IACUC Guidance on Categorizing and Minimizing Pain and Distress

Background, Assumptions and Definitions

The Institutional Animal Care and Use Committee strives to support UCI as a world-class research facility.  As stewards of animal welfare, the committee recognizes its role in facilitating a deeper understanding of the importance of identifying and minimizing pain and distress associated with research procedures in live animals.  This is a complex and dynamic goal that the IACUC actively pursues as it oversees all proposed animal research.


If something is painful to people, it is the humane assumption that it will also cause pain in research animals.   Painful stimuli prompt withdrawal and evasive action to prevent bodily damage.   Pain is a complex experience that is researched worldwide as a means to alleviate it.


Distress in general refers to an aversive state in which an animal fails to cope or adjust to various negative stressors.  Distress can be variable between individuals and different species, and so is more difficult to categorize.

"USDA Pain Categories"

Refer to the columns on the USDA Annual Report of Research Facilities (APHIS Form 7023):

  • Column B:  Animals held only for breeding purposes or for future research use; they have not undergone any research procedures of any kind (with the possible exception of genotyping procedures).  Animals produced in breeding colonies and not used for research purposes (e.g. culled littermates).
  • Column C:  Animals that have undergone procedures considered not painful or only momentarily painful, for which analgesics, anesthesia or other intervention is not required.
  • Column D:  Animals that have undergone procedures, tests, etc., considered more than momentarily painful or uncomfortable and have received appropriate analgesia, sedation, anesthesia, or other methods to mitigate the discomfort.
  • Column E:  Animals that have undergone painful procedures or disease processes without the use of drugs or other methods to relieve the pain/distress.

Most vertebrate animals used or intended for use in research, testing or teaching at UCI will fall into categories C, D, and E.

General Guidance Regarding the Categorization of Pain/Distress

Procedures and disease models that produce more than momentary pain or distress are an unavoidable part of research with animals; such procedures require the appropriate use of sedatives, anesthetics, or analgesics, unless withholding of such agents is scientifically justified in writing and approved by the IACUC.  All potentially painful or distressful procedures must be identified in the Animal Use Protocol; the protocol must include criteria that will be used to assess pain and distress in the animals and describe the actions that will be taken to minimize it, such as the use of anesthesia and analgesic agents, housing and husbandry variations, and identification of alternative humane endpoints.  Consultation with ULAR veterinarians is strongly recommended.

Guidelines and Examples:

Category C

Procedures considered to produce minimal, transient, or no pain and distress when performed by competent individuals using recognized methods.  These procedures typically do not require the use of pain-relieving drugs or other interventions.


  • Intravenous, subcutaneous or intraperitoneal injections
  • Venous blood collection
  • Gastric lavage
  • Stimuli that produce only transient pain and/or are escapable or avoidable (e.g., mild foot shock)
  • Behavior testing without aversive stimuli
  • Specific activities such as wheel running in rodents where the running wheel is an experimental variable.
  • Injection of euthanasia agents

Category D

Procedures considered to produce more than minimal pain or distress, where use of anesthesia or analgesic agents are routinely provided to relieve the discomfort or the animal is humanely euthanized in a timely manner.  In addition to analgesia, husbandry variations such as softened food, water accessible from the bottom of the cage, and extra bedding may be considered to reduce potential pain/distress.


  • All surgical procedures, regardless of whether a body cavity is opened or penetrated and regardless of whether the animal is allowed to recover
  • Wounding or induction of injury/disease states such as burns or radiation exposure, where animals are provided with analgesia/anesthesia according to standard veterinary practices
  • Injection or application of compounds with induce excessive inflammation or necrosis (e.g., bradykinin, Complete Freund’s Adjuvant; infectious agents, etc.) where animals are provided with analgesia/anesthesia according to standard veterinary practices.
  • Disease or injury models associated with pain or distress that can be treated with analgesics, anesthesia, or special husbandry variations or the animal is humanely euthanized in a timely manner.

Category E

Procedures considered to produce more than minimal or transient pain or distress, where the use of anesthetics, analgesics, tranquilizers or other palliative efforts would compromise the experimental design.  Category E procedures require strong scientific justification in the animal use protocol.


  • Toxicological or microbiological testing, where the lethal dose of an agent must be determined (e.g. LD50 studies)
  • Disease research that requires continuation until an animal's death (i.e. "Death as an Endpoint")
  • Untreated controls in pain studies

Additional Guidance

The IACUC at UCI is composed of scientists, veterinarians, and others who carefully review all proposed work with live vertebrate animals to assess the impact of research procedures and their potential for causing pain and distress.  The IACUC acknowledges that it may be difficult to identify and categorize pain and distress in live animals, and will work with individual researchers to make these determinations on a case-by-case basis.

  • Procedures in which brief anesthesia is used only to temporarily immobilize or restrain the animal may be considered Category C.
  • Animals that are removed from the study and euthanized at the first sign of pain or distress as a result of frequent and careful monitoring may be considered Category D.
  • All examples listed under Category D above are considered "Category E" if analgesics must be withheld for experimental reasons.

There may also be situations where the actual pain/distress categories experienced by the animals is different from what was predicted in the animal-use protocol; in these cases, the animals should be reported to the IACUC after use in the category most consistent with the animal's apparent experience of pain (as part of the Annual Progress Report).