Health related to breeding choices

The main health issues for does in our line are mammary tumours and less commonly, pituitary tumours. Our bucks (and does to a lesser degree) tend to age with gradual onset hind leg weakness. This is due to degeneration (demyelination) of the spinal root nerves, which may or may not be associated with chronic progressive nephropathy, but this condition is itself so common in elderly rats as for it to be sensible to treat all aged rats as having it to a degree. I will aim to add articles on all these illnesses to section three of the health pages.

General considerations

No line of rats is ever going to be perfect health-wise, and health outcomes vary from litter to litter depending on the combinations of predisposing genes handed down from each parent. Pet rats are rarely so inbred as to be consistent in the illnesses they succumb to and immune system health is so complex that the susceptibility to an infection is always a possibility. From monitoring the health of litters over the past 8 years I would say that an outcross litter can potentially be 'unhealthy'- as can an inbred one, and within any line of loosely related rats, any pairing can potentially throw up problems that may not be seen in the same way again. Individual litters can be affected by environmental, dietary and microbiological factors preconception, during pregnancy and birth, or in the early weeks of life. Rats are particularly vulnerable to these influences over this period.

One thing that I am convinced of is that many of the problems that we have seen in our own rats have been due to our selection for bigger, chunkier rats - and overfeeding. I cannot justify this course of action, but was probably simply responding to two things: our own measure of what a physically beautiful rat looks like, and our desire not to underfeed when raising whole litters. In attempting to ensure this we have overcompensated and overfed our rats, achieving rapid early growth and early physical maturity. Over recent litters we have tried very hard to feed in a way that produces slow growth into adulthood (without of course ending up with under nourished rats). Of course it will take some time to see what impact this has on mammary lumps in the rats, and overall lifespan.

Mammary/pituitary tumours and neophilic rats

Regarding predisposition to mammary tumours, it does seem to be possible to select for less/later tumours by breeding from particularly neophilic rats. This behaviour (the confindent embracing of new situations) has been shown to have a positive effect on lifespan, aging and the development of mammary and pituitary tumours. This is thought to be due to hormonal interactions, neophilic rats were found to have lower levels of corticoid hormones, while neophobic rats at puberty (when mammary tissue is growing rapidly) experienced more irregular cycles with prolonged “luteal” phases. This suggests a role for prolactin, prolonged progesterone and fewer estrogen surges during this sensitive period for mammary tumor risk. Overall tumour development could be delayed by as much as 6 months in the neophilic (and tumour prone) rats that were studied). The neophobic rats were also found to be 60% more likely to die at any point in their lives than their neophilic siblings. Since these studies were done using very inbred laboratory strains (where genetic differences between siblings are minimal), it is quite possible that neophilia/neophobia are determined as much by early social experiences as genetic predispositon.

Chronic progressive neuropathy detection and prevention

CPN is a degenerative disease of the kidneys seen in virtually all old rats to some degree. Advanced disease causes kidney failure. This disease is greatly influenced by diet and CPN rarely occurs in rats who are fed a low protein, restricted diet. Depending on source studies quote either soya or egg protein as being the least taxing on the kidneys, and levels of around 6 to 8% overally dietary protein are needed to prevent CPN. Moderatly restricted diet (rather than ad lib feeding can also help in thsi regard.

Of course it would be better if we could breed rats that are not prone to CPN regardless of diet, and a recent study showed that urine protein screening in 4 month old bucks could determine their predisposition to CPN in terms of their likelihood to suffer from the disease severely and at an earlier age. By looking at the protein concentrations in the urine of young rats you can predict which ones will go on to develop CPN. Breeding from rats with low protein urine protein concentrations produces significantly more offspring that are less prone to develop CPN themselves. The level of sensitivity of protein testing is still under discussion. Human labstix would not be sensitive enough to pick up the differences in urinary protein necessary.