Refinement of the Equine Influenza model: the benefits of individual nebulisation for experimental infection

Abstract

Equine influenza (EI) is a major respiratory disease of the horse induced by the highly contagious equine influenza virus (EIV). Worldwide, diverse EI vaccines are available commercially. The EIV strain, adjuvant composition and vaccine technology differs greatly. The use of the same EI vaccine throughout the life of a horse is unlikely, due to change of ownership, veterinary practitioner, location and/or vaccine availability. The compatibility of different EI vaccines has rarely been studied but is of great importance as immunisation failure could favour EIV dissemination. Study aims: To determine the impact of mixed EI vaccination on protective immunity to EIV in horses and to measure herd immunity in Hong Kong (HK). Methods: HK has approximately 1,250 race horses with around 450 new horses arriving each year. Imported racehorses must have been vaccinated with an approved EI vaccine, irrespective of their point of origin. All horses are given a new primary course on arrival with a unique ISCOM matrix adjuvanted EI vaccine and are vaccinated twice a year thereafter. EI-specific antibody titres were measured by single radial haemolysis (SRH) assay (A/eq/South Africa/4/03 EIV strain) in 1050+ archived sera collected between 2010 and 2014 from 129+ horses. Objectives: 1) to determine the SRH antibody titre in serums from HK resident horses (i.e. in HK for at least 1.5 year), 2) to evaluate the antibody titre on arrival at HK in relation to their country of origin, 3) to measure the effect of post-arrival mandatory EI primary vaccination and 4) to compare this response with antibody levels measured in HK resident horses. Results: Preliminary results indicates that the average yearly SRH antibody titre for HK resident horses was 113±34mm2 (n1⁄4467), with only 15% of horses below the clinical protection threshold (i.e. 85mm2) and less than 3% of horses below 50mm2. A significant increase was detectable (p-value<0.00004) after each immunisation boost (maximum average titre of 128±42mm2 in October, n1⁄455). The average SRH antibody titre in the week following importation to HK (and prior the mandatory EI vaccination in HK) was 180±40mm2 (n1⁄479), with results in horses originating in Australia significantly higher (207±36mm2, n1⁄410, p-value 1⁄4 0.0281) than horses from New Zealand (171±34mm2, n1⁄442), Great Britain (168±38mm2, n1⁄417) or Ireland (172±31mm2, n1⁄43). SRH antibody titres at the time of importation were maintained for around 75 days (Figure 1) due