms. However, young kittens are more at risk of developing pneumonia (a lung infection). 
  • Coughing, sneezing, fever, lethargy, and nose or eye discharge may occur. 
  • Some cats can remain carriers for several months and are a potential source of infection.
  • It causes kennel cough in dogs and can spread between dogs and cats.  

Mycoplasma Felis

  • It's part of the normal flora, and most likely, an overgrowth occurs when another respiratory infection is present or when the immune system is suppressed. 
  • It mainly causes conjunctivitis, although some sneezing, coughing, nasal discharge, and lethargy may occur. 

Secondary Bacterial Infections

  • The damage caused by a primary upper respiratory infection can allow secondary bacteria to invade.
  • Secondary bacteria infections include Pasteurella, Staphylococcus, E.coli, Streptococcus, and Pseudomonas.
  • They worsen symptoms and delay recovery. 
  • These organisms are suspected when the clear nose and eye discharge associated with a viral infection turns green.  

How Is Cat Flu Diagnosed?

A veterinarian often makes a diagnosis based on the history and physical exam findings. A PCR test can be performed by swabbing the eyes and back of the throat to check for Bordetella bronchiseptica, Chlamydophila felis, calicivirus, herpesvirus, and Mycoplasma felis. 

However, PCR results can be challenging to interpret as they may detect inactivated virus particles used in recent modified-live vaccinations, recently used antivirals or antibiotics may cause organisms to become undetectable, yet they may still be present, and herpesvirus sheds intermittently, so may be missed. Testing will depend on the individual case, finances, and if it will affect the treatment given. It should ideally be performed when a cat is showing active signs of infection.  

How Is Cat Flu Transmitted?

Cat flu is highly contagious and easily spreads between cats. Infectious organisms are shed (released from the body) in secretions from the nose, eyes, or saliva. Transmission can occur via:

  • Direct contact: close interactions with an infected cat that is either showing signs of cat flu or a carrier that's shedding infectious organisms but not showing symptoms.
  • Indirect contact: sharing objects contaminated with an infectious organism, such as toys, food bowls, bedding, or even via people's hands and clothing.   
  • Sneeze droplets: when a cat sneezes, droplets containing the virus can travel up to 4-5 feet.

How is cat flu spread

A cat starts showing signs (the incubation period) 2-10 days after infection. Calicivirus survives in the environment for around 1-4 weeks, whereas herpesvirus survives for less than 24 hours. Most disinfectants kill herpesvirus, whereas calicivirus is more resistant. Dilute bleach (1 part bleach to 32 parts water) is effective against calicivirus but must not come into direct contact with your cat.  

Owners often ask if cat flu is contagious to humans. The viral causes of cat flu, herpesvirus, and calicivirus, are cat-specific and not contagious to humans.

However, there have been (very) rare case reports of people becoming infected with the bacterial causes of cat flu, Bordetella bronchiseptica,1 Chlamydophila felis,2 and Mycoplasma felis.3 Therefore, it's advisable not to rub a cat's infected eye and then rub your own and to wash your hands after handling a sick cat. Consult your doctor if you have a compromised immune system.

Cats and dogs can transmit Bordetella bronchiseptica to each other, known as kennel cough in dogs. While adult cats often have mild or no symptoms, the illness can be more severe in kittens as they can develop pneumonia (a lung infection). Therefore, it is best to keep pets separate if an infection is suspected.

Cat Flu Prevention

A combination vaccine is given for herpesvirus and calicivirus prevention. Protocols will vary between clinics and countries, but typically, a first dose will be given at 8-9 weeks, then repeated every 3-4 weeks until they are 16 weeks of age. An additional vaccine is given one year later, and then a booster vaccination every 1-3 years, depending on their lifestyle.

Vaccinations may not always prevent infection, but symptoms are typically milder if they become unwell. Also, a kitten may already have been exposed but not yet symptomatic prior to starting vaccinations.

Vaccinations are available for Bordetella bronchiseptica and Chlamydophila felis, but they are usually used to help control an outbreak in a group of housed cats. 

When Should You Take Your Sick Cat to the Vet?

