"Prevention is the biggest success": Barriers and enablers to personal biosecurity in the thoroughbred breeding industry.

Employees in the equine industry are at occupational risk of zoonoses such as Hendra virus and equine chlamydiosis through exposure to infected materials. This study aimed to gain a deeper understanding of the views and experiences of employees, and the key drivers of infection control and personal biosecurity (PB) practices in the Thoroughbred breeding industry. Methods: An exploratory qualitative study was conducted in 2018 in New South Wales, Australia using interviews (9) and small group discussions (7). The 29 participants included veterinarians, veterinary nurses, foaling staff, stud managers and laboratory personnel working in a range of equine medicine settings. Interviews and focus groups were recorded, transcribed and analysed manually by at least two members of the research team. An iterative approach was used to derive themes. Results: Five main themes emerged: (i) greater awareness of current and emerging infectious risks promotes use of Personal Protective Equipment (PPE); (ii) currently available PPE is not comfortable, practical or well-suited to equine reproductive work in Australia's hot climate; (iii) creating supportive environments for PB reduces risk of exposure to infectious materials; (iv) strong leadership is required to implement sustainable change in workplace culture and practices; and (v) policy and economic factors play an important role in adopting biosecurity (BS) and PB measures in the workplace. Personnel working in the Australian Thoroughbred breeding industry face unique zoonotic risks in a challenging physical environment. A qualitative approach provided rich insights into social and physical factors motivating BS and PB in this occupational group. There is an opportunity for collaboration between Public Health services and industry partners to develop and implement strategies most likely to be effective in ensuring consistent uptake of PB measures in the workplace.

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.

Information delivery and the veterinarian-horse owner relationship in the context of Hendra virus in Australia.

Hendra virus (HeV) is an emerging bat-borne virus endemic in Australia that can be transmitted from horses to humans and has a high fatality rate for horses and people. Controversy surrounding HeV risk mitigation measures have strained the veterinarian-horse owner relationship. This study aimed to characterise the veterinarian-horse owner relationship in general and also in the context of HeV by analysing data derived from the 'Horse Owners and Hendra Virus: A Longitudinal Study to Evaluate Risk' (HHALTER) study. Australian horse owners were recruited via emails, social media and word-of-mouth for a series of five surveys that were administered online at six-monthly intervals over a two-year period to capture baseline knowledge, attitudes and practices of horse owners regarding HeV and any changes over time. In the current study, descriptive analyses of information sources were performed to understand the use of veterinarians as a HeV information source (Surveys 1 and 5; n = 1195 and n = 617). Ordinal logistic regression analyses were conducted to determine factors associated with the frequency of horse owner contact with a veterinarian (Survey 3; n = 636). This study found a relative increase over the study period in the proportion of horse owners who had used veterinarians as HeV information source in the last 12 months (from 51.9% to 88.3%). Owning more horses, being older, having a 'duty of care' for other people working with horses and deriving the main income from horse related business were factors associated with more frequent veterinary contact. Results suggest that traditional information sources such as workshops, information packs and risk training are likely to be used by horse owners. Smart phone applications should be considered for use in the future and require further investigation for horse health communication. The findings of this study may be helpful in optimising strategies for horse health information delivery.

Gasterophilus flavipes (Oestridae: Gasterophilinae): A horse stomach bot fly brought back from oblivion with morphological and molecular evidence.

