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IV. Humane Endpoints
Federal regulations require researchers to limit any pain and distress their animals might experience to what is unavoidable to carry out meaningful scientific research. The laws state that unrelieved pain and distress should continue only for the duration of a worthwhile experiment that cannot avoid it.
U.S. researchers also are required to report unrelieved pain and distress in certain species of animals to the United States Department of Agriculture. More than 90 percent of animals used in research and testing are rats and mice; in the United States, however, rats, mice, and birds are excluded from the reporting required by the Animal Welfare Act. Estimates are that the total number of research animals suffering unrelieved pain and distress likely exceeds one million each year 1.
Humane endpoints should be sought and used when an animal is so physically or psychologically stressed that its continued use does not add to and indeed detracts from answering the scientific question under study.
An endpoint should be used if the degree of suffering produced by an experimental procedure is higher than anticipated and cannot be justified by the sought-for benefits, or when the level of distress is so high that it is simply immoral to produce it. If suffering can be justified, but an alternate, earlier endpoint can be found, of course this needs to be put into practice 2, 3
Setting a humane endpoint prior to unavoidable pain and suffering is an alternative experimental design that sets out to avoid as much suffering on the part of animal subjects as possible. Such an ending point can be defined as the earliest point at which an animal can be identified as experiencing severe pain, severe distress, or impending death. It would be best to be able to predict this point before it occurs and plan to end the experiment there.
Humane Endpoints: Definition
The term "endpoint" is therefore defined as the point at which an animal's pain or distress is ended or reduced by taking action. Intervention includes killing the animal humanely, stopping a painful procedure, or alleviating the pain or distress with appropriate measures.
In the best case scenario, an experiment is ended when animals begin to show clinical signs of disease, provided this endpoint is compatible with the research objectives. Such an endpoint would be better than using death or a moribund condition as an experiment's end, as this would minimize pain and distress and in fact might avoid unintended bias in the data 5.
Is Death the Best Endpoint?
Research that produces the most pain and suffering includes experiments to find effective new medical procedures for addressing infection and cancer, and these are likely to use death as an endpoint (Preface 6or 7).
Death is rarely required to decide if a new intervention will succeed. Typically, death in lab animals follows from failing to eat and drink. An animal in this state may take several days to die. One way to avoid this burden of suffering is to identify the physical signs that say an animal will die and choose a specific ending point that can be relied on to predict death. This approach will work for a variety of situations, including testing new vaccines or antibiotics, toxicity testing, and virulence assays of microbes or parasites.
For instance, in a rabies prevention test, the fact that animals will start to go in circles at a certain point in the rabies infection was able to serve as a reliable endpoint. Another example is a five degree drop in body temperature in rats exposed to a Staph infection in order to test for effective treatment: the fall in body temperature reliably predicted death by several hours.
Animals found dead in the course of experiments not only represent missed opportunity for implementing alternative and more humane endpoints but possible loss of valuable data as well. Such deaths mark unnecessary suffering and bad science (Morton 8 or 9).
Limit the Unavoidable
Many situations can be cited where pain and suffering is an unwelcome accompaniment, such as in the production of monoclonal antibodies in mice or the use of Freund's adjuvant in making antibodies in lab rabbits. Here humane endpoints are relatively easy to define and can be implemented, for example, by limiting the volume and number of times one collects fluid from a mouse with ascites. Often guidelines for these procedures are available in the literature 10.
Even when designed into a study, endpoints should not be slavishly followed if unpredicted changes in the course of an experiment take place. Judgment and experience must operate. Studies should be halted before the anticipated endpoint is reached if the objectives of the study have been met or if it is clear that the study will not ever meet its objectives.
Endpoints have to be carefully set. Ending an experiment too early-ie, without meeting its objectives-wastes the animals' lives. Ideally, as much information as possible should be obtained from each and every animal, while keeping pain, suffering, and stress to an absolute minimum12.
Seek and Validate Shorter Endpoints
Effective endpoints should not simply be looked up in the literature. This does not allow refinement of endpoints. A pilot study can generate a better, new endpoint, comparing data with prior studies using a previously accepted endpoint.
