Browsing by Author "Dwyer-Leonard, Rebecca Ann"

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    The application of immunological biomarkers and enhanced pathogen detection for the epidemiological characterisation of bovine tuberculosis in African Rhinoceros
    (Stellenbosch : Stellenbosch University, 2024-01) Dwyer-Leonard, Rebecca Ann; Miller, Michele Ann; Goosen, Wynand Johan; Witte, Carmel; Stellenbosch University. Faculty of Medicine and Health Sciences. Dept. of Biomedical Sciences. Molecular Biology and Human Genetics.
    ENGLISH ABSTRACT: African rhinoceros, specifically the black (Diceros bicornis) and white (Ceratotherium simum) rhinoceros, are iconic species that are under threat due to poaching for their horns, range/habitat loss, unbalanced genetic/demographic structure, climate change, and infectious diseases, including tuberculosis (TB). Mycobacterium bovis (M. bovis) infection, a cause of TB, has been identified in African rhinoceros populations in Kruger National Park (KNP), South Africa. An interferon-gamma release assay (IGRA) is routinely applied for testing of individuals earmarked for translocation out of the park, and for general surveillance purposes. However, relatively little is understood about the overall susceptibility and pathogenesis of TB in these species, and its impact on affected populations. This study had four broad aims: i.) to collate information on the epidemiology of Mycobacterium tuberculosis complex (MTBC) infections in African rhinoceros, ii.) to determine prevalence and risk factors for M. bovis infection in KNP rhinoceros, iii.) to assess the impact of refrigeration and delayed stimulation of rhinoceros whole blood on mitogen stimulated interferon-gamma (IFN-γ) production, to increase flexibility in implementation of testing, and iv.) to determine whether MTBC can be detected in nasal swabs from rhinoceros with immunological evidence of infection, as an indication of potential infectiousness. Drawing from existing literature on MTBC infections in other species and contexts, a foundational understanding of TB epidemiology in rhinoceros species was developed. In other species, demographic risk factors include sex and age, with males and adults generally being at higher risk than females and younger individuals. Review of limited historical information reflected similar age- and sex-associated patterns for TB in captive African rhinoceros, with more reports of TB disease in black rhinoceros than white rhinoceros. Intra-species transmission of MTBC in rhinoceros was also considered to be a potential source of infection. Free-ranging rhinoceros in bovine TB (bTB) endemic areas may be exposed to MTBC, likely shed by maintenance hosts in KNP such as African buffaloes (Syncerus caffer), greater kudus (Tragelaphus strepsiceros), or warthogs (Phacochoerus africanus), through shared environmental niches, and resources. Based on previous reports, hypotheses were generated then investigated in a population-based study of M. bovis infection in 437 African rhinoceros in KNP. We determined an estimated overall infection prevalence of 15.4% (95% CI: 10.4-21.0%) based on mycobacterial culture and IGRA results for animals sampled between 2016-2020. Notably, a significant spatial clustering of cases was detected near the southwestern park border, although infection was widely distributed. Multivariable logistic regression models, including demographic and spatiotemporal variables, showed a significant, increasing probability of M. bovis infection in white rhinoceros based on increased numbers of African buffalo herds in the vicinity of the rhinoceros sampling location. Spillover of infection from African buffaloes to white rhinoceros sharing the environment was suspected. There was also a significantly higher proportion of M. bovis infection in black rhinoceros in the early years of the study (2016-2018) than in 2019 and 2020, which coincided with periods of intense drought, although other temporal factors could be implicated. Species of rhinoceros, age, and sex were not identified as risk factors for M. bovis infection. Ante-mortem surveillance for M. bovis infection in the Kruger National Park (KNP) rhinoceros population currently relies on results from (QFT)-Mabtech equine interferon-gamma (IFN-γ) release assay (IGRA). However, the requirement for same-day processing of rhinoceros blood samples for the IGRA is a logistical challenge to performing this test, particularly in remote locations. A pilot study showed that relative concentrations of IFN-γ (based on optical density values) in mitogen stimulated whole blood plasma decreased significantly with increased time blood was stored post-collection and prior to QFT stimulation. These findings support a need for same-day processing of rhinoceros blood samples for QFT-IGRA testing, as per the current practice to ensure optimal test performance. It was previously unknown whether M. bovis-infected rhinoceros could shed mycobacteria in respiratory secretions. Previous studies suggested that subclinically M. bovis-infected rhinoceros may pose minimal transmission risk. However, recent advances that have improved detection of MTBC members in paucibacillary samples prompted further investigation of respiratory secretions from rhinoceros with immunological evidence of infection, to elucidate the potential for mycobacterial shedding. A pilot study detected M. bovis in 14/64 (22%; 95% CI: 13-33%) of the IGRA positive rhinoceros, and none in the IGRA negative rhinoceros (n = 11) studied, suggesting that M. bovis-infected rhinoceros may be a source of infection for other susceptible animals sharing the environment. Overall, these studies address important knowledge gaps related to surveillance and epidemiology of TB in African rhinoceros, specifically, the free-ranging populations in KNP. This has created awareness of the potential threat of this pathogen to the conservation of these species and highlighted important areas for future research that will contribute to understanding the multi-host TB ecosystem in KNP and other complex systems.