  • Even if signs are mild, always contact your vet for advice 
  • Don't delay seeking treatment as signs may worsen and can complicate recovery
  • If a cat has not eaten in 24 hours, that's a clear indication to contact your vet  
  • If symptoms are worsening or not improving, then contact your vet 
  • Always work closely with your veterinary team for the best outcome for your cat

Veterinary Treatment and Medication

Treatment for an upper respiratory infection depends on the severity and duration of the symptoms, whether or not secondary complications are present, and if a bacterial or viral cause is suspected. For example, an adult cat with mild symptoms due to a virus may need minimal treatment. In contrast, a sick kitten or an adult cat with more severe symptoms may require hospitalization and more intensive treatment. Only a vet can determine what treatment is appropriate.  

Treatment is mainly supportive and may include the following: 

  • Antibiotics. May be used if a bacterial infection is suspected (chlamydia, mycoplasma, or bordetella) or if the infection is viral but secondary bacterial infections have likely occurred. Antibiotics can be oral (tablets or liquid) or topical (applied to the eye). Potential indications include a mucopurulent (colored) discharge, lethargy, a reduced or absent appetite, fever, or if signs are worsening or not improving.
  • Antivirals. Often used for more severe or recurring cases of herpesvirus and can be given orally (Famciclovir) or topically (Ganciclovir, Cidofovir, or Idoxuridine). While they slow viral replication, they will not cure a lifelong latent infection.
  • Eye lubrication. Herpesvirus reduces the quality of the tear film, so lubrication can be applied to improve comfort. 
  • Oral anti-inflammatories and pain-relief. Ulcers in their mouth and corneal ulcers hurt and often require pain-relief medication.
  • Saline nasal drops. They help to loosen secretions in the nose, making them easier to expel. 
  • Appetite stimulants. They may be beneficial to boost their appetite if they are not eating. 
  • Fluids. If a cat is mildly dehydrated, fluids may be given subcutaneously (under the skin). If they are moderately to severely dehydrated, hospitalization may be necessary for intravenous fluid therapy (directly into their vein).  
  • Feeding tube. May be required if they are not eating enough because they feel unwell or they have an ulcerated mouth due to calicivirus and eating is too painful.

Home Care

Diligent nursing care at home is essential to supporting your cat while they recover. Keeping them hydrated, eating, clean, warm, and comfortable is important, as is reducing their stress. For further information, read How to Care for a Cat with Flu at Home

Discover the Best Ways to Nurse a Sick Cat With Flu at Home

Sick cats should be isolated until they have recovered if you have more than one cat. Ensure you wash your hands thoroughly after handling them and keep their litter trays, bedding, bowls, and toys separate. However, they will likely continue to shed the virus even after symptoms have resolved. Therefore, ensure the cats in your household stay up-to-date with vaccinations to help minimize the frequency and severity of cat flu.

Isolation may not be necessary if one of your adult cats has a herpesvirus flare-up and your cats have been living together for a while. Your healthy cats are likely already exposed, and isolating your sick cat may cause stress and worsen signs. Always speak to your veterinarian for advice.

How Long Does It Take for a Cat to Recover?

Most uncomplicated cases of cat flu recover within 7-10 days with appropriate treatment and supportive care. However, recovery may take up to 14 days if they are more severely affected.

Prognosis and Long-Term Effects

Although most cats recover from the flu, it can be fatal in young kittens or adults with weakened immune systems. Complicating factors include:

  • Having more than one virus or bacteria
  • Having an additional disease, such as FIV
  • Delayed treatment

Herpesvirus has the potential to cause severe damage to the eyes. Some kittens may be left with lifelong defects, especially if symblepharons form or they have a deep or ruptured corneal ulcer. 

