Pathology · Short notes

Infectious Disease Pathology: Short Notes & Cheat Sheet

Tuberculosis, viral hepatitis, typhoid, malaria, dengue, HIV and cholera, with toxins and virulence factors.

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Short notes

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The five hepatitis viruses

Route of spread predicts chronicity: faeco-oral viruses clear, blood-borne ones can persist.

Tuberculosis and typhoid

Primary TB forms a Ghon focus plus hilar lymph node involvement (the Ghon complex). Reactivation TB forms cavities in the lung apices.
The hallmark is the caseating granuloma with Langhans giant cells. TB can also affect the spine (Pott's disease), meninges and other organs.
Diagnosis: sputum smear (Ziehl–Neelsen), GeneXpert MTB/RIF (also detects rifampicin resistance) and culture. Pakistan has one of the world's highest TB burdens.
Typhoid (Salmonella Typhi) ulcerates Peyer's patches, which can perforate in the third week. Blood culture is the best test in the first week; the Widal test is unreliable.

Viral infections

Hepatitis A and E spread faeco-orally and do not become chronic. Hepatitis E is dangerous in pregnancy.
Hepatitis B spreads by blood, sex and from mother to child. HBsAg means infection, anti-HBs means immunity, HBeAg means high infectivity and IgM anti-HBc means acute infection. A vaccine exists.
Hepatitis C spreads mainly through blood, including reused syringes and unsafe injections. Most infections become chronic, leading to cirrhosis and liver cancer. There is no vaccine, but direct-acting antivirals cure it. Pakistan has one of the highest HCV rates in the world.
Hepatitis D occurs only with hepatitis B.
Dengue (Aedes mosquito, day-biting) causes fever, pain behind the eyes, rash and low platelets. Plasma leakage is most likely around defervescence (days 3–7). NS1 antigen is positive early.
HIV destroys CD4 T cells. AIDS is diagnosed when CD4 falls below 200/µL or an AIDS-defining illness occurs. Pakistan has seen HIV outbreaks linked to reused syringes.

Malaria, cholera and toxins

Malaria is spread by the female Anopheles mosquito. Parasites pass through the liver, then infect red cells.
P. falciparum causes severe and cerebral malaria; P. vivax and P. ovale form liver hypnozoites that cause relapse. Thick films detect parasites; thin films identify the species.
Cholera toxin raises cAMP in gut cells, causing profuse 'rice-water' diarrhoea; oral rehydration is life-saving.
Exotoxins are proteins secreted by bacteria (tetanus, diphtheria, botulinum, cholera) and can be made into toxoid vaccines. Endotoxin is the LPS of Gram-negative bacteria and causes fever and septic shock.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Hepatitis viruses

Virus	Route · chronic?
A	Faeco-oral · no
B	Blood, sexual, perinatal · yes (esp. infants)
C	Blood (unsafe injections) · yes, most
D	Blood; needs HBV · yes
E	Faeco-oral · no; severe in pregnancy

Hepatitis B serology

Marker	Meaning
HBsAg	Current infection
Anti-HBs	Immunity (vaccine or recovery)
HBeAg	High infectivity
IgM anti-HBc	Acute infection
IgG anti-HBc	Past or chronic infection

Memory aids

Vowels hit the bowels: Hepatitis A and E (vowels) spread faeco-orally; B, C and D spread through blood.

Exam traps

A Widal test alone should not be used to diagnose typhoid.
Anti-HBs alone (with no anti-HBc) indicates vaccination, not past infection.
Dengue shock is most likely as the fever settles, not at its peak.
Endotoxin is part of the Gram-negative wall; it is not secreted like exotoxins.

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