The concept of human health is the most central one in medicine and in the health sciences in general [1]. Health is, indeed, the foremost goal of medicine. Health also has a prominent position in many life contexts, is a crucial condition for pursuing a profession, for enjoying leisure activities, and indeed for living a good life in general. For example, health is a formal prerequisite for performing certain tasks or taking up certain occupations, such as that of soldier, police officer or firefighter. More compelling is the place of mental health as a condition for moral and criminal culpability.

Etymologically, health is connected with the idea of wholeness. This is evident in the verb ‘heal’, with the sense of regaining wholeness. The healthy person is a person who is whole in the sense of having all the properties that should pertain to a human being. Health has thus traditionally been viewed as an ideal notion, a notion of perfection that very few people, if any, can completely attain. Today health also sometimes functions as an ideal notion. This is, indeed, the case with the formulation of health by the World Health Organization (WHO) in its initial declaration, published in 1948: ‘Health is a state of complete physical, mental and social well-being and not only the absence of disease or injury’.

The notion of health is the object of scientific study from several points of view and within several disciplines. Besides research by those in medicine, public health, nursing and other paramedical disciplines, other investigations are based in anthropology, psychology, sociology and philosophy. In some of these disciplines the focus is on a particular aspect of the notion: in psychology, the experience of health and illness; in anthropology and sociology, health and illness as factors of social importance. Philosophical analyses of health have often involved an attempt to formulate global definitions of the idea. Thus, in the following, many references will be taken from philosophical theories of health.

Animal Health

If the concept of human health is central in many scientific debates this is not so with animal health [1]. Although animal health is central to veterinary medicine there are hardly any fora where this concept has been subjected to scrutiny. The reasonable diagnosis seems to be that the concept is not viewed by the professionals as problematic.

Thus the general picture is that only a few veterinary textbooks include any explicit definitions of health and disease. This is, indeed, not different from the case in medical textbooks. A major difference, however, is that in some subdisciplines within human medicine and human health sciences there is some systematic discussion concepts of health, illness and disease. Moreover, other disciplines such as anthropology, sociology and philosophy have entered the arena and put the notions of human health, disease and illness under scrutiny. Such a development does not yet exist in the case of veterinary medicine.

Parasites

Parasites are organisms that depend on a host for feeding and reproduction and belong to various unrelated taxa, primarily protozoa, helminths and arthropods [2]. Complex life cycles have often lead to extreme adaptations; nevertheless parasites may harm their hosts and even cause serious disease and death. Transmission of parasite stages can be environmental, nutritional or vectorborne. Among the most important human protozoan parasites in a global context are Leishmania, Plasmodium (both transmitted by bloodsucking arthropods) and Toxoplasma which is soil- or food-borne. Parasitic worms (helminths) relevant for human health include Echinococcus, Toxocara, hookworms (all soil-borne) and Dirofilaria (transmitted by mosquitoes). Various arthropods (ticks, insects) are involved in the transmission of pathogens due to their blood-feeding behaviour. They are especially involved in transmission cycles between animals and humans (zoonotic infections). Research on parasites and their interactions with the host requires suitable animal models. For parasites with a wide host range or those naturally infecting rodent species available as laboratory animals, established models are available. Others, like the human malaria parasites, require sophisticated and often costly genetic manipulation to allow for infection in nonnatural rodent hosts. Alternatively, surrogate models of closely related helminth species in rodent models are used to study human parasites.

Every organism may host several parasite species. Of all animals, humans have the greatest parasite diversity. While coevolution of the parasite with its host leads to adaptation limiting the damage to the host at the cost of parasite control, in a medical and veterinary context many parasites still cause considerable harm and, due to their infectious nature, can threaten human and animal health. Parasites often have complex life cycles which may involve stage conversion, metabolic and morphological changes during development and a switch from one host to another (heteroxenous development). Parasites frequently produce long-living stages that can persist in the environment for months and even years. While certain parasites are specialised on a single host species (stenoxeny), others are generalists (euryxeny) and can infect unrelated host species. Zoonotic infections are characterised by parasite transmission between humans and non-human vertebrates.

