Abstract

1. Introduction

One Health (OH) is a holistic approach which defines the health of humans, animals and the environment as a coherent system. OH as defined by the WHO includes the design and deployment of policies, legislation and research at multidisciplinary level to assure better public health [1]. Hippocrates (460 BC–370 BC), the Greek physician and “father of modern medicine”, mentioned in his text entitled “On airs, waters and places” (400 BC) that the health of humans depends on a clean environment. He emphasized the importance of medical researchers incorporating geographical locations, climate conditions and the quality of drinking water as factors affecting health and susceptibility to diseases. The interdisciplinary approach of OH combines veterinary and human medicine, environmental science, wildlife biology and public health. OH becomes increasingly important as 61% of infectious agents are zoonotic [2]. Several factors influence OH as increasing travel behavior, global trade and change in land use, population growth of humans and animals as well as an increasing use of animal products [3,4]. The idea of OH can be traced back to ancient times, when traditional healers treated both animals and humans. Moreover, plant and animal components (e.g., hooves and bones) were used to cure diseases in humans [5]. Nowadays, comparative medicine, an interdisciplinary field of veterinary and human medicine, integrates animals as an important factor in unearthing the origin of diseases. Therefore, studying the human/animal interface is crucial.

Horses are among the most important animals in human history; they have been used in wars, as a means of transport, and even facilitated work in mines. In the late 19th century, horses played a crucial part in developing the first antidote to cure diphtheria. Since then, the rate of contact between domesticated horses and humans steadily increased. Nowadays, for example, horses play an important role in animal-assisted therapy. Furthermore, the detection of infectious diseases that affect both humans and horses are crucial, especially in cases of highly transmissible diseases. Beside infectious diseases, non-communicable diseases (NCDs) such as skeletal and joint diseases or metabolic disorders are of concern to both. Several risk factors concerning the health of humans and horses exist. Therefore, this article will give an overview of OH with a focus on horses and their relation to the environment and humans, along with the influence of other factors such as climate change (see Figure 1).

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Figure 1

Horse, human and environment interaction pathways. (Symbols: freepic.com).

2. The Horse-environment Relationship

Feral horses (Equus caballus) can be found on every continent of the earth except Antarctica [6], and have an impact on soil and vegetation [7,8], the biodiversity of plants [9], and several animal species, such as reptiles and small mammals [10,11], ants [7], herbivores [12,13] and grassland birds [14]. Nevertheless, among the genus Equus itself, great diversity can be observed [15]. From an OH perspective, horses’ influence on the environment depends strongly on several factors such as plant-animal coevolutionary history, soil development, climate, frequency of grazing, and animal density [10]. Horses play an important role in increasing plant diversity through acting as natural fertilizer and by the dispersal of plant species [9,16]. Accordingly, horses can positively influence the biodiversity of both plants and animals. In southwestern Spain, free-ranging Galician mountain ponies can help in preventing forest fires by eating small plants growing under the shades of long trees. Moreover, the consumption of scrub has a positive influence on the maintenance of threatened heathlands and the variety of species living there. Furthermore, those ponies are the most important source of food for wolves; consequently, this leads to the indirect protection of farmers´ livestock, since wolves do not hunt their animals [17]. The grazing habits of the wild horse have a significant impact on other animal species; for instance, horses consuming Spartina grass destroy the nesting habitats of laughing gulls (Leucophaeus atricilla)—however, as a result, the shore bird diversity will increase [18]. The presence of wild horses leads to a significant reduction of the desert bighorn sheep and elks sharing the same water source, which is evidence of indirect competition [12,13]. Wild horses can have a negative impact on environmental health, for example horses’ trampling leads to an increased soil strength and compaction, which reduces rainwater infiltration [8]. This in return can lead to nutrient and water shortages for plants and trees. Horses´ bark chewing behavior increases the mortality of trees, which results in changes in vegetation composition and structure [19,20]. These examples clearly show the influence of the horse is not linear, but rather depends upon the surroundings. Thus, a negative influence on one species can have a positive effect on another.

3. The Domestic Horse-human Relationship

In contrast to wild horses, domesticated horses live in close contact with humans. The research of Warmuth et al. (2012) discovered that the domestication of horses started approximately 6000 years ago in the Ukraine, southwest Russia and west Kazakhstan [21]. The domestication of animals can be a result of “humans’ innate tendency to focus on life and lifelike processes”, referred to as Biophilia hypothesis by Edward O. Wilson [22]. To demonstrate the strong animal–human bond, a human´s loss of a companion animal—whether natural or by euthanasia—is associated with deep grief and depression [23]; however, compared to other pet animals, horses are frequently sold when not “suitable” anymore [24]. Furthermore, the use of horses strongly depends on the cultural background. In some countries, horses are needed for work, and thus have an impact on the economic status of the owner. In high-income countries, horses are primarily used for sport, breeding, animal assisted therapy, or as companions for leisure. Besides that, horse meat is a common food source, especially in France, Mexico and Argentina [25]. Despite animals (e.g., cattle in Hinduism) playing an important role in religion [26], which directly affects human mental health [27], horses are not part of any religion. Nevertheless, horses have always been deployed as a sign of power. During the Iron Age, a horse would be sacrificed during the burial of its leader [28]. In Roman times, white horses were considered to be holy [29]. Horses were also associated with a goddess, Epona, who represented horsemanship [30].

