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Humane Endpoints in Animal Research

How to Select and Use Humane Endpoints

Examples of Humane Endpoints

Suggestions for the Use of Humane Endpoints in Selected Studies

What is a humane endpoint?

Humane endpoints refer to one or more predetermined physiological or behavioral signs that define the point at which an experimental animal’s pain and/or distress is terminated, minimized or reduced by taking actions such as euthanizing the animal, terminating a painful procedure or giving treatment to relieve pain and/or distress (CCAC). Humane endpoints function as an alternative to experimental endpoints and provide investigators with an effective way to refine their research. The establishment of humane endpoints prior to the start of an experiment allows the investigator to prevent unnecessary animal pain and distress while ensuring accurate and timely data collection.

To be effective, humane endpoints must be clearly defined and based on objective criteria. Non-specific signs of illness such as inactivity, hunched posture or a rough coat are an indication that an animal should be examined more closely. By themselves these signs do not often constitute an endpoint. Familiarity with the animal model in use is necessary to select endpoints that are both humane and scientifically sound. As experience with and data collected from a specific animal model accrue, endpoints can be refined or modified. Further information on humane endpoints may be found here:

UC Davis Center for Animal Alternatives.

USDA National Agricultural Library

Canadian Council on Animal Care

Investigators should include the following when describing how humane endpoints will be used in their IACUC protocols:

  • A precise definition of the humane endpoint(s), including specific assessment criteria. Examples of potential humane endpoints are listed below.
  • The frequency of animal observation and assessment.
Frequency of observations: Normal, healthy experimental animals must be observed at least once a day. Animals in studies involving pain and/or distress will often require more frequent observations to effectively determine the time at which a specific endpoint has been reached. An appropriate monitoring schedule must be specified in the IACUC protocol for each study.
  • The training of personnel responsible for observation and assessment.
  • The action(s) taken when an animal reaches a humane endpoint.

Moribund animals

The term moribund refers to an animal that is near death or in the process of dying. Animals in this state are often comatose (unresponsive and unaware of stimuli) and so beyond awareness of suffering. However, an animal may have experienced much pain and distress prior to reaching a moribund state. Stating that animals will be euthanized when they become moribund is not an appropriate humane endpoint as this may not reduce or alleviate any suffering that the animal will experience. The purpose of identifying endpoints is to prevent or minimize animal pain and distress.

Death as an Endpoint

While certain types of studies have historically used death of the animal as a scientific endpoint, this is now rarely accepted and investigators must present conclusive evidence to support the use of such an endpoint.

How to select and use humane endpoints:

Choose appropriate endpoints that are objective and relevant for the assessment of pain/distress in the species. This may include:

  • Body weight changes
  • External physical appearance
  • Behavioral changes
  • Physiological changes (e.g., body temperature, hormonal fluctuations, clinical pathology, etc…)

Research personnel responsible for observing and evaluating animals must be adequately trained and experienced in the recognition of these signs for the species being used. Especially when using behavioral assessment, personnel must be familiar with “normal” before they can be expected to recognize “abnormal”. Investigators are responsible for ensuring these students and employees are appropriately trained and have the skill and authority to treat or euthanize animals who have reached an endpoint.

Pilot studies (experiments) can be useful in determining endpoints, especially when the effects of an experimental treatment in animals are not well known. They may also function to refine experimental studies by allowing for the establishment of earlier endpoints and provide training for personnel in the recognition of endpoints.


Examples of humane endpoints:

Deteriorating body condition score

  • Objective and easy to use for assessing the condition of animals used in research, especially studies where animals may experience some degree of debilitation as the study progresses.
  • Scoring methods have been developed for many species including mice (Ullman-Cullere, M. Body condition scoring: a rapid and accurate method for assessing health status of mice. Laboratory Animal Science 1999, 49(3):319-323.)

Weight loss

  • Rapid weight loss of 15-20 percent within a few days. This requires frequent monitoring of body weight.
  • Gradual weight loss - over an extended period of time leading to emaciation. The degree of weight loss should be specified in terms of % or quantity (grams, pounds, kg).
  • Note: Certain debilitating conditions such as tumor growth and ascites may mask true weight loss.

The inability to rise or ambulate

  • Correlates with inability to access food or water.
  • Visually obvious, objective and easy to assess.

Tumor size

  • Usually measured as diameter of the mass or percentage of body weight (i.e. greater than 1.5 cm diameter in mice or greater than 10% of body weight)

The presence of labored respiration

  • The animal shows increased respiratory rate and/or effort. Labored respiration is often accompanied by a strong abdominal component to breathing.


  • The skin looses its elasticity. Skin pinched over the back should return to its normal position after it is released. In a dehydrated animal the skin will remain tented.

Ulcerated, necrotic or infected tumors.

  • The presence of large open wounds.

Suggestions for the use of humane endpoints in selected studies:

Chronic studies where some degree of debilitation is expected.

  • Body Condition Scoring
  • Weight Loss
  • Loss of ability to ambulate (inability to access food or water).
  • Labored respiration may be associated with lung pathology or abdominal enlargement placing pressure on the diaphragm.

Experimental Neoplasia

  • Tumor size - should not exceed 10% of body weight (approximately 1.5 cm diameter for a mouse, up to 3.0 cm diameter for a rat).
  • Tumor ulceration, infection, or necrosis.
  • For internal tumors Body Condition Scoring may be more useful.
  • Loss of ability to ambulate (inability to access food or water).
  • Labored respiration may be associated with lung pathology or abdominal enlargement placing pressure on the diaphragm.

Acute Studies

  • Body condition scoring
  • Weight loss of greater than 20%
  • Dehydration
  • Loss of ability to ambulate (inability to access food or water).
  • Labored respiration may be associated with lung pathology or abdominal enlargement placing pressure on the diaphragm.

Experimental Surgery

Many of the same signs as listed above in addition to conditions specific to post-surgical infections or other complications including:

  • Pain, swelling, redness or discharge from surgical incisions.
  • Dehiscense (splitting apart) of surgical incisions.

These signs may not by themselves be endpoints but are medical issues that suggest there are deficiencies in surgical techniques or care that require attention.