Animal impacts: pain
Carbone L. Pain in laboratory animals: the ethical and regulatory imperatives. PLoS ONE 2011; 6(9): e21578. doi:10.1371/journal.pone.0021578
Link to article

ABSTRACT
The paradoxical goal of much of animal-based biomedical research is to model severe human injuries and illnesses without causing severe pain or distress to the animals. Current public policy in most countries calls for treatment or prevention of laboratory animal pain whenever possible. However, the “whenever possible” provision allows for some intentional infliction of untreated pain in laboratory animals when doing otherwise would be expected to disrupt the experiment. Permission to withhold painkillers when their use would interfere with the experiment is codified in public policy, as in the United States Department of Agriculture's 1971 designation of “Category E” painful procedures. Determining which experiments may permissibly cause pain and distress in laboratory animals, and deciding how that pain may be minimized or managed, requires clear ethical reasoning as well as the best available knowledge of animal biology and behavior. This article explores some of the common reasons why some laboratory animals may not receive pain medicines, and discusses some proposals for increasing use of pain medications for them. The policy focus is on the American system, though the general themes apply equally to other countries' laboratory animal welfare rules.