[triceratops]

> The neighbourhood cats keep the mice and hantavirus down. Aircraft are often infested with mice

They aren't doing a good job of keeping them "down" if they're on planes.

[cucumber3732842]

Airports have tons of food (grass environment + human garbage), tons of mouse habitat (grassy areas, outbuildings, machinery), and not many cats.

[triceratops]

It was a joke. If mice are in aircraft they are "up" (in the air), not "down".