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Put the first instance on-screen, then use that as an assumed precedent for why the others disappear over time. You could use the solution of mentioning their deaths in passing, since that was like...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/42083 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Put the first instance on-screen, then use that as an assumed precedent for why the others disappear over time. You could use the solution of mentioning their deaths in passing, since that was likely part of casual conversation, but putting one death on-screen and using that as the precedent is fairly common for mass tragedy. For instance, in _Sword of Truth_ by Terry Goodkind, a plague is released into the country. They are alerted to this plague by one villager bringing the MC into their home to ask what's wrong with their kid, and the MC's brother and the village elder say that the kid has the plague, specifically "tokens" that mean he is going to die soon. The other deaths are then mentioned in passing (besides a death of a central character), such as a mention of prostitutes infecting the border guards that were supposed to be isolated. This was an organic way to 1) show the tragedy, but then also 2) show the effect of the tragedy without taking up too much of the viewer's time.