Public Health

Public health discussions have come to the fore in the last decades, which is laudable because these discussions are focused on prevention of disease rather than only treatment [3]. But in practice, “public health” discussions are so narrowly focused on human health issues that nonhuman animals are dismissed as unimportant.

The connection between “health,” “public,” and “animals” is easy to identify—it is the important notion of “zoonotic disease” or “zoonosis,” defined generally as a disease that can be transmitted from nonhumans to humans. Cases where humans infect nonhuman animals are referred to as “reverse zoonoses.” The root of the word zoonosis is the Greek word zoon , “animal.” Because humans are as truly members of the “animals” category, as is any nonhuman, transmission of disease between humans and nonhumans is basic biological reality. Thus, transmission of diseases from, say, birds to humans is no more astonishing than transmission of diseases between two nonhuman species.

It has been estimated that as many as 75% of the new diseases that have affected humans over the past decade have been caused by pathogens originating from nonhuman animals, in one way or another. These include not only newly emerged diseases, but some older ones as well, usually called reemerging zoonotic diseases. Some of these problems have spread by various means over long distances and become global problems. Even though many zoonoses are preventable (rabies is an example), they cause many deaths and otherwise impact millions of people every year. Tragically, they affect most heavily the poorest segment of the human population. In addition, zoonotic diseases can greatly impact production of food from nonhuman animals. This has created obstacles to national and international trade, and thus drawn the attention of powerful governmental and private interests.

Despite the link between animal welfare and human health, animal welfare issues have been, with few exceptions, notoriously absent from the public health dialog [4]. When the subject of animals does enter discourse on human health, it is usually to highlight how animals are sources of infection for and cause injuries to humans. There has been almost no discourse, however, about the fact that the way in which we treat other animals is often central to how and why humans are injured or catch infections. Moreover, such treatment is central to how and why we face a significant number of health threats. This distinction is important to note. To illustrate this, it helps to take a look at how the public health discipline has traditionally approached the subject of zoonoses— infectious diseases that can be transmitted from non-human animals to humans (and vice versa). There is ample medical literature exploring the variety of zoonoses that can be passed from animals to humans. There is also much medical literature and many studies exploring how and why we contract zoonoses and pass them on to each other. Such work looks predominantly at human–human interactions, weather patterns and the life cycle of infectious agents. When public health practitioners and investigators do turn their attention specifically to animals, it is usually to describe which animals are likely to be infected and thus pose a threat to us. Rarely do we explore deeper than this and ponder whether the nature of our relationships with animals could play a role in whether they become infected with a pathogen in the first place or pass the pathogen on to us.

It is easy to forget that one of the primary purposes of a health system should be to improve health [5]. For decades, debates on health systems have been dominated by discussions of how much they cost to run (typically questioning whether they are aff ordable, as if there was an alternative in a civilized society) or how many resources they require (typically expressed in an arbitrary fashion as people, usually doctors and nurses but not managers or physiotherapists, or facilities and items of furniture, usually hospitals but not primarycare clinics or beds, but not examination couches). The nature of this discourse has meant that health systems have tended to be regarded as a cost to society from which there is little return, instead of as an investment whereby appropriately directed expenditure leads to better health. Indeed, some studies even suggested that more resources were associated with higher mortality, possibly as a consequence of higher rates of discretionary surgery, a finding supported by the observation that death rates fell when doctors went on strike.

AAI

Animal-assisted intervention (AAI) includes several activities in which animals are participants, with the aim of assisting human patients [6]. These activities commonly involve quiet interaction with a dog. This type of interaction is associated with a relaxation effect resulting from sensory input involving four senses; namely olfactory, tactile, auditory, and visual. The patient sees and smells the dog, pets and talks to the dog and its handler, listens to the noises made by the dog and hears the handler’s interpretation of the dog’s behavior, and a decrease in the patient’s serum cortisol levels occurs, leading to a decrease in blood pressure and heart and respiratory rates. This relaxation may be the factor making AAI applicable in high-anxiety contexts. Given that anxiety is typically associated with medical and psychotherapeutic treatment, this mechanism may be the main reason that use of AAI has increased so markedly in recent years.