Species of Gasterophilus Leach are obligate parasites in domestic and wild equids and responsible for cosmopolitan gasterophilosis. Although with only eight species known so far, they have received considerable attention because of their significant veterinary and economic importance. Surprisingly, we found that G. flavipes (Olivier) is a valid species based on morphological characters from male, female and the egg, after spending half a century as a synonym of G. haemorrhoidalis (Linnaeus). In the present study, G. flavipes, G. haemorrhoidalis and G. inermis (Brauer), which are the three closely related species possessing a remarkable mixture of shared morphological characters, are diagnosed and comparatively redescribed; the key to separate adults and eggs are provided, together with a series of high-resolution photographs from all the body parts. COI barcodes do not allow for a separation of G. flavipes, G. haemorrhoidalis and G. inermis, but showed a closer relationship between G. flavipes and G. haemorrhoidalis than the other two combinations, which is consistent with the morphological evidence. Geographically, G. flavipes seems to be common and widespread in the warmer parts of the Palaearctic region. Thus, the epidemiology of gasterophilosis where G. flavipes is known or supposed to occur calls for a more careful veterinarian re-assessment. A decline in the populations of Gasterophilus spp. has been noticed in Europe, but all seven Palaearctic species of Gasterophilus appear to maintain stable populations in Xinjiang (China), which may be explained by a higher biodiversity of equids and less use of anti-parasitic treatments in Xinjiang than in Europe. Our study shows that morphological characters still provide the solid backbone in classification of Gasterophilus at species-level, and updated diagnoses and a key is provided to distinguish G. flavipes, G. haemorrhoidalis and G. inermis, and to facilitate studies of epidemiology, phylogeny and host-parasite co-evolution.

A cross-sectional study of horse owners' awareness and perceived risk of exotic diseases in the United Kingdom.

The international nature of the equine industry provides opportunities for the spread of infectious diseases between countries. While incursions of exotic diseases into the United Kingdom (UK) equine population have been rare, the potential socioeconomic and welfare impacts are a significant concern. However, little is known about leisure horse owners' ability or willingness to prepare for an exotic disease incursion. The objectives of this study were to describe UK leisure horse owners' awareness and perceptions of exotic diseases, and to identify clusters of horse owners characterised by their awareness and perceived risk of exotic diseases. A cross-sectional study of leisure horse owners in the UK was conducted between April and July 2018. Participants (n = 403) completed an online questionnaire with questions pertaining to demographics, experiences with endemic diseases, and awareness and perceptions of exotic diseases. Hierarchical cluster analysis was used to identify groups of participants that were similar in regard to their awareness and perceived risk of exotic diseases. Participants identified a median of 3 (IQR 2-4) exotic diseases, with the most recognised exotic diseases being African horse sickness and West Nile virus. The most frequently mentioned clinical signs that participants thought were associated with exotic diseases included high temperature (57.2%), discharge (46.5%), and lack of energy (41.2%). Hierarchical cluster analysis identified three clusters of participants: 1) those who were aware of exotic diseases and perceived a high amount of risk (n = 78); 2) those who were aware of exotic diseases but perceived a low amount of risk (n = 111); and 3) those who were less aware of exotic diseases and perceived a low amount of risk (n = 214). Efforts to communicate the relevance and consequences of exotic diseases to horse owners should consider the potential difference in receptiveness among horse owners in each cluster. Further investigations are required to determine the implications of horse owners' perceived risk on exotic disease preparedness.

Survey of human-horse relationships and veterinary care for geriatric horses.

OBJECTIVE To assess relationships between horse owners (or lessees) and geriatric or nongeriatric horses and to investigate factors influencing veterinary care decision-making. DESIGN Descriptive, cross-sectional, survey-based study. SAMPLE 2,879 people who owned or leased ≥ 1 horse. PROCEDURES Participants were recruited through social media and online horse forums to participate in an online survey about their horses, including measures of attachment, veterinary care decision-making, and experiences surrounding the death of a horse. Data were collected for primary horses (those respondents interacted with most) and secondary horses. Horses were further categorized as geriatric (≥ 20 years of age) and nongeriatric. RESULTS Geriatric primary horses were considered companion animals, retired, or used as part of a business significantly more frequently, and described as competition horses significantly less frequently, than nongeriatric horses. Geriatric horses were owned or leased significantly longer than nongeriatric horses, but the degree of respondents' attachment did not differ for geriatric versus nongeriatric horses. When respondents reported the death of a horse in the previous year, euthanasia was associated with higher levels of bereavement than death by other means. Death of geriatric horses most commonly followed a chronic illness and was associated with significantly higher levels of bereavement than death of nongeriatric horses. Among factors influencing decisions regarding expensive or long-term medical care, the horse's ability to lead a comfortable life was ranked highest. Respondents with geriatric horses made numerous accommodations for their care. CONCLUSIONS AND CLINICAL RELEVANCE Results provided important initial information about the relationships people have with geriatric horses. Understanding how individuals perceive their horses and how they make decisions regarding complex veterinary care is critical in informing effective client communication.

