Influenza virus infection in cats

Edited October 18, 2015



This is an updated version of the ABCD recommendations on a practical approach to infected and suspected cats. Earlier guidelines were published in J Feline Med Surg 2009; 11: 615-618 (Thiry et al., 2009) and updated in J Feline Med Surg 2013; 15: 537 (Horzinek et al., 2013); the present update has been compiled by Etienne Thiry.


Cats were recently found to be susceptible to the H3N2 and H5N2 influenza viruses, the agents of canine influenza in Asia, the former leading to morbidity and mortality in cats, but only to mild clinical signs for the latter (Feng Hai-xia et al., 2014; Na Lei et al., 2012). Experimental infection with the recent H5N8 influenza A virus was successful, but remained subclinical (Kim et al., 2014).






  • Cats are susceptible to influenza A viruses and can develop disease, sometimes fatal
  • Epidemic spread amongst cats has not been observed, waterfowl is the source of feline infections; however, infected cats can transmit the virus to in-contact cats
  • Risk factors include access to outdoors, particularly to an environment where waterfowl or poultry are present and feeding of uncooked poultry meat
  • Virus transmission from a cat to a person has so far not been reported
  • Fever, lethargy/depression, dyspnoea, conjunctivitis, pneumonia, neurological signs and rapid death in cats should alert the veterinarian and make him confirm a suspicion by laboratory testing.



08 Flu JFMS Fig.1


Infection of cats


Felids can be naturally and experimentally infected with influenza A viruses with different outcomes ranging from subclinical infection to fatal disease. Early observations reported successful but subclinical infection of cats after experimental exposure to the human influenza H3N2 virus responsible for seasonal flu (Hinshaw et al., 1981). Although the susceptibility of cats to influenza A viruses was already known, it became obvious with the evidence of the virulence of H5N1 highly pathogenic avian influenza virus for the domestic cat in 2004 (Kuiken et al., 2004). At the same period, infection of household cats and outbreaks of fatal disease in tigers and leopards were reported from Thailand (Keawcharoen et al., 2004).

In late February/early March 2006, three cats were found dead on the island of Rügen, Germany, and infection with H5N1 virus was established by laboratory tests. Also in March, three cats were found infected but alive in an animal shelter in Graz, Austria (Leschnik et al., 2007). The susceptibility of cat to influenza A viruses was reinforced during the pandemic of influenza A H1N1 in the human population in 2009/2010. Indeed, several cases of respiratory diseases in domestic cats were associated with an infection with the pandemic H1N1 virus strongly suggesting its role in the disease (Sponseller et al., 2010). Nevertheless, neither H5N1 nor H1N1 virus infection spread among domestic cats in Europe (Marschall et al., 2008; Pingret et al., 2010).

The reservoir of all influenza A viruses, members of the Orthomyxoviridae family, is waterfowl. From these birds, viruses spill over to various avian and mammal species (Fig. 1) and eventually adapt themselves to new hosts. In the case of H5N1 and H1N1 viruses, transmission to mammals happens sporadically and may drive to an established infection like H1N1 in humans and pigs. The infection then may cause disease with a high morbidity and a high number of deaths among ill individuals. Humans, mustelids, canids, and felids, including domestic cat, ferret, and dog may be infected and may succumb to the disease (Songserm et al., 2006; Thiry et al., 2007) (A listing of susceptible species is given at :



What do we know?


The following data have been obtained from experimental infections with H5N1 virus (Kuiken et al., 2004; Rimmelzwaan et al., 2006; Van Riel et al., 2006) and observations from field cases of H5N1 and H1N1 infections (Klopfleisch et al., 2007; Leschnik et al., 2007) and clinical observations with the pandemic H1N1 virus (Sponseller et al., 2010; unpublished observations). They reflect the current state of knowledge and will have to be revised and expanded, as additional information becomes available:

  • cats can be infected via intratracheal and oral routes, and, for H5N1 virus by feeding on infected chickens;
  • infection can occur through contact with infected birds for H5N1, and likely with infected owners for H1N1;
  • H5N1 virus can be indirectly transmitted through passive carriage of infected bird faeces in cat’s hair coat;
  • infected cats can transmit the virus to in-contact cats; this is verified for H5N1 and suspected for H1N1;
  • moderate amounts of virus are sufficient to infect a cat;
  • the H5N1 virus is shed with nasal secretions and in faeces; nasal excretion starts 3 days after infection and continues for 4 days or more, but is always limited in duration;
  • the incubation period in experimental H5N1 infections is about 2 days;
  • clinical signs of influenza A virus infection of cats are fever, lethargy/depression, dyspnoea, sero-sanguineous nasal discharge and conjunctivitis. In addition, in H5N1 infection, protrusion of the nictitating membrane, neurological signs like convulsions
  • and ataxia as well as icterus are also observed. When clinical signs occur, the outcome of the disease can be fatal as early as 2 days up to one week after the onset of clinical signs in the case of H5N1 infection;
  • at necropsy, multifocal lung lesions are observed. In the case of H5N1 infection, petechial haemorrhages in the tonsils, mandibular and retropharyngeal lymph nodes, and the liver are also seen;
  • at histology, inflammatory and necrotic lesions are found in the lungs in both virus infections. In H5N1 infected cats, heart, brain, kidneys, liver, and adrenal glands also show the same lesions. Lesions in the small intestine (ganglio-neuritis of the
  • intestinal plexus) are observed in cats that had been fed H5N1-infected chickens. Non-suppurative encephalitis is also observed after H5N1 infection.
  • Subclinical infection does occur in cats, mainly through low infectious dose.




