Recognition of pain and distress in a rat model of mammary cancer

Aims: Mice and rats are the most frequently used animals in experimental protocols performed in the European Union. They have several advantages when compared with other animals, such as their small size, and well-known anatomy, physiology, biochemistry and genetic. This work aimed to define the most adequate humane endpoints to recognize the pain in a rat model of mammary cancer chemically-induced. Material and Methods: Twelve female Sprague-Dawley rat of four weeks of age were obtained from Harlan Interfauna and randomly divided into two groups: MNU (N-methyl-N-nitrosourea) (n=10) and control (n=2). Animals from group MNU received an intraperitoneal injection of the carcinogenic agent MNU, at a dose of 50 mg/Kg. Animals from control group received only the vehicle (saline solution). A list of biological parameters to be evaluated during the experiment was elaborated prior to the study, including: body condition, body weight, food and water intake, posture, coat and grooming, mucosal, eyes, ears and whiskers, mental status, response to external stimuli, hydration status, respiratory rate, heart rate and body temperature. A score from 0 to 3 was attributed for each parameter. Severe alteration in some of these parameters, such as weight loss >20%, severe anemia, moribund or comatous mental status, development of mammary tumors that interfere with animal bodily functions (eat or drink), tumors in contact with cage floor or tumor burden > 10% of the animal body weight (>35 mm in a 250 g rat) were considered indicators of animal sacrifice. The animals were observed twice a day by the same researchers, for 18 weeks. Results: Six out 10 animals (60%) MNU-exposed developed a total of 21 mammary tumors (3.5 tumors/animal). Moderate anemic mucosal (score 2) were observed in one animal from group MNU at the 11th, 12th and 16th week of the experiment. No interference of mammary tumors with animals’ bodily functions or alterations in mammary tumor surface that implied animal sacrifice was observed. At the end of the study, five animals (83.3%) developed at least one mammary tumor > 35 mm. Conclusions: We concluded that the alteration in only tumor dimensions does not imply the animals’ sacrifice. The endpoints should be evaluated together, in order to define the most adequate time for humane animals’ sacrifice