Horse Husbandry and Preventive Health Practices in Australia: An Online Survey of Horse Guardians.

Little is known about the horse health management practices of Australian horse caregivers (owners). This article presents findings from a convenience sample of 505 horse owners who participated in an online survey. No large-scale welfare issues were identified, but there were some areas of potential concern, including owners who did not regularly deworm their horses (4%), a lack of strategic parasite control (3.1%), and a lack of regular dental care (11%). Several participants did not have their horse's hooves regularly shod or trimmed (2%), and 14% had an unqualified person maintain their horse's hooves. One in five owners (19%) did not vaccinate their horses against tetanus. The findings are discussed in relation to current Australian horse health guidelines and traditional sources of horse health information, together with recommendations for providing horse owners with relevant information in relevant forms.

Australian horse owners and their biosecurity practices in the context of Hendra virus.

In recent years, outbreaks of exotic as well as newly emerging infectious diseases have highlighted the importance of biosecurity for the Australian horse industry. As the first potentially fatal zoonosis transmissible from horses to humans in Australia, Hendra virus has emphasised the need to incorporate sound hygiene and general biosecurity practices into day-to-day horse management. Recommended measures are widely publicised, but implementation is at the discretion of the individual owner. This cross-sectional study aimed to determine current levels of biosecurity of horse owners and to identify factors influencing the uptake of practices utilising data from an online survey. Level of biosecurity (low, medium, high), as determined by horse owners' responses to a set of questions on the frequency of various biosecurity practices performed around healthy (9 items) and sick horses (10 items), was used as a composite outcome variable in ordinal logistic regression analyses. The majority of horse owners surveyed were female (90%), from the states of Queensland (45%) or New South Wales (37%), and were involved in either mainly competitive/equestrian sports (37%) or recreational horse activities (35%). Seventy-five percent of owners indicated that they follow at least one-third of the recommended practices regularly when handling their horses, resulting in medium to high levels of biosecurity. Main factors associated with a higher level of biosecurity were high self-rated standard of biosecurity, access to personal protective equipment, absence of flying foxes in the local area, a good sense of control over Hendra virus risk, likelihood of discussing a sick horse with a veterinarian and likelihood of suspecting Hendra virus in a sick horse. Comparison of the outcome variable with the self-rated standard of biosecurity showed that over- as well as underestimation occurred. This highlights the need for continuous communication and education to enhance awareness and understanding of what biosecurity is and how it aligns with good horsemanship. Overall, strengthened biosecurity practices will help to improve animal as well as human health and increase preparedness for future disease outbreaks.

The effect of postsurgical pain on attentional processing in horses.

OBJECTIVE: To investigate the effect of postsurgical pain on the performance of horses in a novel object and auditory startle task. STUDY DESIGN: Prospective clinical study. ANIMALS: Twenty horses undergoing different types of surgery and 16 control horses that did not undergo surgery. METHODS: The interaction of 36 horses with novel objects and a response to an auditory stimulus were measured at two time points; the day before surgery (T1) and the day after surgery (T2) for surgical horses (G1), and at a similar time interval for control horses (G2). Pain and sedation were measured using simple descriptive scales at the time the tests were carried out. Total time or score attributed to each of the behavioural categories was compared between groups (G1 and G2) for each test and between tests (T1 and T2) for each group. RESULTS: The median (range) time spent interacting with novel objects was reduced in G1 from 58 (6-367) seconds in T1 to 12 (0-495) seconds in T2 (p=0.0005). In G2 the change in interaction time between T1 and T2 was not statistically significant. Median (range) total auditory score was 7 (3-12) and 10 (1-12) in G1 and G2, respectively, at T1, decreasing to 6 (0-10) in G1 after surgery and 9.5 (1-12) in G2 (p=0.0003 and p=0.94, respectively). There was a difference in total auditory score between G1 and G2 at T2 (p=0.0169), with the score being lower in G1 than G2. CONCLUSIONS AND CLINICAL RELEVANCE: Postsurgical pain negatively impacts attention towards novel objects and causes a decreased responsiveness to an auditory startle test. In horses, tasks demanding attention may be useful as a biomarker of pain.

