The following are factors that may determine whether swimmer's itch may be a problem on a specific lake at a given time:
- distribution and number of snails that can serve as intermediate hosts
- distribution and number of bird hosts that can serve as hosts for the adult worm
- wind direction
- water currents
- number of hours that people stay in the water
- time of day
- sensitivity of the individual to swimmer's itch.
All of these factors can change on an annual basis.