ISCOM-based equine influenza vaccine: Duration of immunity and randomised clinical trials to assess an accelerated schedule of immunisation and efficacy

Equine Influenza Virus and Vaccines

Equine influenza virus (EIV) is a constantly evolving viral pathogen that is responsible for yearly outbreaks of respiratory disease in horses termed equine influenza (EI). There is currently no evidence of circulation of the original H7N7 strain of EIV worldwide; however, the EIV H3N8 strain, which was first isolated in the early 1960s, remains a major threat to most of the world's horse populations. It can also infect dogs. The ability of EIV to constantly accumulate mutations in its antibody-binding sites enables it to evade host protective immunity, making it a successful viral pathogen. Clinical and virological protection against EIV is achieved by stimulation of strong cellular and humoral immunity in vaccinated horses. However, despite EI vaccine updates over the years, EIV remains relevant, because the protective effects of vaccines decay and permit subclinical infections that facilitate transmission into susceptible populations. In this review, we describe how the evolution of EIV drives repeated EI outbreaks even in horse populations with supposedly high vaccination coverage. Next, we discuss the approaches employed to develop efficacious EI vaccines for commercial use and the existing system for recommendations on updating vaccines based on available clinical and virological data to improve protective immunity in vaccinated horse populations. Understanding how EIV biology can be better harnessed to improve EI vaccines is central to controlling EI.

Novel Vaccine Technologies in Veterinary Medicine: A Herald to Human Medicine Vaccines

The success of inactivated and live-attenuated vaccines has enhanced livestock productivity, promoted food security, and attenuated the morbidity and mortality of several human, animal, and zoonotic diseases. However, these traditional vaccine technologies are not without fault. The efficacy of inactivated vaccines can be suboptimal with particular pathogens and safety concerns arise with live-attenuated vaccines. Additionally, the rate of emerging infectious diseases continues to increase and with that the need to quickly deploy new vaccines. Unfortunately, first generation vaccines are not conducive to such urgencies. Within the last three decades, veterinary medicine has spearheaded the advancement in novel vaccine development to circumvent several of the flaws associated with classical vaccines. These third generation vaccines, including DNA, RNA and recombinant viral-vector vaccines, induce both humoral and cellular immune response, are economically manufactured, safe to use, and can be utilized to differentiate infected from vaccinated animals. The present article offers a review of commercially available novel vaccine technologies currently utilized in companion animal, food animal, and wildlife disease control.

An Evaluation of Three Different Primary Equine Influenza Vaccination Intervals in Foals

In order to evaluate the effect of three different primary vaccination intervals on EI vaccine response, 21 unvaccinated thoroughbred foals were randomly divided into three groups of 7 and vaccinated with three different intervals of primary immunization (i.e., with 1, 2 or 3 months intervals between V1 and V2, respectively). The antibody response was measured for up to 1 year after the third immunization V3 (administered 6 months after V2) by single radial hemolysis (SRH) assay. All weanlings had seroconverted and exceeded the clinical protection threshold 2 weeks after V2 and 1 month after V3 until the end of the study. Significant differences were measured at the peak of immunity after V2 and for the duration of the immunity gap between V2 and V3. The group with one month primary vaccination interval had a lower immunity peak after V2 (158.05 ± 6.63 mm²) and a wider immunity gap between V2 and V3 (18 weeks) when compared with other groups (i.e., 174.72 ± 6.86 mm² and 16 weeks for a two months interval, 221.45 ± 14.48 mm² and 12 weeks for a 3-month interval). The advantage observed in the group with 1 month primary vaccination interval, which induces an earlier protective immunity, is counterbalance with a lower peak of immunity and a wider immunity gap after V2, when compared with foals vaccinated with 2- and 3-month intervals.

Equine influenza vaccination as reported by horse owners and factors influencing their decision to vaccinate or not

BACKGROUND

Equine influenza virus is a highly contagious respiratory pathogen that causes pyrexia, anorexia, lethargy and coughing in immunologically naïve horses. Vaccines against equine influenza are available and vaccination is mandatory for horses that participate in affiliated competitions, but this group forms a small proportion of the total horse population. The aims of this study were to: i) identify the equine influenza vaccination rate as reported in 2016 by horse owners in the United Kingdom (UK); ii) examine the demographics of owners and horses which were associated with significantly lower influenza vaccination rates and iii) explore factors that influence horse owners' decisions around influenza vaccine uptake.

