Pain and Distress Categories


The categories below reflect the potential levels of pain, discomfort, and/or distress in common lab procedures. The examples are not exhaustive and are only to guide PIs and the IACUC. They cannot cover the range of severity and duration of P & D in specific study procedures, which must be considered in assigning categories.
Where appropriate, PIs should assign research groups undergoing different procedures to separate categories. The IACUC determines the final pain category. PIs’ Annual Updates should include any category changes that occurred during the course of the study.



Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage.


Distress is the state associated with conditions which significantly compromise the welfare of an animal, which may or may not be associated with pain, and where the animal must devote substantial effort or resources to the adaptive response to environmental challenges.


Discomfort is viewed as a mild form of distress.


A — The research involves procedures that would be expected to produce no pain or involves momentary, slight pain or discomfort.

  • Breeding
  • Injection
  • Venipuncture (blood sampling)
  • Collection of tissues preceded by approved methods of euthanasia
  • Behavioral testing without stress
  • Toxicity testing in which no pain is expected
  • Naturalistic observations

B — The research involves procedures that are expected to produce minor short-term pain which will be treated with appropriate anesthetics/analgesics or procedures that are expected to produce minor distress.

  • Short-term physical restraint of animals
  • Induction of minor behavioral stress
  • Minor surgical procedures with anesthesia (exposure of blood vessels, implantation of catheters in superficial vessels for chronic studies)
  • Field studies that will involve trapping and releasing wild animals and may include other minor procedures such as banding or tagging
  • Chronic maintenance of animals with a disease or functional deficit that involves minor pain or distress

C — The research involves procedures that may induce moderate pain which will be treated with appropriate anesthetics/analgesics or procedures that may induce moderate distress.

  • Major surgery with anesthesia (both survival and terminal)
  • Inducement of a functional deficit (e.g., inflammation induced by adjuvants and tumor inducement)
  • Toxicological, microbiological, or infectious disease research that requires continuation until significant clinical signs are evident and death is not the endpoint
  • Prolonged physical restraint of animals
  • Prolonged food or water deprivation
  • Induction of more than minor behavioral stress

D — The research involves the potential for pain, and greater than moderate distress which cannot or will not be alleviated through the administration of appropriate anesthetic, analgesic or tranquilizer drugs.

  • Toxicity testing – infectious agent challenge where death is the endpoint
  • Chronic maintenance of a disease or functional deficit where death is the endpoint
  • Studies causing inflammation, tissue damage, or neoplasia that result in morbidity or mortality which is not the result of euthanasia
  • Application of noxious stimuli from which escape is impossible
  • If animals experience more than momentary pain, appropriate anesthetics/analgesics are required for categories B or C. (‘Momentary pain’ is no greater than the severity and duration of common injection pain.)
Category D protocols require consultation with the AV and written justification (which goes in the annual report to USDA).