Standard on Assignment of Animals Into Pain Categories


University of North Carolina at Chapel Hill Standard on Assignment of Animals Into Pain Categories



Per USDA requirements, the University of North Carolinat at Chapel Hill ("UNC-Chapel Hill" or "University") reports an annual census of the number of USDA-regulated animals used in research and teaching, the type of species used, and the number of animals placed in each of four Pain Categories (B, C, D and E). UNC-Chapel has extended use of the USDA Pain Categories to all animals used in research at the institution. The animal care and use application (ACAP) Section 4.3 is a table in which investigators include the estimated animal numbers during the 3 year protocol approval period and categorize animals into appropriate anticipated pain categories. The intent of this document is to provide general recommendations and guidance on proper assignment of animals to the pain categories.


This Standard applies to all personnel involved with animal research at UNC-Chapel Hill.

The UNC-Chapel Hill Institutional Animal Care and Use Committee (IACUC) expects that anyone involved in animal work at the University will comply with this Standard. Requests for exceptions to this Standard must be reviewed and approved by the IACUC.


A submitted ACAP should assign each animal or group of similar animals to the appropriate category, indicating the highest pain/distress category that will apply to the animal at any time while on the protocol. These categories are denoted as Pain Category B, C, D, or E. The severity or duration of the pain/distress or the use of anesthesia/analgesia to alleviate pain/distress will determine the appropriate pain/distress category. Procedures for the purpose of non-research veterinary care should not be included in determining the pain category.

Pain Categories

These examples are not comprehensive lists, but may be helpful in assigning the proper pain category.

Pain Category B:

Animals being bred, conditioned, or held for use in teaching, testing, experiments, research, or surgery but not yet used for such purposes.

  • Typically used for large, long-lived animals such as dogs and non-human primates.  Uncommon in laboratory mice and rats
  • Housing or holding only with no euthanasia
  • Breeding only with no experimental manipulations, genotyping, or euthanasia
  • Observation in natural habitat having no physical contact with animals

Pain Category C:

Animals upon which teaching, research, experiments, or tests are conducted involving minimal or no pain, distress, or use of pain-relieving drugs.

  • Routine injections of non‐toxic, non‐irritating substances
  • Blood collection, including tail nick, tail snip, and venous blood sampling
  • Tattooing, ear punching, ear notching, tail clipping, or ear tagging, genotyping
  • Approved AVMA euthanasia
  • Restraint for non‐invasive procedures
  • Genetically engineered phenotype with unknown clinical health complications, but not expected to cause congenital pain/distress
  • Mild symptoms after inoculation, tumor induction, or infection by a viral/bacterial agent that do not require clinical treatment for pain relief or alleviation of symptoms
  • Observation of animal behavior in research setting, such as locomotion and social interactions
  • Assessment of responsiveness to mild noxious stimuli (e.g., hotplate, Von Frey) in which the animal can escape the stimulus
  • Mild, transient colitis
  • Mild to moderate ascites formation without systemic effects
  • Infrequent electric shocks from which an animal can escape or avoid
  • Cancer studies limited to subcutaneous tumors and conforming to size limits described in the Tumor Burden Standard

Pain Category D: 

Animals upon which experiments, teaching, research, surgery, or tests are conducted involving accompanying pain or for which appropriate anesthetic, analgesic, or tranquilizing drugs are used.

  • All survival and non‐survival surgeries
  • Exsanguination or transcardial perfusion conducted under anesthesia
  • Any post-procedural outcome resulting in evident pain, discomfort, or distress that is treated with anesthetics or analgesics
  • Induced infections or antibody production treated with appropriate anesthesia or analgesia
  • Arthritis models treated with analgesics
  • Footpad injections with concurrent use of analgesics
  • Injection or blood collection by invasive routes such as intracardiac, central vessel, or periorbital sites under anesthesia
  • Physical trauma under anesthesia
  • Single injection of Complete Freunds Adjuvant (Incomplete FA subsequent injections)

Pain Category E:

Animals upon which teaching, experiments, research, surgery, or tests are conducted involving accompanying pain or distress to the animals and for which the use of appropriate anesthetic, analgesic, or tranquilizing drugs would have adversely affected the procedures, results, or interpretation of the teaching, research, experiments, surgery, or tests. A scientific justification for the use of the procedures producing pain or distress on these animals and the reasons why pain and distress relieving drugs or methods cannot be used is required.

  • Persistent, severe colitis
  • Inflammatory conditions and microbial infections with significant systemic effects
  • Application of noxious stimulation, trauma, or electric shock from which an animal cannot escape or which an animal cannot avoid
  • Genetically engineered phenotype that causes pain/distress that will not be alleviated
  • Procedures for which needed analgesics, tranquilizers, sedatives, or anesthetics must be withheld for scientific reasons
  • Exposure to abnormal or extreme environmental conditions (e.g., temperatures, feeding, etc.)
  • Arthritis models causing prolonged pain or discomfort
  • Footpad injections without analgesia
  • Prolonged restraint
  • Forced swim test or other measures of depression
  • Models of neurological or muscular impairment


Requests for exceptions to this Standard must be reviewed and approved by the IACUC.


Pain: “An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage.” ~IASP

Painful procedure: “...any procedure that would reasonably be expected to cause more than slight or momentary pain or distress in a human being to which that procedure was applied, that is, pain in excess of that caused by injections or other minor procedures.”

Distress: Negative state at which an animal is unable to adapt completely to a stressful situation using normal species-specific coping mechanisms. More information can be found here.

Related Requirements

External Regulations and Consequences

University Policies, Standards, and Procedures

Contact Information

Contact Information Table
Subject Contact Telephone Email

Standard Information

Office of Animal Care and Use


Standard Information

DCM Veterinarians



Important Dates

  • Effective Date and title of Approver: July 2018; UNC IACUC
  • Revision and Review Dates, Change notes, title of Reviewer or Approver:

Approved by: UNC IACUC

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Article ID: 132193
Thu 4/8/21 9:25 PM
Mon 4/8/24 11:03 AM
Responsible Unit
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Research-Institutional Animal Care & Use Committee
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Vice Chancellor
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10/18/2024 12:00 AM
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09/10/2019 12:00 AM