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Recognition of Pain and Distress in Rats

Because animals cannot volunteer to participate in medical research, we are ethically constrained to provide humane care, and to alleviate as much pain and distress as is possible in such animals. This is mandated by the PHS Policy on Humane Care and Use of Laboratory Animals. The U.S. Government Principles for the Utilization and Care of Vertebrate Animals Used in Testing, Research, and Training (upon which PHS Policy is based), states that “Proper use of animals, including the avoidance or minimization of discomfort, distress, and pain when consistent with sound scientific practices, is imperative. Unless the contrary is established, investigators should consider that procedures that cause pain and distress in human beings may cause pain or distress in other animals.” The proper use of anesthetic and analgesic drugs helps to alleviate pain and distress during procedures. It is imperative that researchers learn to recognize the signs of pain and distress in mice. Inconvenient or not, the benefit of the doubt must always go to the animal.

The most common signs of pain and distress in rats listed in order of increasing severity, include: (1) ruffled or “spikey” fur (rat looks unkempt); (2) weight loss which may be mild to severe, anorexia, dehydration; (3) ocular discharge which may include porphyrin staining around the eyes; ( 4) lethargy, depression, or reluctance to move; (5) sitting with the back in a hunched position; (6) ataxia (uncoordinated muscle movements), regional or generalized weakness; (7) tremors, which may be intermittent to persistent depending on the condition of the animal; (8) hypothermia characterized by cool extremities; (9) labored respiration; and (10) cyanosis, or a blue tinge to the mucous membranes. Any animals exhibiting combinations of 2 to 3 minor signs, or a single major sign should be euthanized immediately.

Animals in pain and distress may not interact with their cage mates, or may interact with them in a more aggressive manner. They may also become more aggressive towards human handling. Female mice may cannibalize litters in response to pain and distressing situations. Animals may squeal when picked up or when an affected area is touched. Persistent vocalization and crying indicates substantial pain or distress that should be relieved immediately. Moribund animals should be euthanized immediately.