World Veterinarian Quiz 36

Introduction

·         Bovine Tuberculosis is a disease caused by bacteria Mycobacterium bovis. 

·       TB usually affects cattle and all mammals causing a general state of illness, coughing and eventual death.

·     Tuberculosis can be transmitted from animals to humans as well as to other animals also so it is zoonotic disease.

^·          The bacteria M. tuberculosis, that is different the type that usually causes disease in humans. 

·         While M. bovis is a different type of bacteria to causes disease in animals.

·     The name Tuberculosis(TB) comes from the nodules, called “ tubercles ” which form in the lymph nodes of affected animals.

·         They are generally killed at a temperature of 60^oC for 15 minutes

·      The bacteria of M. bovis survive in a wide range of alkalis and acids. And also long periods in moist and warm soil.

·         M. bovis in faeces and they survive 1 – 8 weeks. 

·         The bacteria of M. bovis is killed in sunlight.

·         Bovine tuberculosis is a zoonotic disease and causes tuberculosis in human.

ETIOLOGY 

·         Etiology: Mycobacterium bovis. 

·         Human Tuberculosis : M. tuberculosis, M. africanum (Africa), M. canettii

·         TB in goats : M. caprae

·         TB in avian : Mycobacterium avium

·         TB in small rodents : M. microti

TRANSMISSION

·         Infected domestic animals like cattle, wild animals and badgers

·         Breath

·         Raw Milk (Raw milk, but pasteurisation milk prevents the spread.)

·         Discharging lesions like saliva, urine.

·         Contamination of fodder

·         Respiratory droplets or within dust particles that can travel long distances in the air in TB.

·         Spread by direct wound during slaughter and hunting of animal.

·         Vertical transmission can also reported in infected females.

 

HOST

·         Mycobacterium tuberculosis (TB) has been reported in All domesticated and non-domesticated animals

·         Also Wild mammal species including lions, tigers, leopards.

·         High infection rate in badgers.

·         Badgers are a significant source of Tuberculosis in cattle.

Which animals are more susceptible for TB ?

·         poorly nourished Animals

·         Stressful is also susceptible for TB

·         Younger cows and growing heifers are most at risk.

·         Evidence more in dairy farms animals have a higher risk of infection.

·         In cattle, excretion of M. bovis begins around 87 days after infection occurs.

Pathogenesis

 

·         Cattle,sheep and goats are infected through inhalation of fomites and fluid droplets contaminated with the bacteria or ingestion via the milk or colostrum

 

·         These droplets are deposited in mucus layer of the respiratory system and are subsequently phagocytosis by alveolar and tissue macrophages

 

·         These Macrophages appear to use several routes to spread the M.Bovis across mucus layer and then to regional lymph nodes and the lung.

 

·         In tonsillar mucosa the macrophages are cross the mucosal barrier and migrateor bind with  the tonsil, and spread the bacteria to tonsillar tissues.

 

·         In pharynx mucosae the macrophages encounter and phagocytose bacteria à spread them to local lymphoid tissues through afferent lymphatic vessels to regional lymph nodes such as the retropharyngeal and parotid nodes

 

·         Finally the bacteria reach in bronchi and bronchioles and bacteria are deposited in the mucus layer and phagocytosed by alveolar macrophages and spread to local lymphoid tissues (BALT) through afferent lymphatic vessels to regional lymph nodes like the tracheobronchial and mediastinal nodes and infection occurs

 

·         spread the infection of M.Bovis other regional or system like liver, spleen, lymph nodes, and intestines, by the leukocyte in the blood or lymphatic vascular systems.

 

Clinical signs

·         Bovine TB may be sub acute or chronic, with a variable rate of progression.

·         A small number of animals may become severely affected within a few months of infection and while some animals may take several years to develop clinical signs.

·         The bacteria can also lie in the host without causing disease for a long periods.

·         Cattle usually show no clinical signs to TB unless the disease has affected multiple organ systems and is very advanced which is quite rare.

·         Typically, infected cattle are asymptomatic and are only detected by skin testing or at during slaughter.

