Chapter 29

### Introduction

In its beginning the malady is easier to cure but difficult to detect, but later it becomes easy to detect but difficult to cure.

Niccolò Machiavelli (1469–1527), On Tuberculosis

Bacterial infections can present diagnostic brain-teasers, and a high index of suspicion is needed to pinpoint the diagnosis. Many are rarely encountered, thus making diagnosis more difficult yet demanding vigilance and clinical flexibility.

The list includes:

• tuberculosis

• infective endocarditis

• syphilis

• septicaemia

• the zoonoses (e.g. brucellosis, Lyme disease)

• clostridial infections: tetanus, gas gangrene, puerperal infection, botulism, pseudomembranous colitis

• hidden suppuration: abscess, osteomyelitis

• mycoplasma infections: atypical pneumonia

• Chlamydia infections: psittacosis, non-specific arthritis, pelvic inflammatory disease, trachoma, atypical pneumonia

• legionnaire disease

• Hansen disease

Chlamydia and rickettsial organisms have been confirmed as being small bacterial organisms.

##### Tuberculosis

Tuberculosis (TB), caused by Mycobacterium tuberculosis, still has a worldwide distribution with a very high prevalence in Asian countries where 60–80% of children below the age of 14 years are affected.1 This has special implication in Australia, where large numbers of Asian migrants are settling. The WHO estimates that one-third of the world's population is infected by the tubercle bacillus. It remains a deadly disease with about 2 million people worldwide dying of TB every year and 8 million new cases a year.

Clinical features

TB can be a mimic of other diseases and a high level of suspicion is necessary to consider the diagnosis, especially if there are only extrapulmonary manifestations. There may be no symptoms or signs, even in advanced disease. Ideally patients should be referred early for specialist management.

DxT malaise + cough + weight loss ± fever/night sweats (± erythema nodosum) pulmonary TB

###### Primary infection

The primary infection usually involves the lungs. Transmission is by droplet infection. The focus is usually subpleural in the upper to mid zones and is almost always accompanied by lymph node involvement.

Erythema nodosum may accompany the primary infection (see FIG. 29.1). Primary TB is symptomless in most cases although there may be a vague, ‘not feeling well’ illness associated with a cough. In most people this pulmonary focus heals but leaves some surviving tubercle bacilli, even if it becomes calcified (the Ghon focus).

###### FIGURE 29.1

Classic erythema nodosum involving the legs of a patient with pulmonary tuberculosis

###### Progressive primary tuberculosis

If the immune response is inadequate, progressive primary TB develops, with constitutional and pulmonary symptoms. Rarely, haematogenous spread can occur to the lungs (‘miliary tuberculosis'), to the pleural space (tuberculosis pleural effusion) or to extrapulmonary sites such as the meninges and bone.

###### Latent TB infection (LTBI)
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