In order to access the potential of a vaccine it is essential to have a
basic appreciation of the genetic history of Equine Influenza virus and
understand how the various strains are named.
Two serotypes of Equine Influenza have been identified. They are
H7N7 which was first identified in Prague (also known as influenza
A/equine 1) in 1956 and H3N8 which was first identified in Miami in
1963 (also known as influenza A/equine 2). The H7N7 serotype has
not been associated with an influenza outbreak since 1979 in Italy and
was last reported in horses in Europe and central Asia in 1991.
It may still circulate in some parts of the world as a subclinical
disease . Equine influenza is currently reported as occurring from
one of two major lineages of the H3N8 serotypes known as European or
Eurasion Lineage and American Lineage which has further diverged into 3
sublineages; South American, Kentucky and Florida . Within each
lineage there are a number of strains created by slight antigenic
differences in the hemagglutinin and neuraminidase proteins. Over
40 strains of the American lineage have been identified .
It is important to note that either of the American or European
lineages may appear in any part of the world [1, 10]. For
a strain of the European lineage was identified in Saskatchewan.
The formal name of this strain is A/Eq/Saskatoon/1/90. All
viruses have a formal name defined by the International Committee on
Taxonomy of Viruses as Genus / Affected Host Species / City Where
Strain was Isolated / incremental number of the strain isolated in a
given year / year the strain was isolated. So in the case of
A/Eq/Saskatoon/1/90 the A represents Influenza A genus, Eq indicates
Equine, Saskatoon was were the strain was isolated and it was the first
strain of Equine Influenza isolated in Saskatoon in 1990. Vaccine
companies often abbreviate the formal names when referring to their
equine influenza vaccines or use informal names so the
A/Eq/Saskatoon/1/90 strain may also be referred to specifically as
Saskatoon 90, or generally as influenza A/equine 2 which indicates just
the lineage or simply as H3N8.
Vaccinations function by exposing the immune system of the vaccinated
animal to antigens that are similar or identical to those of the
particular strain of virus. In response to the antigens the
immune system will prepare itself for a future onslaught of the
antigens, ie infection with Equine Influenza virus. The success
and duration of this preparedness depends on the nature of and way in
which the antigens are administered .
Although much is known about the immune
system of vertebrates, its function is poorly understood.
In simple terms the immune system has three mechanisms of response; 1)
the innate component which functions immediately against any type of
invader; 2) the humeral response which is acquired (responds after
exposure to specific antigens) and is heavily reliant on antibodies and
3) cell mediated immunity, also acquired, which takes the longest
time to develop but has more comprehensive effects. In order to
provide the best response to an infection the immune system must
mobilize and coordinate all three mechanisms .
It is known that natural infection of Equine Influenza virus provides
the best protection for the longest time from reinfection by maximally
invoking all three immune mechanisms[4, 9].
Equine Influenza virus vaccinations based on killed viruses tend to
produce short lived antibody dependent responses which must closely or
identically match the specific strain of the virus to which the horse
is exposed [2, 3, 11, 14, 15].
Modified live virus vaccinations contain
live viruses that have been altered so that they do not cause disease
in the animal but still expose the immune system to suitable viral
antigens. The live virus results in virions entering host cells
which results in a broader response from the immune system including
cell mediated and humeral immunity. A modified live virus which
is administered intra nasally has been developed. The intranasal
route has the added benefit of following the same infection route as
natural virus, further enhancing immune response via invoking mucosal
immunity. This vaccine is able to respond to a much broader
variation of strains than killed virus vaccinations [3, 4, 16, 17, 18].
Recombinant vaccines use an innocuous viral
vector, such as canary pox, which does not cause disease in a horse
that has been modified to express antigens associated with a different
disease causing virus. The advantage of these vaccines is that as
functional viruses they stimulate broader and longer lasting immune
responses, including cell mediated immunity, and provide protection
against viral strains related to the expressed antigens. One such
recombinant canarypox vector vaccine has been developed for Equine
Influenza. This vaccine is administered intramuscularly so
although it does generate cell mediated immunity it does not invoke
mucosal immune mechanisms .