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Cage Density Guidelines for Rodent Breeding Colonies

Overcrowded mouse and rat cages represent a significant animal welfare concern. Such cages are noncompliant with Public Health Service (PHS) Policy and our Assurance to PHS. The Guide for the Care and Use of Laboratory Animals states the PHS recommendations for housing densities. In order to standardize housing densities and prevent or eliminate the possibility of overcrowding within cages, the Office of Research and Creative Activities has adopted the following IACUC-approved policy. Cage sizes and animal numbers allowed are listed in the table below. Cage densities exceeding these numbers represent clear policy violation.

Policy:All animals will be housed according to requirements in the Animal Welfare Act and PHS Policy, and guidelines in the Guide for the Care and Use of Laboratory Animals.

Applicable:All faculty, staff, students and employees of Brigham Young University caring for and using animals in research or teaching.

Purpose:Describe the procedures used to provide routine husbandry and to determine appropriate housing density for mouse and rat breeding colonies while ensuring their well-being at all times.

Housing

Cage Capacity for Mice and Rats housed in Open-top cages in ECS and SWKT based on the Guide for the Care and Use of Laboratory Animals, 8th Edition

Species Cage size Weight of Animal Rule of Thumb Number of Animals per cage
Mice LSB Small shoe box 10.5 X 6.5 in 68.25 sq. in. < 10 grams Pups under weaning age 11 pups
. . Up to 15 grams Weaning age pups 8
. . Up to 25 grams Juvenile and Young adult 5
. . > 25 grams Adult 4
. Female + Litter 1 Adult + Litter
Rats LSB Large shoe box 9.5 X 16 in 152 sq. in. < 100 grams Pups (weaning age pups are <50g) and Juveniles 8
. . Up to 200 grams Young Adults 6
. . Up to 300 grams Adult 5
. . Up to 400 grams Older Adult 3
. . Up to 500 grams Old Adult 2
. . > 500 grams Old Adult 2
. Female + Litter 1 Female + Litter

Cage Capacity for Mice and Rats housed in the Life Sciences Vivarium based on the Guide for the Care and Use of Laboratory Animals, 8th Edition

Species Cage size Weight of Animal Rule of Thumb Number of Animals per cage
Mice LSB Small shoe box Mouse Tecniplast 77.66 sq. in. < 10 grams Pups under weaning age 12 pups
. . Up to 15 grams Weaning age pups 9
. . Up to 25 grams Juvenile and Young adult 6
. . > 25 grams Adult 5
. Female + Litter 1 Adults + Litter
Rats LSB Large shoe box Rat Tecniplast 140.12 sq. in. < 100 grams Pups (weaning age pups are <50g) and Juveniles 8
. . Up to 200 grams Young Adults 6
. . Up to 300 grams Adult 4
. . Up to 400 grams Older Adult 3
. . Up to 500 grams Old Adult 2
. . > 500 grams Old Adult 2
. Female + Litter 1 Female + Litter

Setting up Breeding Cages

Breeding cages MUST comply with the space requirements for mice noted on the previous page unless an exception to this policy has been approved by the IACUC. According to OLAW, baby mice must be counted for space purposes when they are active and moving freely around the cage. Depending upon the strain, this is usually at an age of 13 – 14 days. As a result, small cages cannot accommodate breeding trios; the females must be moved to another cage when they become pregnant.

  • Mice are typically bred in male: female ratios of 1:1 or 1:2.
  • If bred in a 1:1 ratio (monogamous breeding), the female should be removed to a separate cage when observed to be pregnant.
  • If breeding is conducted in a 1:2 ratio (harem breeding), each female should be removed to a separate cage when observed to be pregnant. Only one litter is allowed per cage.
  • Date of birth for the litters must be recorded on the cage card by the investigator.

Weaning Age

In a mouse husbandry context, weaning refers to removing a pup from its home pen (rather than to the time a pup stops nursing and starts eating solid food).

  • Weaning age is generally 21 days.
  • Weaning age depends on weanling size and maturity. Although most strains are weaned when they are 21 days old, some benefit from being weaned at a later age.
  • Mice should not be weaned before they are 17 days old without transferring them to a foster mother.
  • Age of weaning is strain specific; C57BL/6 mice develop slower and generally require a weaning age of 24–28 days. Use your best judgement when weaning pups from their mother.
  • A cage is considered overcrowded if a litter remains with the female beyond 28 days of age.
  • Weaning beyond 28 days of age is a special circumstance and requires either written Veterinarian approval (if done sporadically for health reasons) or prior IACUC approval (if done routinely by the Investigator).

The weaning age for rats is 21 days, although the same general rules for mice will apply to rats. Please use you judgement.
Breeding Cage Cards

Each cage must be identified with a cage card. Cage cards are to be attached to the front of each cage with a cardholder. The following information must be on each card:

Investigator’s name:Protocol #:Sex:Delivery Date:

Species:Date Bred:Expected Delivery Date:Date to Wean:

Husbandry

Follow the appropriate SOP for either barrier or conventionally housed mice with the exceptions noted below:

Do NOT change cages containing pups less than 3 days old. Mark the cage so that it can be changed later in the week.If there are pups in a cage, transfer the nest from the old cage to the new one.

Electronic Log or "issues" board

The activities in an animal room are to be recorded daily on the facility’s computer electronic log. Both LARC caretakers and the protocol’s investigator or someone assigned by the investigator, must sign and check the electronic log. The person assigned by the investigator will check the animals and complete the electronic log during the 5 work days of each week. Any “issue” denoted on the log should be addressed immediately. LARC caretakers will sign the daily checklist to signify that the animals have been checked 7 days a week and on holidays.

Temperature and humidity readings are recorded electronically or by LARC staff. Problems should be reported to the Laboratory Animal Manager promptly.

Cage Overcrowding

Overcrowding is a non-compliance. A noncompliance is the failure to comply with a law or a regulation. If you receive a notice of overcrowding on your cage this is a noncompliance issue. Cage density regulations are based on the Guide for the Care and Use of Laboratory Animals, 8th Ed. The BYU Cage Density Policy is located above and on the IACUC website. The following events are considered overcrowding when:

  • a second litter is born before the first litter is weaned
  • a litter isn’t weaned by 28 days
  • two females and two litters are housed in the same cage
  • more than 5 adult mice are housed in the same cage.

If an overcrowded cage is found, the LARC staff personnel will notify the Principal Investigator or Manager of the colony with an “issues” card on the cage and post the notice on the facilities electronic log. Overcrowded cages must be corrected within 48 hours of notice.
Note: If the veterinary staff determines that a cage is severely overcrowded, they may separate the animals immediately to ensure their welfare. The cages will be marked and identified and the PI will be notified.

If LARC personnel determine that an overcrowding problem exists and is a continuing problem they will notify IACUC and appropriate action will be taken. Repeated violations of the cage density policy may result in the loss of “PI maintained colony” privileges.

The IACUC may take any or all of the following actions: Require a written plan from the PI detailing how the overcrowding problem will be eliminated. If a plan is not provided or is not acceptable to the IACUC, the IACUC will consider restriction of the PI's privileges to conduct animal research and will determine whether the repeated overcrowding problem constitutes continuing noncompliance reportable to OLAW.

Applicability: This policy applies to all rodents used in research at Brigham Young University.

Exceptions: Exceptions to this policy are considered to be an exception to the Guide and require a written request submitted to the IACUC and subsequent approval by the IACUC. Adequate scientific justification must be provided for exceptions to the Guide and will be reported, as required, to accreditation and regulatory agencies.

Contact Information: For any additional questions concerning this policy, please contact LARC directly.