Essentially, two forms of AAI are being used in health care contexts. Animal-assisted activity (AAA) consists of animal visits that are usually conducted by volunteer dog-handler pairs. The hallmark of AAA is spontaneous content of the visits; there are no visit-related treatment goals for the person receiving the visits. In contrast, animal-assisted therapy (AAT) is a specific treatment goal-directed activity that is customized for the particular patient.

AAT

During the last century, some health-related treatments and therapies have spawned fierce controversies among their various practitioners [7]. In the medical field, for example, licensed MDs have long questioned the medical benefits of chiropractic, osteopathy, and homeopathy. In the mental health field, psychiatrists have held dominance for many years and have occasionally disputed the professional competence of psychologists and/or social workers to counsel patients. Indeed, some psychiatric organizations have vigorously opposed insurance payments to non-MD mental health workers.

Unlike these medical and mental health fields, animal-assisted therapy (AAT) has been mostly spared from such acrimonious debates. A few factors may account for this lack of controversy. AAT is a relatively new therapy and thus has not been subjected to the type of long-term critical analysis that has been applied to the medical and psychiatric professions. AAT has also not experienced competition from alternative therapies. Since it is a nascent therapy, AAT is still generally unknown among many health and mental health professionals, who therefore have little reason to oppose its therapeutic regimen or fear its possible economic consequences to their own health-related professions.

Nevertheless, some controversies have arisen. Although most studies of AAT indicate its beneficial effects, a few psychologists and animal ethologists have raised questions about both the methodology of these studies and the lack of long-term assessment of this therapy. Probably the most contentious AAT issue has been the treatment of animals.

Transmission and Spread

Much of the current disease profile in humans owes to either the domestication of animal species or their use during our evolution from huntergatherer to agriculturally oriented societies [8]. The human-animal interface is the nexus that permits the cross-species transmission of infectious agents and is represented by a continuum of contacts between humans and animals, either directly or indirectly through their products and their shared environments. ‘The human-animal interface’ is thus a term that encompasses the wider socioeconomic and biological influences of disease transmission and spread, elements which are fundamental to the examination of human-animal infectious disease management. It is the human-animal interface that has arbitrated the transmission of zoonotic diseases and the introduction of novel pathogens into new geographical areas and novel host species. However, while its role in disease transmission is not new, because of globalization, its current ecological dimensions are of a completely different order of magnitude. In essence, modern industrialized society is an important source of the expanded ecological pressure at the human-animal interface. The human-animal interface therefore provides an important conceptual framework for the examination of the public and animal health risks of animalsourced epidemics, and through health policy, their relationship with risk enabling anthropogenic activities. These risk enabling activities include changes in land use, livestock production, chosen routes for economic growth and trade promotion; activities that both foster and enhance zoonosis transmission.

Recent examples of global infectious disease spread, such as Severe Acute Respiratory Syndrome (SARS) and pandemic avian influenza, have negatively impacted on both public health and economies. Such threats have led to a shift to develop policies to respond to these risks, at national, regional and international levels. But because the dynamics, and therefore the risks, of disease emergence differ from location to location, equally important is the integration, within these policy frameworks, of approaches to assessing both the risk’s ‘local’ likelihood and ‘impact’ to ensure, to the extent possible, the appropriateness of policy responses. This is a challenge, for both developing and developed countries, given the myriad interests that contribute to this ‘perfect microbial storm.’ It is reasonable to assume, however, that the risk of infectious diseases, and in particular, pandemic scale infectious disease emergence, is unlikely to abate, and as a result, the public health and animal health communities have to think of emerging infectious diseases, their control and the assessment of their risk of occurrence in completely novel ways.

