JAMES W. YEATES

Cats Protection, Haywards Heath, UK

Over the past two decades, the veterinary field has seen expanding awareness and discussions of animals’ quality of life (QOL). The concept (or perhaps concepts) has been applied to human medical patients, veterinary patients, and public social policymaking. Over this time, discussions on veterinary patient QOL have become increasingly mainstream within the veterinary literature. Anecdotally, this seems to have been reflected in more explicit considerations of QOL within consulting conversations, although doubtless some veterinary surgeons have long been considering, assessing, and improving their patients’ QOL without necessarily using the term. The construct of QOL has been developed primarily by conceptual analysis, extrapolation from human medical literature, and by learnings from researchers trying to develop assessment methods and tools (McMillan, 2000, 2003; Wojciechowska and Hewson, 2005; Scott et al., 2007; Yeates and Main, 2009; Giuffrida and Kerrigan, 2014; Belshaw et al., 2015; Mullan, 2015).

This chapter examines the complex concept of quality of life. An emphasis is placed on the practical application of QOL for animals. We suggest how veterinary surgeons, and animal owners, might better assess and improve animals’ QOL. Arguably, QOL is – or should be – the single most important factor involved in the most consequential decisions made in veterinary medical and nursing practice and policy. It can guide decisions regarding clinical diagnosis, therapeutic options, and elective euthanasia, as well as help us to evaluate societal management practices and to test novel therapies.

7.2 Qualities of Quality of Life

The World Health Organization defines QOL in humans as ‘The individuals’ perception of their position in life in the context of the culture and value systems in which they live and in relation to their goals, expectations, standards and concerns’ (World Health Organization, 1996). At first glance this definition appears to be problematic for use in animals, but it is actually more useful than it may initially seem. What it does is relate QOL to the individual’s perception, motivations, and predictions, and to what matters for that individual. These ideas, as we shall see, are very applicable to animals.

More specifically for animals, QOL has been described as ‘a subset of welfare’ (Wojciechowska and Hewson, 2005; Broom, 2007; Taylor and Mills, 2007). Others have suggested treating it as equivalent (or at least related) to other concepts such as ‘well-being, welfare, happiness, life satisfaction, and contentment’ (see Chapter 2, this volume). However, there is limited consensus on what QOL is in animals (e.g., for dogs, see Belshaw et al., 2015). Indeed, many reports of QOL tools do not adequately describe the concepts of QOL involved (Giuffrida and Kerrigan, 2014; Belshaw et al., 2015). That said, it is reasonable to suggest that we do not need a strict ‘scientific’ definition (Yeates, 2013) so long as the term meets certain criteria to assure that all who use the term QOL are actually referring to the same thing.

In the absence of a consensus definition useful for nonhuman animals (see below), we might consider several characteristics that QOL has as a concept. In broad terms, we might think of QOL as what gives animals’ lives – from the animal’s perspective – value. Operationally, we can describe it as what animals would (most likely) want for themselves.

7.2.1 Evaluative

QOL is, as the name suggests, the quality that life has for that animal. In this sense, the technical term is well connected to the philosophical use of ‘quality’ to refer to a property of something (e.g., the ‘quality of mercy’). More specifically, we are not concerned with just any property of an animal’s life (e.g., whether a life is rational, verbal, or even useful to others). We are concerned with the characteristic, or characteristics, that make life good for that animal.

This view that QOL is inherently evaluative has three key implications. The most evident is that ‘evaluative’ implies some idea of value. There are varying views on what is valuable for animals, including health, resources, security, control, social interactions, and human–animal relationships. Some might be said to have direct, intrinsic value; others are considered indirect for what they cause or represent.

A second, related implication is that QOL is not merely a factual description – it has ethical importance. As such, it is somehow directly relevant to what we should try to achieve or avoid. (How it is relevant is discussed below.) Of course, it is not the only relevant factor, but all else being equal it can provide a guide to our moral interactions with animals.