As an example, a study to test potential toxic action on the nervous system could look for damage to the nerve cells, seeking an endpoint earlier than an extreme indication of neuronal damage, such as loss of the righting reflex.
Chain of Command
The investigator is most aware of the scientific objectives of the study. But endpoints should be decided in consultation with the animal care veterinarian and the animal care and use committee (IACUC)13.
Investigators will, of course, have an in-depth knowledge of previous studies that have been conducted using a particular animal model, either from their own efforts or from knowing the literature. This inherent expertise aids in deciding how to monitor the animals so that the endpoint will be reached when the appropriate people are present and can humanely end the animals' lives or otherwise stop the experiment.
Investigators need to establish a clear chain of command so that the person who will decide to terminate is available and informed. Animals need to be monitored with sufficient frequency, as guided by thorough knowledge of the normal behavior and physiology of the species14.
When is Enough Enough?
A main concern to scientists, particularly cancer researchers, is that killing an animal too soon might miss the opportunity to observe that a sick animal will indeed get better with a new treatment. It is crucial in such cases to find the signs that point to irreversible decline. For instance, in a rat model of brain tumor, researchers were able to determine that a weigh loss for more than six days correlated highly with inevitable progression to death. An endpoint that satisfied the scientific concerns could therefore be established after six days of continued weight loss15.
Why Moribund Doesn't Work
Many research questions require the production of progressive and severe disease states that clearly could cause the deaths of experimental animals. These types of protocols often specify conditions under which preemptive euthanasia will be performed and may state that animals will be euthanized when they become "moribund." People may have varying concepts of this term's implications, however, rendering it poorly defined and arbitrary16.
The animal in a moribund state may be past suffering, actually comatose. Before the animal gets to the point of being moribund, judgments based on accurate observations of the animal could set an earlier endpoint to reduce distress17.
Dispute over consciousness and awareness is ongoing in human medicine and is still the subject of news headlines. In people, as in animals, lack of behavioral response to painful stimuli does not prove lack of consciousness. A state of unresponsiveness can mimic or even be due to neuromuscular blockade, in which someone cannot generate motor responses but remains aware. Clinicians call this condition "locked-in"18. Data collected under those conditions could prove unusually variable or even impossible to interpret, as the moribund state and imminent death might modify important physiologic variables.
What Can Work as Humane Endpoints?
Hypothermia can predict imminent death. Preemptive euthanasia is carried out if an animal's temperature drops below a predetermined value that is invariably linked to impending death. A caveat is that body temperature can be influenced by ambient temperature and by other aspects of the environment, such as the bedding material or the presence or absence of cage mates19.
Monitoring body temperature in small laboratory animals can be accomplished with laser-directed infrared temperature scanners, tympanic infrared thermometers, or implanted thermistor microchips 20.
Another simple but specific endpoint that is obvious and objective is the inability to stand or move about. Hind-limb paralysis in rats and mice, rather than death, can be used as the endpoint in certain clinical models.
It should be noted that some studies may have valid reasons for using spontaneous death as the endpoint. Some may require prolonged clinical maintenance of animals before euthanasia. In other cases, despite conscientious efforts by the research team, objective criteria that predict imminent death and provide a signal for preemptive euthanasia may be difficult or impossible to identify21.
Checklists Useful, Objective
Detailed observational checklists that help to select earlier endpoints have allowed many investigators to move away from death or the moribund state as the endpoint22.
Checking off on lists of signs and symptoms that score an animal's condition provides an objective basis for decisions during a study. The advantages of such checklists are the same as what is available for airline pilots: nothing is taken for granted. Checklists help to focus one's observations, particularly with the smaller laboratory animals where clinical observations are more difficult to carry out than with larger animals—e.g., temperature, heart rate, respiratory rate.
How often should the animals be observed? It is generally accepted that normal, healthy animals should be checked at least daily. As an animal enters a potentially critical period, more frequent observations are necessary. The sensitivity and judgment of the animal care committee will help determine what is acceptable 23.
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Readings and Resources on Humane Endpoints