Environment

Humans and animals all live in some kind of environment [1]. This environment has many parts. First, there is a physical environment, a landscape with natural resources and climate. Secondly, there is a cultural environment, a society with laws and regulations, a political system, customs and other expressions of culture. The cultural environment is particularly salient for humans but it also has a great impact on animals, not least with regard to regulations for human interference with animals. Thirdly, there is a close psychosocial environment consisting of relatives, friends and co-workers in the human case, and of mates, offspring and human carers in the animal context.

This environment influences our lives in many ways, when seen from a logical point of view. First, there is the direct causal influence. There are fundamental positive causal influences from our physical environment; we get our nourishment from nature and we get the necessary warmth from the sun. The climate has a direct physical impact on our bodies, both positively and negatively. The artefacts of human civilization, constituting the cultural environment, are often important positive factors. Houses, for instance, provide us with shelter and protection. Other products of civilization, however, are detrimental. Chemicals in the water and in the air can and sometimes do have disastrous consequences for humans and animals.

A state of welfare is created, affected or annihilated by different combinations of external conditions. To the effect that such a combination of external conditions contributes positively to a person’s or animal’s welfare we call it a state of external welfare. The simple terms ‘welfare’ and ‘illfare’ will in the following denote the internal state of welfare of a human or an animal.

This characterization must now be supplemented. There is an important domain between external and internal welfare. There are not only external conditions of inner welfare, but also a series of internal conditions. It is not only the external world that affects us. We are also, to a great extent, determined by our own physical and mental constitution: how we are constituted of physical and mental elements, our physical and mental strength, our health and our character, as well as our inclinations and interests. Our inner properties are, certainly, continuously affected by the external environment, but there is an important constitutional basis that the environment cannot affect in any other sense than that it can annihilate it.

Human Obligations

Moral systems have evolved, in humans and other species, because cooperation and tolerance are successful strategies, especially in social species [9]. Most people would say that we have moral obligations to humans and animals of other species. If we use a living animal in a way that gives us some benefit, we have an obligation to that animal. It is my view that human behaviour and laws should be based on the obligations of each person to act in an acceptable way towards each other person and to each animal that is used. It is better to base strategies for living on our obligations rather than to involve the concept of rights because some socalled rights can result in harm to others.

With increasing knowledge and increasing efficacy of communication there has been a change in attitudes to people with a broadening of the range of people for whom we have concerns. We also now consider that a wide range of animals deserve moral consideration. One view of animal protection occurs because the animals are considered to have some intrinsic value. For many people, certain animals are valued because of evidence for their cognitive abilities, their awareness, mental aspects of their needs and the feelings that they can have, e.g. pain, fear and pleasure. Animals vary in the extent to which they are aware of themselves and of their interactions with their environment, including their ability to experience pleasurable states such as happiness and aversive states such as pain, fear and grief. The concept of sentience affects our decisions about which animals to protect. A sentient being is one that has some ability: to evaluate the actions of others in relation to itself and third parties, to remember some of its own actions and their consequences, to assess risk, to have some feelings and to have some degree of awareness.

People are more likely to want to protect those animals that are sentient. Human opinion as to which individuals are sentient has changed over time in well-educated societies to encompass, first all humans instead of just a subset of humans, and then also:

• Certain mammals that were kept as companions

• Animals that seemed most similar to humans e.g. monkeys

• The larger mammals

• All mammals

• All warm blooded animals

• All vertebrates

• Some invertebrates

New knowledge about brain function and welfare has tended to show that the abilities and functioning of non- human animals are more complex than had previously been assumed so there should be some re-appraisal of which animals should be protected.