At least 80% of cats exposed to feline herpesvirus never clear the virus and develop a lifelong latent infection, where it remains in the body but is not active. The virus can be intermittently reactivated, known as recrudescence, triggered about one week after a stressful event. Once reactivated, they will shed virus, become infectious to other cats, and may or may not have symptoms. Potential stressors include:

  • An illness 
  • Giving birth and lactating
  • A new pet 
  • If they or their owners travel
  • Going to a cattery
  • A new cat in the neighborhood
  • Guests staying at the home
  • Tension between two cats in the home
  • A lack of resources (litter boxes, scratchers) in the home
  • A lack of enrichment, activity, and play
  • Improper handling
  • Construction work 
  • Any change they don't like!  

Herpesvirus reactivation can also be triggered by medications that dampen down the immune system, such as corticosteroids or chemotherapy. 

During a herpesvirus flare-up, they may have very mild signs of cat flu, such as sneezing, or the only sign they may show is a squinty, watery eye. Both eyes can be affected, but often only one, and it may be the same eye each time. In mild cases, symptoms often resolve within 24-48 hours. 

Feline herpesvirus flare-up in an adult cat - squinty, watery eye
A typical feline herpesvirus flare-up in an adult cat, with one squinty, watery eye

However, eye disease can also become severe in adult cats affected by herpesvirus. Some develop painful corneal ulcers, which can start to 'melt' if they become infected. Less commonly, they develop stromal keratitis, when the cornea reacts and becomes inflamed in response to virus particles within it.

Herpesvirus can also be very destructive to the turbinates (bony structures inside the nose) and the mucous membranes that line them. As a result, some cats can develop chronic post-viral bacterial rhinitis, a condition in which bacteria invade their damaged nasal passages and cause ongoing nasal discharge and congestion. Often these cats are referred to as 'chronic snufflers'.

Advice for Managing a Herpesvirus Flare-up in an Adult Cat

As discussed above, many cats become lifelong carriers of herpesvirus, and some experience flare-ups, often triggered by stress, which usually manifest as conjunctivitis, although sneezing may occur. A mild squinty, watery eye (clear discharge) during a herpesvirus flare-up, with no redness or swelling, may resolve within 1-2 days. However, eye problems can also become severe during flare-ups, for example, corneal ulcers or significant corneal inflammation can develop, which can deteriorate quickly if not managed correctly. 

If your cat has eye issues, always consult your veterinarian to assess if any medications are required, such as antivirals or antibiotics, and check for other underlying health issues. 

To manage a herpesvirus flare-up in an adult identifying and reducing potential stressors is vital. They should also be fed a complete, high-quality diet to support their immune system. In addition, keeping up-to-date with vaccinations may help reduce the frequency and intensity of flare-ups.

Gentle wipes, such as WaterWipes, should be used around the eyes to keep them clean and remove any gunk that has built up in the inner corners. A study found that herpesvirus reduces the quality and stability of the tear film, the thin layer on the eye's surface that provides hydration and protection.4 Therefore, applying lubricating eye drops, 1 drop every 4-6 hours, during a flare-up will help stabilize the tear film and improve eye comfort and health. Eye lubrication should contain hyaluronic acid, such as OptixCare Eye Lube Plus

OptixCare + Hyaluron

OptixCare + Hyaluron

  • Eye lubrication containing hyaluron (hyaluronic acid)
  • Nourish and stabilize the tear film during a herpesvirus flare-up
  • Provides long-lasting comfort for irritated eyes

L-lysine, an over-the-counter supplement, may reduce the frequency and intensity of herpesvirus flare-ups. For example, a study looking at cats with lifelong herpesvirus infections found they had reduced viral shedding following a stressful situation when given lysine.5 In another study, cats receiving lysine had less severe conjunctivitis than those on a placebo.6

Lysine can be given only when they start to show signs of cat flu, when a stressful event is anticipated, or long-term if they have frequent flare-ups. Adult cats receive 500 mg (per cat) twice daily, and kittens receive 250 mg (per kitten) twice daily.

Vetoquinol Viralys L-Lysine Supplement

Vetoquinol Viralys L-Lysine Supplement

  • May help reduce the frequency and severity of feline herpesvirus flare-ups
  • The majority of cats don't notice the powder mixed in with wet food or sprinkled over dry
  • Manufactured in the USA and according to rigorous pharmaceutical standards

Another useful supplement is the probiotic, Fortiflora, which was shown in a study to lessen symptoms associated with chronic herpesvirus infections in some cats.7 It is thought to strengthen their immune system and can be given long-term.

