A quarter of veterinarians do not administer painkillers to dogs or cats during operations, believing the drugs to be unnecessary and unsafe, according to a survey by a group studying animal pain.

"Veterinarians and pet owners need to understand more about the pain animals suffer," Yoshihisa Yamane, leader of the study group, said.

The group plans to create by March a pain index that will give a numerical indication of the pain an animal suffers, judging by their appetite and other biological signs, said Yamane, who is a professor at the Tokyo University of Agriculture and Technology.

The group's study was based on 470 responses to a questionnaire it distributed to veterinarians across Japan.

While all the responding veterinarians acknowledged that animals feel pain, 26 percent said they do not administer painkillers before, during or after operations.

Of that group, 40 percent said administering the drugs is unnecessary and 30 percent cited the danger of side effects.

Many of the respondents said they use anesthesia during castration and sterilization operations, but do not administer painkillers.

Those who use painkillers account for 34 percent. Another 34 percent said they use the drugs only during operations in which they would feel pain under similar circumstances.