RESULTS

Responses from 4837 UK horse owners who were responsible for 10,501 horses were analysed. An overall equine influenza vaccination rate of 80% (8385/10501) was reported. Several owner demographic characteristics were associated with significantly lower (p<0.05) reported equine influenza vaccination rates including: some geographical locations, increasing horse owner age, annual household income of less that £15,000 and owning more than one horse. Horse-related features which were associated with significantly lower reported equine influenza vaccination rates included age ranges of <4 years and > 20 years, use as a companion or breeding animal or leaving their home premises either never or at most once a year. The most common reasons cited for failing to vaccinate horses was no competition activity, lack of exposure to influenza and expense of vaccines. In contrast, the most common underlying reasons given by horse owners who vaccinated their horse were protection of the individual horse against disease, veterinary advice and to protect the national herd. Owners of vaccinated horses had less previous experience of an influenza outbreak or adverse reaction to vaccination compared with owners of unvaccinated horses.

CONCLUSIONS

This study documented a high rate of equine influenza vaccination as reported by owners in a substantial number of horses in the UK, but this does not reflect the level of protection. Sub-populations of horses which were less likely to be vaccinated and the factors that influence each owner's decision around vaccination of their horses against equine influenza were identified, but may alter following the 2019 European influenza outbreak. This information may nevertheless help veterinary surgeons identify "at-risk" patients and communicate more personalised advice to their horse-owning clients. It may also influence educational campaigns about equine influenza directed to horse owners, which aim to improve uptake of vaccination against this pathogen.

Impact of Mixed Equine Influenza Vaccination on Correlate of Protection in Horses

To evaluate the humoral immune response to mixed Equine Influenza vaccination, a common practice in the field, an experimental study was carried out on 42 unvaccinated thoroughbred weanling foals divided into six groups of seven. Three groups were vaccinated using a non-mixed protocol (Equilis^® Prequenza-Te, Proteqflu-Te^® or Calvenza-03^®) and three other groups were vaccinated using a mix of the three vaccines mentioned previously. Each weanling underwent a primary EI vaccination schedule composed of two primary immunisations (V1 and V2) four weeks apart followed by a third boost immunisation (V3) six months later. Antibody responses were monitored until one-year post-V3 by single radial haemolysis (SRH). The results showed similar antibody responses for all groups using mixed EI vaccination and the group exclusively vaccinated with Equilis^® Prequenza-TE, which were significantly higher than the other two groups vaccinated with Proteqflu-TE^® and Calvenza-03^®. All weanlings (100%) failed to seroconvert after V1 and 21% (9/42) still had low or no SRH antibody titres two weeks post-V2. All weanlings had seroconverted and exceeded the clinical protection threshold one month after V3. The poor response to vaccination was primarily observed in groups exclusively vaccinated with Proteqflu-Te^® and Calvenza-03^®. A large window of susceptibility (3–4.5-month duration) usually called immunity gap was observed after V2 and prior to V3 for all groups. The SRH antibody level was maintained above the clinical protection threshold for three months post-V3 for the groups exclusively vaccinated with Proteqflu-Te^® and Calvenza-03^®, and six months to one year for groups using mixed EI vaccination or exclusively vaccinated with Equilis^® Prequenza-Te. This study demonstrates for the first time that the mix of EI vaccines during the primary vaccination schedule has no detrimental impact on the correlate of protection against EIV infection.

The Immunity Gap Challenge: Protection against a Recent Florida Clade 2 Equine Influenza Strain