Purina FortiFlora for Cats

FortiFlora for Cats

  • Probiotics powder supplement
  • Helps boost the immune system and supports intestinal health
  • Has a tasty meaty flavor and can be mixed with food

In summary, if your cat keeps having herpesvirus flare-ups: 

  • Take them to the vet's for a health check 
  • Reduce stress
  • Feed them a high-quality, complete diet
  • Keep their eye(s) clean with WaterWipes and apply OptixCare Lubricating Eye Drops 
  • Administer Fortiflora to support their immune system 
  • Administer L-lysine to reduce the severity and frequency of flare-ups 
  • Keep up-to-date with vaccinations 

Long-Term Eye Issues Can Also Be Due to Chlamydia

Cats can also sometimes get recurring conjunctivitis due to chlamydia, which, unlike herpesvirus, does not cause corneal ulcers (open sores on the surface of the eye) or keratitis (inflammation of the cornea). Typically there is more swelling of the conjunctiva (chemosis) with chlamydia than with herpesvirus. One or both eyes can be affected, and they often improve while on medicated eye drops, only for the signs to return later. To completely clear the infection, a long course of oral antibiotics (typically doxycycline for 3-4 weeks) is required, as well as treatment of in-contact cats since they can be carrying and shedding chlamydia without showing signs.

PCR testing can be performed to check for chlamydia and herpesvirus. However, due to intermittent shedding, herpesvirus is not always detected.

Other Causes of Discharge From the Eyes or Nose

Always take your cat to the vet if they are unwell to establish a diagnosis. Other causes of nasal discharge or congestion include:

  • Fungal infections
  • Nasal tumors
  • Nasopharyngeal stenosis
  • Polyps
  • Foreign bodies
  • Tooth root abscesses
  • Chronic post-viral bacterial rhinitis
  • Allergies
  • Irritants

Causes of a red, painful, or watery eye are not limited to but include:  

  • Corneal ulcers
  • Trauma and scratches
  • Foreign bodies
  • Uveitis
  • Glaucoma
  • Congenital diseases (abnormal eyelids or eyelashes)
  • Blocked nasolacrimal ducts
  • Allergies
  • Irritants


  1. Redelman-Sidi G, Grommes C, Papanicolaou G. Kitten-transmitted Bordetella bronchiseptica infection in a patient receiving temozolomide for glioblastoma. J Neurooncol. 2011 Apr;102(2):335-9.
  2. Wons, J., Meiller, R., Bergua, A., Bogdan, C., & Geißdörfer, W. (2017). Follicular Conjunctivitis due to Chlamydia felis—Case Report. Frontiers in Medicine, 4, 105.
  3. Bonilla HF, Chenoweth CE, Tully JG, Blythe LK, Robertson JA, Ognenovski VM, Kauffman CA. Mycoplasma felis septic arthritis in a patient with hypogammaglobulinemia. Clin Infect Dis. 1997 Feb;24(2):222-5.
  4. Lim, C.C. et al. (2009) ‘Effects of feline herpesvirus type 1 on tear film break-up time, Schirmer tear test results, and conjunctival goblet cell density in experimentally infected cats’, American Journal of Veterinary Research, 70(3), pp. 394–403.
  5. Maggs, D.J., Nasisse, M.P. and Kass, P.H. (2003) ‘Efficacy of oral supplementation with L-lysine in cats latently infected with feline herpesvirus’, American Journal of Veterinary Research, 64(1), pp. 37–42.
  6. Stiles, J. et al. (2002) ‘Effect of oral administration of L-lysine on conjunctivitis caused by feline herpesvirus in cats’, American Journal of Veterinary Research, 63(1), pp. 99–103.
  7. Lappin, M.R. et al. (2009) ‘Pilot study to evaluate the effect of oral supplementation of enterococcus faecium SF68 on cats with latent feline herpesvirus 1’, Journal of Feline Medicine and Surgery, 11(8), pp. 650–654.

Catastrophic Creations