Human-Elephant Contact and the Transmission of Tuberculosis
Zoonoses, or diseases that are transmissible from animals to humans, account for 75% of the emerging infectious diseases that afflict humans and may threaten many of the world’s species; many of which are already in general decline¹. The high risk of transmission of zoonotic pathogens results from human activities that bring humans into closer and more frequent contact with animals: expansion and encroachment on natural habitats, changes in agricultural practices and globalization, development of ecotourism, and access to petting zoos². In particular, these human activities can pose a tremendous risk to humans and both captive and wild elephants alike.
Historically, in Asia and Africa, in addition to the naturally close association between native peoples and elephant families, zoonotic and reverse zoonotic (disease transmission from human to animal) disease emergence was mediated by the close contact involved with captive elephant husbandry in circuses and zoos. Now, both captive and wild elephants as well as humans may face the consequences of increasing contact in everyday life. Although Southern Africa has 25 people per square kilometer, the population growth rates are higher than in South Asia which supports 350 people per square kilometer³. As a result, disease transmission becomes a growing threat as the conflict between humans and wild elephants persists in Asia and Africa, and humans expand into elephant territory and continue to place elephants under their care.
Tuberculosis (TB), caused by bacteria in the genus Mycobacterium, is easily transmitted between and across species, and has become a growing threat to both elephant and human health⁴⁴. TB is known as one of the leading causes of death in humans around the world⁵. Moreover, although TB was described in elephants by Ayurvedic physicians of ancient Asia and implicated in the extinction of the mastodon, TB in modern day elephants was first discovered upon the death of two circus elephants at the end of the 20th century⁶. Since the first reports of TB in captive elephants, a 2011 study notes that the prevalence of TB in captive Asian elephants in the United States is significantly greater than that of captive African elephants in the U.S.⁶ The higher TB occurrence in Asian elephants in captivity may be the result of the historical association between these elephants and humans and emphasizes of the potential role of prolonged, close contact on disease transmission between humans and elephants in countries around the world.
The contact humans maintain with elephants has created a complex pathway, of which little is known, for bidirectional disease transmission between elephants and humans⁷. A TB infection causes symptoms in elephants that may present as another disease, such as chronic wasting disease, or signs may be subtle or absent until the disease is in an advanced stage⁶. Consequently, TB may be challenging to initially diagnose because clinical signs may not be consistent with the disease. Furthermore, the difficulty in diagnosing, treating, and identifying the mechanisms of TB transmission make the disease especially problematic for elephants and their caretakers. This is exemplified by cases of elephant-human transmission of tuberculosis that have been reported in captivity settings. Initial infections of elephants held in captivity are suspected to come from contact with infectious humans. Humans may in turn develop TB when they inhale aerosolized droplets containing TB organisms from an infected human or animal that is actively releasing, or shedding, the bacteria⁸. In 2013, three infected elephants and seven human individuals known to have been in close contact with the elephants were identified at an Oregon zoo. One of the elephants and one of the human contacts were discovered to carry nearly identical isolates of TB. This indicates the passage of TB across the species barrier but does not tell us anything about the direction of transmission or the mechanism⁷.
The incidence of TB has raised concerns for captive elephants and has threatening implications for wild elephants as human populations expand⁹. Reports of wild elephants that have succumbed to tuberculosis have emerged in recent years. The first documented case of fatal tuberculosis in a wild African elephant with past human contact was reported in 201³¹⁰. The research shows that although the source of TB in the deceased elephant remains unknown because TB transmission pathways between elephants and from humans to elephants remain largely speculative, elephants may be able to maintain latent human tuberculosis in the wild for years after their release from human care¹⁰. Despite the lack of documentation on the mechanisms of transmission between elephants, the contraction of TB by this elephant after having closely interacted with humans highlights the risk the elephant may have posed to other wild elephants after its release had the infection been or become active. Additionally, the study may support the assumption that TB transmission may result from intimate and extended association of elephants with other elephants and humans. Moreover, the discovery of 3 infected wild elephants in India highlights the ways TB transmission may occur in wild species. These elephants had negligible contact with captive elephants, there were no releases of captive elephants into the studied area, and they were bulls which are usually involved in conflict with humans¹¹. As a result, although the epidemiology of TB in wild elephants has yet to be elucidated, the 3 bulls that succumbed to tuberculosis may have been exposed to the lethal pathogen from humans during conflict activities¹¹. As contact with humans increases and transmission of diseases, both zoonotic and reverse zoonotic arise, wild elephants and other susceptible wild species may be at increased risk of infection.
