7 Mar 2023
Image: © Zsolt Bota Finna / Adobe Stock
Through movement for competition and breeding, horses form a highly interconnected population that may also be exercised in close contact and often stabled in confined and shared airspaces – culminating in potential for rapid spread of infectious diseases.
Whether these diseases are endemic contagious respiratory infections – such as strangles and equine influenza (EI) – or due to exotic vector-borne pathogens, such as West Nile virus (WNV), they pose an ever-present or potentially emerging threat to the equine industry. Clearly, such disease outbreaks deleteriously affect horse health and welfare, but also have a further-reaching impact on trade, sport, society and economic livelihoods, culminating in a need to control and, ideally, prevent infections and associated disease.
Vaccination has long been the foundation of preventive health care and, indeed, for some diseases – notably EI – is a mandatory condition of entry for many affiliated equestrian competition disciplines. However, a large proportion of the equid population is not constrained by such requirements, and with rising vaccine hesitancy among horse owners, a need exists to reaffirm the benefits and importance of vaccination if we are to maintain protection of the national herd, and minimise the impacts of infectious diseases.
In the first instance, vaccination aims to protect the individual through amelioration of clinical signs and a reduction in the morbidity and mortality associated with infection.
Furthermore, vaccines reduce pathogen shedding and corresponding onward transmission to minimise infection pressure, and the size and extent of outbreaks at the population level. This benefit of vaccination can be shown by numbers from the 2019 UK EI epidemic, where 72% of confirmed cases occurred in unvaccinated animals, while just 18% were seen in horses with an up-to-date vaccination history (Whitlock et al, 2023).
However, due to the imperfect nature of most currently available vaccines, antibody levels decline with time since previous vaccination, driving the need for regular boosters and increasing the cost and commitment required for owners to engage with the programme. Yet, the costs in veterinary treatment, yard closures and lost competition opportunities associated with a disease outbreak are likely to be significantly greater.
We must better educate horse owners and try to break down barriers to vaccination if we are to improve protection through the generation of an enhanced immunity level across much of the horse population. Ideally, this immunity would reach such a plane that the much smaller proportion of the population that remains unvaccinated, for whatever reason, is still protected – a concept known as “herd immunity”.
Considering the horse and its particular environment, circumstances may be present that make certain individuals particularly vulnerable to the risk and consequences of infection, tipping the cost-benefit analysis very much in favour of vaccination. This may be related to horse movement, age, immune status and use, but overall, even horses not falling into any of these risk factor groups should ideally be vaccinated.
Only by achieving herd immunity, with very high vaccine coverage wherever possible, will the most vulnerable remain protected.
Outbreaks of infectious disease can have devastating and potentially far-reaching consequences for horse health and welfare, the economy and trade, equestrian sport and the well-being of owners.
Indeed, surveys produced in response to the 2007 Australian EI outbreak estimated that an average loss of AU$1,000 (£575) per week was experienced by affected horse owners, and losses for businesses were far greater (Hoare, 2011). Despite this, a questionnaire of UK vets found that 86.4% of respondents experienced vaccine hesitancy in owners, and the most commonly cited reasons for this revolved around concerns regarding over-vaccination, side effects and lack of effectiveness (Wilson et al, 2021).
To break down these barriers, we must first understand what motivates clients not to vaccinate and then re-evaluate methods for educating them to facilitate a culture of change.
Social media has undoubtedly developed a significant impact in all our lives and, as a profession, we often find ourselves competing with the ubiquitous online search resource that is “Dr Google”.
The reality of this, when looking at owner attitudes to vaccination, is that reports of adverse reactions and the negative effects of vaccination are far more tangible to owners than any advantages that are conveyed, which, in reality, is the absence of disease (Barrett et al, 2021).
Accordingly, owners are exposed to fear-mongering, which is not helped by the low level of reporting and education with regards to what is an expected side effect, versus a true adverse reaction. Owner perception would often class minor swelling, pain and heat at the vaccination site as an adverse reaction, when in reality, it is in fact a prerequisite to an effective immune response and consequent protection from disease (Bambra et al, 2020).
Increasing owner education and awareness should help combat this misperception, although the difficulty of achieving and maintaining this cannot be underestimated.
As well as the fear of adverse reactions, a lack of perceived risk is another major reason behind vaccine hesitancy in owners. So, vaccines in many species, including humans, are often “victims of their own success”, in a phenomenon sometimes referred to as the “vaccination paradox” – the diseases that vaccines protect against become rarer and the emphasis shifts to relatively more minor adverse effects, leading to a reduction in vaccine uptake and – regrettably, in time – the re-emergence of the disease.
Indeed, a survey of UK horse owners’ attitudes to EI vaccination found that owning a young or old horse, a horse used for breeding or as a companion, horses that never left the home premises, and limited exposure to new horses were all factors associated with lower vaccination rates (Bambra et al, 2020).
Alternatively, horses aged 10 to 14 years old and those that regularly left the home premises received the highest levels of vaccine coverage (Bambra et al, 2020) – presumably because owners perceived these horses to be at greater risk.
Although, as the authors explored previously, some truth remains in this. A population approach dictates the need to vaccinate as many horses as possible, and the young and old are likely to be most vulnerable to severe disease if infected.
Similar arguments against vaccination were cited in numerous surveys carried out in Australia after the rise in Hendra virus (HeV) cases led to an increased push for vaccination.
Cheaper vaccines and increased likelihood of HeV infection were self-reported as key factors that may change owners’ attitudes towards vaccination (Manyweathers et al, 2017). This is interesting as, in a climate of rising equine cases and seven human infections – four of which were fatal (Middleton et al, 2014) – one would have assumed the very real risks – especially to human health – were discernible. Yet, even in high risk areas, only 13% to 20% of the equine population was vaccinated (Wilson and Ward, 2016). Potentially, a discord between perceived severity and perceived likelihood was operating here.
Another important finding in light of the Hendra outbreak was the perception by owners that vets were abusing their professional power to gain financially – many practices refused to treat unvaccinated horses due to the zoonotic risk, but owners interpreted this as increasing profit from vaccine sales (Manyweathers et al, 2017). This highlights the importance of preserving the vet-client relationship and also how vital this rapport is for effective infectious disease risk communication.
The fact that the Australian data suggested horse owners would not reconsider vaccination of their animals if advised to by their vet (Manyweathers et al, 2017) indicates a level of distrust that must be avoided.
In conclusion, reduced vaccination uptake not only puts individual horses at risk, but, more importantly, affects our ability to maintain herd immunity and reduce the likelihood of large-scale outbreaks.
Although the benefits of vaccination may be obvious to veterinary professionals, we must understand the perceptions of horse owners that lead to barriers to vaccination uptake – if we are to educate and ignite a change in behaviour.
Human motivation and views should never be underestimated as an explanation for behaviour, but we must work together to reshape the landscape and enhance protection of horses through vaccination.
