EQUINE INFLUENZA VACCINES - OVERVIEW

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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 [3]. 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 [2].

It is important to note that either of the American or European lineages may appear in any part of the world [1, 10].  For example, 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 [9]. 

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 amount of 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 [9].

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 [15].



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