Introduction

GENERAL. There are many losses in the livestock industry, zoo animals, wildlife, and of course humans as a result of the bacterial disease called tuberculosis (TB). As described in the Eighth Edition of The MERCK Veterinary Manual ^1. “Tuberculosis is an infectious, granulomatous disease caused by acid-fast bacilli of the genus Mycobacterium.  Although commonly defined as a chronic, debilitating disease, TB occasionally assumes an acute, rapidly progressive course.  The disease affects practically all species of vertebrates, and before control measures were adopted, was a major disease of man and domestic animals.  Bovine TB is still a significant zoonotic disease in many parts of the world.  Clinical signs and lesions are generally similar in the various species.” 

The Mycobacterium family of bacteria causes a variety of disease scourges such as tuberculosis, leprosy, cervical lymphadenitis, a swimming pool granuloma, chronic pulmonary diseases, Johne's disease, etc.  

There is a very broad range of species that are susceptible to the tuberculosis causing species. The main species of bacilli that cause disease are Mycobacterium tuberculosis, Mycobacterium bovis, and Mycobacterium avium complex.  M. tuberculosis and M. bovis as they affect animals are the species most referenced in this document.  Other species of the bacilli cause similar diseases in fish, turtles, etc. and they are included in this document.  Since the topic of Johne’s disease in cattle, caused by Mycobacterium avium subspecies paratuberculosis, is the topic of another Animal Welfare Information Center information resource, the articles dealing with this species are included here only if found in birds.  In the literature cited below, there are articles detailing the various aspects of Mycobacterium tuberculosis and Mycobacterium bovis  as they affect domestic and wild ruminants, ferrets, badgers, rodents, elephants, various birds, pigs, deer, coyotes, camels, pumas, fish, dogs, cats, non-human primates, and of course humans world-wide.

THE DISEASE. Tuberculosis is one of the oldest of the recognized diseases in humans and animals.  Egyptian mummies show typical lesions of the disease.  Ancient manuscripts indicate that the disease existed when humans began living in villages.  Even the recognition by humans that contaminated carcasses should not be used for human consumption can be seen  in “early Mosaic laws in the Talmud which classed any animal carcass showing adhesions between the lungs and the lining of the lung cavity as unsatisfactory for edible purposes.” ^2.

It was not until 1882, that the cause of the disease became known. Dr. Robert Koch, the noted German scientist, proved that a microorganism could be isolated from the characteristic tubercules that form during the typical TB disease process. The rod-shaped organism as seen under the microscope varies in size, from about 1 to 4 microns (about 6,000-16,000 laid end to end equals 1 inch). Koch showed that these same “rods” can cause the typical tuberculosis disease when inoculated into another animal.  Although it was originally called Koch’s bacillus, the organism was later given the scientific name of Mycobacterium tuberculosis as it is still known today.

The disease may be contracted in a variety of ways and affects many organs of the body. Most of the time, the bacilli are inhaled via infected droplets from an infected person’s or animal’s lungs, or by ingesting contaminated food, milk or water. 

Once the bacilli get into a susceptible organism, a number of events occur which cause the characteristic disease.  It generally starts in the areas where the exposure occurs.  The lungs are often attacked, but other parts of the body can be affected. When it gets into the lymph system, it can travel to all parts of the body.  Wherever the bacilli lodge, invader-fighting white blood cells are attracted to the bacilli invaders and attempt to ingest them.  The bacillus have a waxy coating that is very resistant to the white cells attempts to digest it.  The invader fighting cells then attempt to isolate bacilli inside a hard nodule and a  “tubercle” is thus formed.  If the wall of the tubercle is dense and effective in encapsulating all the mycobacterial bacteria, there will be little advancement of the disease and the tubercule may even calcify.  But, if the bacilli are not completely contained, a spreading lesion will ensue.  Bacilli may escape from the lesion and move to other parts of the body.  Wherever invader cells lodge, the disease process starts over again.  During the process of the disease development and expansion, the animals become emaciated, lethargic, organs become damaged and the animal weakens and dies.  

