Tuberculosis

Monograph

definition: 

Tuberculosis is an infectious, bacterial, contagious disease affecting humans and many other animal species. It is characterized by its chronic evolution and high polymorphism. It is identified by inflammatory lesions in the form of nodular granuloma known as tubercles.

Significance

Hygienic

It is a major zoonosis. It is however important to distinguish zoonosis tuberculosis caused by Mycobactérium bovis from interhuman tuberculosis caused by Mycobactérium tuberculosis, which is responsible for most of human tuberculosis cases.

Economic

It is a threat to cattle farming as it causes losses in milk production, carcass and offal depreciation (abattoirs harboring the disease are seized), stunted growth and fattening, reproduction disorders, loss of young animals and leads to export embargo.

Situation in America: 

Declared to the OIE en 2004 by the following countries: Argentina, Bolivia, Brazil, Canada, Chili, Columbia, Costa Rica, Cuba, Dominican Republic, El Salvador, Guatemala, Honduras, Mexico, Nicaragua, Panama, Paraguay, Peru, USA, Uruguay and Venezuela.

Susceptible species: 

All domestic and wild vertebrate species including man who can become infected by tuberculosis bacilli.

Etiological agent: 

Tuberculous bacilli are germs from the Actinomycetales group, from the family Mycobacteriaceae, Mycobacterium genus.

They include numerous species. Some are ubiquitous and highly resistant and others are apathogenic, while others are pathogenic for humans and animals. This makes diagnosis difficult.

Mycobacterium bovis is made up of a rudimentary pseudomycelium forming small, static bacilli which are strict aerobes and resistant to acid-alcohol solutions. The ideal growth conditions for the bacilli are: T° = 37°C, pH = 6-6, 5.

Mycobacterium bovis: is the agent responsible for bovine tuberculosis.

It affects bovines and other herbivores but also cats and to a lesser degree, dogs and humans.

Mycobacterium tuberculosis: is the agent responsible for human tuberculosis. It affects man, dogs and to a lesser degree, cats, ruminants and ornamental birds.

Mycobacterium avium: is the agent responsible for tuberculosis in birds and largely contributes to pig tuberculosis.

Methods of transmission
Source: 

Tuberculous subjects (human or animal) are a significant source of bacilli and present early, lasting symptoms. Affected subjects produce large quantities of bacilli in an irregular manner over time.

There are many more people with latent tuberculosis infection who carry and excrete the germs than people with the apparent disease. They are responsible for insidious propagation of the infection.

Virulent matter:

• Discharge, saliva, aerosols

• Milk, excrements (birds), urine (carnivores), genital secretions

• Organs and ganglions presenting tuberculous lesions

• Blood: in the bacteremia phase (rare, temporary), in acute cases

• Muscle and meat: abattoirs seized depending on the type of affected lymph nodes.

Receptivity and susceptibility factors

Greater susceptibility of imported dairy cows than of local or cross-breeds in intensive farming areas.

Animal stress (lactation, gestation, overwork, etc.) may encourage clinical expression of the infection.

Transmission: 

Direct: cohabitation, ingestion of virulent milk by the calf, venereal contamination.

Indirect: on premises, through contaminated feed and water etc.

Human-animal intertransmission exists.

Method of contamination: 

Contamination of animals:

Mainly via respiratory route.

Other routes: digestive, genital, percutaneous, mammary, congenital.

Contamination of humans:

Man is contaminated via respiratory route (inhalation of infectious aerosols) and via digestive route (consumption of infected raw milk).

Symptoms: 

 

Slow, chronic progression over several months or even years. Acute phases aggravate progression.

Symptoms go unnoticed for long periods where the tuberculous animal appears to be in perfect health. At the end of progression general health is affected but there are few characteristic signs.

Incubation period: long, more than 2 months.

General health:

• Young subjects: retarded and unstable growth, sickly appearance

• Adults: poor general condition in severe cases (exhaustion, wasting, muscular atrophy, dull fur, weathering, diarrhea, cachectic, increase in T° prior to progression, progressive anorexia, interrupted rumination, death)

Other symptoms: no other characteristic symptoms apart from specific local signs.

