Tuberculosis

D V COUSINS, H F K A HUCHZERMEYER, J F T GRIFFIN, G K BRU¨CKNER, I B J VAN RENSBURG AND N P J KRIEK

GENERAL INTRODUCTION: MYCOBACTERIA

Mycobacterium bovis infection in cattle

Synonyms: Bovine tuberculosis, tuberkulose by beeste (Afrik.)

Bovine tuberculosis existed in the Mediterranean littoral before classical times. It spread from northern Italy to western Europe and Great Britain and from there infected cattle carried the disease to many parts of the world that had been colonized.^{105, 129} Currently, the disease occurs globally and it appears to be an ever-increasing problem. Even in developed countries, major difficulties are experienced to control and eradicate the disease from cattle. The distribution of the disease and its prevalence in the various countries can be obtained from sources such as World Animal Health compiled by the Office International des Epizooties (OIE) on an ongoing basis,⁵ and others.²⁷ Bovine tuberculosis is an important zoonosis, its prevalence in humans showing a close correlation with the level of infection in the cattle population of a specific country.²⁷

In recent years it has become evident that infection with Mycobacterium bovis, the cause of the infection in cattle, is common in a wide variety of wildlife in various parts of the world. The infection in wildlife is important not only from the perspective of the value of some species of wildlife, some of which are endangered, but because of the role that wildlife maintenance hosts of the disease can play in sustaining the infection in domestic stock.³³ The complex nature of the epidemiology of bovine tuberculosis, and the role of wildlife in sustaining the infection, are becoming clearer as time goes on and have an adverse effect on the efficacy of control measures that are currently in use in developed and developing countries.

The first reference to the occurrence of tuberculosis in animals in South Africa was made by Hutcheon in 1880⁶⁸ and it appears unlikely that bovine tuberculosis existed in South Africa prior to the importation of European breeds of cattle towards the end of the nineteenth century.⁶² In 1902 it was reported that cattle in Natal were free of tuberculosis but that many animals imported from Australia, the Argentine, Madagascar and England, that reacted to the tuberculin test, were destroyed upon arrival in Durban.

Subsequently, the prevalence of the disease in slaughtered cattle remained low in South Africa between 1917 and 1929. Its spread in southern Africa was originally confined to dairy herds but since the early 1970s beef herds have been found to be infected.

Since bovine tuberculosis was first recorded in Zimbabwe in 1908 in a heifer imported from the Western Cape Province, South Africa, only sporadic cases have been diagnosed. A national testing scheme for tuberculosis was instituted from 1950 to 1978. The prevalence remained very low, with one or none of the slaughtered animals per year showing tuberculous lesions.⁶¹

Namibia is considered to be virtually free of tuberculosis in livestock, and all diagnosed cases can be traced back to imported animals that originated mainly from South Africa and Germany.¹²⁸ From 1913 to 1984, of 66 888 head of cattle tested, ¹⁶³ reacted positive to the tuberculin test and 284 gave suspicious reactions. At abattoirs only seven animals were found to be tuberculous.²

Tuberculosis in animals in Swaziland is rarely diagnosed. Its prevalence in cattle is estimated to be below 0,5 per cent and it has not been diagnosed in wild animals.³⁷

Epidemiology

Tuberculosis in cattle is caused predominantly by M. bovis, of which cattle are the maintenance hosts. In New Zealand the possum (Trichosurus vulpecula) and fallow deer (Dama dama), in England the badger (Meles meles), in some European countries swine and deer, and in the USA and Canada bison (Bison bison) and elk (Cervus elaphus) are additional maintenance hosts. In the USA, white-tailed deer (Odocoileus virginianus) have also become infected and are now considered a maintenance host for M. bovis.

Mycobacterium bovis infection has recently been diagnosed in African buffalo (Syncerus caffer), which is considered to be a maintenance host, and various other species of wildlife in some game parks and commercial game ranches in South Africa. Tuberculosis in the water buffalo (Bubalus bubalis) in some countries and free-living African buffaloes in African game reserves may therefore constitute a source of infection when they mingle with livestock on the borders of conservation areas.³³ Kudu (Tragelaphus strepsiceros) in certain areas in South Africa may play a limited role in the transmission of M. bovis to cattle.¹³⁹

Infection with Mycobacterium tuberculosis in cattle is rare, but it may cause reactions in tuberculin-tested cattle. Infection with members of the Mycobacterium avium intracellulare complex (MAIC) is not a major disease problem in cattle but can confound tuberculosis eradication programmes in cattle. Cattle infected with M. avium become sensitized to mammalian tuberculin, which may cause false positive results in the intradermal tuberculin test. In cattle the sensitization to M. avium and M. tuberculosis infections may disappear soon after they are removed from contact with either infected birds or humans infected with M. tuberculosis. Infection and disease due to M. bovis in other susceptible species in contact with cattle, including humans...