4. Horses in the Medical Field

In the 1890, horses played a crucial part in developing the first antidote to cure diphtheria (Corynebacterium diphteriae) in humans [54]. Horse serum is also used as an anti-venom, for example, when humans are bitten by snakes (e.g., Brazilian Bothrops or Crotales) [55]. However, horse serum can cause an allergic reaction in humans named serum sickness, as reported after the introduction of the diphtheria-antitoxin in 1894 and with equine rabies immunoglobulin [56,57]. Wilde et al. (1989) point out the importance of standardizing the purification methods and potency criteria to achieve as low side effects as possible [58].

As an animal model, horses were used in the research of Hepatitis C, since the virus displays great similarities to the equine hepacivirus [59,60]. Furthermore, horses were employed as a model for respiratory diseases (human allergic neutrophilic asthma) [61], orthopedic problems (focal articular cartilage injuries) [62,63], and in defining the causes of depression as an ethological animal model [64].

5. Zoonotic Diseases

Several infectious diseases are known to affect both horses and humans, see Table 1. Zoonotic diseases are those transmitted from horse to human through direct contact (e.g., Hendra virus) or indirect infections due to vehicles such as food products (e.g., Botulism) or vectors like ticks (e.g., Lyme-Borrelioses) and mosquitos (e.g., West Nile Fever) [65]. Zoonotic diseases are mostly an interface between wildlife, animals and humans [66]. Therefore, the transfer rate of zoonotic diseases highly depends on the immune status of the animals and humans, pathogen-control measurements and the contact to wildlife [67]. To monitor the transmission of zoonotic infections, several surveillance systems do exist [66]. Despite many reports on zoonotic diseases, no study has been recorded on reverse zoonosis, where human is the main source of infection to horses.

6. Non-communicable Diseases

Besides infectious diseases, non-communicable diseases (NCDs) such as skeletal and joint diseases, cardiovascular problems, psychological problems or metabolic disorders are of great concern for horses and humans. NCDs are the result of genetic, physiological, environmental and behavioral factors and persist over a long duration of time [103]. Since horses are often kept in stables, physical inactivity may be a man-made reason for skeletal and joint diseases in horses. On other hand, the extensive training of a horse can lead to NCDs, e.g., post-traumatic osteoarthritis [104] and back problems [32]. Psychological disorders can evolve because of inappropriate housing management, weaning methods, and social or feeding restrictions [105,106,107]. Horses can develop stereotypic behavior—such as weaving, wind sucking, crib biting or box walking—when living in a poor and unattractive environment [105,108]. Furthermore, unhealthy diets, especially highly concentrated and sugar-containing diets, can cause diseases such as equine metabolic syndrome [109]. If keeping horses in stables with bad air quality and low hygiene, horses and their keepers can develop respiratory problems, like the chronic obstructive pulmonary disease [110,111]. Other NCDs such as Colitis-X, Caprophagy in foals or Buttress foot can be influenced by humans as well [112]. Since the welfare of farm animals is the human responsibility, several welfare concepts, like “The five freedoms”, exist to prevent management-related diseases [113]. Pain in the lower back, hip joint and hamstring muscles are the most common horse riders’ orthopedic problems [114].

7. Risk Factors

There are various risk factors, such as climate change and antimicrobial resistance, which have a negative impact on the health of both humans and horses. In order to counter this, the U.N. developed the “Sustainable Development Goals” focusing on 17 targets, such as “Zero Hunger”, “Good Health and Well-Being” and “Climate Action” [115], with the ambition to reach a sustainable future for everyone. Even those goals are anthropocentric, from a OH perspective horses are a crucial part in Goal 3 “Good Health and Well-Being”, since this objective aims to reduce the spread of infectious diseases. As already mentioned above, the health of working horses is significant for the income in certain communities. Therefore, horses are also of relevance to Goal 2 “Zero Hunger”.

8. Conclusions

Despite the need of OH several decades ago, only recently has more attention been given. Discussions at the scientific level are urgently needed; however, the involvement of the public and community members at local, national and global levels is essential to assure the implementation of OH. Cooperation between “open-minded” interdisciplinary professions bringing together OH aspects from various research areas will be a strong approach to increasing awareness beyond the anthropocentric perspective. In other words, keeping animals and environment health primarily for the service of human health is no longer valid. If a `radical` OH approach assumes that the health of humans, animals and the environment are of equal value to one another and interactive, then the responsibility of humans becomes immediately apparent, especially in the case of man-made issues. The public must be educated before direct interaction with horses; therefore, legislation with social license to operate the horse is necessary. While the OH approach is often associated with zoonotic infections, other OH aspects are important, especially the NCDs and maintaining a healthy environment as well as the production of effective vaccines. Ultimately, the horse as a crucial part of OH represents a valuable opportunity to demonstrate the constructive interference of OH aspects between animals, the environment and humans.

Author Contributions

Conceptualization: N.S.L., A.A.E.W.; writing-original draft preparation: A.A.E.W., N.S.L.; preparing illustration: N.S.L.; review and editing: A.A.E.W., N.S.L., K.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding. We acknowledge the support of the Open Access Publication Funds of the University of Goettingen.

Conflicts of Interest

The authors declare no conflict of interest.

References