A Trunkful of TB?
A Trunkful of TB?
ABSTRACT & COMMENTARY
Synopsis: Wild, domesticated animals, including elephants, may pose a risk of mycobacterial infection in humans.
Source: Michalak K, et al. Mycobacterium tuberculosis as a zoonitic disease: Transmission between humans and elephants. Emerg Infect Dis http://www.cdc.gov/ncidod/EID/vol4no2/michalak.htm
Editor's Note-This review by Carol A. Kemper, MD, recently appeared in an issue Infectious Disease Alert (1998;17:133-134). The point, of course, is the potential for zoonotic disease transmission among travelers, particularly those who have been living and working abroad over prolonged time periods. But, there is more bad news emerging with the availability of new technologies used in this case to document the epidemiology of M. tuberculosis spread among animals-for example, the technique referred to as "RFLP" (restriction fragment-length polymorphism) in the above review.
With this technique, an incidental variation in the mycobacterial genome, often occurring in noncoding spaces between mycobacterial genes, alters the "cuts" that occur when that genome is exposed to a restriction enzyme. Organisms that contain the polymorphism can be recognized after such a restriction enzyme digest is subjected to further analysis by the Southern blot technique (using a probe composed of a radioactively labeled cloned fragment of DNA which has a complementary sequence to the DNA fragments of interest).
Our readers might wish to take a look at a recent publication by Valway et al entitled, "An outbreak involving extensive transmission of a virulent strain of Mycobacterium tuberculosis" (N Engl J Med 1998;338:633-639). This study addresses an important new concept for Travel Medicine physicians. It considers an issue beyond that of multidrug-resistant tuberculosis, which has been so prominent in the news, and speaks to the problem of mycobacterial virulence and transmission rates, unrelated to underlying HIV infection. The study documents a large outbreak of tuberculosis in two rural counties of Tennessee and Kentucky. Twenty-one patients with tuberculosis were identified. What was remarkable about this outbreak, beside the fact that HIV infection was not contributing to it, was the very brief and casual exposures that lead to the infections. When 338 contacts of the index patient were identified and tested, 68% had positive tuberculin skin tests (³ 10 mm induration). In fact, all contacts of the index patient had a significantly higher risk for a positive skin test. The relative risk for a casual social contact over a few hours was 17.0, and, for a close contact, it was 37.3. DNA-fingerprint analysis, performed at the CDC, ultimately allowed this kind of epidemiological documentation of the outbreak.
In additional and more classical animal experiments, mice were given an aerosol challenge of the organisms responsible for the outbreak, and the numbers of bacilli present in lungs were monitored over time. They demonstrated an extraordinary growth rate in vivo, reaching approximately 107 bacilli per lung after 20 days, in contrast to a standard, typical laboratory strain, which reaches about 104 organisms over the same time period.
Whether other virulent and rapidly growing strains of M. tuberculosis are lurking in both human and animal reservoirs here and abroad is not clear yet, but, for those of us advising travelers to developing countries, it should be obvious that it doesn't take a long period of exposure to transmit some virulent strains of tuberculosis. A short ride in a crowded bus or train might be enough-not to mention a prolonged flight in a confined aircraft cabin.
Following the deaths of two circus elephants within one week of each other in August 1996 due to overwhelming pulmonary Mycobacterium tuberculosis (MTB) infection, the United States Department of Agriculture (USDA) now requires all elephants in the United States, as well as their handlers, to undergo regular screening for M. tuberculosis infection.
The elephants belonged to an exotic animal farm in Illinois and were travelling around the country giving rides to the public. MTB was initially suspected when the first elephant, who had a 10-month history of unexplained weight loss, died suddenly in California while undergoing a routine dental procedure. The second animal also died suddenly while en route home for evaluation. Necropsy of the animals revealed copious truncal and respiratory secretions, extensive pulmonary caseous necrosis, and pleural effusions.
