Cat Flu

From Cat
Cat flu in a 16-week-old Devon rex kitten with bilateral conjunctivitis; case responded to broad-spectrum antibiotics (doxycycline), topical eye therapy, and lysine orally. Courtesy of Dr Jim Euclid
Catflu in a 14 week-old Devon rex kitten with sinusitis and nasal discharge; case responded to 4 x weekly Convenia injections. Courtesy Dr Jim Euclid

'Cat flu' is one of the most common viral diseases of cats and responsible for half the visits to veterinarians by cat owners[1]. Although the term is a misnomer since the influenza virus is rarely involved in the disease, cat flu is synonymous with upper respiratory illness[2]. A number of agents have been incriminated in the disease including, in order of clinical importance:

  1. Feline herpes virus 1 (FHV-1; feline rhinotracheitis virus)
  2. Feline calicivirus (FCV)
  3. Chlamydia (Chlamydophila felis)
  4. Bordetella bronchiseptica (Kennel cough)
  5. Influenza virus
  6. Miscellaneous bacteria, e.g. Moraxella spp

All species of the family Felidae are believed to be susceptible. Intestinal infections of Ancylostoma spp, Dipylidium caninum, and Toxocara cati are diagnosed more frequently in cats with upper respiratory tract disease[3].

FHV-1 and FCV are the main agents and are responsible for acute illness and may also be the cause of recurrent or chronic lesions[4]. FHV-1 has a tropism for nasal epithelium and conjunctival cells and for neurocytes. Trigeminal ganglia are the site for FHV-1 latent infection and viral reactivation during a stress period[5]. Isolation of virus in clinically healthy cats is difficult. FCV, more genetically variant than FHV, is readily isolated by nasopharyngeal swabs in infected cats, and clinical signs usually involve gingivitis and lingual ulcers.

Clinical signs

Feline herpes virus causes upper airways infection in young kittens, sneezing and bronchitis. It can also cause eye infections (conjunctivitis and eosinophilic keratitis). About half the cats presenting with respiratory disease have FHV, about one third have FCV and a few have Chlamydophila felis infection. The clinical importance of catflu is not the initial illness, which usually responds to broad-spectrum antibiotics, but the insidious long-term complications of FHV infection. Although most cats will recover from FHV and FCV infection, a significant majority will have life-long recurrence of viral infection, leading to unrelated disease such as gingivitis, chronic gastroenteritis, chronic cystitis, eosinophilic dermatitis and chronic conjunctivitis. In catteries, catflu can be problematic in that carriers can be unwittingly sold to prospective buyers who alarmingly find symptoms of catflu emerging weeks or months after purchase. Chronic cat flu is of litigious concern for many reputable breeders[6].

The incidence of FHV-1 antibody in most colony cats (breeder's homes, catteries, boarding kennels, etc) is over 70% whereas for household cats the figure is less than 50%. Feline herpesvirus 1 causes acute disease of the upper respiratory tract in the first year or so of life. After an incubation period of 24-72 hours, there is a sudden onset of bouts of sneezing, coughing, profuse serous nasal and ocular discharges, frothy salivation, dyspnoea, anorexia, weight loss and fever. Occasionally there may be ulcers on the tongue. Keratitis associated with punctate corneal ulcers is common. In fully susceptible kittens up to 4 weeks old, the extensive rhinotracheitis and an associated bronchopneumonia may be fatal. Clinically the disease caused by herpesvirus is very similar to that by calicivirus. Profuse frothy salivation and corneal ulcers suggest herpes virus infection, whereas ulcers of the tongue, palate and pharynx suggest calicivirus infections. Infections of cats over six months of age is likely to result in mild or subclinical infection. Pregnant queens may abort, although there is no evidence that the virus crosses the placenta and fatally infects foetuses, and virus has not been isolated from aborted foetuses or placenta. Abortion is thought to be secondary to fever and anorexia.

Calicivirus infection cause lameness and fever in young kittens (these can occasionally be seen after vaccination) but more commonly the kittens present with ulcers in the mouth, sneezing and runny noses. Although the viruses can be isolated by taking a nasal/oral swab and testing this at the lab, most vets normally treat symptomatically, assuming both viruses are involved.


Most kittens with flu are reluctant to eat, so nursing is the most important part of assisting with recovery. Always make sure that the food they are given is warmed (more palatable, and tasty) and that hand-feeding may be required to keep them eating. Bathing the eyes and nose with warm salty water or cold calamine tea will hep stop secondary eye infections. With kittens that have serious sinus infections, placing them in a steamed bathroom will help soften the nasal mucous.

