Simply because the ones you feel moving around can be different pathogens. Then those you can feel are already adult parasites. You won't feel their cysts, eggs and coenuri until you hit them with frqs and feel the resonance.
Even though after sometime that resonance won't be felt any more, it doesn't necessarily mean they are completely gone.
So, it's either some testing equipment or dowsing that's left to use as a more reliable test than just feeling them moving.