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Chapter 3: Respiratory infections 103

wall components interfere with the fusion of the lyso- Secondary tuberculosis
somes with the phagocytic vacuole, so that the bacteria Secondary tuberculosis is a reactivation of infection
can survive intracellularly. occurring in a previously latently infected individual.
The most common pattern of TB is a primary pul- It usually occurs due to reactivation of a healed pri-
monary infection: marylesionduetoaweakeningofthehostdefence,e.g.
The mycobacteria are inhaled into the alveolar spaces diabetes mellitus, malnutrition, immunosuppression

ofthelung.Conditionsforgrowtharemostfavourable (drugs, HIV). It may occur at any time from weeks
just below the pleura in the apex of the upper lobe or up to years after the original infection. Occasionally it
the apical lobe of the lower lobe. Here the inflam- may be due to re-infection from an external source. It
matory process forms the ‘Ghon focus’ usually just differs from primary infection in its immunopathol-
beneath the pleura. Bacteria spread to the draining ogy. The lymph nodes are rarely involved, and there is
lymph nodes at the lung hilum, and excite an immune reactivation of the immune response in the tissues.
response there also. This pattern forms the primary Inthelung,thebacteriahaveapreferencefortheapices
complex with infection at the periphery of the lung (higher pO 2 ), and form an apical lung lesion known
and enlarged peribronchial lymph nodes. as an Assmann focus. It begins as a small caseating
The outcome of the primary infection depends on the tuberculous granuloma, histologically similar to the

balance between the virulence of the organism and Ghon focus, with destruction of lung tissue and cavi-
the strength of the host response (see Table 3.11). If tation. T cells are re-induced by the secondary infec-
the host can mount an active cell mediated immune tion, with activation of macrophages, and exactly as
response the infection may be completely cleared. with primary TB the outcome is a balance between the
In the vast majority of cases there is an intermediate virulence of the infection and the strength of the host

responseandtheactivatedmacrophagesaggregateand immune system.
wall off a central area of caseous (cheese-like) necro- In patients with a vigorous immune reaction there
sis. These are called granulomas or tubercles. Collagen is healing of the apical region with collagen de-
is deposited around these, often becoming calcified. position and eventually calcification by the same
Twenty per cent of these tubercles contain viable my- process as the healing of the primary infection re-
cobacteria, but clinically the infection becomes latent. sulting in a latent apical fibrocaseous tuberculosis
If the immune response is very poor, there is progres- lesion.

sion of the caesating granulomas in the lymph nodes In patients with a poor immune response the le-
which erode into a bronchus or blood vessel caus- sion enlarges with caseous necrosis destroying lung
ing further dissemination. This is called a ‘progres- tissue, thinning of the collagen wall and increasing
sive primary infection’. Occasionally the Ghon focus the risk of erosion of a bronchus or blood vessel.
may rupture through the pleura causing a tuberculous
pleurisy.
Patterns of progressive TB infection
Table 3.11 Predisposing factors towards extension or See Fig. 3.3.
containment of infection. 1 Tuberculous Bronchopneumonia: If an infected
lymph node in primary tuberculosis, or an enlarging
Predisposing to Predisposing
extension of infection to containment secondary tuberculous lesion erodes into a bronchus,
tuberculous caseous material containing live tuber-
Exposure to large numbers Exposure to a small number
of highly virulent of poorly virulent cle bacilli spreads infection throughout that segment
organisms organisms of lung. Coughing disperses these bacilli into the at-
Poor immune system eg Good immune response, e.g. mosphere, transmitting TB to other people. Without
malnutrition, extremes of healthy immunised treatment, extensive caseating lesions develop rapidly,
age, intercurrent disease individual leading to a high mortality. This disease is sometimes
Use of appropriate antibiotics
called ‘galloping consumption’.