Vegetarianism

Consider what humans beings would stand to lose from the widespread adoption of vegetarianism [10]. To choose a world where vegetarianism was morally obligatory for humans would be to choose a world where humans, relative to the actual world, have to give up certain things. The first point to note, however, is that these things do not include life or health. In most environments at least, a healthy life is perfectly possible for a vegetarian, a fact attested to by the existence of millions of healthy vegetarians living in almost all parts of the world. There is no doubt, of course, that meat is a valuable source of nutrition, primarily because it provides all of the amino acids essential for human beings (i.e., amino acids that the human body is not capable of producing on its own). Meat, however, is not essential in this regard since the essential amino acids can also be obtained from suitable combinations of vegetable protein. And while the knowledge necessary for effecting such combinations is, perhaps, not currently widespread – due in large part to the prevalence of meat-eating in our society – this knowledge is in no way abstruse or recondite. The necessary knowledge is, in fact, no more complex than that required to combine suitable amounts of protein, carbohydrate, and fat in one’s diet. Vegetarianism, then, does not ordinarily require humans to give up either their life or health.

If vegetarianism were to become widespread, the principal thing that humans would have to give up would be certain pleasures of the palate. Meat, for most people at least, is often delicious. Some vegetarians actually profess to dislike the taste of meat, others, however, having been persuaded by moral considerations, still dream of rump steaks, and of those heady days when pork ribs would be merrily crackling on the barbecue. While meat is undoubtedly tasty, it is perhaps easy to make too much of this. One who does like the taste of meat dishes is not thereby precluded from finding vegetarian dishes equally appetizing. It is not as if vegetarianism and eating palatable food are mutually exclusive options. To suppose that they are is simply to be ignorant of what it is possible to do with the humble vegetable.

Human health is defined as a high degree of general functionality of the organism, ie undisturbed functionality of the organism, harmony and undisturbed intellectual and biological functions. Human health can also be seen as a state of good physical, mental and social well-being. This term is most often used as a term for absence from illness, trauma, deformities and mental disorders. Some of the health conditions are a balanced diet, physical activity and hygiene, and stable family and social circumstances.

1. L. Nordenfelt. "Animal and human health and welfare: a comparative philosophical analysis." CABI, Wallingford, UK, 2006, pp. 3–4, 47, 159–160.

2. J. Walochnik et al. "Parasitic infections in humans and animals." In: E. Jensen-Jarolim (ed.), Comparative medicine: disorders linking humans with their animals. Springer International Publishing AG, Cham, Switzerland, 2017, p. 178.

3. P. Waldau. "Animal rights: what everyone needs to know." Oxford University Press, New York, USA, 2011, pp. 119–120.

4. A. Akhtar. "Animals and public health: why treating animals better is critical to human welfare." Palgrave Macmillan, Basingstoke, UK, 2012, p. 4.

5. M. McKee. "Health systems and health." In: S. Benatar and G. Brock (eds), Global health and global health ethics. Cambridge University Press, Cambridge, UK, 2011, p. 63.

6. R.A. Johnson. "Animal-assisted intervention in health care contexts." In: P. McCardle et al. (eds), How animals affect us: examining the influence of human-animal interaction on child development and human health. American Psychological Association, Washington, USA, 2011, pp. 183–184.

7. D. Altschiller. "Animal-assisted therapy." Greenwood, ABC-CLIO, Santa Barbara, USA, 2011, p. 39.

8. K. Mwacalimba. "Globalized perspectives on infectious disease management and trade in Africa: a conceptual framework for assessing risk in developing country settings." In: G. Steier and K.K. Patel (eds), International farm animal, wildlife and food safety law. Springer International Publishing AG, Cham, Switzerland, 2017, pp. 42–44.

9. D.M. Broom. "International animal welfare perspectives, including whaling and inhumane seal killing as a W.T.O. public morality issue." In: D. Cao and S. White (eds), Animal law and welfare: international perspectives. Springer International Publishing AG, Cham, Switzerland, 2016, p. 46.

10. M. Rowlands. "Animal rights: moral theory and practice." 2nd ed., Palgrave Macmillan, Basingstoke, UK, 2009, pp. 163–164.