The principal aim of vaccination is to reduce clinical signs of the disease, to shorten the convalescent period and to reduce the likelihood or severity of secondary infections. This will lead to improved animal welfare. Reduced nasopharyngeal shedding of virus has important implications for the spread of infection and is certainly the other major target that should be achieved by vaccination. An ideal vaccine should also provide long-term immunity, an efficient memory response and cross-protection from viruses of different strains. The different strategies of vaccination are presented for Equine Influenza, similar approaches have been developed for Equine Herpes Virus-1.
Vaccines against Equine Influenza can be divided into 6 categories.
1. Inert vaccines:
1.1. Inactivated virus vaccine (category 1): Since the introduction of EIV vaccines in the 1960s, the majority of vaccines commercially available have been inactivated whole virus.
- The main advantages of these vaccines are the absence of pathogenicity, virus replication and subsequent spread between hosts. Influenza virus has been traditionally grown in embryonated hens’ eggs for the preparation of these vaccines.
- Viruses were treated with either formaldehyde or b-propriolactone for virus inactivation.
- This inactivated virus is often mixed with an adjuvant that improves the strength and duration of the immune response.
- Protection induced by conventional inactivated EIV vaccines remains constrained by the requirement of a correct match between vaccine and field strains of EIV.
1.2. Sub-unit vaccine (category 2): Numerous split or subunit vaccines have been developed to replace whole inactivated virus vaccines. A split vaccine is obtained by the disintegration of purified virions using detergents (e.g. triton) or ether, substances afterwards removed. Split vaccines contain the same elements than inactivated whole virus vaccines and generally induced similar adverse reaction. Subunit vaccine contains one or more pure or semi-pure antigens, purified from the pathogen or produced by a baculovirus-expression systems using insect cells.
In the last decade, a whole generation of vaccines have been designed to stimulate a virus-specific immune response aimed to mimic immunity induced by natural infection with EIV, to provide a long lasting protection involving both humoral and cellular immune responses, and therefore to minimise the effect of a strain mismatch.
1.3. Immuno-stimulating complex (category 3): Current subunit vaccines against EIV contain purified HA and NA proteins. The main types of vaccines are:
- Immune-stimulating complexes (ISCOM)-based vaccines. ISCOM particles are spontaneously formed cage-like structures resulting from the combination of antigens with cholesterol, phospholipids and Quillaja saponins.
- ISCOMATRIX vaccine, identical to the ISCOM vaccine except the antigen is not bound within the cage structure of the ISCOM.
1.4. DNA vaccine (category 4): Administration of plasmids DNA could results in expression of the encoded antigen gene in situ. DNA vaccination was shown to stimulate a protective influenza-specific immune response in horses, which was not based on the antibody response only, when compared with conventional inactivated virus vaccines. However, studies have shown some variability in the level of protection from clinical signs of disease and the prevention of virus shedding achieved. Furthermore, the method of administration of DNA vaccines (multiple sites of inoculation and multiple injections for each immunisation) is obviously impractical for veterinary practice.
2. Live vaccines:
A live vaccine is defined as a vaccinal preparation containing live organisms. These organisms can be an attenuated or a modified form of the pathogen itself, or a heterologous organism used as a carrier or vector of the pathogen antigens (live recombinant vector vaccines).
2.1. Live-attenuated virus vaccine (category 5): Immunisation with a live attenuated virus closely mimics natural infection. Antigens are presented to the immune system via both exogenous and endogenous antigen presentation pathways, and vaccines are therefore expected to stimulate an immune response similar to those induced by infection. The live-attenuated virus vaccine against EIV contains a temperature-sensitive (Ts) influenza virus that multiplies efficiently in the cooler environment of the upper respiratory tract where immune responses are induced but does not replicate in the warmer environment of the lower respiratory tract in horses, therefore, avoiding clinical sign of disease.
2.2. Canarypox-based vaccine (category 6): Live recombinant vector vaccines are constructed by inserting selected genes from the pathogen of interest into live, infectious, but non-disease-causing viruses, which may have been genetically modified. With virus-based vector vaccines, viral antigens are expressed and synthesised de novo within the infected cell. Thus, because they are presented via MHC class I (endogenous) and class II (exogenous) antigen-processing routes, the selected viral antigens will stimulate both humoral and cellular immune responses.
A list of currently commercialised vaccines against EIV will be found at http://www.equiflunet.org.uk
Please click the links below to view the vaccine process.