It is highly unlikely for any ASF's in America to carry lassa fever. Not unless they came into contact with infected fecal matter straight from Africa. It is one of the reasons ASF's were banned from importation into the US years ago.

I have also not heard of any zoonotic diseases to infect humans from their own collections of feeders or pets, though it is possible. These are more worrisome in third world countries or places that have big problems with wild rodent populations as pests.

All in all, buy from a good breeder.