Rodent Breeding and Weaning

Rodent Breeding

In an effort to maximize production, minimize stress on the animals, and adhere to animal welfare policies and regulations, it is important to establish an appropriate breeding program. The objective(s) of the study, the peculiarities of the strain(s) involved, and the experience of the individual(s) responsible for maintaining the breeding program must be taken into consideration when developing a breeding program.

Several mating systems can be used, but cages may not house more than one adultmale and twoadult female mice that are left together continuously. Such a system facilitates production of offspring for lines that are difficult to breed and maintain. If not properly managed, this mating system can severely overcrowd the cage and can lead to changes in metabolic and physiologic processes, alterations in disease susceptibility (Broderson and others 1976, Schoeb and others 1982; Vesell and others 1976), and may adversely affect scientific research. In an ideal situation, each pregnant female should be moved to a separate cage to have her litter. If this is not possible, the next best method to help reduce overcrowding is to separate at least one of the moms and her pups immediately after birth. At any time, no more than one male and two females with litters can be housed in a cage. Occasionally, breeding trios will produce large litters that will strain the caging system. If at 14 days there are over 14 animals in a cage the two litters must be separated. If pups are not weaned in a timely manner, severe overcrowding may occur. This is due to postpartum breeding, which will produce another litter when it is time to wean the first litter.

Normal housing of mice allows five adults in a cage. One male and four females may be housed in breeding cages and plugged or pregnant females removed to separate cages. If this system is used no more than two females with litters are permitted to be housed together.

This policy applies to all breeding programs, including transgenic mouse production colonies. Any modifications to this policy must be made to the Animal Care and Use Committee and receive approval. An appropriate scientific justification will be required to obtain an exemption.

Rodent Weaning

Accepted weaning age for most strains of mice and rats, regardless of mating system, is 21 days. However, some genetically altered animals benefit from weaning at a later age. Regardless of the system used or the approved weaning age, it is the PI’s responsibility to ensure prompt disposition of overcrowded cages. Details are provided below:

  1. Animal care personnel/Investigator will record the date of birth on the cage card at the time each litter is born in their respective animal rooms. The litter should not be disturbed immediately after birth; therefore, the number of pups is recorded at the next cage changing. Facility management will ensure cages go no longer than one week before changing.
  2. All litters, regardless of mating system used, must be weaned by 21 days of age unless otherwise specified in the Animal Study Proposal. If a longer weaning time is specified in the Animal Study Proposal, the mother and litter must be separated into a clean cage, or a mating system of one female to one male must be used. In such cases, litters must be weaned no later than 28 days of age.
  3. If animals are not weaned by 22 days of age (or 29 days if approved in the Animal Study Proposal), the investigator/technician is notified by facility management by e-mail, fax, and/or phone call. If there is no response within 24 hours, and if space is available, the facility animal care staff will wean the litter by separating males and females. If space is not available, the investigator/technician is notified a second time by e-mail, fax and/or phone call. If the investigator/technician does not wean the litter, the facility/clinical veterinarian will determine disposition of the litter. In such cases, euthanasia of the new litter may be the only recourse. This same notification system will be used if the number of animals exceeds 14 at 14 days of age.
  4. Weaning procedures performed by the animal care staff are charged to the investigator’s account at the current rate for technical service.
  5. The noncompliance to the policy not only creates numerous management hardships, but also can adversely affect the animals involved. Continued noncompliance will be brought before the NCI ACUC and may result in loss of facility privileges.

Investigators and their staff have the primary responsibility for weaning litters on time. The animal care staff is responsible for identifying overcrowded cages and following notification of the PI, separating animals in overcrowded cages, if not resolved by the investigative staff.

NCI Animal Care and Use Committee - Issued 11/8/2002, Revised 10/2002, Revised 4/2003