Tuberculosis

Zoonosis Update
Tuberculosis
John B. Kaneene, DVM, MPH, PhD, and Charles O. Thoen, DVM, PhD

T

uberculosis is a term that encompasses various diseases caused by bacteria of the Mycobacterium tuberculosis complex, including M tuberculosis, M bovis, M africanum, and other mycobacterial species. Whereas M tuberculosis infection is largely spread from human to human, M bovis infection has beenidentified as a zoonotic disease with most cases of human infection attributable to animal sources. The mycobacteria other than tuberculosis complex (MOTT), which includes M avium subsp avium and M avium subsp intracellulare isolated from animals,1,2 has been isolated from immunocompromised humans (ie, those with human immunodeficiency virus [HIV] infection), but seldom from immunocompetenthumans.3 Recently, there has been increased interest among public health officials in drug-resistant strains of M tuberculosis, M bovis, and M avium because several have been isolated from HIVinfected and nonimmunocompromised humans.4 Mycobacterium tuberculosis is the cause of most of the cases of tuberculosis in humans. Worldwide, more humans die as a result of tuberculosis each year than from anyother infectious disease.4 At present, more than a third of the world’s population is infected with tubercle bacilli and more people are dying as a result of tuberculosis than ever in history. Ninety-five percent of the tuberculosis cases are reported in developing countries, and it has been estimated that the disease results in the deaths of 2 to 3 million people each year. Infection with M bovishas been reported in humans5-10,a,b and causes pulmonary and extrapulmonary disease.11,12 In the United States and other developed countries, extrapulmonary M bovis infections in humans have been almost eliminated following the introduction of food-production procedures such as pasteurization of milk and routine carcass inspection.11,13 However, M bovis infection commonly occurs in less-developedcountries and in specific demographic groups within developed countries in which consumption of unpasteurized dairy products is practiced. Although there is no active surveillance program for human cases of M bovis infection in the United States, most of the reported cases appear localized to states with large immigrant populations from
From A-109 Veterinary Medical Center, College of VeterinaryMedicine, Michigan State University, East Lansing, MI 48824 (Kaneene); and the Department of Veterinary Microbiology and Preventative Medicine, College of Veterinary Medicine, Iowa State University, Ames, IA 50011-1250 (Thoen). Address correspondence to Dr. Kaneene. JAVMA, Vol 224, No. 5, March 1, 2004

countries with recognized M bovis infections in livestock.10,a,b For example, 7% ofmycobacterial isolates from 1,931 cases of tuberculosis in San Diego were identified as M bovis. These infections were associated with ingestion of raw dairy products; 53% of these patients had extrapulmonary disease,10,14 and 33% of isolates obtained from children were M bovis.10,14 Contact with infected animals is another source of M bovis infection for humans and is a recognized hazard for abattoirworkers, veterinarians, and livestock handlers.57,11,15-17 Among such workers who developed the disease, aerosol transmission was considered the most likely route of infection, but there are many occasions on which infection had been spread via cuts and abrasions (eg, butcher’s wart).16 Although many of the primary nonaerosol sources of M bovis infection in humans have been removed in industrializedcountries, there has been an increase in the number of cases of pulmonary infection with M bovis, which may be due to several factors: the lung is the usual site of postprimary M bovis infection, regardless of the site of the primary lesion; cases of pulmonary M bovis infection may be the result of reactivation of previously quiescent (ie, nonclinical) primary lesions; and infection may be the...