Bovine tuberculosis - Symptoms and diagnosis

Bovine tuberculosis is a chronic and slow disease that can affect our cows and that is very important in public health, since it is a zoonosis with ability to be transmitted to humans. The symptoms are mostly respiratory and characteristic of a pneumonic process, although digestive signs can 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 AnimalWised article to learn all about the bovine tuberculosis, what it is, how it is spread, what symptoms it causes and how it is diagnosed.

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  1. What is bovine tuberculosis?
  2. Causes of bovine tuberculosis
  3. Stages of bovine tuberculosis
  4. Symptoms of bovine tuberculosis
  5. Diagnosis of bovine tuberculosis
  6. Treatment of bovine tuberculosis

What is bovine tuberculosis?

Bovine tuberculosis is a chronic bacterial disease in which the symptoms take a few months to appear. Its name comes from the nodular lesions that they cause in affected cows, called "tubercles", in the lungs and lymph nodes. In addition to cows, goats, deer, camels or wild boar, among others, can also be infected.

In addition, it is a zoonosis, which means that bovine tuberculosis can be transmitted to humans through aerosols or by ingestion of contaminated or unhygienic dairy products. It is a disease notifiable to the OIE, in addition to one of the most common diseases of cattle.

Causes of bovine tuberculosis

Bovine tuberculosis is caused by a bacterial bacillus of the complex of Mycobacterium tuberculosis, especially for Mycobacterium bovis, but also Mycobacterium caprae or Mycobacterium tuberculosis much less frequently. They have very similar epidemiological, pathological and ecological characteristics.

Wild animals such as fallow deer or wild boars can serve as bacterium amplifiers and as a source of infection to domestic cattle.

The contagion occurs mainly through inhalation of respiratory aerosols, by secretions (urine, semen, blood, saliva or milk) or ingestion of fomites that convey it.

Stages of bovine tuberculosis

After infection, a primary stage and a post-primary stage occur.

Primary stage

This phase occurs from infection up to 1 or 2 weeks in which 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, killing the macrophage with the mycobacterium, resulting in debris and necrosis. The immune system directs more lymphocytes around the necrosis that become spindle-shaped, sticking together, forming the tuberculous granuloma..

This primary complex can evolve to:

  • Healing: it is not usually the most frequent.
  • Stabilization: more frequent in humans, with calcification of the lesion to prevent the exit of mycobacteria.
  • Early generalization through the bloodstream: when there is no immunity. This can be rapid, with miliary tuberculosis occurring, with the formation of numerous tuberculous granulomas on all sides, small and homogeneous. If it occurs slowly, heterogeneous lesions appear because not all mycobacteria emerge at the same time.

Post-primary stage

It happens when there is specific immunity, after reinfection, stabilization or early generalization, where the bacterium spreads to adjacent tissues via the lymphatic pathway and by rupture of the nodules.

Symptoms of bovine tuberculosis

Bovine tuberculosis can have a course subacute or chronic, it must take at least a few months for symptoms to appear. In other cases it can remain latent for a very 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.
  • Drop in milk production.
  • Fluctuating fever.
  • Painful and intermittent dry cough.
  • Lung sounds.
  • Shortness of breath.
  • Rib pain.
  • Diarrhea.
  • Soft spot.
  • Lymph node enlargement.
  • 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 symptoms are very general and indicative of several processes that can affect cows, such as:

  • Upper respiratory diseases.
  • Aspiration pneumonia lung abscesses.
  • Contagious bovine pleuropneumonia.
  • Bovine leukosis.
  • Actinobacillosis.
  • Mastitis.

Therefore, the 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 mycobacterium in a sample with Ziehl-Nelsen staining under the microscope. This is very specific but not at all sensitive, which indicates that if mycobacteria appear, it can be said that the cow has tuberculosis, but if they are not seen, we cannot rule it out.
  • Bacterial culture: it is not routine, just as a check, being very slow. Identification is done with PCR or DNA probes.

For his part, laboratory diagnosis It includes:

  • Indirect elisa.
  • Elisa postuberculinization.
  • Tuberculinization.
  • Interferon gamma test (INF-y).

The tuberculin test It is the indicated test to detect it directly in the cow. This test consists of the injection of bovine tuberculin, a protein extract of Mycobacterium bovis, through the skin of the neck table, and measurement at 3 days from the injection site to change the thickness of the fold. 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 with 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 measures between 2 and 4 mm without clinical signs, and it is negative if it is less than 2 mm and has no symptoms..

Thus, the official diagnosis of bovine tuberculosis consists of:

  • Culture and identification of the mycobacterium.
  • Tuberculinization.

Treatment of bovine tuberculosis

Treatment is not advisable. It is a notifiable disease that in Spain is undergoing an eradication campaign due to the danger of contagion and because the treatment would be very expensive, insidious and prolonged, and what is sought is to permanently end the disease. All positive animals must be euthanized.

There is only treatment for human tuberculosis, and also a vaccine. The best prevention to avoid contracting bovine tuberculosis is milk pasteurization of these animals before being ingested, as well as good management and control of cattle.

Also, in addition to the control of the farms, a tuberculosis screening program with official diagnostic tests and inspection of viscera lesions at the slaughterhouse to prevent their meat from entering the food chain.

This article is merely informative, at AnimalWised.com we do not have the power to prescribe veterinary treatments or make any type of diagnosis. We invite you to take your pet to the vet in the event that it presents any type of condition or discomfort.

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