A third implication of QOL being evaluative is that it requires evaluators – or, at least, valuers – who use data to generate some assessment. Because our focus here is animal QOL, it is important to note that this does not necessarily imply that the individual forms such evaluations through some conscious and explicit cognitive process, i.e., animals do not need to comprehend the concept of QOL for their lives to have quality. Animals evaluate specific aspects or factors affecting their lives while we might serve as proxies to evaluate their lives on their behalf (we shall consider below how these different evaluative processes might be used together to base our proxy evaluations on animals’ first-person evaluations). So, this aspect of QOL does not require the ability to provide verbal self-reports of one’s QOL – the preferred method for evaluating QOL in conscious adult humans – it does restrict QOL to animals for whom aspects of their lives can have ‘value’ based on these positive and negative evaluations. This in turn would suggest that QOL is limited to sentient animals. Rocks and trees do not have a QOL, but tree squirrels, rock doves, rockfish, tree frogs, and rock pythons do.

7.2.2 Broad

The value of a life can range from severely negative to extremely positive: QOL can be good, poor, or indifferent. QOL goes beyond those facets linked directly to survival and maintenance (e.g., eating, drinking, toileting, and grooming) to also consider less immediate matters (e.g., play, exploration, and social engagement). It includes both aspects of ‘coping’ and, even more positively, ‘flourishing’. It covers animals’ prudential motivations to avoid (e.g., suffering) and to achieve (e.g., satisfaction). It covers what animals should be free from, and what they should have opportunities for (Table 7.1).

QOL also has breadth as a holistic concept. It relates to or is constructed from many different domains or facets that are potentially valuable (Wojciechowska and Hewson, 2005; Mullan and Main, 2007; Taylor and Mills, 2007; Hielm-Björkman et al., 2009). Considering QOL is therefore helpful to ensure that an array of aspects is considered holistically, in proportion to their importance for each animal. It is also well worth bearing in mind that the different facets interact in complex ways, so that the value of one facet may depend on others, for example, the interplay between stressors and previous exposure or learning.

QOL is also broad insofar as it covers an extended period of time. An animal might have good or bad welfare at any given point, but QOL is essentially considered over a longer interval. Indeed, we might characterize it as animal welfare over time (see Chapter 2, this volume). This extended temporal nature makes it important to consider duration and chronicity alongside intensity.

7.2.3 Individualistic

It is generally considered that QOL relates to what is valuable for the individual animal (McMillan, 2003). The individual’s perspective and perception has been described in the literature in animals for both welfare (Green and Mellor, 2011) and QOL (Wiseman-Orr et al., 2006). This individualism links to the consideration of positive states, insofar as these might vary more between individuals. In general, we see widespread uniformity in the dislike of injury or disease, whereas, in contrast, sources of enjoyment exhibit a wide variation among animals. This individualism also reflects the complex interactions between personalities, preferences, previous experiences, development, and learning, which results in two animals faring quite differently in the same circumstances and responding differently to analogous stimuli.

7.2.4 Mental

There is one (and perhaps only one) aspect of animals’ lives that (i) definitely and intrinsically has value; (ii) covers a wide range, multiple facets, and extends over time; and (iii) is completely personal to the individual: the individual’s mental states (including their etiologies and symptoms). Intrinsically, the affective experience of pain is worth avoiding, and conscious desires are worth seeking. As noted earlier, while these mental states are subjective experiences, they may also be viewed as subjective evaluations. In other words, the value of an animal’s life relates to that animal’s subjective experiences.

These subjective experiences appear to play a predominant, if not exclusive, role in how animals evaluate aspects of their lives. Two subjective processes seem inherently evaluative: affective and motivational (see Berridge, 1996; Berridge and Robinson, 1998). Affect is the subjective experience of reward or its opposite: in everyday terms, ‘liking’ and ‘disliking’, such as pleasure and pain, and enjoyment and suffering. Many authors have suggested QOL represents the balance of pleasant and unpleasant feelings (see Chapter 2, this volume). Motivation is the subjective preference for a particular outcome: in everyday terms, ‘wanting’ to achieve or avoid given stimuli or outcomes. Pleasure and reward imply positive evaluations; displeasure and aversion imply negative evaluations. To rephrase Nagel’s (1974) classic question of ‘What is it like to be a bat?’, QOL can be viewed as ‘How enjoyable it is to be a bat?’.

