Bovine tuberculosis is a chronic and slow disease that can affect our cows and is of great public he alth importance, since it is a zoonosis with ability to be transmitted to the human being The symptoms are mostly respiratory and characteristic of a pneumonic process, although digestive signs may also be seen. The responsible bacterium belongs to the Mycobacteium tuberculosis complex and can affect many animals, especially ruminants, herbivores and some carnivores.
Keep reading this article on our site to find out everything about bovine tuberculosis, what it is, how it spreads, what symptoms it causes and how it is diagnosed.
What is bovine tuberculosis?
Bovine tuberculosis is a chronic bacterial disease in which symptoms take a few months to appear. Their name comes from the nodular lesions they cause in affected cows, called "tubers", in the lungs and lymph nodes. In addition to cows, goats, deer, camels or wild boars, among others, can also be infected.
Also, it is a zoonosis, which means that bovine tuberculosis can be transmitted to humans through aerosols or by ingestion of contaminated or unsanitized dairy products. It is a Notifiable OIE, as well as one of the most common diseases of cattle.
Causes of bovine tuberculosis
Bovine tuberculosis is caused by a bacterial bacillus of the Mycobacterium tuberculosis complex, especially by Mycobacterium bovis, but also Mycobacterium caprae or Mycobacterium tuberculosis with much less frequency. They present very similar epidemiological, pathological and ecological characteristics.
Wild animals such as fallow deer or wild boar can serve as amplifiers of the bacteria and as a source of infection for domestic cattle.
Contagion occurs mainly through inhalation of respiratory aerosols, secretions (urine, semen, blood, saliva or milk) or ingestion of fomites that carry it.
Stages of bovine tuberculosis
After infection, a primary stage and a post-primary stage occur.
Primary stage
This phase occurs from infection to 1 or 2 weeks when specific immunity begins. At this time, when the bacteria reach the lung or lymph nodes, cytokines begin to be released by the dendritic cells that attract macrophages to try to kill the bacteria. Cytotoxic T lymphocytes then appear and kill the macrophage with the mycobacteria, resulting in debris and necrosis. The immune system directs more lymphocytes around the necrosis, which become spindle-shaped and stick together, forming the tuberculous granuloma.
This primary complex can evolve to:
- Healing: Not usually the most frequent.
- Stabilization: more frequent in humans, with calcification of the lesion to prevent the exit of the mycobacteria.
- Early generalization through the bloodstream: when there is no immunity. This can be rapid, with miliary tuberculosis occurring, with the formation of numerous small and homogeneous tuberculous granulomas everywhere. If it occurs slowly, heterogeneous lesions appear because not all the mycobacteria come out at the same time.
Post-primary stage
Occurs when there is specific immunity, after reinfection, stabilization or early generalization, where the bacterium spreads to adjacent tissues via the lymphatics and by rupture of the nodules.
Symptoms of bovine tuberculosis
Bovine tuberculosis can have a subacute or chronic course, and it must take at least a few months for symptoms to appear. In other cases it can remain dormant for a long time and in others the symptoms can lead to the death of the cow.
The most frequent symptoms of bovine tuberculosis are:
- Anorexy.
- Weightloss.
- Fall in milk production.
- Fluctuating fever.
- Painful and intermittent dry cough.
- Lung sounds.
- Respiratory difficulty.
- Rib pain.
- Diarrhea.
- Weakness.
- Increase in the size of the lymph nodes.
- Tachypnea.
- Caseous necrosis of tuberculous lesions, with a pasty and yellowish consistency.
Diagnosis of bovine tuberculosis
The presumptive diagnosis is based on the symptoms of the cow. However, the symptomatology is very general and indicative of several processes that can affect cows, such as:
- Diseases of the upper respiratory tract.
- Lung abscesses due to aspiration pneumonia.
- Contagious bovine pleuropneumonia.
- Bovine leukosis.
- Actinobacillosis.
- Mastitis.
Therefore, symptoms can never be a definitive diagnosis. The latter is achieved with laboratory tests. The microbiological diagnosis can be obtained by:
- Ziehl-Nelsen stain-Search for mycobacteria in a Ziehl-Nelsen stained specimen under the microscope. This is very specific but not sensitive at all, which indicates that if mycobacteria appear, it can be confirmed that the cow has tuberculosis, but if they are not seen, we cannot rule it out.
- Bacterial culture: it is not routine, only for verification, as it is very slow. Identification is done with PCR or DNA probes.
For its part, the laboratory diagnosis includes:
- Elisa indirect.
- Elisa post-tuberculinization.
- Tuberculinization.
- Interferon gamma test (INF-y).
The tuberculin test is the test indicated to detect it directly in the cow. This test consists of injection of bovine tuberculin, a Mycobacterium bovis protein extract, through the skin of the neck table, and measurement at 3 days from the injection site for change in skinfold thickness. It is based on comparing the thickness of the pinch in the area, before and after 72 hours of application. It is a test that detects type IV hypersensitivity in an animal infected by mycobacteria of the bovine tuberculosis complex. The test is positive if the thickness is greater than 4 mm and if the cow has clinical signs, while it is doubtful if it is between 2 and 4 mm without clinical signs, and negative if it is less than 2 mm and without symptoms.
Thus, the official diagnosis of bovine tuberculosis consists of:
- Culture and identification of mycobacteria.
- Tuberculinization.
Treatment of bovine tuberculosis
The treatment is not advisable This is a notifiable disease that is subject to an eradication campaign in Spain due to the danger contagion and because the treatment would be very expensive, insidious and prolonged, and what is sought is to put an end to the disease definitively. Any positive animal should be euthanized.
There is only treatment for human tuberculosis, and also a vaccine. The best prevention against contracting bovine tuberculosis is pasteurization of the milk of these animals before it is ingested, as well as good management and control of cattle.
Also, in addition to the control of the farms, a tuberculosis detection program is carried out with official diagnostic tests and by inspecting viscera for injuries at the slaughterhouse to prevent their meat from entering the food chain.
