My first go-to to learn about avian diseases and other issues is usually the Merck Veterinary Manual. It says LL is vertically transmitted through the albumin or yolk, and that vertically-infected chicks don't aquire antibodies and remain viremic for life...
After learning my first flock of pullets came carrying MG several years ago, I read and researched everything I could possibly find on the disease. I found much more info overseas than in the U.S., since MG (and MS) are endemic in many foreign commercial flocks. There was a long-term study by...