7.3 QOL and Other Concepts: What QOL is Not (Only)

Recognizing that many factors may affect QOL, it is also useful to consider those factors which QOL does not directly consider.

7.3.1 Quality versus quantity

We commonly differentiate between quality and quantity of life. In one sense, QOL is absolutely not about death, birth, or longevity (although these might be co-affected by the same factors). QOL is independent of any idea of sanctity of life. Rather, QOL is a matter of what each animal ‘gets out of’ its life from that animal’s point of view. So, in fact, QOL provides a way of assessing whether a specific quantity of an animal’s life is, or is expected to be, good (see Fig. 7.1). Life is valuable where the QOL is good, and worth avoiding where the QOL is poor. So improving QOL is always good. Increasing quantity of life is only good if the expected QOL is good.

Pain is an important facet of QOL (Brown et al., 2009; Belshaw and Yeates, 2018), but clearly QOL is much more than pain. QOL is about the overall – net – value of life. Pain constitutes just one form of suffering; there are many others, such as fear, anxiety, boredom, frustration, loneliness, and the like. Plus, as considered above, good QOL is not simply the absence of negatives: it also involves positive aspects of life such as pleasure, enjoyment, engagement, achievement, and flourishing.

7.3.3 Health

Health is important for animals’ QOL (Wiseman-Orr et al., 2006; Reid et al., 2013), but QOL encompasses more than health (Smith et al., 1999). It is not its physical state, nor is it restricted to an animal’s biological ‘function’. It also is not its ‘normality’ – another commonly used factor in health and welfare assessments. In contributing to an animal’s QOL, some health problems may be less important than some other aspects of life. Most importantly, an animal’s QOL depends on how it is affected by, and responds to, any pathologies or physiological disturbances. Two animals (or humans) may have the same degree of disease or pathology, yet experience much different impacts on their QOL.

However, even allowing for individual variation in responses to health disorders, unless health – including mental health – is at least adequate, an animal is unlikely to have a high quality of life. Severely poor health can be an important determinant – as well as indicator – of poor QOL. However, because of the prominent involvement of QOL in veterinary medicine there is a real risk of ignoring (or at least deemphasizing) QOL factors that are not health related. We should recognize that ‘health related’ and ‘nonhealth related’ are interrelated characteristics and it is impossible to disentangle the QOL effects of a disease from all the other impacts on a patient’s QOL, including emotional factors, environment, genetics, etc. In the absence of clear demarcations between what facets of QOL are health related or not, we risk overlooking relevant aspects of QOL that would be included in a more holistic consideration of QOL.

7.3.4 Relationships and instrumental value

The value that QOL denotes is independent of any instrumental value of animals’ lives to humans (e.g., as a source of eggs, progeny, meat, as an experimental subject, or even as a companion). An animal’s QOL is the value of its life for that animal, not for the owner. An owner might consider an animal valuable for its usefulness and performance, and an owner might value their relationship with the animal in terms of affection, affinity, and companionship. But although these factors may be linked to other factors or signs that affect QOL, they are not themselves directly relevant. For example, a beloved or valued animal might actually have a lower QOL than another animal that is less loved, but better cared for, by its owner. Measures of human–animal relationships (HAR) are decidedly not a proxy for QOL, and Serpell (2019) has recently detailed the difficulties that these relationships create in obtaining an accurate assessment of a pet’s QOL.

7.4 What Affects QOL

7.4.1 Inputs and indicators

QOL can be affected by many different aspects of animals’ lives. In a way, the lack of clarity over exactly what constitutes QOL (e.g., objective and subjective aspects) has meant that previous work is often unclear whether what is being assessed is a constituent of QOL, a factor of QOL, or an indicator of QOL.

