In a setting I am working on there is a civilization based in a tropical floodplain that’s dependent on paddy field agriculture to maintain its population. Problem is that all the standing water and moist soil creates the perfect conditions for mosquitoes, and those buggers spread absolutely awful diseases that kill millions of people every year. The people in my setting do not have access to modern pesticide, vaccinations, spraying and cannot simply drain the water away due to their agrarian economy. The few things in their favor are a very high degree of social cohesion and a centralized state that’s obsessed with maintaining cleanliness as a religious mandate.
How can a pre modern society significantly reduce the effects of mosquito borne diseases when removing the water supply and pesticide sprays are not an option?