08 Flu JFMS Fig.2




Risk considerations: questions and answers


1. How can cats become infected?


With H5N1 virus:


Precondition is that the cat lives in a region where one or more H5N1 virus-infected birds have been identified by laboratory tests; if this condition applies, the following risk factors must be considered:
  • the cat has access to outdoors;
  • the cat lives in an environment where waterfowl are present;
  • the cat has contact with free-range or indoor poultry;
  • the cat has been fed uncooked poultry meat;
  • the cat has been in close contact with an H5N1-infected cat.



With H1N1 virus:
The cat lives in a household where human cases of influenza are identified or highly suspected. A close contact with an H1N1 infected cat is also a risk factor.

2. How can a cat be infected by another cat?

Close contact with an H5N1- or H1N1-infected sick cat during the first seven days of infection; although inapparent infection might occur for a limited period, persistent influenza virus infections have not been reported. Passive carriage of H5N1 virus by bird’s faeces in the cat’s hair coat is also possible.



3. How may a cat transmit the infection to a person?

To date (October 2011), no virus transmission from a cat to a person has been reported.
However, it should be remembered that:
  • a H5N1 virus which has infected a cat is already adapted to a mammalian species; H5N1 viruses isolated from humans exhibited increased virulence for mammals (Maines et al., 2005);
  • a H1N1 virus that infects a cat has a human origin and is therefore already adapted to humans;
  • the virus is excreted by the oral and respiratory route (saliva, nasal secretions), and for H5N1 virus, by the urines and in the faeces;
  • the level of excretion is high enough to allow in-contact cats to become infected;
  • in view of the habitual close contacts between cats and their owners, an influenza A virus-infected cat can probably infect a human being;
  • the risk of infection and disease for human beings can presently not be predicted.



4. When should veterinarians suspect an ill cat to be infected with influenza A virus?

Before expressing a suspicion, the potential risk must be evaluated according to the case history and circumstances (see under point 1.);
  • if a risk is assumed, a clinical assessment should be carried out and the following signs should be expected: fever, lethargy/depression, dyspnoea, conjunctivitis, pneumonia and rapid death; neurological signs have also been reported but only with H5N1 virus;
  • The differential diagnosis should exclude other infections leading to similar systemic and respiratory signs, including infections with feline herpesvirus, feline calicivirus, and Bordetella bronchiseptica;
  • clinical signs alone are not diagnostic; diagnosis should always be confirmed by laboratory testing.



5. How should samples for laboratory diagnosis be handled?

Some general rules apply whenever influenza A virus infection is suspected. In case of a suspected H5N1 infection, the authorities should be notified according to the national regulations. The diagnostic laboratory should be contacted for detailed instructions.
To take oropharyngeal, tracheal or nasal swabs, follow the steps below:
  • Label plastic sample tubes using an alcohol-proof ink marker;
  • Transfer the samples to the tubes, immersed in virus transport medium or saline solution (not bacterial transport medium); close tightly;
  • Swab the outside of the tubes with alcohol to reduce the risk of infection for the laboratory staff;
  • Ship the sample tube safely enclosed in plastic bags to the diagnostic laboratory or, in case of suspicion of H5N1, to the national reference laboratory according to procedures defined by the authorities;

Post mortem samples of lung and mediastinal lymph nodes should be kept and shipped in 10% formol saline.

It is not recommended to perform an in-house influenza detection test.



6. Which measures should the veterinary practitioner take if a case of influenza A virus infection in a cat is suspected?

For his/her personal protection:
  • Minimise all physical contact with the cat, avoid scratching and biting;
  • Avoid contact with saliva and respiratory secretions, faeces and urines;
  • Wear gloves, a mask, and protective eyewear when handling the cat;
  • sedation of the cat is recommended before taking samples if the cat is not cooperative;
  • Use standard medical disinfectants for surface decontamination.


For the protection of veterinary practice staff:

  • the suspected cat should be kept in isolation in a cage at the veterinary clinic;
  • veterinary practice staff who is suspected of influenza virus infection should avoid contact with cats; in every occasion, the staff will minimise the risk of transmission to the cats by washing hands frequently and by covering coughs and sneezes.
  • For the protection of the owners and their family, and other animals of the household:
  • at the owner’s house, the cat should be confined to a separate room (before bringing it to the clinic);
  • physical contact with the cat must be minimised when owners are suspected of influenza A virus infection; owners should wash hands frequently and cover coughs and sneezes;
  • litter trays, bowls, baskets and other potentially contaminated objects should be disinfected using a hypochlorite solution (household bleach);
  • rooms to which an infected cat had access before the visit to the veterinarian should be thoroughly cleaned using a household detergent.



7. What can owners do to minimise the risk for their cat to acquire an influenza A virus infection?

  • Closely follow the development of the epidemiological situation in the national and local media;
  • Considering H5N1 infection, do not feed uncooked poultry meat to cats;
  • Considering H5N1 infection, if many deaths occur amongst wild birds, cats should be kept indoors until further information about the cause is available.
  • Considering H1N1 or another human influenza A infection, minimise the risk of virus transmission to cat by avoiding physical contact when a human infection is suspected.




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