An epidemiological investigation was launched. The farm originally housed 18 Asian and two African elephants, 13 of which lived in one large barn, which was in direct communication with a general social area and some of the handlers' housing-although there was a separate ventilation system. The elephant necropsies were apparently done on the farm and were attended by many of the handlers. Initial PPD skin tests were positive in eight of 22 handlers (36%), and three more converted their skin tests during the investigation (50% infection rate). One handler (who had attended the necropsy) had an abnormal chest radiograph; he proved to have smear-negative, culture-positive pulmonary tuberculosis.
Of the remaining elephants, a single live elephant had a positive mycobacterial culture of truncal secretions. Pulmonary MTB was also posthumously diagnosed in a fourth elephant who died in 1994. RFLP analysis demonstrated similarity between the isolates obtained from the four elephants and the handler, although the direction of the transmission could not be confirmed. Based on the presence of the organism in the elephants' truncal secretions, the USDA has recommended that screening cultures be obtained from this site. And, because you apparently cannot skin test an elephant, empiric antituberculous treatment was given to all of the elephants on the farm, although subsequent cultures remained negative.
COMMENT BY CAROL A. KEMPER, MD
Zoonotic transmission of M. bovis from animals to humans can occur with an extensive number of mammalian species, such as cattle, horses, deer, elk, buffalo, goats, swine, and bats; the greatest risk in these cases is posed by human contact with endemically infected wildlife species. For example, wild badgers have been implicated as a residual reservoir of infection for both humans and cattle in England and Wales, where eight patients younger than 30 years of age have been recently diagnosed with primary M. bovis infection (Hardie RM, Watson JM. Epidemiol Infect 1992;109:22-33).
On the other hand, zoonotic transmission of M. tuberculosis appears to be less common, and the direction of the transmission is not always clear. Nonetheless, exposure to both organisms remains a present-day occupational hazard for employees of zoos, nature parks, and abattoirs. A seal trainer developed pulmonary tuberculosis following the deaths of three seals in a marine park in Australia; molecular analysis confirmed the similarity of the mycobacterial isolates, although the direction of transmission could not be confirmed (Thompson PJ, et al. Am Rev Respir Dis 1993;147:164-167). Seven of 24 zookeepers converted their skin tests following exposure to a Southern white rhinoceros infected with M. bovis, but none developed clinical illness (Dalovisio JR, et al. Clin Infect Dis 1992;15:598-600).
Other animals, such as dogs, which live in close contact with humans, may be at greater risk for acquiring human infection than vice versa. In a survey of 15,272 canine necropsies in the Metropolitan New York area from 1962 to 1978, only eight (0.05%) cases of TB were identified (Si-Kwang Liu, et al. J Am Vet Med Assoc 1980;177:164-167). All eight dogs were symptomatic with anorexia, weight loss, lethargy, vomiting, and leukocytosis, and radiographic studies revealed pleural and pericardial effusions, ascites, and hepatomegaly. Given that each of the dogs had a history of close contact with a human known to have TB, the dogs most likely acquired their disease from their humans.
One of our local veterinarians became quite concerned about the possibility of zoonotic transmission of tuberculosis following her unsuccessful attempts at direct CPR on a client's favorite pet sailfin lizard. Necropsy of the lizard subsequently revealed caseous necrosis involving numerous organs, including the lungs. Although mycobacterial cultures were negative, we were able to reassure her that lizards are probably too cold-blooded to support infection with MTB.
On the local scene, one of the elderly female elephants at the San Francisco Zoo is being treated for pulmonary TB. She recently made front page news because of continued gastrointestinal intolerance to rifampin requiring administration of the drug by rectal suppository. (How large is a rifampin suppository for an elephant?) As a result, she has developed very pink cheeks, a curiosity which does not go unnoticed by visiting children. (Dr. Kemper is Associate Director, AIDS Program, Division of Infectious Diseases, Santa Clara Valley Medical Center.)
Subscribe Now for Access
You have reached your article limit for the month. We hope you found our articles both enjoyable and insightful. For information on new subscriptions, product trials, alternative billing arrangements or group and site discounts please call 800-688-2421. We look forward to having you as a long-term member of the Relias Media community.