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to study the infectivity of avian influenza ( h5n1 ) in dogs , we inoculated 3 spf beagles ( hsdcpb : dobe ; harlan nederland , horst , the netherlands ) , 16 weeks of age , with 10 median tissue culture infectious doses of influenza ( h5n1 ) ( a / chicken / gxla/1204/2004 ) .
half of the dose ( 0.5 ml ) was administered intranasally and the other 0.5ml intratracheally .
body temperature and health status of the dogs were monitored twice a day during the first 5 days after challenge and once a day thereafter .
excretion of virus was monitored daily in swabs from rectum , oropharynx , and nose .
the presence of influenza ( h5n1 ) virus in these swabs was studied by inoculation into embryonated eggs and by real - time reverse transcription 
, we used the forward primer ai - m - f45 ( 5-cttctaaccgaggtcgaaacgta-3 , reverse primer ai - m - r251 ( 5-cactgggcacggtgagc-3 ) and taqman probe ai - m - tqmn1 ( 5-6fam - ctcaaagccgagatcgcgcaga - xt - ph ) ( tibmolbiol , berlin , germany ) . a calibration curve consisting of serial dilutions of a standard batch of influenza ( h5n1 ) virus with a known median 50% egg infectious dose ( eid50 )
in this dog , virus was demonstrated by pcr in nasal swabs taken on days 1 through 4 after challenge and by virus isolation in embryonated chicken eggs on days 2 and 3 after challenge ( table 1 ) .
quantification by real - time pcr indicated that the amount of virus present in the nasal swabs corresponded to 2.03.2 log10 eid50 . * pcr , real - time pcr of nasal swabs from dogs ; egg , embryonated chicken eggs . ;
serum samples collected at days 7 and 14 after challenge were tested for antibodies against h5n1 subtype in an influenza a nucleoprotein - blocking elisa as well as in a hemagglutination - inhibition assay .
an antibody response against influenza ( h5n1 ) was detectable in 1 dog at day 7 after challenge . in all dogs
an antibody response against influenza ( h5n1 ) was demonstrated in both assays at day 14 after challenge ( table 2 ) . postmortem examination on day 14 after virus challenge showed no gross pathologic or histopathologic changes in the respiratory tract and other organs .
considering the time of sampling after challenge , transient histopathologic changes may have occurred unnoticed .
* antibodies were measured in a nucleoprotein - blocking elisa and in the hemagluttination - inhibition ( hi ) assay at different days postinoculation .
hi titers are presented as the reciprocal to the highest serum dilutions completely inhibiting agglutination of chicken erythrocytes by influenza ( h5n1 ) virus .
influenza ( h5n1 ) viruses bind with their hemagglutinin surface proteins to cell surface oligosaccharides terminating in sialic acid 2,3 galactose ( sa2,3gal ) ( 4 ) . in humans and cats , influenza ( h5n1 ) virus predominantly attaches to the lower part of the respiratory tract where the sa2,3gal receptors are present ( 5 ) . to study the attachment pattern of influenza ( h5n1 ) in the respiratory tract of dogs
, we performed binding experiments with labeled influenza ( h5n1 ) virus on formaldehyde - fixed and paraffin - embedded tissue sections .
we collected tissues directly from a euthanized control dog of the same breed as that used for the infection experiment and fixed them in 4% buffered formaldehyde solution for several weeks .
virus was grown in 10-day - old embryonated eggs and inactivated with formalin . after being purified by sucrose gradient centrifugation
staining was performed by incubating tissue sections with fitc - labeled virus and detecting the fitc - label with a peroxidase - labeled rabbit anti - fitc . to create a negative control ,
staining was performed without prior incubation with labeled virus or after prior incubation with fitc alone .
furthermore , as a specificity control , preincubation with maackia amurensis lectin , which specifically binds to sa2,3gal  terminated oligosaccharides , was performed .
moderate particulate binding of influenza ( h5n1 ) to canine nasal mucosa , tracheal epithelium , and alveoli was observed .
this staining pattern was also found after binding of fitc - labeled m. amurensis lectin to canine respiratory tract tissues .
binding of fluorescein isothiocyanate  labeled influenza ( h5n1 ) virus to formaldehyde - fixed , paraffin - embedded tissue slides of dog respiratory tract tissues .
right panel shows blocking of virus binding by competitive binding of maackia amurensis lectin to sialic acid 2,3 galactose .
our results demonstrate that dogs are susceptible to infection with avian influenza ( h5n1 ) virus and can shed virus from the nose without showing apparent signs of disease .
moreover , receptors for avian ( h5n1 ) virus are present not only in the lower part of the respiratory tract of dogs but also in their trachea and nose , which are potential portals of entry for the virus . influenza virus infection of dogs was first reported in 2004 ( 6 ) .
influenza ( h3n8 ) of equine origin caused outbreaks in greyhounds in florida and has since been found in dogs in > 20 us states ( 7 ) .
the course of experimental infection of spf dogs with subtype h5n1 resembles that of the experimental infection of dogs with the subtype h3n8 ( 6 ) : all dogs seroconverted , and some excreted virus without obvious disease .
in contrast to the experimental outcomes , natural infections with influenza ( h3n8 ) resulted in serious illness , death , and widespread infection for dogs . this finding warrants special attention to the potential course of avian influenza ( h5n1 ) infection in dogs . therefore , dogs contact with birds and poultry should be avoided in areas with influenza ( h5n1 ) outbreaks to prevent possible spread of virus and human exposure to influenza ( h5n1 ) virus that might have been adapted to mammals .
inoculation of influenza ( h5n1 ) into beagles resulted in virus excretion and rapid seroconversion with no disease . 
 binding studies that used labeled influenza ( h5n1 ) showed virus attachment to higher and lower respiratory tract tissues . 
 thus , dogs that are subclinically infected with influenza ( h5n1 ) may contribute to virus spread .