Nevertheless, we can identify a wide range of aspects that might affect, or be affected by, QOL. Proximately, we can consider inputs over a given period, such as diet, environment, social companionship, and care. Ultimately, these in turn depend on owners’/stockpersons’ compassion, expertise, and resources. Animals’ health can be both causes of QOL (e.g., disease may diminish QOL) and indicators of QOL (e.g., feline interstitial cystitis resulting from stress [Stella et al., 2011]). Indeed, so can most factors insofar as variations in intakes (e.g., appetence) or interactions (e.g., explorations) may be symptomatic of QOL changes. As we said above, facets need to be considered holistically, and with regard to how valuable those inputs are, as indicated by how each animal responds.

We might base our QOL assessments on animals’ evaluations without necessarily reflecting their actual views naively. For example, one approach is to think of QOL in terms of what animals prudentially should be motivated to avoid or achieve, i.e., what would they want if they were perfectly rational and informed. This does not mean that QOL is the satisfaction of whatever motivations animals express. Animals (including humans) are not perfectly rational and happen to have many desires that conflict, for example, a dog’s expressed motivation to eat chocolate might conflict with an (implicit) motivation to avoid the suffering from theobromide poisoning (cf. Yeates, 2018). However, if the dog were informed on the dangers of consuming chocolate, this motivational conflict would be, for that dog, more easily resolved. This approach is crucial in avoiding flawed QOL assessments for animals (and humans, for that matter) in deprived living conditions, who, because they know no better life, view their own QOL as satisfactory (the ‘it’s-all-he-knows’ situation). This is the ‘Happy Slave Problem’ (Phillips, 2006), and the implications for deprived animals is that it is possible that because these animals have an incomplete knowledge of what is possible, the animal may not be aware of a gap between his current experience and the possibilities of a life he would find more desirable. Using a measuring stick for QOL that emphasizes desires, we might imagine asking the animal if he has everything he wants – and the answer is yes. But if the animal based its response on a fully informed view, the answer would be no.

7.4.2 Evaluating multiple inputs

When making an overall evaluation of an animal’s QOL, we typically desire to reach a single assessment based on a broad range of all facets of an animal’s life, extended over a period of time. Sometimes, one facet may be so bad or good that we do not need to consider others. Severe suffering may preclude the possibility of significant enjoyment, effectively making other aspects of the animal’s life irrelevant to whether the life is worth living. An animal with an extended aggressive bone tumor can be expected to have a poor QOL whether or not his bedding is comfortable. (This does not mean that his bedding should not be made more comfortable – we might still want greater precision of assessment to mitigate some of his suffering.) The situation where the negativity of one facet ‘overshadows’ all other aspects of QOL, however, appears to be the exception rather than the rule.

The most common situation asks us to combine or aggregate all facets into a single score. Human patients may perform this aggregation for themselves. However, nonhuman animals cannot assess and report their QOL, and we cannot observe it objectively. We must therefore infer QOL from observations about all of an animal’s positive and negative experiences over the specified period of time. This aggregation adds a difficulty of combining seemingly incommensurable data into one overall evaluation (including the problem of QOL predictions, which is discussed in Section 7.6). One drawback of combining all facets into a single score is the potential for ‘hiding’ inter-observer variation in how individual facets are evaluated while risking worsened reliability. In addition, numerical scores may be arbitrary if the scale is not linked to a fixed comparison point (Craven et al., 2004).

Because different factors have different levels of importance to different individual animals, a QOL evaluation that gives the same weight of every facet to every animal may not result in an accurate QOL assessment. For example, if one dog seems fine being left home alone all day and another dog exhibits great distress when alone, then a score for ‘level of human companionship’ that was weighted equally between the two dogs would show the same effect on the total QOL score, when in fact the overall QOL would be impacted much differently between the dogs. One approach to overcome this problem is to assign ‘weights’ to different QOL facets based on presumed importance of that facet to that individual animal, and then aggregate these weighted facet scores into an overall evaluation (McMillan, 2003; Morton, 2007; Budke et al., 2008). As logical as this approach appears, the precise method for determining weights clearly involves much subjectivity, and, hence, error. These weighting approaches have been questioned in the human field (Trauer and Mackinnon, 2001).