 

The usual clinical signs include:

·         Weakness,

·         Loss of appetite and weight,

·         Fluctuating fever,

·         Dyspnoea,

·         Intermittent hacking cough,

·         Signs of low-grade pneumonia,

·         Diarrhoea,

·         Enlarged and prominent lymph nodes.

·         Tubercular mastitis is one of the common conditions in M. bovis infection.

Macroscopic Pathology

·         Characteristic gross lesion of an animal infected with bovine TB is the presence of “tubercles” within the body

·         A tubercle is a white nodule usually 1mm-2cm in diameter within a lymph node or organ. Commonly found in the thoracic cavity

·         The centre of the granuloma is usually necrotic and calcified.

·         Moderate splenomegaly seen.

·         Chronic pneumonia with or without regional lymph node involvement.  

·         Caseous necrosis is prominent in the granulomas of bovine tuberculosis

MICROSCOPIC Pathology

·         Granulomatous inflammation is the hallmark of infection with

·         Lymphocytosis and monocytosis observed in case of TB

 

Diagnosis

·         Base on clinical sign and macroscopic and microscopic lesions  

·         Sample collection:

Sputum, milk, uterine discharges, pleural and peritoneal fluids, urine, faeces and Tissue.

·         Microscopical examination

o   Z-N  stain(Ziehl-Neelsen method)-  The tubercle bacilli appear in clumps as slender rods stained pink in a blue background.

o    In milk samples, the organisms are commonly seen in epitheloid cells.

·         Cultural examination: Culture isolation of the causative agent from fresh tissue or other specimens

M. bovis isolates - Stone brink's medium

·         Animal inoculation

M. bovis- Guinea pigs, Rabbits

M. tuberculosis- Guinea pigs, Rabbits

M.avium- Chicken,Rabbit

·         Tuberculin Test (Delay type Hypersensitivity test)

o   In live cattle, pig, deer tuberculosis is usually diagnosed in the field with the tuberculin skin test.

o   Tuberculin / purified protein derivative (PPD)- is a extracts of M. bovis that is used in skin testing in animals and humans to identify a tuberculosis infection.

o   Purified protein derivative (PPD) is complex mixture of protein, lipid, carbohydrate, nucleic acid

 

o   Intradermal test:

The side of neck shaved à Skin fold Measure by vernier caliper à0.1 ml of tuberculin (PPD) is injected i/d at sides of neck. (caudal fold of tail, vulvar lip) à After 72 hours edematous swelling 

àindicates positive reaction.

àThickness of swelling 3mm is doubtful and 4mm is positive.

·         I/D comparative cervical: The side of neck shaved

o   Skin fold Measure à0.1 ml of tuberculin (PPD) of bovine and 0.1 ml tuberculin of avian is injected i/d into different side of neck.(12 cm apart)

o   After 72 hours edematous swelling  indicates positive reaction.

o   Thickness of swelling 3mm is doubtful and 4mm is positive at the site of bovine PPD injected

o   False positive reaction- Tuberculin test may be attributed to sensitization to mycobacteria other then M. bovis. So comparative i/d test use in preference to single i/d.

o   False Negative – Test perform before 30 days post infection.

o   Some cattle unresponsive referred to as anergy

o   Immunosuppression may contribute to the  inability to respond to the tuberculin test

 

·         Ophthalmic: One drop of tuberculin is dropped into conjuctival sac and again 3 drop after 48 hr. After 24 hour of second instillation, the eye should show purulent conjunctivitis in a positive.

·         PCR

·         ELISA

TREATMENT

•      On December 28th, 1908 the French bacteriologists Albert C. Calmette and Guérin notified a loss of virulence of M. bovis when cultured in bile containing media.(Bacille Calmette-Guerin)

•      A live attenuated vaccine (BCG) is the current vaccine for tuberculosis

•      BCG name due to  Bacille Calmette-Guerin

•      Isoniazid, Rifampicin, Pyyrazinamide & Ethambutol drugs are used.

 

 References:

Pathologic Basis of Veterinary Disease Expert Consult

6th edition

By: James Zachary, M. McGavin