Page 172 - Concise Pathology for Exam Preparation ( PDFDrive )

7 Infections 157

• The lesion spreads by peripheral extension and may have satellite lesions (pseudo
buboes). It is generally not accompanied by inguinal lymphadenopathy.
• Untreated cases are characterized by development of extensive scarring, often associated
with lymphatic obstruction and lymphoedema (elephantiasis) of external genitalia.
Morphology
• Main pathology is an ulcer accompanied by abundant granulation tissue, which on gross
examination appears as a soft, fleshy, painless mass.
• Active lesions are marked by epithelial hyperplasia (pseudoepitheliomatous reaction).
A mixture of neutrophils and mononuclear inflammatory cells is usually present at the
base of the ulcer.
• Culture of the organism is difficult, so morphologic examination of smears or biopsy are
the mainstay of the diagnosis.

Tuberculosis
• It is caused by Mycobacterium tuberculosis, a slender, aerobic rod which belongs to the
genus Mycobacterium.
• Mycobacteria possess a waxy cell wall composed of mycolic acid, which makes them
acid fast.
• ‘Infection’ with M. tuberculosis must be differentiated from ‘disease’. Infection indicates
mere presence of the pathogenic organisms, which may or may not cause clinically
significant disease.
• Mycobacteria spread from person-to-person via airborne droplets containing organisms
from an active case to a susceptible host.
• Primary tuberculosis is usually asymptomatic; although it may sometimes cause fever
and pleural effusion. The primary focus undergoes spontaneous healing by fibrosis
and/or calcification in most individuals; however, progression of the disease can occur
in a few.
• Viable organisms may remain dormant in such lesions for decades. Reactivation of the
infection occurs when the person’s immune defences are lowered.
Pathogenesis
• The entry of M. tuberculosis into macrophages occurs through endocytosis and is
influenced by several macrophage receptors such as mannose receptors (that bind
lipoarabinomannan or LAM) and complement receptors (that bind the opsonized
organisms).
• Mycobacteria replicate within the macrophage and block formation of phagolysosome
by inhibition of calcium signals as well as recruitment and assembly of proteins
that cause formation of the phagolysosome (Flowchart 7.1).
Primary Tuberculosis
• Primary tuberculosis develops in individuals who are previously unexposed or unsensi-
tized to M. tuberculosis.
• Initially, only a nonspecific inflammatory reaction is evident, followed 2–3 weeks later
by a positive skin test, which is due to a specific granulomatous parenchymal response.
The latter manifests as a tubercle which could be with or without caseation.
• Primary tuberculosis can involve the following sites:
• Lung
• Intestine
• Skin
• Oropharynx
• Lymphoid tissue/tonsil
• In areas of high tuberculosis transmission, primary pulmonary tuberculosis has a high
incidence in children. Most commonly involved areas are the middle and lower lung
zones because most inspired air is distributed to them.

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