Tuberculosis (TB) is the most common fatal infectious disease in humans worldwide. There are different tuberculosis pathogens that are categorised as part of the Mycobacterium tuberculosis complex (MTC). The most common pathogen causing tuberculosis in humans is Mycobacterium (M.) tuberculosis. The bacterium can be inactivated by pasteurisation (brief heating to 72 °C); however, it is insensitive to dehydration or cold.

Tuberculosis is widespread worldwide, especially in Africa, Asia and Latin America. According to estimates by the World Health Organisation (WHO), more than a third of the world's population is infected with tuberculosis(M. tuberculosis). Every year, 1.6 million people die from the infection and around 9 million are newly infected.

In Europe, the bovine tuberculosis pathogen(M. bovis) was greatly reduced after the Second World War, as a result of which many countries received the officially recognised status "free of bovine tuberculosis". Austria's cattle herd received the status "officially recognised free of bovine tuberculosis" from the EU in 1999, and since then this tuberculosis pathogen has no longer been detected in any Austrian cattle herd. With the new Animal Health Law of the European Union, Austria has the status of "disease-free" with regard to infections with the Mycobacterium tuberculosis complex(M. bovis, M. caprae and M. tuberculosis).

Since 2008, however, M. caprae has been transmitted between red deer and cattle in individual areas of the provinces of Tyrol and Vorarlberg, and M. microti has also been detected sporadically in wild animals, cattle, cats and New World camelids. However, these detections are irrelevant for Austria's recognised free status.

Humans are the only relevant reservoir for M. tuberculosis. For mycobacteria that can be transmitted from animals to humans, such as M. bovis and M. caprae, cattle, wild boar, goats or wild ruminants (especially red deer) are the pathogen reservoir. The reservoir for M. microti is formed by voles and shrews, for example.

Whether an infection occurs depends on the frequency and intensity of contact, the amount of inhaled or orally ingested pathogens and the physical condition of the affected person. Infection usually occurs through inhalation of fine droplets with the air breathed, which are released during coughing and sneezing by persons suffering from overt tuberculosis. Open pulmonary tuberculosis refers to diseases in which pathogens can be detected in the sputum. Transmission through raw (unpasteurised) milk from infected cattle is possible in principle.

Infection from animal to animal occurs preferentially by the aerogenic route through inhalation of fine air droplets containing the pathogen, which are coughed up by diseased animals. However, it can also occur through contact or orally, e.g. via contaminated feed in feed mangers and salt licks.

The time from infection to the onset of the disease can range from a few months - especially in young children - to many years and decades.

Humans: The disease most commonly occurs as pulmonary tuberculosis with the main symptom of coughing. If mycobacteria are also coughed up, it is referred to as open pulmonary tuberculosis. Other symptoms are unspecific and may include tiredness, weakness, loss of appetite and weight loss, swollen lymph nodes, slight fever, especially in the afternoon, and night sweats, with a worsening general condition.

If the infection is fought off by the immune system and the pathogens are sealed off in nodules (tubercles), for example, the pathogens may still reactivate years later and the disease may break out again as secondary tuberculosis.

As a result of infection with bovine tuberculosis pathogens, mainly manifestations outside the lungs (extra-pulmonary forms) occurred before the implementation of control programmes and the pasteurisation of milk.

Animal: Chronic pulmonary tuberculosis in cattle mainly manifests itself in progressive coughing and slowly deteriorating general condition. However, disease processes can also occur in other organs. In cattle, tuberculosis can remain latent or subclinical for years.

An infection with mycobacteria should also be considered in camelids if they are in poor condition and emaciated. Respiratory symptoms may occur.

In cats, symptoms such as emaciation, deterioration in general condition, coughing and (palpable) circumferential enlargements in all parts of the body occur. Therapy-resistant skin changes have also been observed.

Infected red deer often show no specific symptoms in the early stages of the disease. In advanced cases, red deer may be in poor condition, emaciated and weakened.

Since the pathogens are difficult to reach with the drugs, the therapy takes several months and the risk of mycobacteria developing resistance is particularly high. In cases of confirmed tuberculosis, patients must therefore be treated with a combination therapy consisting of several special antibiotics, so-called antituberculotics. The duration of treatment is correspondingly long (over months) in order to avoid possible relapses.

Tuberculosis is a notifiable animal disease. Control focuses on the culling of infected animals.

Since there is no effective vaccination against tuberculosis, the most important measure is to detect ill persons as soon as possible and to treat them effectively.

