Infection of Cats with H5N1 Avian Influenza Virus
World Small Animal Veterinary Association World Congress Proceedings, 2006
Marian C. Horzinek1; Etienne Thiry2
1Professor Emeritis, Veterinary Research Consult, Bithoven, The Netherlands; 2Professor, Université de Liège, Faculté de Médicine Vétérinaire, Liège, Belgium

This document has been compiled by Etienne Thiry (Liege/B), with the assistance of Diane Addie (Glasgow/UK), Herman Egberink (Utrecht/NL), Katrin Hartmann (Munich/D), Hans Lutz (Zurich/CH) and Hervé Poulet (Lyon/F). It is published under the authority of the Advisory Board on Cat Diseases (ABCD), a committee dedicated to developing and issuing guidelines for the prevention and management of feline infectious disease. The initiative is supported by animal health care company Merial.


The H5N1 subtype of Avian Influenza Virus type A, a member of the Orthomyxoviridae family, occurs primarily in birds. Transmission to mammals happens sporadically, and the infection then may cause disease with a high morbidity and a high number of deaths among ill animals. Humans, primates, rodents, lagomorphs, mustelids and felids, including the domestic cat may be infected and may succumb to the disease. A listing of susceptible species is given at

Infection of Cats

Felids can be naturally and experimentally infected with H5N1 virus. In February 2004, infection of household cats was reported from Thailand (WHO, 2004); also from that country, two outbreaks of fatal disease in tigers and leopards have been published (Keawcharoen et al., 2004; Thanawongnuwech et al., 2005). In February/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 2006, three cats were found infected but alive in an animal shelter in Graz, Austria. First experimental evidence for the pathogenicity of H5N1 avian influenza virus for the domestic cat was obtained by Kuiken et al. (2004).

A statement from the WHO (28th February 2006) reads: "There is no present evidence that domestic cats play a role in the transmission cycle of H5N1 viruses. To date, no human case has been linked to exposure to a diseased cat. No outbreaks in domestic cats have been reported. Unlike the case in domestic and wild birds, there is no evidence that domestic cats are a reservoir of the virus. All available evidence indicates that cat infections occur in association with H5N1 outbreaks in domestic or wild birds."

The following data have been obtained from experimental infections (Kuiken et al., 2004; Rimmelzwaan et al., 2006); 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 the intratracheal and oral routes, and by feeding them infected chickens.

 Infection can occur through contact with infected birds.

 Infected cats can transmit the virus to in-contact cats.

 Moderate amounts of virus are sufficient to infect a cat.

 The virus is shed with nasal secretions and in faeces; nasal excretion starts 3 days after infection and continues for 4 days or longer.

 The incubation period in experimental infections is about 2 days

 Clinical signs are fever, lethargy/depression, dyspnoea, conjunctivitis; when clinical signs occur, the outcome of the disease is mostly fatal within one week. Also icterus has been observed.

 At necropsy, multifocal lung lesions and petechial haemorrhages in the tonsils, mandibular and retropharyngeal lymph nodes, and the liver are seen.

 Histologically, inflammatory and necrotic lesions are seen in the lungs, heart, brain, kidneys, liver and adrenal glands. Lesions in the small intestine are observed in cats that had been fed infected chickens.

Risk Considerations--Questions and Answers

1. How may a cat become infected after contacts with birds or poultry products?

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 lives in an environment where aquatic birds are present.

 The cat has access to outdoors.

 The cat has contact with free ranging or indoor poultry.

 The cat has been fed uncooked poultry meat.

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

 Close contact with an H5N1 infected, sick cat is required at least during the first seven days of infection; although inapparent infection might occur for a limited period, persistent H5N1 virus infections have not been reported.

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

To date (March 2006), virus transmission from a cat to a person has not been reported. However:

 A H5N1 virus which had infected a cat is already adapted to a mammalian species; viruses isolated from humans exhibited increased virulence for mammals (Maines et al., 2005).

