A perceived low risk of their horses being infected by the Hendra virus is a key driver in Queensland owners deciding against vaccinating, fresh research has shown.
Owners who opt against vaccination against Hendra might consider their location leaves them at little risk from the disease or that their management practices minimise the chances of infection, the study team from the University of Queensland has reported.
Hendra is a virus carried by Australia’s native fruit bats that is capable of causing severe respiratory and neurological disease in horses and humans.
Since 2012, a vaccine has been available for horses which has been promoted by authorities.
Uptake has been limited and spill-over events of Hendra virus infection in horses continue to occur.
Joerg Henning and his colleagues conducted an online, questionnaire-based cross-sectional study of 376 horse owners belonging to a variety of different equestrian clubs in Queensland to identify risk factors for non-vaccination against Hendra virus.
The researchers found that 162 of the owners – that’s 43.1% – had opted against vaccination while 56.9% had horses that were currently vaccinated against the virus.
A total of 52 risk factors were evaluated in the study, with the biggest factors associated with non-vaccination being a perceived low risk, either due to location or management practices; the level of perceived risk of Hendra virus infection in humans; horse owners’ tendency toward non-vaccination of their household pets; non-vaccination against strangles disease in horses; handling of more than three horses each week; and the perceived attitude that vets had a high motivation of making money from the vaccination.
Horse owners were more likely to vaccinate against Hendra virus if their horses were used for dressage, show jumping or eventing, the authors reported in the open-access peer-reviewed journal PLOS ONE.
The study also identified horse owners’ concerns about side-effects and what some consider a lack of evidence on the vaccine’s effectiveness.
They found no significant differences in the level of the self-assessed knowledge about Hendra virus between horse owners vaccinating or not vaccinating, with 57.6% of Hendra virus vaccinators saying they had a very high to high knowledge, 35.6% indicating moderate and 6.8% indicating low to very low knowledge, compared to 46.9%, 44.1% and 9.1% of non-Hendra vaccinators, respectively.
Discussing their findings, the researchers said it was not surprising that many owners opted against vaccinating against Hendra, with many owners perceiving the vaccine to be experimental, hurriedly introduced, lacking data on effectiveness, with a high incidence of adverse effects, with unknown effects on horses used for breeding and during pregnancy, and accompanied by the attitude held by many owners that the risk of Hendra infection in horses was low.
“Many more respondents reported adverse effects following Hendra vaccination than for tetanus and strangles vaccination, but it is possible that those who observed adverse effects following Hendra vaccination were more motivated to reply to the survey, which may have created a potential selection bias.
“However, the difference between observation of adverse effects following Hendra virus vaccination and the perception that Hendra virus vaccination is associated with adverse effects, often of a severe nature, requires further investigation.”
Public correspondence from vaccine manufacturer Zoetis has indicated that reports concerning post-Hendra adverse vaccination effects indicated they were uncommon, and severe adverse events were rare. Veterinarians had derived a similar view.
“These facts would appear to be in contradiction to the reported incidence of adverse effects by respondents in this survey.
“There are a number of very strident opponents to Hendra virus vaccination presenting their views via social and other media; their views would appear to have considerable influence on a significant number of horse owners and there appears to be little doubt that their dire warnings and descriptions of adverse post-Hendra vaccination events has created a perception amongst horse owners that the vaccine commonly causes adverse effects, often of a serious and sometimes fatal nature.
“However, the use of social media is similar between Hendra vaccinators and non-Hendra vaccinators, but Hendra vaccinators seem to trust more the advice provided by veterinarians.”
They continued: “One concern highlighted by this study was the number of non-vaccinators who consider making money to be a main motivator for veterinarians recommending and conducting Hendra vaccinations.
“This suggests a level of distrust of veterinarians and the veterinary industry as a whole. Alternatively, it may represent a failure of communication between veterinarians and horse owners.
“Numerous respondents voiced distrust towards Zoetis showing concern that they are financially motivated rather than driven to produce a suitable vaccine that meets horse owner requirements. Due to these concerns it is likely that horse owners would be highly critical of studies produced by Zoetis and it is recommended that independent research be undertaken.
“Veterinarians who promote Hendra vaccination may also be viewed by horse owners in a similar light.”
One of the most significant risk factors identified was the perception by owners that their horses were at low risk of becoming infected or horse owners were unsure about the Hendra virus risk to their horses.
“While owners consider that the risk of horses contracting Hendra is low, it is likely that the Hendra vaccine uptake will remain low.”
The researchers suggested that the relatively small number of recent cases may have reduced the level of concern about the risk of horses becoming infected.
Respondents who had not vaccinated their horses against Hendra virus expressed concern about the effectiveness of the vaccine, reflecting the obvious argument of ‘why vaccinate if it does not work’.
“Long-term studies are required to establish the safety, efficacy and appropriate booster vaccination frequency. Such studies would improve public confidence and may lead to improved vaccine uptake.”
The researchers said horse owners represented a diverse cross section of the population and therefore it was not surprising they expressed a wide range of views and knowledge.
However, the survey highlighted serious misconceptions concerning the severity of Hendra virus infections in both horses and humans.
“That fact that a considerable number of responders indicated that Hendra infection was not a severe disease in either species is very concerning as it is indicative of a failure by statutory authorities and the veterinary profession to educate horse owners about the true nature of the disease, which possibly has led to a percentage of horse owners rejecting the need for Hendra virus vaccination because they do not regard the disease as a serious risk.
“The lack of awareness of the potential serious consequences of strangles infection adds support to the conclusion that there is an issue with communication and education, and perhaps trust, between horse owners and veterinarians.
“The fact that a significant number of surveyed horse owners’ expressed distrust of the motives of veterinarians is also very concerning. Development of a trust and close relationship of horse owners with veterinarians is the key strategy for the prevention of Hendra virus infection in horses.”
Distrust, they wrote, created uncertainty, which can be exploited by others, such as anti-vaccinators, to espouse their own agenda.
“Although the vaccine release was expedited for altruistic reasons, i.e. protection of humans and horses from a lethal disease, it was seen by many horse owners as a money-making exercise.
“The survey demonstrates that underlying public misconceptions and distrust of motives were not adequately addressed prior to release of the Hendra virus vaccine, which despite best intentions accompanying the vaccine’s release, meant that the vaccine uptake was less than expected, and accompanied by considerable horse owner opposition.”
The study team comprised Henning, Kailiea Arianna Goyen, John David Wright, and Alexandra Cunneen.
Goyen KA, Wright JD, Cunneen A, Henning J (2017) Playing with fire – What is influencing horse owners’ decisions to not vaccinate their horses against deadly Hendra virus infection? PLoS ONE 12(6): e0180062. https://doi.org/10.1371/journal.pone.0180062