14| VOLUME 79 ISSUE 1 | MAY Reducing Nematopsis spp Infection of Panaeus Vannamei Shrimps Post Larvae by Using a Mixture of Natural Antimicrobials Igori BALTA 1,2,3 , Ioan PET 3 , Patrick WARD 4 , Adelina VENIG 5 , Todd CALLAWAY 6 , Nicolae CORCIONIVOSCHI 1,2,3* and Lavinia STEF 3* 1 Bacteriology Branch, Veterinary Sciences Division, Agri-Food and Biosciences Institute, Belfast BT4 3SD, Northern Ireland, UK; 2 Faculty of Animal Science and Biotechnologies, University of Agricultural Sciences and Veterinary Medicine, Cluj- Napoca 400372, Romania; (I. B.); (N. C.). 3 Faculty of Bioengineering of Animal Resources, Banat University of Agricultural Sciences and Veterinary Medicine - King Michael I of Romania, 300645 Timisoara, Romania; 4 Auranta, Nova UCD; 5 Faculty of Engineering and Management, University of Oradea, 410087, Oradea, Romania; 6 Department of Animal and Dairy Science, University of Georgia, Athens, GA, United States; * Corresponding authors: N. Corcionivoschi: nicolae.corcionivoschi@afbini.gov.uk; L. Stef: lavi_stef@animalsci-tm.ro RESEARCH ARTICLE Abstract The aim of this study was to evaluate the use of a mixture of natural antimicrobials (maltodextrin, sodium chloride, citric acid, sodium citrate, silica, malic acid, citrus extract, and olive extract) to prevent Nematopsis spp. infestation of shrimp and to explore the biological mechanisms involved in preventing infection and improved performance. Our results show that the natural antimicrobials were able, in vitro, to reduce (p < 0.0001) the ability of N. messor, N. quadratum and N. annulipes to adhere to CHSE-214 epithelial cells. In vivo the natural antimicrobial mixture reduced (p < 0.0001) their gut colonisation, increased the (p < 0.05) shrimp survival rates and (p < 0.05) and also the relative growth rate and shrimp size. Moreover, the average daily gain, the total weight gain and the feed conversion rate were all improved over the 30 d trial. The beneficial effects on shrimp performance were associated with a significant increase (p = 0.0004) in the abundance of probiotic-like bacteria (Lactobacillus spp.). This positive change in gut bacterial composition was correlated with a significant increase in the expression of genes related to digestion or the immune response suggesting that natural antimicrobials can improve the performance of shrimp in vivo. Keywords: Nematopsis spp., natural antimicrobials, growth performance, probiotic bacteria, gut immunology INTRODUCTION Over 6000 species of Apicomplexa gregarines, single-celled and obligate intracellular protozoan organisms, can cause severe disease in humans and livestock (Seeber and Steinfelder, 2016) but they can exclusively infect aquatic an terrestrial invertebrates (Rueckert et al. 2019). They are also known as microbial eukaryotes being causing agents of toxoplasmosis and malaria, and emergent diseases (e.g. cryptosporidiosis) (del Campo et al. 2019) . Among these, Nematopsis spp., a parasitic protozoa, is able to cause significantly higher rates of infestations in shrimps (90%) and leads to poor performance and to the characteristic white faeces syndrome (WFS) (Sriurairatana et al. 2014). This syndrome is very damaging to Panaeus vannamei cultures as it was reported to reduce their survival rates to as low as 20-30% being associated to the occurrence of major intestinal dysbiosis due to low microbiota diversity in affected shrimps compared to un-infected shrimp populations (Hou et al. 2018). Nematopsis Received: 16 March 2022 Accepted: 24 March 2022 Published: 14 May 2022 DOI: 10.15835/buasvmcn-fst:2022.0007 © 2022 Authors. The papers published in this journal are licensed under the Creative Commons Attribution- NonCommercial-NoDerivatives 4.0 International License