animals Article Pilot Investigation of Anti-Salmonella Antibodies in Oral Fluids from Salmonella Typhimurium Vaccinated and Unvaccinated Swine Herds Alessia De Lucia 1 , Shaun A. Cawthraw 2 , Richard Piers Smith 2 , Rob Davies 2 , Carlo Bianco 3 , Fabio Ostanello 1, * and Francesca Martelli 2   Citation: De Lucia, A.; Cawthraw, S.A.; Smith, R.P.; Davies, R.; Bianco, C.; Ostanello, F.; Martelli, F. Pilot Investigation of Anti-Salmonella Antibodies in Oral Fluids from Salmonella Typhimurium Vaccinated and Unvaccinated Swine Herds. Animals 2021, 11, 2408. https:// doi.org/10.3390/ani11082408 Received: 15 July 2021 Accepted: 12 August 2021 Published: 14 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Department of Veterinary Medical Sciences, School of Agriculture and Veterinary Medicine, via Tolara di Sopra 50, 40064 Ozzano Emilia, Italy; alessia.delucia3@studio.unibo.it 2 Animal and Plant Health Agency, Woodham Lane, New Haw, Addlestone KT15 3NB, UK; shaun.cawthraw@apha.gov.uk (S.A.C.); richard.p.smith@apha.gov.uk (R.P.S.); rob.davies@apha.gov.uk (R.D.); francesca.martelli@apha.gov.uk (F.M.) 3 Animal and Plant Health Agency Lasswade, Pentlands Science Park, Bush Loan, Penicuik, Midlothian EH26 0PZ, UK; carlo.bianco@apha.gov.uk * Correspondence: fabio.ostanello@unibo.it Simple Summary: The consumption of pork meat is responsible for a significant number of outbreaks of salmonellosis in people. Surveillance in pig herds is constrained by the cost-effectiveness and efficiency of sampling methods. The last decade has seen significant advances in the routine use of pool samples, including oral fluids (OFs). This study aimed to investigate the OF collected passively via chewed sampling ropes as a potential sample type for assessing anti-Salmonella antibodies in two Salmonella-vaccinated (V) and two non-vaccinated (NV) farrow-to-finish pig farms, comparing the results with the Salmonella shedding of tested animals. Sows in the V farms were vaccinated prior to farrowing. Pooled faecal and OF samples were collected from sows and their offspring. Salmonella was isolated with direct bacteriological methods. A commercial ELISA assay was adapted to detect IgG and IgA antibodies in OF. Overall, a higher Salmonella prevalence was observed in the NV farm and in the offspring (76.3%) compared to sows (36.4%). The protocol used to test anti-Salmonella IgA in pig OF samples was found to lack sensitivity and specificity. At herd level, IgG is the most reliable isotype for monitoring Salmonella specific antibody via OF. Abstract: Oral fluid (OF) can be a simple, cheap and non-invasive alternative to serum or meat juice for the diagnosis and surveillance of important pathogens in pigs. This study was conducted on four Salmonella Typhimurium-positive farrow-to-finish pig farms: two Salmonella-vaccinated (V) and two non-vaccinated (NV). Gilts and sows in the V farms were vaccinated with a live, attenuated vaccine prior to farrowing. Pooled faecal and OF samples were collected from the sows and their offspring. Salmonella was isolated according to ISO6579–1:2017. In parallel, IgG and IgA levels were assessed in OF samples using a commercial ELISA assay. Salmonella was detected in 90.9% of the pooled faecal samples from the NV farms and in 35.1% of the pooled faecal samples from the V farms. Overall, a higher prevalence was observed in the pooled faecal samples from the offspring (76.3%) compared to the sows (36.4%). IgG antibodies measured in V farms are likely to be related to vaccination, as well as exposure to Salmonella field strains. The detection of IgA antibodies in OF was unreliable with the method used. The results of this study show that IgG is the most reliable isotype for monitoring Salmonella-specific antibody immunity in vaccinated/infected animals via OF. Keywords: antibodies; ELISA; oral fluid; pigs; Salmonella 1. Introduction Non-typhoidal salmonellosis is regarded as one of the most important food-borne zoonotic diseases, causing ill health and high disease-related costs in people [1,2]. The Animals 2021, 11, 2408. https://doi.org/10.3390/ani11082408 https://www.mdpi.com/journal/animals