Rodent and Ectoparasite-Borne Disease Surveillance Support During the Balikatan Exercise in 2024 and 2025.

Zoonotic diseases are a primary threat to military readiness during deployments in the Asia-Pacific region. To characterize these risks, we conducted a multi-site, multi-year 'One Health' biosurveillance study during the Balikatan (BK) 2024 and 2025 exercises in the Philippines, sampling rodents, ectoparasites, and the environment at four key training locations on Luzon Island. A comprehensive biosurveillance study at four BK training sites revealed distinct, evolving, and site-specific zoonotic disease risks that require tailored health protection strategies. The risk is most acute at Clark Air Base, a critical dual-threat hotspot with an active circulation of several pathogenic strains of Leptospira in environment and rodents near the animal support facility and a direct threat from Rickettsia-positive chiggers. Other sites show escalating vector-borne disease threats including Fort Magsaysay Military Reservation transforming into a high-risk area for rickettsial diseases and Camp Aquino showing an emerging scrub typhus risk on top of its persistent Leptospira circulation. Even limited surveillance at Naval Operating Base Subic confirmed a mixed-threat environment where both Leptospira and rickettsial pathogens are co-circulating. Crucially, the detection of environmental Leptospira during the dry season is a key predictive indicator for a probable surge in transmission risk during the subsequent wet season, demanding proactive mitigation efforts.