Citation: Wanlop, A.; Angeles, J.M.M.; Macalanda, A.M.C.; Kirinoki, M.; Ohari, Y.; Yajima, A.; Yamagishi, J.; Ona, K.A.L.; Kawazu, S.-i. Cloning, Expression and Evaluation of Thioredoxin Peroxidase-1 Antigen for the Serological Diagnosis of Schistosoma mekongi Human Infection. Diagnostics 2022, 12, 3077. https:// doi.org/10.3390/diagnostics12123077 Academic Editors: James P. Landers, Melinda D. Poulter, Jamila S. Marshall and Rachelle A. Turiello Received: 17 November 2022 Accepted: 5 December 2022 Published: 7 December 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). diagnostics Article Cloning, Expression and Evaluation of Thioredoxin Peroxidase-1 Antigen for the Serological Diagnosis of Schistosoma mekongi Human Infection Atcharaphan Wanlop 1 , Jose Ma. M. Angeles 2 , Adrian Miki C. Macalanda 3 , Masashi Kirinoki 4 , Yuma Ohari 5 , Aya Yajima 6 , Junya Yamagishi 7 , Kevin Austin L. Ona 8 and Shin-ichiro Kawazu 1, * 1 National Research Center for Protozoan Diseases, Obihiro University of Agriculture and Veterinary Medicine, Obihiro 080-8555, Japan 2 Department of Parasitology, College of Public Health, University of the Philippines Manila, Manila 1000, Philippines 3 Department of Immunopathology and Microbiology, College of Veterinary Medicine and Biomedical Sciences, Cavite State University, Indang 4122, Philippines 4 Department of Tropical Medicine and Parasitology, Dokkyo Medical University, Tochigi 321-0293, Japan 5 Department of Veterinary Medicine, Rakuno Gakuen University, Ebetsu 069-8501, Japan 6 Would Health Organization Regional Office for Southeast Asia, New Delhi 110011, India 7 International Institute for Zoonosis Control, Hokkaido University, Sapporo 001-0020, Japan 8 College of Medicine, University of the Philippines Manila, Manila 1000, Philippines * Correspondence: skawazu@obihiro.ac.jp Abstract: Schistosoma mekongi, a blood fluke that causes Asian zoonotic schistosomiasis, is distributed in communities along the Mekong River in Cambodia and Lao People’s Democratic Republic. Decades of employing numerous control measures including mass drug administration using praziquantel have resulted in a decline in the prevalence of schistosomiasis mekongi. This, however, led to a decrease in sensitivity of Kato–Katz stool microscopy considered as the gold standard in diagnosis. In order to develop a serological assay with high sensitivity and specificity which can replace Kato–Katz, recombinant S. mekongi thioredoxin peroxidase-1 protein (rSmekTPx-1) was expressed and produced. Diagnostic performance of the rSmekTPx-1 antigen through ELISA for detecting human schistosomiasis was compared with that of recombinant protein of S. japonicum TPx-1 (rSjTPx-1) using serum samples collected from endemic foci in Cambodia. The sensitivity and specificity of rSmekTPx-1 in ELISA were 89.3% and 93.3%, respectively, while those of rSjTPx-1 were 71.4% and 66.7%, respectively. In addition, a higher Kappa value of 0.82 calculated between rSmekTPx-1 antigen ELISA and Kato–Katz confirmed better agreement than between rSjTPx-1 antigen ELISA and Kato–Katz (Kappa value 0.38). These results suggest that ELISA with rSmekTPx-1 antigen can be a potential diagnostic method for detecting active human S. mekongi infection. Keywords: Schistosoma mekongi; Schistosoma japonicum; recombinant antigen; ELISA 1. Introduction Schistosomiasis, also known as bilharzia, is a parasitic disease caused by blood-dwelling flukeworms or flatworms, belonging to the class Trematoda and genus Schistosoma [1]. This remains a serious public health concern in humans worldwide, with more than 230 million people infected with Schistosoma spp. and 800 million individuals at risk of infection [2,3]. Human schistosomiasis is mainly caused by S. mansoni, S. haematobium, S. intercalatum, S. japonicum and S. mekongi [4]. S. japonicum and S. mekongi are the two species causing Asian zoonotic schistosomiasis. S. japonicum is endemic in the Philippines, the People’s Republic of China and parts of Indonesia [5,6]. On the other hand, S. mekongi is found in Cambodia and Lao People’s Democratic Republic (Lao PDR) [1]. It has been reported that these two species are closely related based on morphological and molecular studies [7]. Diagnostics 2022, 12, 3077. https://doi.org/10.3390/diagnostics12123077 https://www.mdpi.com/journal/diagnostics