  • Antibiotics - broad spectrum antibiotics such as Amoxil®, Antirobe® or Clavulox®. However, Vibravet® is not normally recommended because it causes yellowing of the kitten's teeth, although the drug does cause clinical improvement in symptoms while the kittens are on this medication. Zithromax has shown to resolve symptoms in kittens suffering from flu symptoms. The recommended dose is 80mg orally once or twice a week for 3-6 weeks, or until symptoms resolve.
  • Mucolytics - drugs which soften the mucous such as Mucohex® or Bisolvon® are often used in conjunction with antibiotics, but their effects are variable. Use of mucous softening therapy such as steam therapy does have some benefits anecdotally. Kittens placed in a steaming bathroom while the owner showers have shown to ease breathing in severely dyspnoeic kittens.
  • Eye ointments are commonly used when conjunctivitis is present. Broad-spectrum antibiotic drops such as Chloromycetin or Chloropt improve clinical symptoms associated with conjunctivitis. It is not recommended that eye ointment containing corticosteroids be used as this often exacerbates the infection. Zivorax®, an anti-viral eye ointment, has been used with some success, but must be applied 6-8 times a day. Kittens that are seriously ill with cat flu require intensive treatment and many that do recover are quite often left with recurring bronchitis.
  • Antiviral therapy - Lysine is commonly used at 100mg per kitten daily (500mg daily for adult cats) until symptoms resolve, which may take up to 4 weeks. NB: It is advisable not to use lysine on pregnant queens.
  • Immune stimulants - Astragalus (a herb similar to echinacea and a proven immunostimulant in humans AIDS patients) is become more popular as an alternative immun-stimulant as shows promise in improving clinical symptoms associated with cat flu. The recommended dose for cats is 75 mg orally once daily for adult cats.


In at risk cats, vaccination (using modified live vaccines) is most important to prevent this disease and it is important to ensure that Queens are fully vaccinated prior to mating to give the kittens the highest protection at birth from maternal antibodies. Although vaccination helps to reduce the risk of cat flu, this disease can still be seen in vaccinated cats.

All cats and kittens must be vaccinated against cat flu from an early age (around 6 weeks). Once cat flu is suspected in a kitten, it is strongly advised to quarantine the cat until at least a week after symptoms have disappeared. It is always advisable when selling a kitten to advise the purchaser that the kitten has had cat flu. Although mild doses of cat flu are not normally a problem, if the new owner is considering showing the cat at a cat show, the risk of spread to other cats can be a problem. Because the virus can be spread from nasal discharges and sneezing, isolating the sick cat is important - perhaps housing it in a separate room altogether. The use of disinfectants on the kitten's bowl, bedding, etc is vital. Also, washing of hands and using shoes and aprons in the kitten's room are the best way of stopping spread of the virus from one area of the house to another. Separate food bowls and litter trays must also be considered.

Commercial core vaccines contained either live attentuated or inactivated viral antigen for protection against feline calicivirus and feline herpes virus[7]. Latest vaccine protocols recommend the following core vaccine schedule:

  • Kittens - week 8, 12 and 16 (F3)
  • Adult - 12 months after last kitten vaccination
  • Triannual - every 3 years until 10 years.

Latest vaccine protocols (.pdf)

A recent study has shown that vaccination of latently FHV-infected cats does not lead to reactivation of the latent virus, nor clinical disease. Sixteen months after superinfection, the cats were temporarily immunosuppressed, and assessed for reactivation of FHV. Mild signs were observed in all cats. Interestingly, only the wild type virus was identified; there was no evidence of mutant virus reactivation. Thus, it was not clear that the mutant virus was able to establish latency. The clinical disease observed was due to the reactivation of the latent wild type strain[8].

Breeder obligations with 'cat flu' and kittens sold under 'warranty'

Herpes virus normally infects kittens prior to or immediately after birth and kittens usually shows symptoms from birth onward, although many symptoms appear at the 6-14 week age period when maternal immunity begins to wane. Contributing stressors such as weaning, rehousing, air travel, etc., usually exacerbate any subclinical disease and quite often the first symptoms of the disease occur when the kitten is sold to a new owner. The fact that the symptoms appear after sale does not exclude the breeder female cat as the origin of the infection, regardless of her immune or vaccine status. 'Carrier’ cats are a well recognised syndrome in catteries and is a problem that is difficult to eradicate.

Adequate remuneration of out-of-pocket expenses for buyers of infected kittens is a moral requirement if a diagnosis of Herpes viral infection is made by a veterinarian. This assumes that the breeder is made aware of the problem by the buyer within an acceptable period after sale (usually up to 4-6 weeks post-purchase). Most breeders sell kittens with a ‘warranty’, covering part or all of any veterinary health expenses that can be attributed to preventable infectious (particularly herpes/calicivirus virus, Chlamydia, FIP) or hereditary diseases.


  1. Thiry, E (2006) Feline viral rhinotracheitis. Clinical virology of the dog and cat. Maisons-Alfort: Edition du Point Veterinaire. pp:91-97
  3. Bech-Nielsen S et al (1980) Feline respiratory tract disease in Louisiana. Am J Vet Res 41(8):1293-1298
  4. Zicola, A et al (2009) Feline herpesvirus 1 and feline calicivirus infections in a heterogenous cat population of a rescue shelter. JFMS 11:1023-1027
  5. Gaskell, RM & Povey, RC (1982) Transmission of feline viral rhinotracheitis. Vet Rec 111:359-362
  6. Richter M, Schudel L, Tobler K et al: Clinical, virological, and immunological parameters associated with super-infection of latently with FeHV-1 infected cats, Vet Microbiol 138:205, 2009
  7. Gore TC, et al (2006) Three-year duration of immunity in cats following vaccination against feline rhinotracheitis virus, feline calicivirus, and feline panleukopenia virus. Vet Ther. 7(3):213-22
  8. Richter M, et al (2009) Clinical, virological, and immunological parameters associated with superinfection of latently with FeHV-1 infected cats. Vet Microbiol 138(3-4):205-216