A Systematic Review of Recent Advances in Equine Influenza Vaccination

Abstract

Equine influenza (EI) is a major respiratory disease of horses, which is still causing substantial outbreaks worldwide despite several decades of surveillance and prevention. Alongside quarantine procedures, vaccination is widely used to prevent or limit spread of the disease. The panel of EI vaccines commercially available is probably one of the most varied, including whole inactivated virus vaccines, Immuno-Stimulating Complex adjuvanted vaccines (ISCOM and ISCOM-Matrix), a live attenuated equine influenza virus (EIV) vaccine and a recombinant poxvirus-vectored vaccine. Several other strategies of vaccination are also evaluated. This systematic review reports the advances of EI vaccines during the last few years as well as some of the mechanisms behind the inefficient or sub-optimal response of horses to vaccination.

Keywords: equid; equine influenza; vaccine.

Figure 1

(A) Schematic of equine influenza virus (EIV) evolution. All main sub-lineages and EIV strains reported in this review are indicated; (B) Recent World Organisation for Animal Health (OIE) recommendations from the OIE expert surveillance panel on EI vaccine composition.

Figure 2

(A) EIV structure; (B) The reproductive cycle of influenza A virus. The virus binds to receptors on the surface of the cells (1) and is internalized into endosomes (2). Modification of pH in both the endosome and the virus induces fusion and uncoating of the virus (3). vRNP are released into the cytoplasm and imported into the nucleus where their replication takes place (4). mRNA are produced and exported into the cytoplasm for protein synthesis (5). This is controlled by NS1. These viral proteins will either assist replication of the virus and formation of vRNP into the nucleus (6), or form new viruses at the cell surface (7). Progeny viruses are assembled and bud from the cell membrane (8). CF = complement fixing; VN = virus neutralising.

Figure 3

Systematic review process.

Figure 4

Different types of EI vaccines.

Figure 5

Immuno-Stimulating Complex adjuvanted vaccines (ISCOM) and ISCOM-Matrix vaccine, uptake and major histocompatibility complex (MHC) presentation. Purified EIV antigens (e.g., HA and NA) are directly mixed with Quillaja saponin, cholesterol and phospholipids to produce ISCOM-based vaccine (1) or mixed with preformed ISCOM to produce ISCOM-Matrix-based vaccine (2). Exogenous pathway and antigen presentation by MHC class II molecules. The antigen/ISCOM/ISCOM-Matrix complex is engulfed in phagosomes (3) and enter the exogenous pathwary of antigen presentation after degradation by proteases into short peptides in acidified endosomes (4). The MHC-class II-peptide complex is recognised by the T cell receptor (TCR) of antigen-specific CD4⁺ T lymphocytes. Cross-presentation of antigen by MHC class I molecules. The antigen/ISCOM/ISCOM-Matrix complex could enter the cell through interaction with the cell membrane, phagocytosis, endocytosis, or similar process (5). The antigens are translocated to the cytosol and reach the endogenous pathway of antigen presentation (6). The complex MHC class I-b2m-peptide is recognised by the T cell receptor (TCR) of an antigen-specific CD8⁺ T lymphocyte.