Mycobacteria

Question 1

Mycobacterium spp. classification

Answer 1

Gram+, non-motile, non-sporulating, aerobic or microaerophilic coccobacilli

Question 2

Structure ?

Answer 2

thick wall, made up of mycolic acid (environmental and antimicrobial resistance)
glycolipids induce macrophage response and prevent lysosomal degradation
peptidoglycan and glycolipids act as immune adjuvants

Question 3

Name the two main mycobacteria species

Answer 3

M. bovis and M. tuberculosis

Question 4

Pathogenesis ?

Answer 4

can survive and proliferate in macrophages, prevents phagolysosome fusion.
macrophages persist at site of infection, causing a type I immune response

Question 5

What is the tuberculoid granuloma ?

Answer 5

a dynamic structure, prevents spread to other sites and hosts, and cell-mediated responses kill infected macrophages, forming a caseous centre
bacteria within tubercles can remain dormant for long periods until something disturbs the immune-response

Question 6

Gross pathology

Answer 6

the tubercle is a classic gross lesion
circumscribed, encapsulated, pale, yellow-white nodules

Question 7

Histopathology

Answer 7

granulomatous inflammation (forms a capsule of lymphocytes and neutrophils) and a central area of caseous necrosis +/- mineralisation
large ones can liquefy and resemble abscesses. fibrosis increases with time

Question 8

Main hosts

Answer 8

cows, deer, pigs, humans

Question 9

Reservoir hosts

Answer 9

depends on location, known to be badgers in the UK

Question 10

Dead-end hosts

Answer 10

humans (can be infected through contaminated milk, and can not transfer)

Question 11

Clinical signs ?

Answer 11

mostly none. Typical lesion is chronic disease with caseating granulomas in the lung and lymph nodes (where depends on route of infection)

Question 12

Describe the respiratory lesions seen

Answer 12

Retropharyngeal, tracheobronchial and mediastinal LNs have lesions.
Lung lesions only seen in 10-30% of cattle, in caudal lobes.
Tubercles can erode into the pleura and spread in the pleural cavity

Question 13

How does the disease appear in deer ?

Answer 13

superficial lymphadenitis that drains to the skin surface
less able to contain infection
suppurative inflammation more prominent than caseous necrosis

Question 14

How does the disease appear in carnivores ?

Answer 14

discrete tubercles and MNGCs uncommon, instead see granulation tissue with scattered macrophages

Question 15

Diagnosis ?

Answer 15

Difficult to isolate in culture if no lesions (takes a long time)
PCR !!!

Question 16

Control ?

Answer 16

complicated by wildlife reservoirs
active and passive surveillance :
- active = routine testing of all animals via a single intradermal comparative cervical tuberculin skin test (SICCT). +ve animals culled
- passed = examination of suspect infections, clinically or at slaughter

also biosecurity and purified protein derivatives (given to produce hypersensitivity reactions in animals infected)