DIAGNOSIS. Tuberculous lesions can not always be seen or palpated, so clinical signs are not reliable for a diagnosis.  The intradermal tuberculin test is still the most important diagnostic test for TB.  Radiography is useful for imaging lesions in non-human primates and small animals. Discharges and sputum can be examined microscopically, but diagnosis other than the tuberculin test requires culturing tissue samples on selective media which can take 4-8 weeks.  The intradermal test made from mycobacterial antigen is the one most used for large animals. A positive reaction includes skin swelling. The test is not perfect as there are cross reactions with other strains of mycobacteria and false negatives may occur under certain conditions.  Current research efforts are trying to find improved diagnostic methods. An interferon based assay has recently been developed and has proven beneficial. Other tests are also being evaluated.

CONTROL MEASURES. The trend toward intensive agriculture has made control more difficult. Also, the presence of wild animals as reservoirs of the Mycobacterium (e.g. badgers in the UK, brush-tail possums in New Zealand, and white-tailed deer in the US) make it very difficult to eradicate the disease.  In general, there are three approaches currently used to the control the disease in domestic animals: 1) test for TB with the intradermal tuberculin test and either slaughter, 2) segregate or 3) treat with drugs.  The culling of reacting presumed infected animals is the only assured approach to eradicating the disease. This is difficult where pastured animals are infected by exposure to diseased wild animals.  Hygienic measures to reduce contamination of facilities is also useful.  The use of drugs is discouraged due to exposure of humans to animals being treated, possible development of drug resistant strains, and the expense to producers. The efficacy of a live vaccine made from the attenuated strain of Mycobacterium tuberculosis BCG (Bacilli Calmette-Guerin) has proven variable and use of this vaccine may confound interpretation of current diagnostic tests. Improved vaccines are currently being evaluated in research trials.

Eradication of TB in animals has been a long term goal of the U. S. Department of Agriculture.  The reader may be interested to note that a campaign to eradicate bovine tuberculosis in the United States was inaugurated as far back as May of 1917. ^2. At that time, the “course of action was the testing with tuberculin of all the dairy and breeding cattle in this country.  On November 1, 1940, about 23 ½ years later, all 3,071 counties in the US, and Territories of Puerto Rico, and the Virgin Islands, are rated as modified accredited areas, signifying that bovine tuberculosis among the cattle in such areas has been reduced to less that 0.5 %.”  The campaign required approximately 232 million tuberculin tests and retests, and the slaughter of 3.8 million tuberculous animals.  Most states have some incidence, Wight reports, but it is interesting that he also notes that human tuberculosis levels dropped as a result of reduction in the exposure to contaminated animals, meat and unpasturized milk.  In general, the eradication program has been successful and today, less than 0.002% of cattle are infected with M. bovis.  However, the zoonotic potential of the disease is still an issue today in this country and more serious in many other countries. 

Mycobacterial diseases seem to have been causing disease for eons and there does not seem to be anything in the modern arsenal of drugs and vaccines to stop it-- yet.  Hopefully, some of the current research will finally yield some breakthroughs and the losses of animal and human life will be dramatically reduced in the not too distant future. 

Resources used above:

1. Aiello, Susan E. and Asa Mays (eds.) The Merck Veterinary Manual, 8th edition. Whitehouse Station, N. J. The Merck & Co, Inc. 1998, p. 537-539.

2. Wight, A.E.; Lash, E.; O’Rear, H.M.; Crawford, A.B. Important General Diseases Common to Several Species. Part 2. Tuberculosis and Its Eradication. Keeping Livestock Healthy, Washington, D.C. U.S. Department of Agriculture. 1942 Yearbook of Agriculture.

I would like to thank Dr. W. Ray Waters of the USDA, Agricultural Research Service’s, National Animal Disease Center in Ames, Iowa for his review and clarifying text changes of the introductory material in this publication.