MYCOBACTERIA

A General Introduction has been added to each disease chapter in an attempt to give a brief updated overview of the taxonomic, biological and other characteristics of the virus family or group of bacteria /protozoa that cause disease in livestock and, where relevant, involve wildlife. As the text of the three-volume book Infectious Diseases of Livestock is currently under revision the Editors are aware that there are inconsistencies between the updated introductions to chapters and the content of the chapters themselves. Once the chapters have been updated – a process that is currently underway – these inconsistencies will be removed.

Introduction

Tuberculosis is a chronic contagious disease caused by infection with certain acid-fast bacterial species of the genus Mycobacterium. It affects many vertebrate animals, domestic and wild (Table 1), and in bovine species it is typically manifested by the formation, in various tissues, but particularly in the lungs and lymph nodes, of granulomas known as tubercles that consist of a core of caseous necrotic tissue surrounded by a zone of granulomatous inflammation.³⁴ The term ‘tubercle bacillus’ was introduced by Robert Koch who first described the causative agent of tuberculosis in humans in 1882. This organism has subsequently been found to be only one of many very different acid-fast bacilli. For a long time the term ‘tubercle bacillus’ has been applied to not only what is today known as the M. tuberculosis complex^{8, 81} but also to distinct species such as M. avium. It is therefore necessary to define the term ‘tubercle bacillus’ whenever it is used.^{14, 30} The characteristic appearance of the lesions, however, differs according to animal species and the respective Mycobacterium sp. with which it is infected. Before effective control measures were adopted, it was one of the major diseases of humans and domestic animals. The disease in humans still remains one of the most important notifiable infectious diseases in the developing world, and is the single biggest public health problem in South Africa,⁴ hugely complicated by an HIV/AIDS epidemic of shattering dimensions.⁵⁴ The various veterinary control measures applied at present in South Africa are aimed at eradication of the disease in livestock and the containment of M. bovis infection in wildlife within the infected game reserves.

For many years tuberculosis appeared to prevail over the entire globe,²⁶ and annually caused more deaths in humans than all of the so-called scourges and wars put together. The records of the Parsees, the Mosaic Laws, the Talmud and other ancient documents all refer to a malady that can be identified as tuberculosis of humans.³⁴ Much of the present knowledge of tuberculosis is based on the initial work of scientists like Laennec and Villemin. Robert Koch first described the causative agent in 1882, and tuberculin, which proved to be a valuable diagnostic tool for the detection of the disease, was developed by him in 1891 for its supposedly curative value.^{42, 43}

Aetiology

The mycobacteria are aerobic, considered Gram-positive (although not readily stainable by the Gram’s staining method), non-motile, non-spore forming, straight or slightly curved rods, 1,5 to 4,0 μm long and 0,3 to 0,5 μm wide.^{17, 84} Their cell walls have a high lipid content, which accounts for their resistance to acids, desiccation and most disinfectants, and also for their slow growth and hydrophobic characteristic in fluid media. Once mycobacteria have been stained with carbol fuchsin, they cannot be decolorized by acid-alcohol — thus their name ‘acid-fast bacteria’. When isolation is attempted, the resistance afforded by the high lipid content of the cell wall allows treatment of samples with acid or alkali to kill microbial contaminants without destroying significant numbers of the mycobacteria in the process.

The genus Mycobacterium consists of well over 120 species, most of which are environmental saprophytes that exist and multiply in a wide variety of substrates such as soil, water and plants, domestic and wild mammals, and birds.^{5, 32, 46, 63, 87} Some of these saprophytic mycobacteria (opportunistic pathogens) may cause ‘opportunistic’ infections in animals and humans, but others never cause disease.³¹ Certain members of the genus, such as those that make up the M. tuberculosis complex (M. tuberculosis, M. bovis, M. africanum, M. microti and M. canetti), which are also known as ‘tubercle bacilli’ and which cause tuberculosis in humans and animals, and M. leprae which is the cause of leprosy in humans, are obligate parasites and are usually transmitted only by infected mammalian hosts. They are also susceptible to a number of chemotherapeutic agents, while the opportunistic pathogens are often resistant to chemotherapeutic agents and are not contagious.⁸⁵

The mycobacteria of the M. tuberculosis complex are characterized by a 99,9 per cent similarity at the nucleotide level and identical 16S rRNA. They differ widely with regard to phenotypes and host tropism. Mycobacterium tuberculosis, M. africanum and M. canettii are human pathogens. Mycobacterium microti causes disease in rodents while M. bovis has a very wide host spectrum, and it was therefore thought for many years that the cause of human tuberculosis evolved from M. bovis. However, the results from a recent study indicate that the common ancestor of the tubercle bacilli resembled modern M. tuberculosis or M...