Abstract

Introduction

Recent findings of Mycobacterium tuberculosis in captive elephants have been a source of great concern to the public, zoo and circus communities, and federal/state authorities. The following is a synopsis of recent cases and the actions that have been taken to address this problem.

Background

In August 1996, two female Asian elephants, aged 46 and 26, died while on the road with a circus. One, “Joyce,” had exhibited marked weight loss for several months and had been tentatively diagnosed with a dental malocclusion. She was anesthetized for a dental procedure but never recovered from anesthesia. Upon necropsy, widespread granulomatous lesions involving 80–90% of the lung tissue were found. Acid-fast organisms were appreciated on histopathology; the organisms were identified on culture as M. tuberculosis.

Since there was no established procedure to deal with tuberculosis in elephants, the U.S. Department of Agriculture (USDA) and the American Association of Zoo Veterinarians assembled a joint panel to review the situation and make recommendations for action. In September 1996 the panel met and observed the affected elephant herd at its home facility. During this meeting, the panel learned of the previous history of tuberculosis (TB) in this herd: one elephant had died of confirmed TB in 1994, and yet another had died of TB in 1983. Before testing was initiated, an adult Asian breeding bull died suddenly at the facility. Necropsy showed no signs of TB and trunk cultures were negative. Of the 18 remaining elephants, 16 appeared clinically normal; two exhibited weight loss.

Methods and Findings

Since many zoo veterinarians felt that the tuberculin skin test alone was an unreliable indicator of disease status in elephants, multiple test methods—including skin testing, trunk cultures, blood tuberculosis (BTB), and serum ELISA assays—were utilized in order to gather as much data as possible from the animals tested. In addition, all animal handlers were tested by the local health department. Many (50%) had positive skin tests; one had a positive sputum culture and chest radiograph.

Most of the elephants in this affected herd were positive on two or more assays; one that had exhibited significant weight loss was culture-positive for M. tuberculosis. Since there were little historical data on the accuracy of these assays in elephants, it was decided to classify individual animals as “high-risk” or “low-risk” based upon both test results and level of exposure to known-positive animals. Fourteen elephants were classified as “high-risk” and four as “low-risk” (Table 1).

Table 1. Classification of 18 members of an elephant herd as at low or high risk for M. tuberculosis infection, based upon results of tuberculin skin testing, BTB and ELISA testing, and level of exposure to known-positive animals

¹#15 had a negative chest x-ray (young animal 3-years-old)
#17 & 18 were clinically thin
#18 was positive for M. tuberculosis on trunk culture
²0.1 ml PPD Bovis injected intradermally in the posterior aspect of the pinna
³Bovine (avian) - reacted to bovine (avian) mycobacterial antigen on the lymphocyte transformation portion of the BTB assay
⁴An equivalent response is one in which the test sample responds equally to both the bovine and avian antigens
⁵Serum ELISA assays performed at Colorado State University (CSU) and Iowa State University (ISU)

Affected animals were treated using protocols based on those prescribed for treatment of human tuberculosis. Sensitivities showed that the M. tuberculosis cultured was susceptible to most of the commonly used human anti-tubercular drugs. For high-risk group animals, multi-drug therapy with isoniazid (INH), rifampin, and pyrazinamide (PZA) was chosen because of its effectiveness in humans and because all the drugs could be given orally. The low risk group was placed on preventive treatment with INH and rifampin for twelve months. PZA was added for the first two months of therapy. Vitamin B₆ (pyridoxine) was also prescribed to counter the effects of INH and prevent possible peripheral neuropathy.

During treatment body weights, liver function, bacterial trunk cultures, and diagnostic blood tests were monitored for each risk group. In addition, travel restrictions were instituted to safeguard the health of the animals and prevent further spread of disease.

Results

By February 1997, the one culture-positive elephant had converted to culture-negative and was gaining weight. As of July 1997, all elephants were tolerating chronic medication with no ill effects reported. INH tends to be unstable in solution and may bind with glucose to become inactive. As a result, some difficulty was encountered when INH was mixed with a syrup. Questions remain as to what drug levels (serum or dosage) can be considered therapeutic in elephants.

Other Cases

CA-1. Texas - In January 1997, a 26-year-old African female died after a long history of respiratory illness and weight loss. Skin test was negative in November 1996. Acid-fast bacteria were identified on histopathology. Necropsy revealed consolidated, “wooden-like” lungs. M. avium was cultured from lungs at necropsy. Handlers and stablemate were skin test negative.

CA-2. California - In March 1997, a 30-year-old Asian female died of Salmonella. Necropsy was incomplete, but both lungs were possibly infiltrated with firm granulomata. Cut surface of the lung was calcified. M. tuberculosis was isolated from a calcified mesenteric lymph node. BTB assay was positive for M. avium.

CA-3. California - June 1997, a 29-year-old Asian female previously housed with Case #2 was trunk culture positive for M. tuberculosis. Skin testing was negative, BTB was positive for M. bovis, and two separate serum ELISAs (one performed at Colorado State University and one at Iowa State University) were positive. Therapy was initiated with the three drug regimen described above. Difficulty was encountered in gaining acceptance of oral medications. This elephant had been exposed to at least seven other elephants (four at a private exhibitor, and three at a public zoo) during the 12 months prior to testing. Testing and treatment were initiated for each of these in-contact elephants. The four elephants at the private exhibitor were dosed by oral syringe with INH and rifampin, and exhibited side effects ranging from poor appetite and lethargy to colitis. The serum drug concentrations in these animals showed were much higher than were obtained in previous studies and the dose was reduced by half. After dose reduction, no further adverse side effects were appreciated. New drug level trials are ongoing. An attempt at medicating by suppository is also being conducted.

CA-4. California - In July 1997, a 30-year-old Asian female that was exposed to Case #3 was trunk culture positive for M. tuberculosis. Skin testing was negative and BTB positive for M. bovis. This elephant was exhibiting a trunk discharge.

Table 2 presents a summary of all test results from all M. tuberculosis culture-positive elephants (as of 1 August 1997).

Table 2. Summary diagnostic testing of all culture-positive cases of M. tuberculosis in elephants (current outbreak, as of 1 August 1997)

¹ISU = Iowa State University; CSU = Colorado State University

Analysis

In June 1997, the draft TB protocol utilized in the first outbreak was reviewed by members of the National TB Working Group for Zoo & Wildlife Species. Given the information that had been gathered since the protocol’s inception, the response from the group was that more work needed to be done to validate diagnostic tests such as the BTB, ELISA, and skin tests before they were used to make decisions regarding treatment and travel restrictions. As this is being written, a new approach to the management of the disease in elephants is being considered, involving culture as the key diagnostic procedure. Given the great expense of treatment and the possible adverse side effects, it was felt that until other tests have been validated only a definitive test, such as culture, along with level of exposure to known positive cases should be used to determine which animals should undergo drug therapy.

Results from drug trials have been inconsistent and more work remains to be done to finalize drug therapy regimens. At this time it is recommended that at least two drugs to which the organism is sensitive be used for treatment. In most cases isoniazid and rifampin would be the initial drugs of choice. A third drug, such as pyrazinamide, would be beneficial as well, especially for animals known to be shedding the organisms. Using humans as a model, current recommendations are to treat culture-positive cases for at least 12 months with isoniazid and rifampin, with PZA added during the first 2 months of treatment. Exposed animals should also be treated either prophylactically or therapeutically, depending on the risk factors involved.

The proposed testing and treatment protocol discussed by the TB Working Group is undergoing further revision at this time. Until it is finalized, any new outbreaks will be handled on a case-by-case basis.