A 32-year-old male agricultural worker from rural Chhattisgarh presents to the primary health centre with fever, chills, and sweating for 3 days. He reports working in paddy fields during the monsoon season. On examination, temperature is 39.5°C, spleen is palpable 2 cm below the costal margin, and liver edge is felt 1 cm below the costal margin. Peripheral blood smear shows ring forms and trophozoites. He is started on chloroquine. After 48 hours, fever persists and parasitaemia increases. Which epidemiological feature of malaria in this region best explains the treatment failure?