During the 2007 equine influenza outbreak in
Australia, respiratory disease in dogs in close contact with infected
horses
was noted; influenza (H3N8) virus infection was
confirmed. Nucleotide sequence of the virus from dogs was identical to
that
from horses. No evidence of dog-to-dog transmission or
virus persistence in dogs was found. Respiratory disease in dogs caused
by type A influenza virus was first noted in racing
greyhounds in Florida in January 2004. This subtype H3N8 virus has a
presumptive
but unidentified equine origin. The geographic extent
of infection in racing greyhounds and in pet dogs suggest that this
virus has become enzootic to the United States. In the
United Kingdom, pneumonia in dogs and influenza (H3N8) virus have been
retrospectively linked, and subtype H3N8 infections
have been identified serologically in dogs likely to have been in close
contact with horses during the 2003 outbreak of equine
influenza. A 78-bp segment of the hemagglutinin (HA) gene identified
in dogs with pneumonia had complete homology with
local equine strains. Unlike the situation in the United States, no
evidence
of continuing circulation of an influenza virus of
equine origin in the canine population has been found in the United
Kingdom.
In Australia, in late 2007, an outbreak of equine
influenza virus (EIV) infection occurred in horses. During this
outbreak,
respiratory disease was noted in dogs of various ages
and breeds that were kept near infected horses. Investigations were
undertaken to exclude influenza virus infection. The
first reported case was in a dog near a large stable; the dog became
inappetant and lethargic and had had a slight nasal
discharge and a persistent cough for several days. Over the next 2–3
weeks,
dogs in or near stables with infected horses,
including dogs whose owners were handling infected horses or dogs that
were
only housed with infected dogs, were examined. Samples
were also collected from dogs kept with horses at 5 other locations
20–60 km from the first case. Of the 40 dogs,
examined, 10 had clinical signs consistent with influenza (anorexia,
lethargy,
and, for some, a harsh cough that persisted for
several weeks). All affected dogs recovered.
Nasal swabs and serum were collected from each of the
40 dogs; 23 were seropositive according to influenza type A blocking
ELISA and hemagglutinin inhibition (HI) assay using
A/equine/Sydney/2007 virus as antigen (Table). HI titers were 16–256
(geometric
mean 122). Results were discordant for 5 dogs: for 2,
HI titer was high but ELISA results were negative; for 3, ELISA results
were positive but HI titer was negative. These
discrepancies may have been resolved had later sampling been possible.
Convalescent-phase
serum samples were collected 14–16 days later from 26
of the dogs; seroconversion was noted for 4 of the 5 dogs with
discordant
ELISA and HI results. Testing of 19 dogs 2 years later
showed no change in HI titer, although ELISA results were negative
for each. Each seropositive dog had been in close
proximity to EIV-infected horses but not always in direct contact. No
evidence
of lateral transmission was found for dogs that did
not have contact with horses.
Nasal swabs from 1 clinically healthy dog had a
positive result in an influenza A real-time reverse transcription–PCR
assay
on 2 consecutive days. The dog remained clinically
healthy and was seropositive (titer 64) on day 16 after the first
positive
swab was collected. Attempts to isolate virus from
these swabs were unsuccessful. Nucleic acid sequencing was conducted for
the HA, neuraminidase (NA), and matrix (M) genes
amplified by PCR from the RNA purified from 2 samples from this dog
(A/canine/Sydney/6525/2007
and A/canine/Sydney/6692/2007) and from a nasal swab
from an infected horse (A/equine/Sydney/6085/2007) in the same stable
(GenBank accession nos. GU045761–GU045769). Sequences
were aligned with representative sequences from GenBank by using Clustal
W (
www.clustal.org)
before phylogenetic trees with
bootstrapping were generated (n = 1,000; random seed n = 111) with
MegAlign
(Lasergene; DNAStar, Madison, WI, USA). Complete
nucleotide homology was found for each of the HA, NA, and M gene
sequences
from the 2 dogs and the sequence from the infected
horse in the same stable (A/equine/Sydney/6085/2007).
When influenza subtype H3N8 sequences from horses and
dogs were compared with other subtype H3N8 sequences in GenBank, the
HA, NA, and M sequences were most similar to strains
A/equine/Kanazawa/1/2007 and A/equine/Ibaraki/1/2007, which were
isolated
during the 2007 equine influenza outbreak in Japan.
The HA, NA, and M gene sequences from the dogs in Australia were
positioned
on separate clades of the phylogenetic trees, as
opposed to those from subtype H3N8 viruses from dogs in the United
States,
which all grouped closely together. Researchers in
Japan have described transmission of EIV from 3 experimentally infected
horses to 3 dogs individually housed with each horse.
Their findings were mostly consistent with ours, but there were some
differences. Both studies showed direct linkage
between active influenza virus infection in dogs and horses. Because
some
naturally infected dogs were only in the vicinity of
stables and not in direct contact with horses, we believe that EIV may
be readily transmitted from horses to dogs in close
proximity. The mechanism of spread remains unclear, although in the
United
Kingdom aerosol transmission was believed to be a
major means of spread to dogs. Studies conducted during the equine
influenza
outbreak in Australia indicate that the levels of
virus excretion from horses not previously exposed to the virus can be
extremely
high. Although humans readily spread virus from horse
to horse, either directly during handling or by fomite transmission,
human transmission of EIV to dogs that were not in the
immediate vicinity of infected horses was not found. Similarly,
dog-to-dog
transmission was not found when infected dogs were
transported and kept with other dogs in urban locations where there was
no opportunity for contact with horses.
Although clinical signs were not observed for any of
the dogs in Japan, >35% of the naturally infected dogs in Australia
exhibited
clinical signs, some quite severe and protracted.
Nevertheless, virus was rarely detected in nasal secretions of the dogs
in Australia, and there was no evidence of horizontal
transmission to other dogs. The lack of clinical signs in experimentally
infected dogs may be because of the small numbers of
dogs or because of inoculum attenuation after passage in embryonated
chicken eggs. That the experimentally infected dogs in
Japan also had lower HI titers than did naturally infected dogs may
be relevant. Finally, when 19 of the dogs in Australia
were tested 2 years after infection and without opportunity for
reexposure,
with only 1 exception, the HI antibody titers had not
changed. This finding supports the interpretation that antibodies
detected
in dogs in the United Kingdom had been acquired during
the equine influenza outbreak several years earlier. The nucleotide
gene sequences encoding the 2 surface proteins (HA and
NA) and the M protein from the infected dog in Australia matched those
from the horse with which it had contact and did not
have any of the nucleotide changes that have been identified in viruses
from dogs in the United States. Such changes may be
critical to, or a consequence of, the adaptation of EIVs to dogs and may
play a role in enhancing the infectivity of these
viruses for dogs because there is no evidence of continuing circulation
of virus in dogs in Australia.