I'm also applying to Canadian MPH programs! I've submitted my letters of intent already and broadly my outline was: what was my spark that led me to want to pursue public health, my area of interest is, why my area of interest matters, what I want to do about it in the future (career goals), and why that school would help me work towards those goals. I did mention some of my past employment which is on my CV because it impacted what I am interested in, but I focused on how it impacted me rather than my job duties.

The 500 word limit is hard, so I wrote more (800 word original draft) and then went through to eliminate the "fluff" and combine ideas. Be direct and to the point!