IACUC Policy: Assigning USDA Pain Categories
Policy # IBT-105.00 IACUC Approval: January 27, 2014

The Guide to the Care and Use of Laboratory Animals recommends scales of pain or invasiveness are utilized to aid in the preparation and review of protocols.  As a result, all vertebrate animals used for research or teaching must be assigned to a USDA pain and distress category regarding the protocol under which they are used. Procedures that could cause pain and distress in humans should be assumed to cause pain or distress in other animals. This document provides definitions and examples of the USDA pain and distress levels to ensure that animals are listed on their protocol under the correct USDA pain and distress category.

This policy applies to all animal care and use subject to oversight by Texas A&M Health Science Center animal facilities and labs.

The Principal Investigator (PI) is responsible for using the guidelines in this policy to assign the appropriate USDA pain and distress category.
The IACUC is responsible for reviewing all protocols and verifying that animals are placed in the correct pain category according to policy.

Pain – a complex experience that typically results from stimuli that damage or have the potential to damage tissue; such stimuli tends to cause prompt withdrawal and evasive action. Pain is a stressor.
Distress – is defined by the guide as an aversive state in which an animal fails to cope or adjust to various stressors with which it is presented, this may not induce an immediate and observable pathologic or behavioral alteration making distress difficult to monitor.


I. Assigning Pain and Distress Categories 

  1. Principal Investigators should assign each animal listed on a protocol to one of the following USDA pain and distress categories: B, C, D or E. Defined below.
  2. Record each animal under the highest pain and distress category that applies at any time during the animals’ enrollment to the study, even if it is for a short duration of time.
  3. Do not include non-research related veterinary care in determining USDA pain and distress category.
  4. If a procedure is done on an animal (e.g. tail snip or euthanasia), list the animals as Category C or greater. This includes animals used for breeding if they are later euthanized. List breeding animals as Category B only if no procedures are performed, including euthanasia.
  5. Genetically engineered animals
         a. Place animals in category C if the phenotype by the genetic alteration is unknown. Amend the category once the investigator or veterinary staff recognizes phenotype-related pain or distress.
         b. Place animals in category D if the phenotype is expected to cause pain or distress that will be alleviated by IACUC approved methods.
         c. Place animals in category E if the phenotype is expected to cause pain or distress that will not be alleviated. Describe any new information regarding the phenotype, including adverse events, and adjust the pain and distress category as necessary during annual review.
  6. The following are examples of studies that may fall under D or E, depending on the severity or length of time:
        a. Ocular or skin irritancy testing
        b. Prolonged/chronic restraint, stress experiments, paralysis
        c. Food or water deprivation beyond that necessary for ordinary pre‐surgical preparation
        d. Application of noxious stimuli such as electrical shock that the animal cannot avoid or escape

 II. Definition and Examples of USDA Pain and Distress Categories

Category B

Category C

Category D

Category E

Animals being bred, acclimatized, or held for use in teaching, testing, experiments, research, or surgery but not yet used for such purposes. Non‐invasive observation only of animals in the wild.

Animals that are subject to procedures that cause no pain or distress, or only momentary or slight pain or distress and do not require the use of pain‐relieving drugs.

Animals subjected to potentially painful or stressful procedures for which they receive appropriate anesthetics, analgesics and/or tranquilizer drugs.

Animals subjected to potentially painful or stressful procedures that are NOT relieved with anesthetics, analgesics and/or tranquilizer drugs. Withholding anesthesia/analgesia must be scientifically justified in writing and approved by the IACUC.





 1. Animals being bred or housed, without any research manipulation (including tail snips or euthanasia), prior to transfer to another protocol

2. Observation of animal behavior in the wild without manipulating the animal or its environment

 1. Holding or weighing animals in teaching, outreach or research activities

2. Observation of animal behavior in the lab after manipulation of the environment

3. Ear punching of rodents

4. Breeding and tail snips in mice ≤ 21 days old

5. Peripheral injections, oral gavage, blood collection or nonsurgical catheter implantation

6. Feed studies, which do not result in clinical health problems

7. Live trapping

8. Positive reward training or research

9. Tattooing


 1. Survival surgery (thoracotomy, craniotomy, laparotomy).

2. Non-survival surgical procedures (organ/tissue removal, thoracotomy, perfusion under anesthesia, etc.)

3. Laparoscopy or needle biopsies

4. Retro‐orbital blood collection with anesthesia

5. Surgical placement of catheters for blood collection or administration

6. Induced infections or antibody production \

7. Tail snips with anesthesia in mice >21 days old

8. Tumor growth (not debilitating), toxicity tests, ascites production (Note: Animals will be sacrificed when showing clinical signs or morbidity).

 1. Toxicological or microbiological testing, cancer research or infectious disease research that requires continuation after clinical symptoms are evident without medical relief or require death as an endpoint.

2. Any procedures for which needed analgesics, tranquilizers, sedatives, or anesthetics must be withheld for justifiable study purposes (inflammation, trauma, burns, analgesic testing, etc




  1. Animal and Plant Health Inspection Service, USDA. US Animal Welfare Act (AWA 1990) and Regulations (PL-89-544, as amended, 7USC Ch. 54) 2008. CFR Title 9, Subchapter A - Animal Welfare. U.S. Government Printing Office, Washington, D.C.
  2.  National Research Council. Institute for Laboratory Animal Research. 2011. Guide for the Care and Use of Laboratory Animals. Public Health Service, Bethesda, MD.
  3. Public Health Service Policy on Humane Care and Use of Laboratory Animals http://grants.nih.gov/grants/olaw/references/phspol.htm.
  4. Silverman, Jerald, et.al. 2000. The IACUC Handbook: The Basic Unit of an Effective Animal Care and Use Program. Baltimore, MD.

5. ARENA/OLAW Institutional Animal Care and Use Committee Guidebook, 2nd Ed. 2002.

Version 00 -  Initial approval – January 27, 2014