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Mice respond to a cage change (CC) with altered activity, disrupted sleep
and increased anxiety. A bi-weekly cage change is, therefore, preferred
over a shorter CC interval and is currently the prevailing routine for
Individually ventilated cages (IVCs). However, the build-up of ammonia
(NH3) during this period is a potential threat to the animal health and
the literature holds conflicting reports leaving this issue unresolved. We
have therefor examined longitudinally in-cage activity, animal health and
the build-up of ammonia across the cage floor with female and male C57BL/6
mice housed four per IVC changed every other week. We used a multicentre
design with a standardised husbandry enabling us to tease-out features
that replicated across sites from those that were site-specific. CC induce
a marked increase in activity, especially during daytime (~50%) when the
animals rest. A reduction in density from four to two mice did not alter
this response. This burst was followed by a gradual decrease till the next
cage change. Female but not male mice preferred to have the latrine in the
front of the cage. Male mice allocate more of the activity to the latrine
free part of the cage floor already the day after a CC. A behaviour that
progressed through the CC cycle but was not impacted by the type of
bedding used. Reducing housing density to two mice abolished this
behaviour. Female mice used the entire cage floor the first week while
during the second week activity in the latrine area decreased. Measurement
of NH3 ppm across the cage floor revealed x3 higher values for the latrine
area compared with the opposite area. NH3 ppm increases from 0-1 ppm to
reach ≤25 ppm in the latrine free area and 50-100 ppm in the latrine area
at the end of a cycle. As expected in-cage bacterial load covaried with
in-cage NH3 ppm. Histopathological analysis revealed no changes to the
upper airways covarying with recorded NH3 ppm or bacterial load. We
conclude that housing of four (or equivalent biomass) C57BL/6J mice for 10
weeks under the described conditions does not cause any overt discomfort
to the animals.
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