2. Epidemiology

### 2.1. The etiological agent

Mycobacteria belong to the order Actinomycetales, family Mycobacteriaceae. The genus Mycobacterium includes Mycobacterium tuberculosis and Mycobacterium avium complexes, other pathogenic Mycobacteria, and numerous species of saprophytic microorganisms present in soil and water. The etiologic agents of mammalian tuberculosis are Mycobacterium tuberculosis, the main cause of human tuberculosis; M. bovis, the agent of bovine tuberculosis; and M. africanum, which causes human tuberculosis in tropical Africa. This last species has characteristics halfway between those of M. tuberculosis and M. bovis. M. bovis is the principal agent of zoonotic tuberculosis. The distribution of M. bovis and M. tuberculosis is worldwide. M. africanum is prevalent in Africa, but it has also been isolated in Germany and England. M. africanum strains phenotypically related to M. tuberculosis are nitrase positive and are found in Western Africa while those which are similar to M. bovis are nitrase negative and are isolated more frequently in Eastern Africa.

The genus mycobacterium is phenotypically characterized as a facultative intracellular microbe, non-capsular, non-spore forming, non-motile, obligate aerobic, and thin-rod bacteria, usually straight or slightly curved having a length of 1–10 length and width of 0.2–0.6 μm. Its cell wall is rich in lipids, that is, mycolic acid, a thick waxy coat responsible for acid fastness, hydrophobicity, greatly contributing to bacterium resistance to many disinfectants, common laboratory stains, antibiotics, and physical injuries [14, 15].

M. bovis is a member of the mycobacterium tuberculosis complex (MTC) and based on 16S ribosomal RNA sequence studies it shared more than 99.95% of identity with other members of MTC [3, 16]. The MTC includes five mycobacterium species, M. tuberculosis, M. canettii, M. africanum, M. microti, M. bovis, and two subspecies—M. caprae and M. pinnipedii [17]. In the environment M. bovis can survive for various months especially in cold, dark, and moist conditions. The survival period varies from 18 to 332 days at 12–24C which is dependent on sunlight exposure. It is found that M. bovis best survives in frozen tissue and there are adverse effects of tissue preservatives, that is, sodium tetraborate on viability [18]. It has been found that the culture of the organism can be done for approximately 2 years in samples that are stored artificially [14, 18].

#### 2.2. Host range

agricultural activities and contribute to the livelihoods of around 70% of the world's population living in poverty. Livestock is central to survival strategies of poor families, can serve as a repository of a family's wealth, and may be sold as an emergency source of cash, in some settings; their ownership is linked to social status or may also be important for ceremonial, cultural, and religious significance. Due to unavoidable interaction of man and animals; zoonotic diseases remain a genuine threat to public health. Zoonotic diseases are the diseases or infections which are naturally transmitted between animals and humans, for example, tuberculosis, brucellosis, leptospirosis, and so on. In most cases, animals play an essential role in maintaining, distributing, and actually transmitting the infection up to varying degrees into the nature. One of the economically significant zoonotic diseases worldwide is bovine tuberculosis (TB) because of serious public health consequences, high cost of eradication programs mainly in developing countries, and trade restriction on animals and their products [1–4]. It is said to be the leading cause of death by infectious diseases [5]. World Health Organization (WHO) classified bovine tuberculosis among seven neglected zoonotic diseases having the potential to infect man [6].

M. bovis mainly affects cattle, which are the most important animal reservoirs and can be established in wildlife. The link between animal and human tuberculosis has long always been known to be strong, as shown by the works of Villemin [7] and Koch [8], which demonstrated the cross-adaptability of the tubercle bacilli from one species to another and pointed out the danger that tuberculosis could be transmitted from animals to humans [7]. The infection currently poses a major concern in human populations in developing countries, as humans and animals share the same microenvironment. It has been estimated that zoonotic transmission of M. bovis is responsible for 10–15% of new human TB cases in developing countries [9]. Bovine TB has been largely eradicated from herds in the developed world by animal TB control and elimination programs, that is, test-and-slaughter programs have drastically reduced the incidence of disease in both animals and humans [10, 11]. However, in developing countries, animal TB is widely distributed

The human burden of disease cannot be reduced without improving the standards of food safety and controlling bovine TB in the animal reservoir. As with other zoonotic diseases, zoonotic TB cannot be controlled by the human health sector alone. Animal health and food safety sectors must be engaged to address the role of animals in maintaining and transmitting M. bovis. The present review highlights chronic multi-species zoonotic TB, its diagnosis prevention and con-

Mycobacteria belong to the order Actinomycetales, family Mycobacteriaceae. The genus Mycobacterium includes Mycobacterium tuberculosis and Mycobacterium avium complexes, other pathogenic Mycobacteria, and numerous species of saprophytic microorganisms present in soil and water. The etiologic agents of mammalian tuberculosis are Mycobacterium tuberculosis, the main cause of human tuberculosis; M. bovis, the agent of bovine tuberculosis; and M. africanum,

trol, veterinary public health challenges, and strategies to combat this important disease.

and control measures are not applied or are applied sporadically [12, 13].

2. Epidemiology

2.1. The etiological agent

24 Basic Biology and Applications of Actinobacteria

The most important causes of bovine TB in cattle are M. bovis and M. caprae, both of which cause infectious diseases [19–22]. M. bovis has one of the broadest host ranges of all known pathogens and has been diagnosed worldwide. Cattle are considered to be the true hosts of M. bovis [23]. However the isolations of M. bovis has also been detected from domestic animals like buffaloes, sheep, goats, pigs, equines, camels, and so on, along with other animals like deer, antelopes, bison, wild boars, primates, llamas, kudus, elephants, foxes, mink, ferrets, rats, elands, tapirs, elks, sitatungas, oryxes, addaxes, rhinoceroses, possums, ground squirrels, badgers, otters, seals, hares, moles, raccoons, coyotes and lions, tigers, leopards, and lynx [23, 24]. The natural movement of these reservoir animals increases the spread of the disease to domestic animals [25]. M. caprae has been reported in many European countries such as Austria, France, Germany, Hungary, Italy, Slovenia, and the Czech Republic. A disease caused by M. caprae is not substantially different from that caused by M. bovis and the same tests can be used for its diagnosis [26].

