[Vocabulary] Surgeon, health aide, at

[Kazuo]

Hello!

An excerpt from For Haitian-American Doctors, a Call to Return (International Herald Tribune)

And from Boston to Miami, so many Haitians staff the best American hospitals, from surgeon down to health aide — often sending remittances to struggling relatives in their homeland — that Americans of Haitian descent are torn between pride and pain at the contrast with Haiti’s own dysfunctional health care system.

1. <from surgeon down to health aide>:
Why are the words “surgeon” and “health aide” used with no determiners and in the singular?

2. <at the contrast with>
Can the “at” be replaceable with “in”? How does the meaning differ in that case?

Thanks in advance

 

[susiedqq]

Meaning;

And from Boston to Miami, so many Haitians staff the best American hospitals, from staff positions of surgeon down to staff postitons of health aide — often sending remittances to struggling relatives in their homeland — that Americans of Haitian descent are torn between pride and pain at the contrast with Haiti’s own dysfunctional health care system.

(could have been with the contrast of Haiti's own . . . )