Vaccination is one of the most effective tools for limiting the impact of equine influenza (EI). The humoral immunity established following a primary vaccination course can decrease significantly between the second (V2) and third immunisations (V3), leaving some horses insufficiently protected for several weeks. This so-called “immunity gap” poses a challenge to all EI vaccines. During this period, the EI infection of vaccinated animals may be followed by marked clinical signs and virus shedding. However, several EI vaccines have been shown to stimulate equine influenza virus (EIV)-specific cell-mediated immunity, which is likely to play a role in protection against EIV infection and/or mitigate the clinical and virological signs of EI. Reducing the interval between V2 and V3 has been shown to be counterproductive to longer-term immunity. Further research is needed to define and address the “immunity gap” in horses. This study aimed to measure the level of protection induced by a whole inactivated, ISCOMatrix adjuvanted, EI and tetanus vaccine (Equilis Prequenza-Te) when challenged during the immunity gap (i.e., immediately before the recommended boost immunisation, more than 5 months after V2) using infection with a recent heterologous Florida Clade 2 (FC2) equine influenza virus (EIV) strain. This vaccine was tested in a Welsh mountain pony model. A group of seven ponies was vaccinated twice, 4 weeks apart. The protective antibody response was measured and ponies were challenged, along with 5 unvaccinated control ponies, by experimental infection with the FC2 A/eq/Northamptonshire/1/13 EIV strain, 158 days (around 5.2 months) after V2 and their clinical signs and virus shedding were monitored. EI serology was measured by single radial haemolysis (SRH) and haemagglutination inhibition (HI). Clinical signs and virus shedding (measured by qRT-PCR and hen’s egg titration) were compared with controls. All vaccinates had detectable, low SRH antibody titres and most had detectable, low HI titres. Significant clinical and virological protection was observed in vaccinates (p < 0.05), supporting the good performance of this vaccine against a recent EIV strain. In this study, the impact of the immunity gap in ponies was limited after primary vaccination with this whole inactivated, ISCOMatrix adjuvanted EI and tetanus vaccine (Equilis Prequenza-Te) when infected several months after V2 with a recent FC2 strain, which is representative of EIV circulating in the EU.

Equine Vaccines: How, When and Why? Report of the Vaccinology Session, French Equine Veterinarians Association, 2016, Reims

To date, vaccination is one of the most efficient methods of prevention against equine infectious diseases. The vaccinology session, which was organised during the annual meeting of the French Equine Veterinarians Association (AVEF) at Reims (France) in 2016, aimed to approach three subjects of importance for the equine industry. Vaccination against three major equine diseases were used as examples: equine influenza (equine influenza virus), rhinopneumonitis (equine herpes virus 1/4), and tetanus (Clostridium tetani neuro-toxin). (1) Emergency vaccination: while it has been very successful to reduce the impact of equine influenza epizooties and it is also recommended for tetanus in case of surgery and accident, the benefit of emergency vaccination against equine herpes virus 1/4 remains arguable; (2) Compatibility of equine vaccines from different brands: despite being a frequent concerns for equine veterinarians, little information is available about the compatibility of equine vaccines from different commercial origins. The consequence of mixing different equine vaccines targeting the same disease is believed to be limited but scientific evidences are sparse; and, (3) Laps vaccination and vaccine shortage: they could have serious consequences in terms of protection and their impact should be evaluated on a case by case basis, taking into account the risk of contact with the pathogen and the effect on herd immunity.

Refinement of the equine influenza model in the natural host: A meta-analysis to determine the benefits of individual nebulisation for experimental infection and vaccine evaluation in the face of decreased strain pathogenicity

Equine Influenza (EI) is an important respiratory disease of horses caused by H3N8 equine influenza viruses (EIV). Vaccination is a key strategy to prevent or control this disease. However, EIV undergoes continuous antigenic drift and whilst numerous EI vaccines are commercially available worldwide, an accurate evaluation of their efficacy is frequently required through clinical trials conducted in the natural host. Room nebulisation is one of the chosen methods to challenge horses during EI vaccine studies. A potential decreased pathogenicity observed with recent Florida Clade 2 (FC2) EIV isolates have increased the heterogeneity of the clinical response and virus shedding measured after infection by room nebulisation, which reduced the statistical power of studies. Our objectives were to compare clinical and virological parameters following experimental infection with several different EIV strains and to confirm that individual nebulisation is a model refinement that prevents an increase of the number of animals per group. This study is a retrospective comparison and meta-analysis of clinical and virological results collected from 9 independent EIV infection studies in the natural host. Naïve Welsh mountain ponies were experimentally infected by room or individual nebulisation with FC2 EIV strains, including A/equine/Richmond/1/07 (R/07), A/equine/East Renfrewshire/11 (ER/11), A/equine/Cambremer/1/2012 (C/12) and A/equine/Northamptonshire/1/13 (N/1/13). The retrospective meta-analysis confirmed a decreased pathogenicity of the EIV ER/11 and C/12 strains when compared with R/07. Experimental infection by individual nebulisation improved the clinical and virological parameters induced by recent FC2 strains, when compared with conventional room nebulisation. In conclusion, individual nebulisation offers a better control of the challenge dose administered and a greater homogeneity of the response measured in control animals. This in turn, helps maintain the number of animals per group to the minimum necessary required to obtain meaningful results in vaccine efficacy studies, which adheres to the 3Rs (Replacement, Reduction and Refinement) principles.