EQUINE | Equine Disease Update
spring is approaching and their influenza season is al-
most over. The influenza season happens every year
during autumn, winter, and early spring months, and
the influenza viruses that circulate each season tend
to be the usual suspects: influenza A/H1N1, A/H3N2,
and influenza B viruses. have been bird flu viruses. In 1989 in northern China
a strain of bird flu was positively confirmed to cause a
large-scale disease outbreak in horses. Its subtype was
also H3N8. Was that coincidence or is there some
unique characteristic of the H3N8 subtype that makes
it more apt to infect horses?
However, there are many other influenza viruses in
the world. The ‘H’ and ‘N’ varieties (called “subtypes”)
of influenza A viruses now go up to H18 and N11 with
the recent discovery of new subtypes in bats. Most of
these subtypes are rare in mammals but common in
wild waterfowl. The only influenza subtypes that have
been confirmed to infect horses naturally are H3N8
and H7N7, and the horse-adapted H7N7 viruses ap-
pear to have disappeared from horses nearly 40 years
ago. Those questions remain unanswered. It is known
that the molecular receptors present on the surface
of horse tracheal respiratory cells are a little different
from other species, which could be a contributing fac-
tor to the specificity of the influenza viruses that infect
horses. Work in the author’s laboratory suggests that
this not the entire explanation and other factors must
be involved.
Does this mean that horses cannot be infected by
influenza viruses from other species of animals? The
answer is no, possibly they can be. Transmission of
influenza viruses between different species definitely
occurs.
A related question is whether influenza viruses can be
transmitted from horses to other mammals. That an-
swer is definitely yes. About 15-20 years ago, H3N8
horse flu infected dogs in the USA and has persisted
in dogs ever since. Can it jump from dogs back into
horses?—maybe, although once the virus adapted to
dogs, it became much less adapted to horses.
Humans, swine, dogs, cats, whales, seals, and some-
times other mammals such as mink have occasionally
been infected by influenza viruses from birds. This was
long thought to happen only rarely, but since 1997 in
southeast Asia there have been annual occurrences of
humans contracting bird flu subtypes such as H5N1 or
H7N9 and these cases are often lethal. Almost all of
these cases have been dead-end transmissions, mean-
ing that each case appears to be a separate event with
very little sign that they are capable of spreading from
human to human. Are humans at risk of infection from equine influenza
viruses? Experimental infection of human volunteers
conducted 50 years ago suggests that infection can
happen but the result is mild or even sub-clinical.
Humans with horse exposure do sometimes develop
antibodies against equine influenza virus. The only
report of a suspected naturally occurring clinical dis-
ease from equine influenza virus in a human never ac-
tually demonstrated the presence of equine influenza
virus in that patient.
Can bird flu viruses infect horses? The answer is most
likely yes. One piece of evidence is that the H3N8
subtype was not always circulating in horses; it first
appeared in 1963, and its genetic ancestors seem to The message for readers is: Transmission of influenza
viruses from one species to another can happen and
sometimes does happen. If your horse is sick with the
flu, take elementary biosafety precautions such as
washing hands and clothes or equipment that came
• Volume 19 no 4 • December 2017 •
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