Elephant Tuberculosis, A View From the Inside.
Everything about elephants is big, from their size to their popularity in mainstream media. Add in a well-known human pathogen and you have a news story of, excuse the term, “elephantine proportions.” Elephant health stories that make the lay news are often alarmist, factually inaccurate and driven by animal extremists – elephant tuberculosis reports are no exception. Tuberculosis, a zoonotic disease caused by Mycobacterium tuberculosis (Mtb), has become one of the leading issues of elephant health and well-being. We (the captive wildlife veterinary community) have learned a lot about this disease in just the last 20 years, but compared to what is known about Mtb in humans, our knowledge is still in its infancy.
What do we know?
African and Asian elephants are both susceptible to infection by Mtb complex organisms. Since 1996, elephant tuberculosis has emerged in the United States as a disease primarily of Asian elephants (Elephas maximus). The Asian elephant has lived in close association with humans in Asian range countries for thousands of years. Elephants have been working animals, as well as spiritual and cultural icons. This close partnership is likely responsible for the exposure of the Asian elephant to a disease for which humans are considered the primary reservoir. Incidental reports of Mtb-like disease in the Asian elephant go back thousands of years (Chalke 1962). However, the confirmation of Mtb in elephants only occurred very recently and is the result of the application of modern veterinary medicine and diagnostics. A decade and a half of routine testing and monitoring in the United States is teaching us a great deal about Mtb in elephants.
One of the biggest issues facing the veterinary practitioner and the public health official asked to consult on human elephant interactions is the question of transmission. There have been 2 well documented human exposures to Mtb through working with elephants. (Murphree 2011, Zlot 2016). In both instances what we know about human to human transmission appears to have held true for zoonotic transmission related to elephants: Mtb is transmitted through close, prolonged contact with a person (or animal in these cases) that is shedding the organism (Heymann 2008). Other routes of Mtb exposure in humans have not been documented or are not considered significant (Heymann 2008). Casual contact, such as petting or riding an elephant or indirect exposure to an elephant, has never been shown to be a risk factor for Mtb transmission. These are the basic differences between occupational exposure risk vs. public health risk. Occupational risks can be handled through routine staff testing, routine preventative health testing of elephants, and protocols for staff to don personal protective equipment (PPE) where applicable.
There is documented elephant to elephant transmission of Mtb within herds. In these instances the rule of close prolonged contact holds true also, as affected animals are typically long term companions of carriers, share the same barn, and have trunk to trunk contact. In other instances, where elephants were implicated in Mtb infections without prolonged close contact, fomite transmission was postulated but no conclusive evidence was found (Vogelnest 2012).
Is tuberculosis in elephants an epidemic?
Using the Webster’s dictionary definition, an epidemic is characterized by very widespread growth or affecting a disproportionately large number of individuals at the same time. As a veterinarian working with elephants, I would say tuberculosis is not an epidemic so much as a slow insidious disease that has only come to the awareness of the veterinary community in the last several decades. Prevalence studies from 1997- 2011 have shown an average annual Mtb point prevalence of 5.1% in the living captive U.S. Asian elephant population. For the same time period, the point prevalence of Mtb in captive African elephants in the U.S. was 0% (Feldman 2013). Most of the Asian elephants alive today in the U.S. have some risks factors associated with higher Mtb infection in humans, such as adult to geriatric age and exposure to or time spent living in countries where Mtb infection is more prevalent. The majority of Asian elephants in the U.S. are adult animals in the mid 40’s or older and were imported from range countries such as India and Thailand up until the mid 1970s.
Another factor affecting the disease in elephants is the difficulty in accurately diagnosing Mtb. Similar to latent Mtb infection in humans, early infection in elephants is insidious and can have little to no clinical signs. Chest radiography is impossible in elephants due to their large size, and the tuberculin skin test (TST) is inaccurate. This leaves the trunk wash culture (TW) which is the elephant equivalent to the human sputum sample. Similar to the human sputum test, the TW test has high specificity because a positive Mtb culture is definitive, but low sensitivity because it may take several tries to get a positive culture. Humoral based tests are helpful diagnostic tools but have not been validated in the living population of elephants. Other diagnostics such as a qPCR on trunk wash fluids are still being developed and offer the hope of increasing sensitivity of TW samples. Currently, most veterinarians working with elephants are using multiple diagnostic tests as well as herd history to make informed judgments about which elephants may be infected and need treatment. Knowledge of the efficacy of treatment of elephants is also in its infancy but early studies hold promise that treatment quickly ceases shedding of the Mtb organism into the environment (Backues 2015).
What are we doing?
The captive wildlife community is very dedicated to researching and eradicating this disease in our captive elephants and acting as a resource for information to protect human health and facilitate the best care for elephants in the U.S. and worldwide. To that effort, The Stakeholders Task Force on Management & Research Priorities of Tuberculosis in Elephants was formed in 2011. This group includes veterinarians, elephant managers, public health specialists, epidemiologists, pharmacologists, physicians and other professionals working with elephants in zoos, circuses, and private facilities. ‘The Stakeholders’ have made a multi-year effort to cooperate with USDA, State Veterinarians, and State Public Health Veterinarians and have developed useful, evidence based, and easy to follow guidelines for dealing with elephant tuberculosis. The ‘Recommendations for the Diagnosis, Management, and Treatment of Tuberculosis in Elephants’ was produced in 2015 and is available via link at AAZV.org. This is a useful guide for veterinarians, elephant managers, and public health officials dealing with elephants and will serve as an accurate source of information for those groups and the general public. These recommendations serve as a living document to be updated regularly as the science and knowledge of Mtb in elephants advances through good management, medical surveillance, and scientific cooperation. The Stakeholders intend to continue to identify research priorities as well as learn more about potential risks and Mtb transmission pathways to further refine these recommendations for diagnosis, management, and treatment of tuberculosis in elephants.
This and other articles were featured in Volume 9 Issue 1 of the One Health Newsletter.
Written by: Kay Backues, DVM, DACZM
Backues KA, Robbe-Austermann S, Isaza R. Documented cessation of mycobacterial shedding with antibiotic treatment in a Mycobacteria tuberculosis positive Asian Elephant (Elephas maximus) by serial culture and direct Real-Time polymerase chain reaction testing of trunk wash samples. Proceedings AAZV 2015: 151.
Backues KA, Wiedner E. (eds) Recommendations for the Diagnosis, Treatment and Management of Tuberculosis (Mycobacteria tuberculosis) in Elephants in Human Care. 2015. Publication of the Stakeholders Task Froce on Management & Researhc Priorities of Tuberculosis in Elephants. AAZV.org
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Vogelnest L, Hust F, Vinette HK, et al. Diagnosis and management of tuberculosis (Mycobacterium tuberculosis) in an Asian elephant in Australia. In: Mycobacterial diseases of wildlife 2012;242.
Zlot A, Vines J, et al. Diagnosis of Tuberculosis in Three Zoo Elephants and a Human Contact – Oregon, 2013. MMWR Morb Mortal Wkly Rep. 2016 Jan 8;64(52):1398-402.