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Sampling at Rack Level
for Potential Contaminant Pathogens:
The health status of laboratory rodents is a principle concern for those responsible for their care. While regular observation of the clinical status of maintained animals is necessary, colony health is also frequently defined by periodic determination of the health status of immune-competent animals placed within the environment or exposed to soiled bedding from animals on the same rack.
Such programs of sentinel animal surveillance require thoughtful consideration of the sampling frequency, cage position and types of pathogens that may be identified by such procedures. Alternatives, or supplements, may afford cost-effective methods to this approach. While the pitfalls of ‘false positive’ and ‘false negative’ results are difficult to avoid altogether, some screening methods can afford simple approaches for the detection of potential contaminants.
To provide direction, we at Thoren Caging would like to present two sites to accomplish comprehensive monitoring at the rack level. While not intended to replace the use of sentinel animals or other direct methods to survey the health status of Individually Ventilated Cage (IVC)-maintained rodents, these procedures may be useful in some limited circumstances. Swab samples may be taken from the proximal surfaces of the rack exhaust, collecting from the filtered air exhausted from all cages on a single side of the rack. Swabs may then be used to provide source material for suitable microbiological assessment.
It is important, however, to recognize that by unique design, air leaving each Thoren IVC is filtered by the cage top filer before entering the rack exhaust plenum. This feature provides a barrier between the IVC and the rack downstream exhaust manifold. As such, any fomites are retained within the cage.
Collection of Samples from the Exhaust Site
(See Figures 1, 2 and 3)
At the double-sided rack exhaust plenum transition (a), the Thoren clamps (b) may be opened and the area swabbed by a suitable method, sterile, if necessary.
The exhaust hose itself may also be unclamped to allow a swab of the inner surface of the exhaust hose directly.
If a single site is desired to monitor the entire rack, the pre-filter within the exhaust fan box (c) will sample air that will have been drawn from all IVC on the entire rack. To do this, the two wing nuts that fasten the prefilter plate may be removed to access the prefilter. The prefilter itself or the containment surfaces may then be swabbed.
See Figure 3 below.
Collection of Samples from an
Individual Shelf Plenum (See Figure 4)
If a particular pathogen is detected at the rack level, as described above, an attempt to localize the source may be pursued. The exhaust at each shelf may be tested. First, turn off the rack blowers for a short period. After establishing which end of the shelves is most proximal to the rack exhaust plenum, unclamp each shelf one at a time and pull the shelf out part way.
Obtain the swab sample for each shelf from the surface at the interior of the shelf and number accordingly. Be sure each shelf is replaced in position and properly clamped. The rack blowers must then be restarted.
Understanding the airflow direction within the Thoren IVC rack is essential for proper interpretation of any sampling data obtained. By design, air exhausted from individual cages first passes through the cage top filter and
and into the shelf plenum through the Exhaust Air Orifice located above the filter.
See Figures 5a, 5b and 6.
The cage top filter prevents many potential contaminants from ever reaching the exhaust channel of the shelf. Exhaust air filtration at the cage level is a unique feature of the Thoren IVC Racks.
Please note that if cage positions are left empty on a rack, air entering the exhaust channel of the shelf will have come from room air, confounding the results of a rack-specific intent.
Cage Air Flow
Shelf Plenum Supply and Exhaust
Located Above the Cage Top Filter
Finally, although somewhat cliché, “Absence of Evidence is not Evidence of Absence”. Thus, health surveillance results must be interpreted in the context of the methods used.
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