#### 2.3. Transmission

Transmission of M. bovis can occur between animals, from animals to humans, and vice versa and rarely in between humans [27]. The transmission of M. bovis between animals occurs mainly through aerosols. Transmission through other routes like cutaneous, congenital, and genital routes has also been reported. Close contact among animals and intensive breeding increase the rate of transmission [28]. Other factors like long survival periods of the organism in the environment also contribute to an increased risk of infection [29, 30]. Suckling calves can get the infection through consumption of infected milk. The infected bull semen may transmit diseases through artificial insemination [18].

blood vessels. Clinical signs vary with the involvement of the lung manifested through cough, dyspnea, and other signs of low-grade pneumonia which can be induced by changes in temperature or manual pressure on the trachea. Digestive tract involvement is manifested by intermittent diarrhea or constipation, extreme emaciation, and acute respiratory distress may

Zoonotic Tuberculosis: A Concern and Strategies to Combat

http://dx.doi.org/10.5772/intechopen.76802

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M. bovis infection in humans has similar clinical forms as those caused by M. tuberculosis [27, 34, 44]. Most of the studies have suggested that the common clinical manifestation of M. bovis infection in man is associated with the extra-pulmonary form of the disease; however, about half of the post-primary cases involve the lung which is responsible for human-tohuman transmission of tuberculosis due to M. bovis [13, 31, 44, 47]. The primary infection of the organism in the intestine may heal or it may progress in the intestines or disseminate to other organs [48]. Cervical lymphadenopathy, intestinal lesions, chronic skin tuberculosis, and other non-pulmonary forms are particularly common [13]. Infection due to M bovis in humans usually has a prolonged course and symptoms generally takes months or years to appear. Sometimes, the bacteria remain dormant in the host without causing diseases [23]. The common clinical signs of zoonotic TB include loss of appetite, diarrhea, weight loss, intermittent fever, intermittent hacking cough, large prominent lymph nodes, weakness, and so on.

Young children infected with M. bovis typically have abdominal infections and older patients suffer from swollen and sometimes ulcerated lymph glands in the neck [49]. Pulmonary disease is more common in people with reactivated infections [50] and this would occur only when some of the animals had active tuberculosis [32]. The symptoms may include fever, cough, chest pain, cavitation, and hemoptysis [50]. The pulmonary form of tuberculosis occurs

Bovine TB affects the national and international economy in different ways. It is extremely difficult to determine the economic impact of bovine TB on livestock production. The presence of bovine TB infection in livestock reduces the livestock productivity and economically devastates the cattle industry especially the dairy sector. Some losses are related to the animal production, marketing, or trading of the animals as well as the cost involvement while implementing surveillance and control programs. These losses are also extremely important when endangered wildlife species get involved [51, 52]. The direct productivity losses due to bovine TB can be categorized into "on-farm" losses and losses after the slaughtering of animals. On-farm losses consist of the losses from decreased milk and meat production, the increased reproduction efforts, and replacement costs for infected cattle while losses during slaughter consist of the cost of cattle condemnation and retention, with the loss from condemnation being essentially the purchased value of a slaughter animal and the loss [51]. Along with the direct productivity losses, bovine TB has profound economic consequences on international trade; it affects access to foreign markets due to import bans on animals and animal products

occur during the terminal stages of tuberculosis [26].

less frequently and is usually occupationally related [44].

4. Zoonotic TB: a concern

3.2. In humans

Humans acquire the M. bovis infection from cattle directly by erogenous route or through the direct contact with material contaminated with the secretions of an infected animal or the herd [31, 32]. The individuals at risk are farm workers, zookeepers, milkers, animal dealers, veterinarians, abattoir workers, meat inspectors, autopsy personnel, laboratory personnel, and owners of potential tuberculous pets [33–35]. People in these occupations may develop pulmonary tuberculosis from M. bovis and in turn put other humans and susceptible animals at risk [36, 37]. Indirectly, man acquires the disease from animal sources by consumption of unpasteurized infected milk and ingestion of meat and meat products from slaughtered infected cattle [13, 38–41]. Therefore tuberculosis can be foodborne also [42]. The consumption of contaminated milk products possesses more risks than infected meat products because badly infected carcasses are mostly condemned and meat is generally thoroughly cooked [43]. People suffering from M. bovis tuberculosis can retransmit the infection to cattle; however, this is not common [44].

#### 2.4. Geographic distribution

Zoonotic TB is distributed globally and is more prevalent in most of Africa, parts of Asia and of the Americas except Antarctica, Caribbean islands, parts of South America and Australia, Iceland, Denmark, Sweden, Norway, Finland, Austria, Switzerland, Luxembourg, Latvia, Slovakia, Lithuania, Estonia, the Czech Republic, Canada, Singapore, Jamaica, Barbados, and Israel [45]. Although most of the developed countries have reduced or eliminated bovine TB from their cattle population, however, the disease is still present in the wildlife of United Kingdom, Canada, the United States, and New Zealand [23]. Eradication programs are in progress in other European countries, Japan, New Zealand, the United States, Mexico, and some countries of Central and South America where it has been eradicated by following strict test-and-slaughter policies [46].
