Mycobacterium bovis in U.S. feral swine – history, diagnostics, and comparative experimental infection
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Bovine tuberculosis (bTB), caused by Mycobacterium bovis, has a long history on the Hawaiian island of Moloka’i where disease spillover from infected cattle into feral swine was identified in the 1980s, cattle were temporarily removed from the island, and disease spillback from feral swine into cattle was subsequently confirmed in the early 1990s. This ignited a 10-year-long bTB surveillance study of Moloka’i wildlife, including feral swine, axis deer, and mongoose from 1999-2008. The last bTB positive cow on Moloka’i was identified in 1997 followed by 26 consecutive years of negative annual testing. That is, up until 2021 when a recent epizootic bTB outbreak on Moloka’i confirmed that several cattle are infected with a strain of M. bovis with single-nucleotide polymorphism (SNP) pattern homologous to 2008 isolates taken from feral swine. The debate remains whether Moloka’i feral swine fit the criteria of a spillover host, an amplifier host, or a wildlife maintenance host for bTB. Determination of bTB host status is complex, requiring extensive epidemiological data from naturally infected populations and is crucial in bTB-disease mitigation and eradication efforts. We aimed to identify surveillance and diagnostic challenges of bTB and infer bTB-host status of genetically distinct populations of U.S. feral swine. Diagnostic test performance for bTB was retroactively compared between a wildlife surveillance study from Moloka’i feral swine (1999-2008) and routine U.S. cattle surveillance (1999-2010). Additionally, experimental infection of U.S. feral swine from distinct genetic backgrounds provides insights into bTB host status potential by highlighting differing pathophysiology and immunology. Tissues from Moloka’i feral swine and cattle originating from the United States of America were submitted to the National Veterinary Services Laboratories (NVSL) in Ames, IA for bTB testing. Mycobacterial culture (current gold standard) from abattoir cattle samples identified more bTB-positive cases than formalin-fixed paraffin-embedded (FFPE) polymerase chain reaction (PCR), while for field sample collections from Moloka’i feral swine the opposite was observed. A total of 63,764 cattle samples were received by NVSL compared with 452 feral swine submissions. bTB was confirmed by culture and/or PCR in 741 cases for cattle (1.16% positive) and 15 cases for feral swine (3.32% positive). Results and disparities between histopathology, PCR, and culture were investigated. Culture identified 39 additional bTB cases from cattle that were missed by PCR while, for feral swine, PCR identified more bTB cases (n = 13) than culture (n = 8). Histopathology identified more lesions as mycobacteriosis compatible than were verified by culture or FFPE PCR for both cattle and feral swine samples but proved to be a valuable and rapid initial screening test in the bTB diagnostic decision-making tree. A multimodal approach to bTB diagnostics, utilizing histopathology, culture, and PCR in parallel, was beneficial in identifying and correcting discrepant results for both cattle and feral swine submissions. Eurasian wild boar descendants from Texas and Polynesian swine from Hawaii were experimentally infected with M. bovis to compare disease susceptibility, pathology, and immunology. A low dose of M. bovis produced tuberculous lesions within 5/6 (83%) Moloka’i and 3/4 (75%) Texas pigs, with distinct lesion profiles and host immunity. Longitudinal sampling of peripheral blood throughout infection indicates highly variable humoral and cell-mediated immune responses as evidenced by antibody production on dual path platform (DPP) and interferon-gamma (IFN-γ) production from peripheral blood mononuclear cells (PBMCs) restimulated with various mycobacterial antigens. Moloka’i-origin swine mounted cell mediated and humoral immune responses to bTB infection on while infected Texas-origin pigs had undetectable antibody responses against MPB83 and ECH. These results suggest unique immune responses and lesion profiles in genetically distinct U.S. feral swine experimentally infected with M. bovis. Feral swine are susceptible to disease caused by M. bovis and diagnostic test performance varies with species and method of collection. The historical information and inherent differences observed between the two groups of pigs used for this study support the hypothesis that Moloka’i feral swine are acting as a maintenance host for bTB and contribute to disease spillback events into domestic livestock.