A 45-year-old Eritrean asylum seeker in the UK presents with recurrent episodes of high fever every 5-7 days, each lasting 2-3 days. During a febrile episode, a thick blood film shows spirochaetes. He has evidence of body louse infestation. After treatment with a single dose of doxycycline, he develops sudden-onset rigors, hypotension, and tachycardia within 2 hours. What is the most likely explanation?