Tuberculosis alone causes more deaths worldwide than any other infectious disease. It is estimated that about one-third of the worlds population is infected and is at risk of developing the disease.
The World Health Organization estimates that 4.4 million people worldwide are infected with both Mycobacterium spp. and HIV and predicts one million deaths every year amongst this population.
Tuberculosis is primarily a respiratory disease and transmission of infection is mainly by the airborne route. The aetiological agents of tuberculosis in mammals are a heterogeneous cluster of organisms included in the so-called Mycobacterium tuberculosis complex, comprising Mycobacterium tuberculosis, M. bovis, M. microti and M. africanum.
Infections due to M. tuberculosis are confined almost exclusively to humans and captive primates (OReilly and Daborn, 1995). There is ample evidence that although other animals are susceptible to M. tuberculosis infection, they represent in most cases end hosts that are only sporadically infected by contact with tuberculous humans.
M. microti was first isolated in an unusual form of mammalian tuberculosis in field voles (Microtus agrestis) in 1937 (Wells, 1937). Since then, several isolations have been published (Wells, 1953; Wells and Wylie, 1954). Besides field voles, wood mice, bank voles and shrews are also naturally infected. M. microti is of low pathogenicity, very large doses being required to produce a progressive disease in laboratory animals (OReilly and Daborn, 1995).
M. microti, although pathogenic for animals other than voles and small mammals, does not pose any pathogenic threat to human beings (OReilly and Daborn, 1995).
M. africanum has characteristics intermediate between those of M. tuberculosis and M. bovis (Grange and Yates, 1989). It has occasionally been isolated in tuberculous lesions in domestic or wild animals, and its significance as a zoonotic agent is therefore considered modest.
Tuberculosis due to M. bovis has a worldwide distribution and represents one of the most important public and veterinary health problems (OReilly and Daborn, 1995). It is of greatest concern in cattle even though it can occur in humans and many domesticated and wild animal species. Goat and swine are highly susceptible to infection, while sheep and horses are more resistant.
Human beings are usually infected by cattle, but other sources of infection have been reported. An association between scrofula (tuberculosis cervical lymphadenopathy) and human consumption of cows milk was hypothesized early in the nineteenth century (Reviewed in Grange, 2001).
M. bovis seems to be less virulent than M. tuberculosis in human beings. This assumption comes from the findings that there is a very low incidence of human-to-human transmission and the likelihood of endogenous reactivation occurring years or decades after the initial infection.
Tuberculosis due to M. bovis is indistinguishable from that due to M. tuberculosis, considering clinical, radiological and pathological features. M. bovis however, usually infects humans through contaminated milk and is characterized not by a pulmonary infection but by a cervical lymphadenopathy as the result of colonization and infection of milk-borne bacilli in the tonsil or pharynx. Pulmonary tuberculosis due to M. bovis is quite rare, with most cases confined to rural areas and probably a result of airborne infection from diseased cattle.
While human infections due to M. bovis are now very rare in developed nations, little is known about what is happening in the developing world. There is a lack of information regarding human and bovine incidence of tuberculosis and the test-and-slaughter policy is performed only in a few countries. Where information is available, tuberculosis due to M. bovis accounts for a percentage of cases ranging from 0.4 to 8 percent, showing that M. bovis is an important factor in human tuberculosis (Grange, 2001).
Infection due to Mycobacterium spp. may not be expressed as overt tuberculosis for decades after infection in about 10 percent of cases. It depends on the balance between the bacillus and the immune system. In this respect immunosuppressed persons are more likely to develop active tuberculosis after infection with Mycobacterium of whatever origin. Thus the potential impact of an AIDS pandemic or HIV infections in humans, on the epidemiology of human tuberculosis due to M. bovis is of great concern, especially in developing countries where bovine tuberculosis is still a major problem. Information about M. bovis tuberculosis in the human population of such areas is still limited and a complete scenario is unknown, but the few data available indicate that the cases reported in developing nations may be the tip of an iceberg. More extensive epidemiological studies, and the institution of control measures where indicated, are therefore urgently required.
PREVENTION OF INFECTION
To minimize the risk of contracting bovine tuberculosis it is important that the following rules are borne in mind:
avoid direct contact with infected animals;
drink pasteurized milk or milk products;
undergo vaccination using a BCG vaccine in areas with a high prevalence of bovine tuberculosis.