when death is certain or predictable, before they become moribund, rather than allowed to die and possibly be cannibalized by cagemates or allowed to undergo autolysis. Tissue autolysis, especially in rodents and other small mammals, can reduce the opportunity to evaluate some aspects of a study. Euthanasia can avoid or terminate unnecessary and severe pain and distress and allow for a complete necropsy. Developing uniform methods to assess morbidity as part of the protocol can contribute to the validity and uniformity of experimental data.

Regardless of the cause of morbidity (i.e., spontaneous or experimentally induced illness, pain, or distress), euthanasia should be considered. No guidelines are available that provide specific information on when to euthanatize animals in pain or distress, and such guidelines would be unrealistic, because animal species, types of studies, experimental needs, and end points vary so widely. Nevertheless, for studies in which death of animals is a necessary part of the protocol, efforts should be made to determine an acceptable end point when euthanasia may be performed to prevent or minimize unnecessary prolongation of pain and distress.


Some factors to consider in deciding whether to perform euthanasia are weight loss; emaciation; failure to gain weight (in a growing animal); severe pain that cannot be controlled; inordinate tumor growth or ascites; prolonged self-trauma; generalized alopecia caused by disease; prolonged diarrhea for which treatment is precluded in the protocol; coughing, wheezing, or severe nasal discharge; shallow and labored breathing; prolonged lethargy associated with rough hair coat, hunched posture, abdominal distention, or impaired movement; severe anemia or leukemia; icterus; CNS signs, such as convulsions, paralysis, paresis, tremors, and progressive head tilt; uncontrolled hemorrhage; urinary dysfunction (polyuria or anuria); lesions that interfere with eating or drinking; infectious disease (in terminal phases); hypothermia; and impairment of function, disablement, and other behavioral and physiologic signs of distress.

Information on signs of pain, distress, morbidity, and moribund condition in previous chapters of this volume should be used to determine when euthanasia is appropriate and justified. Another factor in the decision is the differentiation between a condition from which animals might recover and a moribund condition that is likely to progress to death. Principal investigators, study directors, attending veterinarians, and institutional animal care and use committees (IACUCs) should collaborate in determining general policies regarding end points of studies and when to perform euthanasia. In specific instances when euthanasia should be considered for humane reasons, attending veterinarians should consult with principal investigators and study directors in making a final decision. The IACUC at the University of Texas M. D. Anderson Cancer Center has considered this issue for rats and mice used in the center's programs (Tomasovic et al., 1988). Such guidelines, applied case by case, provide a rationale for determining whether potentially lethal

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