"We've learned to live with it"-A qualitative study of Australian horse owners' attitudes, perceptions and practices in response to Hendra virus.

Hendra virus causes sporadic zoonotic disease in Australia following spill over from flying foxes to horses and from horses to people. Prevention and risk mitigation strategies such as vaccination of horses or biosecurity and property management measures are widely publicised, but hinge on initiative and action taken by horse owners as they mediate management, care and treatment of their animals. Hence, underlying beliefs, values and attitudes of horse owners influence their uptake of recommended risk mitigation measures. We used a qualitative approach to investigate attitudes, perceptions and self-reported practices of horse owners in response to Hendra virus to gain a deeper understanding of their decision-making around prevention measures. Data presented here derive from a series of in-depth interviews with 27 horse owners from Hendra virus 'hot spot' areas in New South Wales and Queensland. Interviews explored previous experience, perceptions and resulting behaviour as well as communication around Hendra virus. All interviews were recorded, transcribed verbatim and analysed in NVivo using thematic analysis. Analysis revealed four major themes: perception of Hendra virus as a risk and factors influencing this perception, Hendra virus risk mitigation strategies implemented by horse owners, perceived motivators and barriers of these strategies, and interaction of perceived risk, motivators and barriers in the decision-making process. Although Hendra virus disease was perceived as a serious threat to the health of horses and humans, individual risk perception diverged among horse owners. Perceived severity, likelihood and unpredictability as well as awareness and knowledge of Hendra virus, trust in information obtained and information pathways, demographic characteristics and personal experience were the main factors influencing Hendra virus risk perceptions. Other key determinants of horse owners' decision-making process were attitudes towards Hendra virus risk mitigation measures as well as perceived motivators and barriers thereof. Horse owners' awareness of the necessity to consider individual Hendra virus risk and adequate risk management strategies was described as a learning process, which changed over time. However, different perceptions of risk, barriers and motivators in combination with a weighing up of advantages and disadvantages resulted in different behaviours. These findings demonstrate the multifactorial determinants of cognitive mediating processes and facilitate a better understanding of horse owners' perspectives on preventive horse health measures. Furthermore, they provide valuable feedback to industry and government stakeholders on how to improve effective risk communication and encourage uptake of recommended risk mitigation measures.

A survey of UK prescribers' experience of, and opinions on, anthelmintic prescribing practices for livestock and equines.

The aim of this study was to determine practices, attitudes and experiences of UK prescribers of anthelmintics for horses and livestock. A questionnaire was sent by direct email to groups licenced to prescribe these medicines. These were veterinarians, Suitably Qualified Persons (SQPs, registered with the Animal Medicines Training Regulatory Authority) and veterinary pharmacists. The survey was also advertised through social media. It comprised questions relating to demographics, training experiences, current prescribing practices, as well as personal opinions on anthelmintic selection, diagnostics and anthelmintic resistance. A total of 193 veterinarians and 326 SQPs were included in final analysis. Pharmacists were excluded from detailed analysis due to the low numbers that responded (n=3). The results indicated that SQP participants were more likely to receive post-certification parasitology training than the veterinarians, and that both channels consulted similar sources for information about helminths and their control (paper articles in journals, online sources). The SQP participants stated a higher frequency of face-to-face interactions with clients/customers (96.1%) than the veterinarians (76.4%), who stated a higher frequency of telephone interactions (55.1% and 73.5%, respectively). Veterinarians were more likely to state that there were specific factors that limited interactions with their clients (54.1%) than SQPs (19.6%), such as competition from other suppliers. SQP participants considered a wider range of factors as important when deciding on which anthelmintic to recommend (i.e. knowledge of specific parasites, knowledge of specific anthelmintics, discussion of measures to avoid anthelmintic resistance and time to talk with clients/customers); however, the veterinarian participants were more likely to consider the results of diagnostic tests. While discussions about anthelmintic resistance were stated with similar frequency in both groups, less frequent were specific discussions about anthelmintic sensitivity testing. In-house faecal egg count analysis was more likely to be available from those that prescribed anthelmintics for equines alone, compared to prescribers who dispensed anthelmintics for livestock alone or livestock and equines. The SQP participants indicated that they felt a large number of organisations were responsible for ensuring that anthelmintics are used responsibly, whilst veterinarian participants were more likely to place responsibility on the prescribers alone. Taken together, these findings provide an insight into how prescribers of anthelmintics in the UK interact with their clients/customers before and at the point of sale and act as a unique source of information on how best practice advice pertaining to sustainable helminth control is disseminated by the various prescribing channels.

