Choosing Health

Health issues play a major factor in making wise breeding choices. Breeding to improve the health of a line is not always as simple as it may seem. Each rat is a product of its ancestors and its environment.

Breeding a healthy rat may not mean that you are breeding from a healthy line.

There are cases when breeding from rats that are apparently healthy can result in serious problems. This can be due to either genetic or environmental issues. Temperament also plays a role in the health of a rat.

Genetic factors that may cause health problems in a line include things such as: simple hidden recessives, spontaneous mutations, and genetic incompatibility (polygenetic factors).

Environmental factors that may cause health problems in a line include things such as: exposure to pathogens or agents that cause or exacerbate illness, environmental cleanliness, temperature control, stress, and nutrition.

Genetic problems and environmental factors can influence or mask each other. An example of this is mycoplasma respiratory infection.

A rat that is genetically prone to myco flare-ups might not have many or might not have any if his environment is kept clean and relatively free from stressors.

On the other hand, a rat that wouldn’t normally be prone to flare-ups might have problems due to poor husbandry.
This includes stressors such as: being kept on pine or cedar bedding, unclean conditions, urine build-up (ammonia), exposure to pathogens, loneliness, severe temperature extremes, constant light or darkness, or poor nutrition.

Temperament also plays an important role in the health of a rat and can be influenced by either or both genetics and environment. Studies have shown that rats who are fearful when young, and maintain fearfulness into adulthood, have more health issues and shortened life spans. This may be a result of higher levels of stress hormones.

Genetic Issues

Genetic problems not only compromise the health of your rats but also perpetuate the problems for future generations. In nature, genetic defects are kept at a minimum through natural selection. In an environment where only the strong and healthy can survive a genetically defective rat most likely would not live long enough to breed. In the hobby there is very little natural selection.

Often test breeding can help determine if there is a genetic problem in a line. This can be accomplished through the use of close inbreeding of the line either by mating siblings and/or breeding offspring to the parents. Just as you can use inbreeding to set or show the best genetic traits you can also use it to determine the undesired traits in the line.

When a genetic health issue arises in a line the response may be to outcross (breed to unrelated rats) to rid the line of the faulty gene(s). Unfortunately, this method can allow the problematic genes to be perpetuated. If that is the case then it can reoccur in subsequent generations. This is not always immediately apparent when one is dealing with recessive, incompletely dominant, or polygenetic genes.

Out crossing rats with genetic disorders can, at times, mask the problematic genes and possibly spread them throughout the gene pool.

On the other hand a regime of careful out crossing followed by inbreedings, to test the line, may help to rid or lessen the problem. Offspring from these breedings should be pet placed and tracked while the line is being tested.

Note: Keep in mind out crossing can also bring in new genetic health problems.

Some Examples of Undesirable Genetic Traits

  • Short lifespan
  • High propensity for tumors- often, but not exclusively, hormonally driven tumors such as mammary and thyroid
  • Poor immune systems- inability to effectively fight disease
  • Megacolon (genetic aganglionic)- congenital colon disorder associated with certain white markings that causes extreme, and usually fatal, colon distention
  • Heart disease- cardiovascular problems such as congestive heart disease
  • Diabetes- a disorder where the body is unable to correctly process blood sugar
  • Hydrocephaly- abnormal accumulation of cerebral spinal fluid in the ventricles of the brain
  • Blood coagulation defects – compromised clotting factors
  • Chronic respiratory disease- weak respiratory system
  • Lactation issues- inability to lactate or inadequate milk supply
  • Birth defects- such as neural tube defects
  • Genetic obesity

Environmental Factors and Health

It is also important to inquire about the environment and care of the colony the rat(s) are coming from.

Breeders who consistently medicate their entire colonies can mask health problems.

Rats kept on pine or cedar can end up with liver and lung damage. Inquire about the rattery’s nutritional practices.

Proper early nutrition and the nutrition of the pregnant doe is very important in determining the long term health of a pup rat.

