Sentinels are a cost-effective way of monitoring mouse colonies for the presence of pathogens. Until recently, generally two types were used: dirty-bedding and direct contact. Recently,1,2,3 a third type has been proposed: air-exhaust sentinels. To be informative, sentinels must be properly used. Sometimes, the best option may be to test the mice in your colony directly. If sentinels are used, here are brief descriptions, advantages and disadvantages of each type.
Dirty-bedding sentinels are most effectively used to detect pathogens transmitted primarily by fecal-oral contamination, such as Rotavirus, Mouse Hepatitis Virus, Reovirus, and Helicobacter. Their advantage is that only one or a few mice are needed to monitor multiple cages at once. Bedding samples from several cages are placed into the sentinel cage. To allow enough time for a putative pathogen to be transmitted, we expose the sentinel to the bedding for at least a month. This is potentially a disadvantage if newly arrived mice are routinely quarantined and must be released as soon as possible. Under such circumstances, it may be more appropriate to test some recently arrived mice directly. After sufficient exposure, the sentinels are tested for the presence of pathogens by diagnostic methods such as serology, parasitology and PCR.
Direct contact sentinels are best used to detect pathogens that can infect mice by a variety of different routes, including direct contact, aerosol, urine, and fecal-oral. To prevent unwanted pregnancies and minimize fighting, we use castrated male sentinels, placing them directly into cages with the colony mice. To allow enough time for a putative pathogen to be transmitted, we keep the sentinels and colony mice together for at least six weeks (standard protocols recommend a minimum of four weeks). We then test the sentinels for pathogens by the same methods used to test dirty-bedding sentinels. The disadvantage of cage-contact sentinels versus dirty bedding sentinels is that more of them are required.
Air-exhaust sentinels have recently1,2,3 been proposed for monitoring mice housed in individually ventilated caging (IVC) systems. The sentinel is placed in a special cage called a Bioscreen system (BioZone, Inc., Fort Mills, SC). The cage is connected directly to pre-filtered exhaust air from the IVC rack, directly exposing the sentinel to the air supply from all cages in the rack. Many mouse pathogens can be transmitted in this manner.
Whichever type of sentinels used, they must be clean and pathogen free. Otherwise, a colony may mistakenly be determined contaminated.
References
1 Lipman NS, Homberger FR. 2003. Rodent quality assurance testing: use of sentinel animal systems. Lab Anim (NY) 32:36-43.
2 Compton SR, Homberger FR, Paturzo FX, MacArthur-Clark J. 2004a. Efficacy of three microbiological monitoring methods in a ventilated cage rack. Comp Med 54:382-92.
3 Compton SR, Homberger FR, MacArthur-Clark,J. 2004b. Microbiological monitoring in individually ventilated cage systems. Lab Anim (NY) 33:36-41.