Pulmonary: are the most frequent symptoms (in 80% of cases). Asymptomatic followed by breathing difficulties, coughing, fetid, yellowish discharge.

Intestinal: Very rare. Asymptomatic or chronic enteritis.

Mammary: in the advanced stages, hypertrophy, hard, lumpy appearance.

Genital: chronic metritis in females, orchitis in males.

These four locations are the most dangerous in terms of transmission. Tuberculous bacilli are massively excreted in discharge, milk, faeces, semen and pus.

Other locations: serous membranes, liver, spleen, lymph nodes (tracheobronchial, retropharyngeal, especially mediastinal lymph nodes), bones, meninges, muscle. Constant tuberculous adenopathy.

Lesions
Macroscopic lesions: 

Three types of tuberculous lesions are observed according to their diffuse or isolated character.

- Tubercules: nodular granuloma.

At the start of progression these are grey and translucid and the size of a pin-head. They then grow and their centre becomes caseous and gradually calcifies.

At the end of progression the encysted lesion takes on a fibrous aspect.

- Infiltration: widespread exsudative diffuse lesions in the affected organ (lung)

- Effusion: in serous cavities, associated with pleuresy, pericarditis and peritonitis. Inflammatory, serofibrous or serous-hemorrhagic exsudate rich in lymphocytic cells.

Microscopic lesions: 

- Tuberculous follicle: basic, specific lesion. Homogenous necrotic centre (caseous material). Lesion evolution: calcification of caseous material with peripheral fibrosis.

Diagnostics
Clinical diagnosis: 

Difficult and insufficient.

Live animals: frequent apparent infection, absence of specific symptoms, laboratory tests necessary.

Autopsy or abattoir: detection of macroscopic lesions (tuberculs) or microscopic lesions (tuberculous follicle). Sufficiently specific for a firm diagnosis.

Differential diagnosis: 

Bovines: actinomycosis, actinobacillosis (lymphatic, pulmonary, osseous), polyadenitis, lymphoid leucosis, genital brucellosis, serous membrane tumours.

Ovines – Caprines: strongylus-induced bronchopneumonia, parasital hepatitis, caseous disease located in the pulmonary or hepatic lymphatic system.

Porcines: cervical adenitis caused by Corynebacterium pyogenes, abscessed bronchopneumonia, purulent vertebral osteomyelitis secondary to caudophagia.

Equids: longworm, echinococcosis, tumors, bronchopneumonia and hepatitis may all mimic tuberculosis. Differential diagnosis is very difficult. Laboratory diagnosis is necessary.

Laboratory diagnosis: 

• Detection of the pathogenic agent

Histology: Using the Ziehl-Neelsen staining technique. Detection of a varying number of acid-resistant intracellular, pleomorphic bacilli, within the tubercles.

Bacterioscopy: Using crushed tuberculous organs or organ impressions. Ziehl Neelsen method (staining reveals the acid-alcohol resistant character of the bacilli) or the auramine 0 method (fluorochromes absorption by mycobacteria).

Culture: decontaminated soiled samples (saprophytes). Slow growth colonies (up to several months). Smears, staining. Identification: culture properties +/- inhibitor presence, enzyme activity, analysis of mycolic acid from the bacterial lining.

Identification: direct hybridization with a specific DNA probe, amplification by PCR

M. bovis strain typing: Pulsed field electrophoresis (restriction profiles), analysis of restriction fragment length polymorphism, spolygotyping analysis.

• Immunological diagnosis

Detection of cell immunity:

In vivo using a tuberculin test, in vitro through interferon gamma detection.

Detection of humoral immunity:

ELISA test.

Treatment: 

Treating animal tuberculosis is both hazardous and dangerous and should be proscribed. Treatment would be costly with unpredictable results. The use of antimycobacterial products in animals would above all impose the risk of selecting resistant germs highly dangerous for human health.

Prophylaxis: 

Necessary for two reasons:

- Hygiene: to eliminate sources of contamination for man

- Economic: reduce losses for farmers

Several countries are aiming to eradicate the disease with simultaneous action for "tuberculosis infection" and the "tuberculosis disease".

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