Chapter 43 Tuberculosis in Elephants

Tuberculosis (TB) is an ancient disease of both humans and animals. Tuberculous scarring has been observed on bones from 21 of 48 mastodon skeletons recovered in North America (dating to the last Ice Age), and deoxyribonucleic acid (DNA) from the human form of TB has been isolated from a 17,000-year-old bison bone.²⁴ Tuberculosis and its treatment in elephants was described by Ayurvedic physicians in Asia more than 2000 years ago.^28,⁴⁰

A case of TB in an elephant at the London Zoo in 1875 was the first published report in modern times,¹⁷ although the archives of the European Elephant Group indicate an even earlier case in an Asian bull named “Hans” who died in 1802 from TB at Jardin des Plantes, Paris, at age 18 years.¹⁶ Numerous case reports appeared in the literature in the twentieth century.^*

I examined medical records of 379 elephants from 1908 to 1992 and found only eight cases of TB.⁴² This retrospective study’s failure to reveal the significance of TB for elephants may be attributed to the sample population, which included elephants from Elephant Species Survival Program (SSP)–participating zoos in North America, but not privately owned elephants (from which records were not available). In addition, surveillance for TB among elephants even in zoos lacked uniformity and consisted largely of intradermal testing, now known to be inaccurate. Also, routine necropsies of elephants and evaluation of ill elephants for the presence of TB were not performed.

The year 1996 is often heralded as the date that TB “emerged” as a disease of concern for elephants, with outbreaks of TB in elephants in North America.^4,^43,⁴⁴ Subsequent outbreaks in Europe have since been reported.^18,^34,⁴⁵

This chapter references TB in humans because this most closely provides a model for the disease as it is observed in elephants. Significant progress has been achieved since 1996 in understanding TB in elephants, and although these elephants have benefitted from the knowledge of how TB acts in humans, certain aspects of diagnosis and treatment unique to elephants have evolved.

DEFINITION

Tuberculosis is caused by bacteria in the genus Mycobacterium, which comprises more than 100 species. Mycobacteria infect a broad range of species, including humans, nonhuman primates, domestic and nondomestic ungulates and carnivores, marine mammals, psittacine birds, reptiles, and fish. Species susceptibility to specific mycobacteria varies.⁴⁶

In mammals the term tuberculosis defines disease caused by Mycobacterium tuberculosis–complex organisms. The M. tuberculosis complex includes M. tuberculosis, M. bovis, M. africanum, M. microti, M. canetti, M. caprae, and M. pinnipedii. A vaccine strain derived from M. bovis (M. bovis BCG) is sometimes listed as a separate member of this complex.

The term mycobacteriosis describes disease caused by nontuberculous mycobacteria (NTM), also called “atypical mycobacteria” or “mycobacteria other than TB” (MOTT). Most NTM are saprophytes found in soil or water, but they may occasionally cause disease in humans and animals. Mycobacterium elephantis, a rapidly growing, newly described mycobacterium, was isolated from a lung abscess of an elephant that died of chronic respiratory disease.⁶¹ This same organism was isolated from 10 human sputum samples and one human lymph node specimen in Canada; however, there was no epidemiologic link between these reports.⁶³

ETIOLOGY

Mycobacterium tuberculosis (M. tb) is the predominant disease-causing agent in elephants, although TB cases have been caused by M. bovis. Mycobacterium szulgai, an uncommon NTM species, has been associated with fatal disease in two African elephants.³⁰ Mycobacterium avium is often isolated from elephants⁵⁰ but has not been associated with clinical disease.

EPIDEMIOLOGY

Frequency and Distribution

Both African and Asian elephants are susceptible to TB. I am aware of 34 known cases of TB affecting 31 Asian and 3 African elephants in the United States between 1994 and 2005.¹⁵ Five cases were reported in Europe in 2002,³⁴ with two subsequent cases in 2005.⁴⁵ More frequent reporting of the disease in Asian but not African elephants may reflect closer human contact related to the use of Asian elephants for performances and rides. Likewise, the vast majority of cases occurring in female elephants may be a result of their higher number in captivity and the greater likelihood of human contact with cows than bulls. The disease occurs in captive elephants in Asia as well.⁶^–¹⁰ ^,47 ^,54 ^,55 There are no reports to date of TB in wild elephants.

Reservoirs

The reservoirs for M. tb and M. bovis are infected humans and cattle, respectively.²⁵ Transmission may occur via respiratory or alimentary routes. Feces, urine, genital discharges, milk, and feed or water may contain contaminated droplets. In elephants, M. tb has been isolated from respiratory secretions, trunk washes, feces, and vaginal discharges. The M. avium-intracellulare group can survive in the environment and, unlike the M. tb– complex group, does not need a host to survive. Thus, infection is not always acquired from another infected animal. Clinical disease in elephants from M. avium has not been reported.

PATHOGENESIS

Infection versus Disease

Infection with mycobacterial organisms does not necessarily imply active disease. Several possible scenarios may result after exposure to an infectious source (Table 43-1). Tuberculosis is estimated to infect latently one third of the global human population and causes approximately 3 million deaths yearly. Latent TB infection (LTBI) in humans is characterized by a positive intradermal test but a lack of clinical disease and no evidence of active shedding of live bacilli. In LTBI, bacterial organisms are sequestered but may reactivate at a later date. Individuals with LTBI are a reservoir for future active cases, and although only an estimated 4% to 10% of latently infected humans with normal immune status will develop active TB during their lifetime, the identification and treatment of individuals with LTBI and at high risk for activation remain an effective means of control.⁴⁸

Table 43-1 Possible Scenarios after Exposure to Active Tuberculosis (TB)

Scenario	Pathogenesis	Tuberculin Skin Test Status (In Humans)
1	All bacteria are killed, and no disease results.	Negative
2	Bacteria multiply, and clinical disease results (primary TB).	Positive
3	Bacteria become dormant and never cause disease (latent TB).	Positive
4	Latent organisms reactivate and cause active disease.	Positive