In 2022, 372 human cases of tuberculosis were reported to the Epidemiological Reporting System (EMS) (as of 06.10.2023), which corresponds to 4.09 cases per 100,000 population. Of these, 281 cases were microbiologically confirmed as belonging to MTC. Five cases caused by M. bovis were detected.

In Austria, bovine tuberculosis is a notifiable animal disease. Austria has been recognised as free of bovine tuberculosis since 1999. From May 2000, the nationwide testing of ruminants using tuberculin tests was discontinued; the disease is monitored in the course of ante-mortem and post-mortem inspections.

Since 2008, there has been a transmission of M. caprae infection between red deer and cattle in individual areas of the provinces of Tyrol and Vorarlberg during the grazing and alpine pasture period due to the use of the same grazing areas by cattle and red deer. To determine the situation in the cattle population, special testing and monitoring areas (in accordance with the Bovine Tuberculosis Ordinance) are therefore officially designated in these regions every year. In these areas, cattle are tested for tuberculosis before and after the grazing period using a tuberculin test (simultaneous test). These tests are adapted to the epidemiological situation identified and, if necessary, appropriate area adjustments are made.

In 2023, the bovine tuberculosis pathogen M. caprae (Lechtal genotype) was detected in the NRL for bovine tuberculosis in two cattle from two farms in Vorarlberg (districts of Bludenz and Dornbirn) following abnormalities in the post-mortem inspection at the abattoir. M. microti was isolated from an alpaca from Styria.

Persons who have close contact with patients with open (i.e., infectious) tuberculosis are particularly at risk. In recent years, there has been a worrying increase in tuberculosis with multidrug-resistant (insensitive at least to the two antituberculotics isoniazid and rifampicin) strains of the pathogen.

After droplet infection, small foci of inflammation usually form in the lungs within the following three to six weeks and encapsulate into nodules (tubercles) (primary infection).

An active infection, usually a reactivation, begins with general symptoms, especially night sweats, increased temperature, fatigue, weight loss, lack of appetite, general feeling of illness. In pulmonary tuberculosis, tissue loss can lead to so-called cavern formation in the lungs. Symptomatic of the disease for this is massive, often bloody sputum, these patient:inside are highly contagious. In rare cases, tuberculous meningitis (meningitis) can occur. One speaks of a miliary tuberculosis when there is a spread via the bloodstream with diffuse infestation of several organ systems, usually also with lung involvement.

The aim of every tuberculosis diagnosis is the cultural detection of the pathogen. This is the only way to identify the resistance pattern and to clarify the outbreak. Due to the danger potential of the Mycobacterium tuberculosis complex, a laboratory with safety level 3 (BSL-3) is required for this purpose. Samples are incubated in the laboratory for up to eight weeks due to the slow growth of the pathogens. Molecular biology methods such as nucleic acid amplification techniques (NAT) shorten the diagnosis time and provide information on the resistance behavior of the pathogen.

Tuberculin testing:

To detect infection without disease, the tuberculin skin test can be performed using the Mendel-Mantoux method. Here, the immunological reaction to injected pathogen components is tested. The test becomes positive as early as six weeks after infection. Increasingly, this skin test is being replaced by the so-called interferon-ɣ-release assay (IGRA), a blood test.

Interferon-ɣ-Release Assay (IGRA):

The IGRA test is a diagnostic tool to detect an infection with tubercle bacilli that has occurred. Especially to diagnose latent tuberculosis, the IGRA test is the diagnostic tool of choice. The test becomes positive 2 to 6 weeks after infection. It should be noted that the test cannot distinguish between active and latent tuberculosis. Basically, the IGRA test is used to detect infection with tubercle bacilli in individuals who have had contact with a tuberculosis case (environmental testing), who come from a high-endemic area (geographic region with increased incidence of disease), or before planned immunosuppressive therapy.

The diagnosis of active tuberculosis is usually made by pathogen detection by microscopy, molecular biology methods and culturing (culture), and clinical and radiological assessment. The IGRA test is not designed to prove treatment success; it may remain positive even after successful therapy.