The emergence of tuberculosis in captive animal facilities gives insight into the avenues for tuberculosis to arise in wild elephant populations. Zoonoses such as TB in captive and wild elephants and human conflict with wild elephants threaten biodiversity as the elephants decline⁹. The health and security of elephants is of significant importance to their conservation. As a result, there arises a global emphasis on both elephant and human safety.
Works Cited
¹Magwedere, K., Hemberger, M. Y., Hoffman, L. C., & Dziva, F. (2012). Zoonoses: a potential obstacle to the growing wildlife industry of Namibia. Infection ecology & epidemiology, 2, 10.3402/iee.v2i0.18365. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3474136/
²Chomel, B. B., Belotto, A., & Meslin, F. X. (2007). Wildlife, exotic pets, and emerging zoonoses. Emerging infectious diseases, 13(1), 6–11.Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2725831/
³Thekaekara, Tarsh (2017). Can Elephants and Humans Live Together? The Guardian. Retrieved from https://www.theguardian.com/environment/2017/mar/06/can-elephants-and-humans-live-together
⁴Mikota, S. K. (2008). Review of tuberculosis in captive elephants and implications for wild populations. Gajah, 28, 8–18. Retrieved fromhttps://www.asesg.org/PDFfiles/Gajah/28-08-Mikota.pdf
⁵Kanabus, Annabel (2018). Information About Tuberculosis. TBFACTS.ORG.Retrieved from https://www.tbfacts.org/tb-statistics/
⁶Mikota, S. K., & Maslow, J. N. (2011). Tuberculosis at the human–animal interface: An emerging disease of elephants. Tuberculosis, 91(3), 208–211. Retrieved from https://www.sciencedirect.com/science/article/pii/S1472979211000308
⁷Zlot, A., Vines, J., Nystrom, L., Lane, L., Behm, H., Denny, J., … & DeBess, E. (2016). Diagnosis of tuberculosis in three zoo elephants and a human contact — Oregon, 2013. MMWR Morb Mortal Wkly Rep, 64(52), 1398–402. Retrieved from https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6452a2.htm
⁸Backues, K., Wiedner, E. (2017). 2017 Recommendations for the Diagnosis, Treatment, and Management of Tuberculosis (Mycobacterium tuberculosis) in Elephants in Human Care. NASPHV.Retrieved from http://www.nasphv.org/Documents/ElephantTB_NASPHV.pdf
⁹Miller, M. A., Buss, P., Roos, E. O., Hausler, G., Dippenaar, A., Mitchell, E., van Schalkwyk, L., Robbe-Austerman, S., Waters, W. R., Sikar-Gang, A., Lyashchenko, K. P., Parsons, S., Warren, R., … van Helden, P. (2019). Fatal Tuberculosis in a Free-Ranging African Elephant and One Health Implications of Human Pathogens in Wildlife. Frontiers in veterinary science, 6, 18.Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6373532/
¹⁰Obanda, V., Poghon, J., Yongo, M., Mulei, I., Ngotho, M., Waititu, K., . . . Alasaad, S. (2013). First Reported Case of Fatal Tuberculosis in a Wild African Elephant with Past Human–Wildlife Contact. Epidemiology and Infection, 141(7), 1476–1480. doi:10.1017/S0950268813000022. Retrieved from https://www.cambridge.org/core/journals/epidemiology-and-infection/article/first-reported-case-of-fatal-tuberculosis-in-a-wild-african-elephant-with-past-humanwildlife-contact/6F0A065FD2F16D50AB7E6A899415862F
¹¹Zachariah, A., Pandiyan, J., Madhavilatha, G. K., Mundayoor, S., Chandramohan, B., Sajesh, P. K., Santhosh, S., … Mikota, S. K. (2017). Mycobacterium tuberculosis in Wild Asian Elephants, Southern India. Emerging infectious diseases, 23(3), 504–506. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5382741/