7.5 Using QOL

Before applying the concept of QOL to animals, some general issues are important to consider.

7.5.1 Using proxy assessments

Insofar as QOL can be considered as the subjective assessment of how one’s own life is faring, this requires us to tap into each individual animal’s own evaluations. As previously mentioned, the assessment of communicative human patients is usually based on asking individuals to evaluate their own QOL. The individual has access to their own subjective states, can make relative assessments across positive and negative states, decides trade-offs, and can aggregate assessments across multiple domains.

Might animals meaningfully assess their own QOL? We currently lack convincing evidence that they can, at least explicitly. This would require not only that the animals communicate their QOL assessments, but also that we can clearly comprehend what they are communicating. Fortunately, however, the use of QOL with animals (and, for that matter, pre-verbal human infants and other noncommunicative humans) does not require that animals explicitly assess and communicate their own QOL. It need not be the individuals themselves who make the overall QOL assessment even if the concept is based on their subjective evaluations of QOL facets. Animals’ inabilities to articulate a QOL means that we serve as proxy assessors for them with the goal of producing an objective evaluation based on their subjective states.

Ultimately, we rely on observable data from which to infer QOL. We cannot ‘see into’ animals’ minds, and need to use what we can ‘see’ such as their circumstances, pathophysiology, and behavior, analyzed using analogical methods. This is, of course, true for humans too – but those data might include their verbal self-reports (which we assume are more reliable representations of their actual mental states). We might usefully consider factors that are observable and representative of the animals’ own subjective perceptions, such as their expressed preferences and decisions, and their apparent degree of satisfaction with those choices.

7.5.2 Measuring quality of life

As an inherently evaluative concept, we do not have a ‘natural’ unit of measurement for QOL. So, while many facets can be measured quantitatively, overall QOL cannot be counted, measured, or calculated using methods like those we employ for phenomena such as blood pressure or body temperature. This is one of the reasons why it is difficult to establish any cut off point between ‘acceptable’ and ‘unacceptable’ QOL.

We might give up on quantification and judge animals’ overall QOL qualitatively. We use everyday words such as ‘good/poor’, or ‘high/low’. We use explicitly ethical words such as ‘acceptable’ and ‘unacceptable’. We might explicitly consider whether an animal’s continued life is of sufficient net quality overall to be ‘worth living’ (FAWC, 2010; Yeates, 2011).

Alternatively, we could try to use numerical representations of our relative assessments. For example, one approach to calibrating the numbers might be to evaluate QOL relative to quantity of life. The dividing line between a ‘life worth living’ and ‘life worth avoiding’ could provide a zero point, as the representation of a QOL that has the same value as nonexistence. We could explicitly evaluate periods of time in terms of their relative value, for example, whether it would be worthwhile for the animal to make a trade-off between two periods. For example, if 1 month of pain would be considered comparable to 1 year of a different suffering, then we could argue the value of the former is, pro rata, 12 times worse than the latter. Together, these could give us a ratio scale for QOL.

7.5.3 Minimizing bias and error

As human proxies, there will be biases in our assessments (Serpell, 2019), and in our effort to be ‘objective’ we should strive to recognize and overcome any such biases. Systematic approaches can help identify and eliminate invalid assessment methods that might, for example, prioritize or ignore certain facets of QOL that are important for humans but not necessarily for nonhumans. It is important here to note that humans’ self-assessments of QOL – which would seem to be tautologically unassailable in terms of accuracy – have been shown to be subject to biases and changing contextual factors. Poor intraobserver reliability occurs as an individual’s situation changes, suggesting that assessments are at least partly contextual. For example, human patients may shift their self-assessed responses over time (Breetvelt and Van Dam, 1991; Sprangers, 1996; Sprangers and Schwartz, 1999), as they age, or if they assess injuries as less bad than they previously seemed. This may be due to recalibration (to different benchmark or thresholds) or reprioritization (of the various factors in their life). The crucial point is that if these errors can occur when humans assess themselves, then they may be expected to present similar problems in owners’ assessment of animals (McMillan, 2007).

