Recessive Hairlessness: The "True Hairless" Rat

Theresa Lee
From the RMCA web site, April 2003


In existence today, there are only three genes known to cause recessive hairlessness in the rat. In the scientific community, these genes are denoted rnu (Rowett nude), fz (fuzzy), and shn (shorn), and their loci have been mapped to chromosomes 10, 1, and 7, respectively. The genotypes, rnu/rnu, fz/fz, and shn/shn, are hairless, while the genotypes, Rnu/rnu, Fz/fz, and Shn/shn, are not hairless. As we shall see, it is very important you know the genotype of your hairless rat.

rnu/rnu rats were first identified in 1953 in a lab in Scotland. Hairless rnu/rnu rats have no thymus. In other words, they are missing a significant part of their immune system. This makes them exceptionally vulnerable to infections, most often of the respiratory tract and of the eye. The average life span of a hairless rnu/rnu rat is 9 months.

fz/fz rats were first identified in 1976 in a lab in Pennsylvania. Hairless fz/fz rats are prone to dental malocclusion, and up to about 50% of male and 75% of female hairless fz/fz rats will have to have their teeth trimmed regularly just to be able to eat. The leading cause of death in the hairless fz/fz rat, however, is progressive kidney failure which begins at about one year of age. In germ-free conditions, all male and 80% of female fz/fz rats will die of kidney failure. The average lifespan of a hairless fz/fz rat is 17 months if male, 20 months if female.

shn/shn rats were first identified in 1998 in a lab in Connecticut. Hairless shn/shn rats also suffer from catastrophic kidney disease. All shn/shn rats die of severe kidney abnormalities by 14 months of age. The average life span of a hairless shn/shn rat is 10 months.

All three hairless rats, rnu/rnu, fz/fz, and shn/shn, have curly whiskers and may undergo cyclic growth, loss, and re-growth of patches of hair. All three types of hairless rats are fertile and have normal sized litters, but all three types of hairless mothers have trouble raising their babies to weaning.


In the scientific community, hairless rats are identified by their genotype. These genotypes are rigorously identified by extensive backcrossing and outcrossing, at the expense of hundreds upon hundreds of rat lives. In the pet community, hairless rats are currently only identified by their phenotype, and if hairlessness is observed to be recessive, it is often generically labeled as "true hairless." But this is not enough. Following from all of the above, it is imperative that we also know the genotype of our hairless rats, as there are serious health ramifications associated with each genotype.

Can we determine the genotype from pleiotropic effects? No, not really. The most common pleiotropic effects anecdotally associated with the pet hairless rat recessive phenotype are susceptibility to infection and trouble rearing young. This would seem to indicate that many pet hairless rats have genotype rnu/rnu, but susceptibility to infection can strongly depend on environment, and trouble rearing young is common to all three genotypes.

Is it possible, then, to determine the genotype? It is possible to make a very good guess. First, let's assume that the only two recessive hairless mutations in the pet populations are either rnu or fz. This is a reasonable assumption as rnu and fz mutants are readily available from lab suppliers, while shn is not. Any breeder who decides that hairless is suddenly en vogue can simply purchase a couple of rnu or fz mutants to start their hairless lines. I'm sure many of these babies end up at the pet store, and, alternatively, I have seen many reputable breeders start their hairless lines from hairless rats they happened upon at the pet store.

With this assumption, genotype may be inferred from blood tests. With no thymus (athymic), the hairless rnu/rnu rat will have nonexistent or low levels of T lymphocytes for life and immunoglobin IgG and blood lymphocyte levels which decrease with age. These results will not be seen in the hairless fz/fz rat which has a thymus (euthymic). The hairless fz/fz rat, however, has a diseased kidney, and kidney function in hairless fz/fz rats should be routinely monitored, beginning from about one year of age and on. Treatment for kidney disease should be begun when appropriate.

I must stress, unless you either have papers that show your hairless came from a lab line or your hairless has had the appropriate series of blood tests, you cannot even begin to guess the genotype of your hairless rat. Many breeders use the rnu/rnu or fz/fz genotype labels, but, again, only with paperwork from a lab or blood work done can they justifiably use these labels. I have also seen many breeders use genes like hr and nu to denote recessive hairlessness in their rats, but, unlike in the mouse, no mutations on these genes are known to cause recessive hairlessness in the rat.


One final point: having a hairless rat in your colony can affect the health of your entire colony. The hairless rnu/rnu rat, with his severely suppressed immune system, can be a carrier of many pathogens. Sendai and SDA normally do not exist as chronic carrier states in euthymic rats, but both do exist as chronic carrier states in hairless rnu/rnu rats. With no thymus, hairless rnu/rnu rats are physiologically unable to generate the antibodies necessary to fight off sendai and SDA and, if ever exposed, will always shed these viruses. No length of quarantine can change that. With just one chronic carrier in your colony, even with a no rats in/no rats out/no new litters policy, you will be maintaining an enzootic infection. And any other rat in your colony can be an acute carrier, as, while there is acquired immunity to sendai and SDA, this acquired immunity lasts only a few months. Once acquired immunity is lost, re-exposure results in re-infection. With just one chronic carrier in your colony, any other rat in your colony can be an acute carrier, no length of quarantine can change that.

Missing immune function, kidney disease, and sendai and SDA infections which don't go away. Despite all of this, many North American rat clubs continue to hold shows, continue to stress looks over health, and continue to put the lives of many, many rats at needless risk.

Every rat deserves a loving home, and I would not hesitate to take in a hairless rat in need. It should, however, be very clear from all of the above that there are special, very serious genetically-determined needs and precautions needed for taking in and caring for a hairless rat.

I am grateful to FG Ferguson, Professor of Veterinary Sciences, for his helpful review of this article.


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