Investigating attentional processes in depressive-like domestic horses (Equus caballus).

Some captive/domestic animals respond to confinement by becoming inactive and unresponsive to external stimuli. Human inactivity is one of the behavioural markers of clinical depression, a mental disorder diagnosed by the co-occurrence of symptoms including deficit in selective attention. Some riding horses display 'withdrawn' states of inactivity and low responsiveness to stimuli that resemble the reduced engagement with their environment of some depressed patients. We hypothesized that 'withdrawn' horses experience a depressive-like state and evaluated their level of attention by confronting them with auditory stimuli. Five novel auditory stimuli were broadcasted to 27 horses, including 12 'withdrawn' horses, for 5 days. The horses' reactions and durations of attention were recorded. Non-withdrawn horses reacted more and their attention lasted longer than that of withdrawn horses on the first day, but their durations of attention decreased over days, but those of withdrawn horses remained stable. These results suggest that the withdrawn horses' selective attention is altered, adding to already evidenced common features between this horses' state and human depression.

Management Strategies Aiming to Improve Horse Welfare Reduce Embryonic Death Rates in Mares.

The objective of this retrospective study was to evaluate the effect of management strategies aiming to improve animal well-being on pregnancy and embryonic death (ED) rates. Breeding records of a cohort of 1206 Thoroughbred mares brought to a stallion station facility, to be bred with the stallions housed there, were evaluated during ten breeding seasons. Mares were blocked according to management strategies in two groups: Stress and Relax. Strategies used to improve animal well-being (Relax group) were as follows: stopping the teasing routine, reducing or eliminating stall confinement, reducing the number of mares per group and maintaining herd stability during the breeding season. In barren mares, the pregnancy rate was higher in the Relax group (91.8%) when compared to the observed in Stress group (84.7%). However, no difference in pregnancy rates were observed (Stress = 85.2% vs. Relax = 86.2) in foaling mares. ED rate was higher in barren and foaling mares of the Stress group mares (25.5% and 26.8%, respectively) compared with the Relax group (16.1% and 14.7%, respectively). No significant differences were observed on foal heat pregnancy rate between groups; yet, the embryo loss on foal heat was significant reduced in Relax mares (Relax = 8.7% vs Stress = 24.5%). In conclusion, management strategies aimed to reduce social stress can reduce early pregnancy losses and the average cycles per pregnancy, improving reproductive performance in mares.

Rapid animal welfare assessment: an archaeological approach.

The welfare of an individual depends on its capacity to overcome suboptimal conditions in its environment; otherwise, its physical and psychological health becomes compromised. A situation that clearly indicates lack of control of the environment is the expression of abnormal behaviours, such as stereotypies. This study aimed to verify the well-being of police horses using a new rapid form of welfare assessment: an archaeological approach. To this end, we sampled and quantified marks found on the stables, deposited as a result of abnormal behaviour. We cross-referenced these physical marks with veterinary records of diseases, such as colic, known to be associated with stress. A total of 46 horses were sampled and the results showed a significant medium-strength, positive correlation between bite mark frequency on stable doors and the incidence of colic. A weak significant positive correlation was found between length of scratch marks (from pawing) and the incidence of lameness. We conclude that these marks reflect the accumulated expression of abnormal behaviour and can provide rapid insight into the welfare of individual animals.

Overview of the industry and social impacts of the 2007 Australian equine influenza outbreak.

The equine influenza (EI) outbreak occurred at the worst time of the year as far as the horse industry was concerned. All horse sports and horse breeds had events planned in the spring, including those relating to qualification for the 2008 Beijing Olympic Games. These were all disrupted and many were cancelled. The social and industry impacts were extensive, and included difficulties related to communication, animal welfare, vaccination, movement restrictions, economics, as well as the psychological stresses experienced by those involved, especially those for whom their primary source of income was horse related.

