Stressed animals are more susceptible to secondary illnesses and can have elevated hormone levels for up to a week post-transportation. This initial stress response is common to all mouse strains, and it is therefore recommended that mice are not enrolled in study until at least five days, or ideally up to twelve days, after arrival, after which time most animals have been properly acclimated.
While some researchers may intuitively think that nurturing the new animals through added handling and providing treats may remedy the stress associated with this process, it is in fact the opposite. Research animals, specifically the super immunodeficient NOG portfolio models, are especially delicate, and over-nurturing them creates even more elevated stress levels. This review will highlight proper ways to care for and acclimate a new cohort of research animals as well as provide specific recommendations from veterinarians and animal care technicians.
Stress in Mouse Research ModelsNot all mouse strains will experience and display stress in the same ways. Males and females differ in their baseline stress markers, inbred rodent strains differ in most behavioral responses to stress, and genetically engineered models could respond in varying ways depending on the type of mutation(s) they carry1.
Signs of Stress in MicePhysiological signs of stress in mice include increased blood levels of glucocorticoids and catecholamines, changes in immune system composition and function (immunosuppression), altered food and water intake, weight gain or loss, shifts in sleep/wake cycles, modified composition of their commensal microbial communities, and various behavioral problems including aggression, self-trauma, and repetitive behaviors without purpose (stereotypes)2.
A specific group of mice that are prone to experiencing higher levels of stress in most situations the CIEA NOG mouse® and next generation NOG models. These mice lack mature T, B, and NK immune cells. These strains can also be engrafted with human immune systems, as with the huNOG, huPBMC-NOG and huNOG-EXL, in which case they have been subjected to the additional stress of the engraftment procedures and possibly even whole body irradiation. While these properties make them extremely useful in preclinical studies, the animals require special care and treatment while being raised and when acclimating to the customer's research facility.
Inducing Stress by Over-NurturingA new cohort of research models is exciting, and the researchers involved might want to study the new animals very carefully once they arrive at their facility to ensure they are eating, drinking, and not displaying any signs of illness. While visual monitoring and inspections are beneficial, certain behaviors associated with monitoring their health can induce stress in the animal.
The amount of handling a mouse experiences is a key contributing factor to increased stress levels. Ideally, mice should be handled as little as possible and kept in the same room for the duration of their stay in your facility. Moving mice to a new location, with different odors, air flow, and watering methods, further elevates stress levels in the animals.
Causes of Human Interaction-Induced Stress
- Too much handling
- Excessive weigh-ins
- Increased amounts of treats for the purposes of enrichment
- Changes in cagemates or cage setup
- Changes in water source
- Changes in food type/source
- Changing aspects of the facility room: introducing new odors, different lights, or changes in ventilation and temperature
Proper Enrichment Helps Animals AcclimateEnvironmental enrichment, with regards to animal husbandry, refers to the addition of objects or changes in cage conditions that aim to improve the welfare of laboratory animals3. Many different kinds of enrichment are beneficial to the health and well-being of mouse models. Environmental enrichment enables the animals to express species-specific behaviors such as nesting, resting, exploring, foraging, and gnawing.
Nesting material is often considered the most important enrichment, as it allows mice to thermoregulate while they are resting and express their species-typical nesting behavior. Different kinds of paper, which may or may not be purchased pre-shredded, are some of the most popular choices, as well as cotton squares the animals can shred themselves.
Chew sticks, tubes, various types of huts, lofts, cage dividers and plastic u-shaped swings are other popular choices usually used in addition to nesting material. The super immunodeficient huNOG and huNOG-EXL mouse models have shown a positive response to enrichment provided by Shepherd shacks®. These paper structures allow the animals to hide and nest, and are also able to be safely broken down and chewed on4.
Proper Acclimation of Laboratory Mouse ModelsTaconic Biosciences' veterinarian Dr. Julita Ramirez-Komo and veterinary nurse Emily Shako recommend a twelve-day acclimation period for research mice before starting any experiments. This helps the animals adapt to their new environment and reduces morbidity and mortality.
When necessary, handling should be slow and gentle, and animals should be housed on a lower part of the rack away from doors, traffic, and equipment such as biosafety cabinets or cage change stations.
The animals will also be transitioning over to water from gel hydration in transport. Automatic watering lixits should be regularly toggled, or easier 'training lixits' can be used to help the animals learn to drink from them. Supplemental gel hydration may be required while the animals are adjusting to drinking from the water source available at the customer's facility.
A gradual transition from their current feed to the customer's feed of choice is also recommended.
Additional Concerns for NOG MiceAdditionally, NOG portfolio models should be unpacked from the transport container immediately upon arrival at the facility by experienced animal care staff and carefully evaluated before being placed in housing. NOG mice which have been experimentally manipulated, such as via irradiation and engraftment of a human immune system (huNOG and huNOG-EXL), normally have a mild scruffy coat appearance, mild kyphosis, and pale integument which make the mice appear as hunched, scruffy, and pale. Even with these issues, the animals should still appear bright, alert, and active. If the animals appear slow, lethargic, and severely hunched and/or pale, veterinary staff should be contacted to evaluate the animals.
They should be offered a supplemental hydrogel, gel diet such as Nutra-Gel Diet™, or given sterile saline subcutaneously under direction from veterinary staff and reevaluated to determine if there has been improvement or of the animal is declining and should be euthanized.