PA10.1-5 | Infections & Infestations — Summary & Reflection

REFLECT

Before you move to the summary, take 3 minutes to write (or think through) answers to these reflective questions:

1. You are looking at a blood smear from a patient with fever. You see enlarged RBCs with Schüffner's dots and one ring form per cell. The patient is from Northeast India. What is the species, and what additional complication should you warn about regarding dormant liver-stage forms?

1. A young child in a rural area develops new seizures. CT scan shows a ring-enhancing lesion with a small eccentric bright spot inside. What is the pathological basis of the 'bright spot'? What does the ring of enhancement represent?

1. Think of a leprosy patient who develops sudden worsening of skin lesions and acute nerve pain after starting treatment. What immune event is occurring? What is the risk if untreated?

1. A COVID-19 patient's D-dimer is 8,000 ng/mL (normal <500). What pathological process does this signify? Why is heparin used in such patients?

These questions mirror the style of clinical viva and OSPE stations in your final examination. Revisit them after the summary.

KEY TAKEAWAYS

Key takeaways from this module:

• Infectious disease pathology = tissue reaction pattern + causative class + specific pathogen features. Master the five reaction types (suppurative, granulomatous, cytopathic, eosinophilic, necrotizing) and you have a framework for any new pathogen.

• Malaria (P. falciparum): PfEMP1-mediated sequestration explains absent schizonts in peripheral blood, cerebral malaria, and placental malaria. Haemozoin = malarial pigment. Blackwater fever = massive haemolysis + haemoglobinuria + ATN.

• Cysticercosis (T. solium): Eggs (not cysts) are the infective form for humans in cysticercosis. Live cyst = vesicular stage (minimal reaction); dead cyst = colloidal/granular stage (seizures). CT showing calcified nodule = old disease.

• Leprosy: The entire spectrum is determined by the host's CMI response, not the bacterium. TT = Th1, compact granuloma, no AFB, strong lepromin, early severe nerve damage. LL = Th2 anergy, foamy macrophages, numerous AFB, negative lepromin, late diffuse nerve infiltration. Fite (not Ziehl–Neelsen) stain for M. leprae.

• Tissue reactions: CMV = owl-eye inclusions; HSV = Cowdry A inclusions; Rabies = Negri bodies; Schistosoma = Symmers' pipe-stem fibrosis; Cryptococcus = India ink capsule, mucicarmine stain; PCP = GMS stain (cup-shaped organisms, no budding).

• COVID-19: ACE2 entry → type II pneumocyte injury → DAD (hyaline membranes = exudative phase) → ARDS. Cytokine storm (IL-6-driven) + endothelial injury → microthrombi → ↑D-dimer. Lymphopenia is a characteristic CBC finding. RT-PCR remains the diagnostic gold standard.