Tehran University of Medical Sciences Journal of Arthropod-Borne Diseases 2322-1984 19 2 2025 11 11 173 182 Samira Firooziyan Pathogens and Vectors Research Center, Cellular and Molecular Medicine Research Institute, Urmia University of Medical Sciences, Urmia, Iran, Department of Medical Entomology and Vector Control, School of Public Health, Urmia University of Medical Sciences, Urmia, Iran Ahmadali Enayati Department of Medical Entomology and Vector Control, School of Public Health and Health Sciences Research Center, Mazandaran University of Medical Sciences, Sari, Iran Ehsan Rikhtegar Department of Internal Medicine, School of Medicine, Taleghani Hospital, Urmia University of Medical Sciences, Urmia, Iran, Urmia Deputy for Health Affairs, Urmia University of Medical Sciences, Urmia, Iran Tahereh Sadat Asgarian Department of Vector Biology and Control of Diseases, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran Mohammad Mehdi Sedaghat Department of Vector Biology and Control of Diseases, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran 2025 02 26 2025 11 02 Background: The body louse spreads diseases such as epidemic typhus and louse-borne relapsing fever and has shown resistance to various insecticides. While deltamethrin is used to treat infestations in some countries, research on its ef­fec­tiveness against body lice is limited. This study assessed the susceptibility of body lice to deltamethrin using a bioas­say. Methods: Body lice specimens were collected from an addiction treatment camp in Urmia City, West Azerbaijan Prov­ince, Iran. They were treated in 5 cm Petri dishes lined with 1 ml of various concentrations of deltamethrin (0.01, 0.02, 0.04, 0.08, 0.16, 0.32, 0.64, 1.28, and 2.56 ppm) in acetone. Thirty lice were tested per concentration, with mortality recorded after 24 hours. Lethal concentrations of 50% and 90% (LC50 and LC90) were calculated using Minitab and compared with SPSS. The regression line was plotted in Excel 2013. Results: A positive correlation was observed between the concentrations of deltamethrin and probit mortality in the regression analysis. The calculated LC50 and LC90 values for deltamethrin against body lice were 0.11 ppm and 2.15 ppm, respectively. Conclusions: These findings suggest that the body lice population was susceptible to deltamethrin, suggesting its poten­tial as an alternative treatment, pending further clinical studies. https://jad.tums.ac.ir/index.php/jad/article/view/1836 https://jad.tums.ac.ir/index.php/jad/article/download/1836/696