Per Amboss, atovaquone and azithromycin used for BOTH mild/moderate cases and severe/resistant cases. The only difference is that azithromycin is given via IV for severe cases, whereas PO azithromycin is used for mild cases.
They indicate that quinine PLUS clindamycin is used as an alternative regimen for severe cases.
UTD also supports Amboss in stating the preferred regimen is atovaquone (PO) + azithromycin (IV) in severe cases. They state that the alternative regimen (quinine+clinda) is associated with greater adverse effects.
https://www.uptodate.com/contents/babesiosis-treatment-and-prevention?search=babesiosis&source=search_result&selectedTitle=2%7E64&usage_type=default&display_rank=2#H772674862
Source: AMBOSS - https://next.amboss.com/us/article/P70WNh#Y915dad9e85344993ec64abcc84001e4a
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