Date First Issued:
10/5/11-DC
Approved by: Approved by UH
IACUC:
Revised:
Purpose:
This policy is to establish criteria for fighting cases, cage mate aggression, and recommendations for treatments. Rodent fighting does have different levels of severity and will be monitored by the LAS Veterinary Staff on a case by case basis.
Group housing is generally preferred to individual housing since the latter is often associated with “isolation stress.” However group housing has its own set of stressors, primarily incidences of aggression, particularly in male groups. Many factors can cause aggression within group housing such as: strain, age, group size, environment, and housing multiple males in a cage with a female(s) present. While female aggression is not common, certain strains can show aggressiveness to determine social status. 1
If fighting within group housing is not monitored or dealt with, it can lead to serious injury and may ultimately lead to death if group members are not separated and treated. Fighting lesions can also affect physiological parameters, as well as create unrelieved psychological stress in mice, which may affect experimental results.
2
Procedure/Evaluation:
The LAS Vet Staff will determine the severity of fighting within group housing and recommend treatment based on the classifications of mild, moderate or severe aggression. These classifications are determined by behavioral observation and the evidence of defensive wounds.
In all cases, the LAS Vet staff will write up injury reports and notify the PI and PI’s Lab.
Mild Aggression - Upon observation, the Vet Staff may not see actual fighting within the cage or only see fighting if the cage is not being disturbed. There may be some barbering present or whisker trimming in rodents (especially in cases of females present). You may also notice pinpoint superficial lesions or “bite markings” along the tail and back. For Mild Aggression cases, close monitoring will be recommended and the PI will have the option of separating the aggressor or not.
Moderate Aggression - Upon observation, the Vet Staff will see fighting within the cage and will be able to determine the aggressor during the evaluation. There will be 1 or more wounded rodents with superficial to dermal lesions seen along the back of the animal, the rump area, tail, ventral aspects along the abdomen, or behind the front and rear limbs. These lesions may be scabbed or moist with or without blood present. For Moderate Aggression cases, LAS will require the PI to separate the aggressor at a minimum and may or may not recommend treatment of antibiotics or analgesics. If there is multiple males and a female(s) in the cage the PI will have to separate all other males to other cages except 1 male and female(s).

Severe Aggression - Upon observation, this cage will have constant fighting seen along with vocalization, even after disturbing the cage. Animals in the cage will have superficial to muscular lesions on 10% or more of body, limbs and tail. Swelling will be present along with fresh blood on the animal or cage. The wounded animals may show signs of difficult mobility in the front or hind limbs and may not be able to move about the cage in a normal manner. There may also be trauma to the genitals such as males missing there penises or unable to urinate on their own. For Severe Aggression cases, LAS will separate the aggressor immediately and contact the PI by phone and email regarding the wounded animals. If euthanasia is required, the veterinary staff will give the PI a deadline for euthanasia. If euthanasia is not required, the wounded animal will need antibiotics and/or analgesics.
Preemptive Options
Rodent aggression or fighting cases can be contained with careful observation from the
LAS staff and from the PI/ PI ‘s Lab. This standard operation procedure offers minimal guidelines when determining levels of aggression on a case by case basis. Other options can be considered to alleviate aggression and include the following:
1.
Increased environmental enrichment.
2.
Transferring a small amount of dirty bedding into a clean cage at the time of change out or weaning. *Note mice are scent driven and this allows for mice to acclimate to a new cage.
3.
Male littermates (mice born to the same mother on the same date) housed together post weaning will often not fight, however this isn’t guaranteed. *Note this is not effective if numbers 4-6 are also not followed.
4.
Do not house multiple males in a cage with 1 or more females.
5.
Do not group house males that have been used for breeding.
6.
Do not switch males into different cages with other males.
Overall the best treatment for these aggression cases is separation when needed.
If rodent aggression is commonly seen within certain strain groups, the PI may consider consulting the LAS Veterinarian to discuss their options. Lesions secondary to fighting can cause pain or distress, and whenever possible this should be avoided to benefit the animal’s welfare.
For more information on aggression in rodents please contact:
Sylvia Kondo, LAS Director and University Veterinarian sylviak@hawaii.edu
Michael Wong, LAS Veterinarian wongmich@hawaii.edu
Dani Carico, LAS Vet Technician caricod@hawaii.edu
Regina Lozano, LAS Vet Technician ralozano@hawaii.edu

Cited References
1.
Brain P. Life Sciences, 1975. What does individual housing mean to a mouse?
< www.sciencedirect.com/science/article/pii/002432057590017X >. Accessed 2011 Oct 5.
2.
Kaliste-Korhonen E. and Eskola S. The Royal Society of Medical Press Journal, 2000
Apr 1. 189-198. Fighting in NIH/S male mice: consequences for behavior in residentintruder tests and physiological parameters
<la.rsmjournals.com/content/34/2/189.full.pdf>. Accessed 2011 Oct 5.