 This virus is excreted by the respiratory route 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 infected cat can probably infect a human.

 The risk of infection and disease for humans can presently not be estimated.

4. When should a veterinary practitioner suspect an ill cat to be infected with H5N1 virus?

 Before expressing a suspicion, the potential risk must be evaluated according to the answers to the anamnestic questions above.

 If a risk is assumed, a clinical assessment must be performed and clinical signs as given above should be observed: fever, lethargy/depression, dyspnoea, conjunctivitis, rapid death; also neurological signs have been reported.

 The differential diagnosis should exclude other infections leading to similar systemic and respiratory signs, as caused by feline herpesvirus and calicivirus and by bacteria (Bordetella bronchiseptica, Chlamydophila felis, also Mycoplasma).

 Clinical signs can only result in a probability diagnosis, which must be confirmed by laboratory testing.

5. How should samples for laboratory diagnosis be taken and handled?

The authorities must be notified as specified by the national regulations and the diagnostic laboratory is contacted for detailed instructions. There are some general rules:

To take oropharyngeal, nasal, and rectal swabs or faecal samples:

 Plastic tubes should be labeled using an alcohol-proof ink marker.

 The samples are transferred to the tubes, which are tightly closed.

 The outside of the tubes is swabbed with ethyl alcohol to reduce the risk of infection for the receiving personal.

 The material safely enclosed in plastic bags is shipped 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 must be taken by the veterinary practitioner when a case of H5N1 virus infection in a cat is suspected?

For own protection:

 Physical contact with the cat must be minimized, scratching and biting avoided.

 Gloves, mask and goggles (protective eyewear) should be worn when the cat is manipulated.

 Sedation of the cat is recommended before taking samples.

 For surface decontamination, a standard medical disinfectant is used.

For protection of attending personnel and other animals:

 In the veterinary clinic, the suspected cat is kept in isolation in a cage.

For protection of owners and relatives:

 In the owner's house, the cat has to be kept in a separate room.

 Physical contact with the cat must be minimized, scratching and biting avoided.

 Litter trays, bowls, baskets and other potentially contaminated objects must be disinfected using a hypochloride solution (bleach).

 Rooms where the cat had access before the visit to the veterinarian should be thoroughly cleaned using a household detergent (which is expected to inactivate influenza virus).

7. What can owners do to minimize the risk of an H5N1 avian influenza virus infection?

 The development of the epidemic must be followed in the national and local media.

 Feeding of uncooked poultry meat to cats must be avoided.

 If many deaths occur amongst wild birds, cats should be kept indoors until further information about the cause is available.

Relevant Web Sites

 World Health Organisation:

 World Organisation for Animal Health:

 European Commission, Animal Health and Welfare:


1.  Keawcharoen et al., Avian influenza H5N1 in tigers and leopards. Emerg. Infect. Dis., 2004, 10, 2189-2191.

2.  Kuiken et al. Avian H5N1 influenza in cats. Science, 2004, 306, 241.

3.  Maines et al. Avian influenza (H5N1) viruses isolated from humans in Asia in 2004 exhibit increased virulence in mammals. J. Virol., 2005, 79, 11788-11800.

4.  Rimmelzwaan et al. Influenza A virus (H5N1) infection in cats causes systemic disease with potential novel routes of virus spread within and between hosts. Am. J. Pathol., 2006, 168, 176-183.

5.  Thanawongnuwech et al., Probable tiger-to-tiger transmission of avian influenza H5N1. Emerg. Infect. Dis., 2005, 11, 699-701.

6.  WHO, Avian influenza A (H5N1)--update 28: reports of infection in domestic cats, 20 February 2004.

Speaker Information
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Marian C. Horzinek
Veterinary Research Consult
Bithoven, The Netherlands

Etienne Thiry
Université de Liège
Faculté de Médicine Vétérinaire
Liège, Liège, Belgium