Systematic follow-up investigation of NSP seroreactors and in-contact cattle and buffaloes for foot-and-mouth disease virus using probang sampling.

Abstract

Background: Foot-and-mouth disease (FMD) is a highly contagious and economically devastating disease of cloven-hoofed animals. In India, the FMD Control Program has been ongoing for the last two decades. A 3AB3 nonstructural protein (NSP)-based indirect ELISA test is used for population serosurveys to differentiate between infected and vaccinated animals (DIVA). In the present study, a systematic follow-up investigation of the NSP seroreactors and in-contact bovines was carried out from rural cohorts as well as an organized farm in Haryana, India to identify the carrier or neoteric animals. No FMD outbreak was reported from Haryana, a Northern state of India in 2022 and NSP reactivity has also consistently been under 10% for the last five years (2018-2022).

Results: Bovines from ten villages of district Hisar, Haryana, demonstrated 5.3% (20/377) [cattle (11.3%; 12/106) and buffaloes (3.0%; 8/271)] FMDV 3AB3 NSP reactivity. Out of those 20 NSP reactors, nine months later, two buffaloes were randomly screened. Both were found negative for NSP reactivity as well as for FMDV in oropharyngeal fluid (OPF) by reverse transcription-multiplex polymerase chain reaction (RT-mPCR) using 1D/2B gene-specific primers. Further screening was done in a herd of regularly vaccinated cattle (n = 11) of an organized farm with no history of FMD outbreaks for more than a decade. All the susceptible animals were vaccinated with FMD + Haemorrhagic septicemia + Black Quarter combined oil adjuvanted vaccine. An NSP reactivity of 36.7% (4/11) in cattle calves 2-4 months after vaccination indicated either the exposure of animals to FMDV or the presence of residual NSPs in the vaccine. None of the OPF samples collected twice from these cattle at intervals of 36-44 days were found to be positive for FMDV with RT-mPCR. The observed NSP seropositivity could be linked to either false positive reactions or evidence of past exposure and virus elimination during OPF sampling. Nearly all animals exhibited protective antibody titers (≥ log₁₀ 1.65) against the structural proteins of FMDV serotypes O, A, and Asia-1 by Solid Phase Competitive ELISA (SPCE) indicating the effectiveness of vaccination.

Conclusion: The present study provided a preliminary follow-up investigation to assess the status of NSP seroreactors to establish the circulation of FMDV in the animal population, if any, so that the effectiveness of the ongoing vaccination program could be assessed and potential disease-free zones could be identified.