7.5.4 Tool validation

Practicing veterinarians seeking to select the best assessment tool(s) for use in their patients will understand the preferability of choosing tools that have been validated. The immediate problem, however, is that there are few such tools (yet) available (current QOL assessments tools is discussed in Section 7.6.2). The lack of a ‘gold standard’ (Sharkey, 2013) as an independent and objective point for comparison makes it challenging to definitively validate newly developed QOL assessment tools. Consequently, many of the published tools are only partially validated (Belshaw et al., 2015). Also important for the practitioner to keep in mind is that there are important reasons for using QOL assessment tools that do not require robust validation. For example, Yeates et al. (2011) proposed that a valuable use of QOL assessments is to promote discussions and facilitate decisions between veterinarians and pet owners, which requires a comprehensive overview of all aspects of the animal’s life rather than precise scores. This was not intended to be validated by conventional measures, since the concepts were grounded in each owner’s (and practitioner’s) own conceptualizations and context.

7.5.5 Recognizing assessment limitations

When using tools, we should understand the underlying psychometrics and evaluative aspects – never using a score from an algorithm we cannot access and understand. One of our most important risks is mistakenly thinking a particular tool is generating an assessment of overall QOL when it actually only covers a significantly restricted range of facets, such as health or pain. We should certainly ensure that the impacts of pain and poor health are considered in QOL assessments, but also remember that QOL assessments are not limited to facets that are affected by health. At the same time, we should also recognize that our assessments are based on our viewpoints and values. They are never definitive evaluations of animals’ QOL. Never should we believe we have ‘determined’ an animal’s QOL with precise accuracy, especially when others’ opinions disagree. However, even with such limitations it is still important to assess QOL as well as we can.

7.6 Applications of QOL Thinking

Broadly speaking, QOL can be used in four ways: screening, assessing, predicting, and decision-making. Of course, we might employ multiple uses at the same time (e.g., using current assessments to stimulate screening discussions or inform predictions). Indeed, we probably should use multiple applications; for example, euthanasia discussions can be useful to move from screening to assessments, to predictions, to decisions.

One important basic question is who should do the evaluating. Veterinary surgeons and nurses may have a wide range of knowledge and experience, and seem more ‘objective’. Owners may have biases from misplaced anthropomorphism or their emotional investment, but also have direct empathetic knowledge of their animals over an extended period in their everyday environment. Where possible, evaluations should be the product of all relevant views, with each party helping others to explore, identify, interpret, and evaluate different facets and form an overall collective evaluation. Indeed, each should be reliant on the others. For example, veterinarians often are reliant on owners’ assessments in the home environment, while owners are reliant on veterinarians to predict disease progression. Veterinarians can help owners to explore each of the below applications, guiding them through screening to a decision.

7.6.1 Screening

QOL screening can help look for a wide variety of potential issues, among which health is a primary but not the only factor. Screening is particularly useful for identifying treatment side-effects or new conditions over time. The benefits of using QOL for screening is that it can help us consider a wide range of facets rather than simple screening for narrow predetermined concerns. Screening is most important when outcomes are uncertain, for example, in novel treatments or assessments for new health problems. It is important not to use screening tools for overall assessments, unless we are confident they are comprehensive and we are clear how we are formulating our overall assessments.

7.6.2 Assessment

Assessment of QOL can target overall QOL or only particular facets of QOL. There are now a number of tools available for assessing overall QOL and are compiled in Table 7.2 (also see Hewson et al., 2007 and Belshaw et al., 2015 for reviews in dogs). We might make these assessments as a comparison (e.g., to an earlier time of life or to another treatment group) or against some particular predefined criteria. Many QOL assessment tools ask people to rate animals’ QOL subjectively (e.g., Mellanby et al., 2002; Craven et al., 2004; Brown et al., 2007). Others ask assessors about multiple facets, with or without some overall mathematical assessment.

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