Mycobacterium tuberculosis (M.tb) infection is a threat to elephant health and conservation and human public health worldwide.1,2 Cases of tuberculosis (TB), the disease caused by M.tb in susceptible individuals, have occurred on multiple continents and affected both wild and captive Asian and African elephants.3 Humans are the natural host for M.tb and main source of transmission to elephants.4 Infection can then spread from elephants to other elephants, to other mammals, and possibly back to humans.5–8 Zoonotic and anthropozoonotic spread of M.tb infection is of particular concern given that some countries with high human TB burdens experience a high degree of human-elephant interaction for tourism or cultural purposes.3 Additionally, cases of TB due to multidrug-resistant M.tb have been documented in captive Asian elephants.9 Asian elephants with TB develop similar clinical signs of disease as do humans (eg, weight loss, inappetence, lethargy, coughing); have a similar pulmonary pathology (eg, granulomas with necrosis); and are treated with the same antituberculosis drugs.10–12 Rapid diagnosis, isolation, and treatment of M.tb-positive elephants are important to preserve the health of elephants and susceptible mammals in their proximity.
The current gold standard for detecting M.tb infection in Asian and African elephants is trunk wash culture.12 Trunk wash samples are collected by instilling sterile saline into the trunk, elevating the trunk to allow for deeper distribution of the liquid, and having the elephant forcefully blow the liquid into a plastic bag.12 This technique has several limitations, including the time required to train animals for sample collection, the time required to obtain culture results given that M.tb is a slow-growing organism (up to 12 weeks), the fact that elephants shed M.tb intermittently and therefore a negative culture cannot rule out infection, and the possibility of bacterial and fungal contamination given the variety of tasks for which elephants use their trunks.13,14 Serologic tests to detect serum M.tb antibodies are also available, including the Dual Path Platform VetTB Assay (Chembio Diagnostic Systems Inc), which has demonstrated high sensitivity and specificity for detecting M.tb infection in Asian and African elephants months to years before diagnosis could be made with trunk wash culture.15 However, investigation into new or complementary serologic testing is warranted, as knowledge gaps remain about elephants’ immune responses to M.tb bacilli and infected animals often show no clinical signs until the disease has significantly progressed, making diagnosis more challenging.10,16
Identification of serum biomarkers of M.tb infection in elephants may improve testing capabilities, allowing for earlier detection, isolation, and/or treatment of M.tb-positive elephants, which would reduce risk of M.tb transmission. Given that Asian elephants develop similar TB pathology to humans and that human TB has been studied much more extensively than elephant TB, emerging diagnostic biomarkers for human pulmonary TB may have potential application in Asian elephants. The biomarkers chemokine (C-X-C motif) ligand 1 (CXCL1), matrix metalloproteinase 8 (MMP8), interleukin-10 (IL-10), interferon-γ (IFN-γ), and tumor necrosis factor-α (TNF-α) have been shown to be significantly elevated in the serum of Diversity Outbred mice infected with M.tb compared to noninfected mice, and this research recently showed translational relevance to humans.17 In this study, we sought to determine (1) if these 5 biomarkers could be detected in serum from Asian elephants, and (2) if the serum concentrations of these 5 biomarkers are significantly elevated in M.tb culture–positive elephants compared to M.tb culture–negative elephants. We predicted that these biomarkers would be both detectable and significantly elevated in serum from M.tb culture–positive elephants compared to M.tb culture–negative elephants.
Funding was received from the Tufts Elephant Conservation Alliance (TECA) Elephant Conservation Fellowship and the Summer Research Training Program offered by Cummings School of Veterinary Medicine at Tufts University.
The authors declare that there were no conflicts of interest.
Funding sources did not have any involvement in the study design, data analysis and interpretation, or writing and publication of the manuscript.
We thank the collaborating zoos for their willingness to share banked elephant serum samples to make this project possible.
The findings of this study were previously presented in poster format at the National Veterinary Scholars Symposium on August 4–6, 2021 and the American College of Veterinary Pathologists 2021 Annual Meeting on October 30–November 2, 2021. Work for this study was performed at Cummings School of Veterinary Medicine at Tufts University.
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