Stopping a Line

There are issues that would cause a breeder to stop a line. But not all issues necessitate such a drastic measure. With breeding rats much of the decision making depends on looking at the whole picture. Much has to do with when the problem occurs (age), severity of the problem, cause of the problem, and how often the problem shows up.

All of these factors need to be considered before making the decision of whether or not to continue the line.

Ending a Line

The sharing of health and temperament information is vital to the fancy. This includes contacting owners of offspring, co-breeders, and other breeders with related rats in the event there is a problem or a concern.

An ongoing open line of communication between all parties will ensure that important information is shared. This will help the breeders involved have more data thus enabling them to make informed and responsible breeding choices.

As a breeder it is important to make potential adopters (for both breeding and/or pet homes) aware of any rats that may carry or have genetic disorders.

An owner who becomes aware of any health or temperament issues in their rat should immediately contact the breeder with that information.

Geographic Considerations

Another thing to keep in mind is that in different countries/areas some lines can have certain problems associated certain genes where in other geographic areas the problems do not seem to occur (or are not as severe).

A good example of this is the blue rats. Although they seem to be the same basic genotype for color there are color associated health issues such as clotting deficiencies seen in Australian blues that haven’t shown up (or been as extreme) in other countries.

Choosing Healthy Breeding Rats

When choosing rats for breeding one should research to learn both the strengths and weaknesses of the line. Ask for and research pedigrees. Find out if the relatives are being tracked for health (with breeders who cull heavily health issues in the line may not be apparent). If possible, privately talk to other people who also have rats from these lines. Check the NARR (North American Rat Registry), if ancestors are registered, for health and temperament issues.

It is best to avoid breeding a rat with any health problems.

The rats you chose to breed should be free of any signs of illness. Once a rat has been ill, even if he or she recovers, it may be a sign that their immune system is not as hearty as it should be and that the rat is not a proper breeding choice.

Often people are tempted to breed a rat that is not robust but is visually attractive or has an exceptional temperament. This only perpetuates poor health in the offspring and can lead to more problems down the road as the babies are placed. It is not fair to the rats or to the future owners.

Traits to help determine wellness:

  • Bright eyes
  • Lack of porphyrin discharge from eyes or nose
  • Clear prolonged respiratory function
  • Healthy coat and skin
  • Well aligned teeth
  • Absence of parasites
  • Good appetite
  • Appropriate energy level
  • Appropriate weight/size for age
  • Good temperament
  • No “infection odor”
  • No diarrhea
  • No bloating

Delayed Breeding

Delayed breeding is an ideal way to help determine health in rats that are from unknown, or little known, lines.
Too often a rat at 4 months will seem healthy only to develop severe health issues later. It is a setback to a breeder to have a litter of 4 month old babies and see their mom developing mammary tumors or severe respiratory issues at less than a year of age.

Waiting until the rats are a little older will provide more time to evaluate their health before breeding.

Female rats can be bred for the first time at up to a year of age with no apparent ill effects. Most male rats can be successfully bred for most of their lives although as they age they may produce smaller litters.

For more information refer to the Breeding Age article in the Rat Guide.

  1. Silver, C. (2003, December 24). Worried Sick: The High Price of Being Fearful. Retrieved December 18, 2008, from


Links to


The Rat Guide and its affiliates accept no responsibility for misuse or misunderstanding of its information. This guide in whole or part, exists solely for the purpose of recognizing and understanding the care and illnesses in the pet rat. Please seek advice and treatment from a qualified Veterinarian if your rat is ill.

2000 - 2023 by Karen Grant RN. All rights reserved.
All other written and visual materials used by permission of specific authors for the sole use of the Rat Guide. Please visit our Privacy Policy for details.
Brought to you by KuddlyKorner4u
See Logos page for linking to the Rat Guide.
Contact us here: Rat Guide Team
Please note: Rat Guide email is not checked daily. Send e-mail to if you have an urgent medical problem with your pet rat. When possible, it is always best to take your rat to a qualified rat veterinarian.