Post-Deployment Screening of Thailand Military Units Deployed to South Sudan from 2023 to 2025 Reveals High Rates of Sub-Microscopic P. falciparum Malaria.

Peacekeeping operations in sub-Saharan Africa continue to be impacted by malaria both in-country and among returning service members. The Royal Thai Army (RTA) deploys an engineering company to Juba and Rumbek, South Sudan to conduct peacekeeping operations as part of the UN Mission in South Sudan (UNMISS). Each deployment is approximately 12 months long. The unit is given doxycycline one week before travel before switching to the UN-provided mefloquine during deployment and for four weeks after returning. The RTA routinely conducts post-deployment screening for malaria by microscopy and PCR for units returning from UNMISS. High rates of prophylaxis failure were observed from both during-mission and post-deployment screening cases, with cumulative malaria attack rates of 11.4% (31 cases out of 271 personnel), 18.2% (49 cases out of 270 personnel), and 23.1% (63 cases out of 273 personnel) for 2023, 2024, and 2025, respectively, with 98% of cases being due to P. falciparum. Furthermore, post-deployment screening revealed high rates of sub-microscopic and sub-clinical parasitemia with 40% of all malaria cases being identified as asymptomatic during post-deployment screening, and 61% of those asymptomatic cases being detected by PCR only. While factors underlying the high prophylaxis failure rate, as well as the high rate of sub-microscopic and sub-clinical parasitemia are unclear, these findings highlight the limitations of relying on clinical symptoms or microscopy for detecting malaria in military units returning from endemic regions and underscore the importance of unit-wide post-deployment molecular screening.