For someone with amebiasis (asymptomatic or not), would symptoms appear before it causes harmful affects like damaging the brain?

Yes, almost always. This is another in your string of obviously anxiety-driven questions, sufficient to suggest you have a rather serious obsession with amebiasis and other intestinal infections, despite apparently having no symptoms, and saying nothing about the usual risk factors (like sex with other men, travel in the tropics, etc). There's little point in continuing to ask Consider counseling.

It is strongly urged that you obtain counseling for your emotional issues. Have found psychological interventions, employing cognitive-behavioral therapies have been of extreme value.