USES TO SCIENTIFIC RESEARCH

A TINY ANIMAL STORY

Your rat's Latin name is Rattus norvegicus.He now comes in innumerable colors and styles but he was once a browncreature living wild in southern Asia (not Norway). He found plentyof food and shelter near people, and by 1340 had become a pest thatmoved to all habitable areas in sailing ships. He was quite anunwelcome visitor. He arrived in Europe about 1346, and soon, over 23million people became ill – 20-40% of the population. But it wasn'tthe rat's fault, he was as sweet and gentle as any of God'screatures; it was just a flea he was carrying and the poor sanitationof the times. By 1856, folks were experimenting with rats in France.Living this life of ease, in cages, the rat's genes changed and hebecame quite mellow. Finally, a (white) litter was born inSwitzerland, around 1906. Its descents were sent to Philadelphia PAwhere the Wistar Scientific Institute used its bloodline to developmore new strains of rats than ever before or after. As I said, theserats didn't join us alone. They brought along old "friends" when theyjumped ship. Friends they had had since the dawn of time. Besides theplague, those friends included the Mycoplasma, some viruses andbacteria that are now making your rat sneeze.

Life Span

Rats were designed to live 3-4 years. Some genetic strains livelonger than others – certain ones no more than 1-1 1/2 years. As inpeople, female rats tend to live longer than male rats. It has alsobeen discovered that limiting their protein and caloric intake aspups increases the length of their lives.Rat Strains and GeneticsI come from a research laboratory background, so I know labrat strains best. Some are Out-bred. That is, bred to maintain thelargest pool of genes possible. It takes about 5 generations and 100pairs to establish an outbred strain. A few of the common outbredstrains include the Sprague Dawley, Wistar and Long Evans rat - oftencalled a hooded or piebald rat.Other strains are Inbred. They result from 20 or more generations ofbrother to sister matings. These strains have the smallest possiblegene pool and so are very ,very alike in all respects. By thefortieth generation, they are 99.5% genetically alike. Scientistslove them, because they are such predictable cookie cutter rats. Themost frequently used inbred strains include the Fisher 344, the Lewisrat, the Brown Norway rat and the Wistar-Furth rat.

The root of your rat's problem is a very tiny organism – smaller thana bacteria-but a bacteria none-the-less, but bigger than a virus. Itis named Mycoplasma pulmonis. In articles it is often just called MRMfor Murine Respiratory Mycoplasmosis. It lives in mice, guinea pigs,and rabbits as well as rats but doesn't seem to be as big a problemin them. It can even exist in your nasal passages where it does noharm. It was lurking there from the day of your rat's birth;clinically silent but causing a slowly progressive disease from thenose to the lungs. Although we think primarily about its effect onthe lungs, it occasionally causes genital infections, sterility andarthritis as well. It is the most important and common disease ofrats.

Rats used in research colonies are obtained from various commercialand institutional sources. It is important that the Mycoplasmastatus of these animals is known and that the rats are housed byvendor or in groups with a similar microbial (bacterial) status. Icheck them frequently by attempting to grow Mycoplasma from swabs ofthe nasal cavity, middle ear, trachea, and uterus-oviduct (remember,this bug hangs out frequently in the reproductive system). We also doblood tests that tell us if Mycoplasm has slipped in. One suspicioussign is when the rats start having smaller weaker litters. Olderfemales seem more susceptible to this reproductive form of thedisease. It can also show itself as creaky, puffy and arthritic legsin older animals.