Factors associated with high psychological distress in horse industry participants during the 2007 Australian equine influenza outbreak and evidence of recovery after 1 year.

This short report summarises data on the prevalence of high psychological distress in horse industry participants during the 2007 Australian equine influenza (EI) outbreak and the statistical analysis undertaken to identify the factors associated with this high psychological distress. In addition, follow-up data collected 1 year later are reported briefly.

An insider's view of the lockdown at Moonbi during the Australian equine influenza outbreak.

Participants in the 1- and 2-day Tamworth Dressage Club (TDC) annual dressage championships arrived in Moonbi, New South Wales (NSW) as reports of the equine influenza outbreak surfaced on 25 August 2007. Three horses who had attended a competition in Maitland the weekend before had been showing flu-like symptoms and the next day the site was declared a suspect premise. On 27 August, infection was confirmed and the site was placed in total quarantine. Although most participants had travelled long distances to compete and did not have provisions for them or their horses, the quarantine was not lifted until 3 October, having a great social and personal impact on all those trapped in Moonbi.

A survey of aged horses in Queensland, Australia. Part 2: Clinical signs and owners' perceptions of health and welfare.

OBJECTIVE: To describe the prevalence and risk factors for clinical signs of disease and owner-reported health or welfare issues of aged horses in Queensland, Australia. METHODS: Owners of horses were contacted via an equestrian organisation in Queensland and asked to complete a questionnaire about their horses aged 15 years and older, to gain information about clinical signs observed in the horse and disease history. Owners were asked to identify health or welfare issues they felt were important in aged horses. RESULTS: Owners were able to identify many clinical signs of disease, with 83% of horses having at least one reported clinical sign of disease. Muscle stiffness (33%), lameness (28%), ocular discharge (22%), change in hair coat (22%) and skin problems (21%) were the most commonly reported signs. Increasing age was a risk factor for the presence of clinical signs of disease. Ponies had an increased risk of both laminitis and hirsutism, whereas Thoroughbreds had an increased risk of stereotypies, wounds, weight loss and musculoskeletal signs. Owners reported a known disease or disorder in their horses less frequently than the clinical signs of disease. Owners reported weight loss (28%), arthritis/lameness (24%) and teeth/dental care (18%) as the most important health issues, followed by welfare issues (8%). CONCLUSION: Owners demonstrated concern for the health and welfare of their aged horses, but the lack of appropriate recognition of the significance of clinical signs and limited veterinary care suggest a need for increased client communication and veterinarian-client interaction.

Partners with bad temper: reject or cure? A study of chronic pain and aggression in horses.

BACKGROUND: Experiencing acute pain can affect the social behaviour of both humans and animals and can increase the risk that they exhibit aggressive or violent behaviour. However, studies have focused mainly on the impact of acute rather than chronic painful experiences. As recent results suggest that chronic pain or chronic discomfort could increase aggressiveness in humans and other mammals, we tested here the hypothesis that, in horses, aggression towards humans (a common source of accidents for professionals) could be linked to regularly reported vertebral problems of riding horses. METHODOLOGY/PRINCIPAL FINDINGS: Vertebral examination and standardized behavioural tests were made independently on the same horses. Here we showed that most horses severely affected by vertebral problems were prone to react aggressively towards humans (33/43 horses, chi-square test, df=1, χ(2)=12.30, p<0.001), which was not the case for unaffected or slightly affected horses (9/16 horses, chi-square test, df=1, χ(2)=0.25, p>0.05). The more affected they were, the fewer positive reactions they exhibited (rs=-0.31, p=0.02). CONCLUSIONS/SIGNIFICANCE: This is to our knowledge the first experimental evidence of such a link between chronic discomfort/potential pain (inferred from the presence of vertebral problems) and aggression, suggesting that chronic painful experiences may act in ways similar to those of acute experiences. Chronic discomfort or pain may often be overlooked when facing "bad tempered" individuals, whether humans or animals. This experimental study confirms the importance of including chronic discomfort or pain as a major factor in interpersonal relations and models of aggression.