Last modified on March 21, 2002

Pathogen-free barrier housing and facility management

Subclinical infections with microbial factors were tremendous problems in laboratory animal science until the advent of gnotobiotic technology in the 1950s as an offshoot of "germ-free life" research. In rats, mycoplasmosis and hemobartonellosis were particularly difficult to diagnose, yet had subtle to profound effects upon research projects. Some definitions are in order:

Axenic or germfree rats = those with no flora; born by hysterectomy and reared in germfree barriers

Gnotobiotic rats = those with more than one known bug; sometimes axenic to begin with and then known microbes are added back

Barrier-reared (BR) or defined-flora (DF) = rats reared from conception as gnotobiotes; generally act as breeding stock to produce gnotobiotes

Specific pathogen-free (SPF)= rats freed of specific pathogens by the use of gnotobiotic technology (because true gnotobiotes often differed significantly from "normal")

Conventional rats (CV)=animals which usually maintain an assortment of pathogens as either latent or overt infections{3584}

Barrier facilities have varying levels of security, depending upon the maintenance of the barrier and the quality of microbiologic monitoring. The determinants of the security of the barrier include microbial definition of the animals, methods for processing materials into the barrier, entry of personnel, animal housing systems, environmental systems and monitoring practices. In the U. S. there is little standardization of terminology between vendors, so the consumer is left to exercise considerable judgment in the purchase of supposedly clean animals. In the U.K. a voluntary system of designated lab animal quality grading based on evaluation by an independent laboratory has been developed.{3584}

Personnel are the weakest link in any quarantine program. Those who have had access to animals of undefined health status should be kept out of a clean facility for a period of time. People must adhere strictly to protective clothing requirements to avoid becoming fomites or vectors.{3700}

Animals should be shipped in filtered boxes and received by trained personnel who can make housing decisions. For SPF colonies, rodents from approved vendors with good track records might be accepted without quarantine; however these animals should still be subjected to periodic monitoring for adventitious agents. Some advocate quarantine of all mice, because even vendors may have outbreaks occasionally. Animals should enter barriers following embryo transfer or cesarean rederivation or rigorous testing. If they originate from non-vendors, health data for the previous 12 months should be examined.{3700}

Two to four weeks is usually adequate for mouse quarantine, to allow for seroconversion. Testing may involve Theiler's encephalomyelitis virus, MHV, MVM, PVM, Sendai, Reo3, LCMV, ectromelia, mouse K virus, mouse adenovirus, EDIM (rotavirus) and Mycoplasma pulmonis, as well as pinworms and fur mites. Some authors also recommend PCR on fecal pellets for Helicobacter spp. Sentinels should be housed in direct contact if possible; if not, they should be exposed to soiled bedding for 3 weeks. Mice should be quarantined on an all-in-all-out basis.{3700}

Rederivation by embryo transfer is the gold standard of disease eradication. Hysterectomy may not remove pathogens transferred transplacentally. Embryo collection and implantation should be performed by different operators in different locations.{3700}

Good barrier facilities will utilize periodic renewal of the microbial status of barrier animals by rederivation and hand rearing of axenics. If periodic renewal is not practiced, the quality of the animals inevitably degrades such that they are not different from conventional animals.{3584}

BR and SPF rats live longer, grow faster, have larger litters, utilize food better and mature at heavier weights. They are more uniform and more resistant to stress. Their WBC counts and blood glucose are lower, and their lungs weigh less than those of CV rats. CV rats, often infected with murine respiratory mycoplasmosis (MRM), have been characterized in their pulmonary function. They are in a state of compensated cardiac function as a result of pulmonary complications of MRM. The right ventricle is usually hypertrophic, and they have lower blood pressure than SPF rats. Lifespan is longer in BR rats than in CV rats; the median age at death is 30 vs. 14 months.{3584}

Microbiological monitoring of BR rats includes testing sterilization procedures, culturing room surfaces and everything that comes into the barrier. Periodic (quarterly) necropsy of ¼ to ½% of the population is necessary, and should include culture for bacteria and Mycoplasma, serological testing for viruses, parasite exams and histopathological search for evidence of infectious diseases.{3584}

Comments? Send an email to janet.rodgers@vet.ox.ac.uk