In an IGRA test, the subject's blood is incubated in a blood collection tube coated with Mycobacterium tuberculosis specific antigen (ESAT6 and CFP10). If the subject's immune system has been sensitized to tubercle bacilli (i.e., the subject has had previous contact with tuberculosis pathogens), stimulation of the responsible T lymphocytes results in the production of interferon-gamma, which is measured. The Quantiferon TB Gold Plus test requires 4 ml of blood. It is not necessary to be fasting for the blood draw. Special blood collection tubes are used and filled with 1 ml each. Alternatively, lithium heparin tubes may be used for blood collection and the blood subsequently transferred (within 16 hours) to the Quantiferon Plus tubes. After blood collection, the tubes must be swirled 10 times. Transport to the laboratory must occur within 16 hours of blood collection at room temperature (22 °C ± 5 °C). The samples are then incubated at 37 °C for 16 to 24 hours and can then be stored at 4 - 27 °C for a maximum of 3 days. After incubation, the tubes are centrifuged and can be further processed or stored at 4 °C for a maximum of 4 weeks.

A positive test result indicates that contact with Mycobacterium tuberculosis has occurred. However, the positive IGRA result does not allow any statement about the degree of activity of a disease or even the necessity of a therapy. It is not possible to distinguish whether the person tested has only a latent infection or whether an active disease has developed. Therefore, a positive IGRA test result always requires further diagnostic clarification. A negative result argues against TB infection having occurred and thus against the presence of latent tuberculosis.

Imaging techniques:

Radiographic diagnosis can identify characteristic images of pulmonary infestation, but differential diagnosis cannot exclude some other pulmonary diseases. Therefore, the diagnosis is usually confirmed by combining several investigative procedures.

Bacteriological diagnostics:

Detection of mycobacterial nucleic acid provides an initial finding within hours. The time-consuming cultural detection of MTC bacteria confirms the diagnosis of tuberculosis. The advantage of cultural detection is the possibility of testing the mycobacteria for their sensitivity to specific antimicrobial drugs (resistance testing). Obtained isolates are typed by molecular biology.

Molecular biology diagnostics:

In accordance with the latest standards, samples are analyzed by whole genome sequencing (WGS). This allows identification of matching strains and epidemiological clarification of infection chains. In addition, WGS allows the detection of resistance genes and facilitates species assignment within the Mycobacterium tuberculosis complex.

The causative agents of tuberculosis in humans and animals are closely related mycobacterial species, which are summarised as the Mycobacterium tuberculosis complex. This complex comprises the described species Mycobacterium (M.) tuberculosis, M. africanum, M. canettii, M. bovis, M. caprae, M. pinnipedii (seals), M. mungi (mongoose), M. orygis (antelope), M. suricattae (meerkat), Dassie Bacillus (rock hyrax) and M. microti (mice, secondary hosts e.g. cats, red deer, wild boar, camelids, cattle).

Chronic pulmonary tuberculosis in cattle mainly manifests itself in a progressive cough and slowly worsening general condition. However, disease processes can also occur in other organs. In cattle, tuberculosis can remain latent or subclinical for years.

In red deer, tuberculosis is a chronic disease. Clinical symptoms, if present, are often non-specific. The route of infection is usually oral or aerogenic. If the disease generalises to the head, thorax or abdomen, pathogens can be excreted in large quantities and - with the potential for transmission to other animal species - contaminate the environment. Winter feeding of wild animals is problematic: transmission is encouraged by pathogen transmission and lower natural mortality as well as accumulations of animals in the feeding area.

Diagnostics in animals

The early detection of infected cattle is an important point in the fight against bovine tuberculosis and depends on in vivo tests such as the skin and g-interferon test. In Tyrol and Vorarlberg, cattle in certain risk areas for infections with M. caprae (special testing and monitoring areas) must be tested annually using a tuberculin test (simultaneous test). In addition, a blood test (g-interferon test) can also be carried out. If the test is non-negative, the cattle are slaughtered for diagnostic purposes. A direct examination of tissue samples using MTC PCR enables a rapid diagnosis.

Camelids can also be tested for MTC disease using a tuberculin test. A subsequent antibody test increases the detection rate of infected animals.

Tissue samples from red deer killed during hunting are also analysed for the presence of the bovine tuberculosis pathogen M. caprae and other mycobacteria of the Mycobacterium tuberculosis complex using MTC PCR and bacterial culture. The selection of the animals to be sampled is based on a sampling plan.

The identification of the MTC species and genotyping of the culturally isolated bacterial strains is carried out using various molecular biological methods (RD4 PCR, DNA strip technology, MIRU VNTR analysis). Due to the classification as a risk group 3 pathogen, the cultivation of tuberculosis pathogens may only be carried out in the safety level L3 laboratory, the Centre for Biological Safety at the National Reference Laboratory for Bovine Tuberculosis in Mödling.