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1. CHOLERA

Etiology

• Serogroup O1 is subdivided into two serotypes: Inaba and Ogawa
• Two biotypes: Classic (thought extinct) and El Tor (current pandemic strain)
• Serogroup O139 (emerged 1992, southeastern India): a derivative of El Tor O1 with a distinct capsule; clinically indistinguishable but immunologically cross-reactive immunity does NOT occur between O1 and O139

Epidemiology

• Cholera is native to the Ganges delta of the Indian subcontinent
• Seven global pandemics since 1817; the current (7th) began in Indonesia in 1961 (El Tor biotype)

• 95% of reported cases come from Africa and Asia; true burden estimated at 1-4 million cases/year with 20,000-140,000 deaths annually

• Transmission: primarily ingestion of fecally contaminated water; contaminated food and person-to-person transmission also contribute; no animal reservoir
• Infectious dose is relatively high but markedly reduced in hypochlorhydric individuals, antacid users, and after meals
• In endemic areas: predominantly a pediatric disease; outbreaks peak in warm, wet seasons
• New introductions: affects all ages equally; often precipitated by war, displacement, and breakdown of public health infrastructure (Haiti 2010, Yemen 2016)

Features of Pathogenesis

1. Colonization: V. cholerae ingested → survives gastric acid → colonizes the small intestine using the toxin-coregulated pilus (TCP), whose synthesis is co-regulated with cholera toxin by the master regulator ToxR (a cascade of regulatory proteins responding to environmental signals and quorum sensing)
2. Cholera toxin (CT) structure:
   • B subunit (pentamer): binds to GM1 ganglioside on intestinal epithelial cells
   • A subunit (monomer): delivered to cytosol; the active A1 fragment irreversibly ADP-ribosylates the Gs-alpha regulatory protein of adenylate cyclase
3. Consequence: Gs-alpha is locked in its active (GTP-bound) state → constitutive activation of adenylate cyclase → massive increase in intracellular cAMP → activation of protein kinase A → phosphorylation of CFTR (Cl⁻ channel) and inhibition of Na⁺/H⁺ exchangers → net secretion of Na⁺, Cl⁻, K⁺, HCO₃⁻, and water into the intestinal lumen
4. Result: Profuse, watery "rice-water" stool (colorless, odorless, containing mucus flecks) - up to 1 liter/hour in severe cases → rapid, profound dehydration, hypovolemic shock, metabolic acidosis, hypokalemia

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Clinical Presentation

• ~75% of infections: asymptomatic (important reservoir)
• ~20%: mild-moderate diarrhea
• ~5%: cholera gravis (severe, life-threatening)

1. Painless, profuse watery diarrhea - the hallmark; "rice-water" stool
2. Vomiting (effortless, without nausea)
3. Rapid-onset dehydration: intense thirst, dry mouth, sunken eyes, decreased skin turgor, oliguria, muscle cramps (from hypokalemia)
4. Hypovolemic shock: hypotension, weak/absent pulse, altered consciousness, cyanosis
5. Metabolic derangements: metabolic acidosis (HCO₃⁻ loss), hypokalemia, hyponatremia

Degrees of Dehydration (Harrison's, Table 173-1)

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Emergency Therapy for Dehydration/Hypovolemic Shock

• Ringer's lactate (preferred) or normal saline if RL unavailable
• 100 mL/kg in the first 3 hours (or 6 hours in children <12 months); start rapidly then slow
• Total 200 mL/kg in the first 24 hours
• Continue until strong pulse and normal mental status restored
• Then transition to oral rehydration solution (ORS)

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Diagnosis of Cholera

• Dark-field microscopy of stool: rapid darting ("shooting-star") motility of comma-shaped vibrios; immobilized by O1 or O139 antiserum (rapid presumptive test)
• Culture: TCBS agar (thiosulfate-citrate-bile salts-sucrose) - V. cholerae O1 produces yellow colonies; alkaline peptone water (pH 8.5) is used for enrichment
• Serotyping with O1 and O139 antisera (Inaba, Ogawa)
• PCR: for rapid confirmation and epidemiologic typing
• Rapid antigen tests: available in field settings

Differential Diagnosis

• ETEC (enterotoxigenic E. coli): similar profuse watery diarrhea, heat-labile toxin also stimulates adenylate cyclase; distinguished by culture
• Rotavirus/Norovirus: more common in children; often with vomiting first, less voluminous diarrhea
• Cryptosporidium: profuse watery diarrhea especially in immunocompromised
• Other Vibrio spp. (V. parahaemolyticus): often seafood-associated, self-limited
• Non-cholera secretory diarrheas: VIPoma, carcinoid (no epidemic context)

Principles of Treatment

1. Fluid and electrolyte replacement (primary treatment - reduces mortality from ~50% to <1%)
   • Severity-based: ORS for mild-moderate; IV Ringer's lactate for severe
   • Monitor ongoing losses and replace continuously
2. Antibiotics (adjunct - reduce duration and volume of diarrhea):
   • Doxycycline: single dose 300 mg (adults) - first choice where susceptible
   • Azithromycin: 1 g single dose (adults), 20 mg/kg (children) - preferred in pregnancy and for resistant strains
   • Ciprofloxacin: 1 g single dose (adults); fluoroquinolone resistance emerging in some regions
   • Tetracycline: 500 mg QID × 3 days
   • Antibiotics reduce stool volume by ~50% and shorten illness by ~50%
3. Nutritional support: early refeeding after rehydration (especially in children)
4. Zinc supplementation (children): 20 mg/day × 10-14 days reduces severity and recurrence
5. Public health measures: safe water, sanitation, oral cholera vaccines (Shanchol, OCV)

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2. MALARIA

Etiology

Epidemiology

• 3.2 billion people at risk worldwide; ~228 million cases/year; ~405,000 deaths/year
• 94% of deaths in the WHO African Region; 67% of all deaths in children <5 years
• Endemic in tropical/subtropical regions; P. vivax extends into temperate zones
• Vector: female Anopheles mosquito; >30 species implicated
• In non-endemic countries, malaria is mainly seen in:
  • Immigrants and returned travelers (especially VFRs - "visiting friends and relatives")
  • These individuals often do not seek prophylaxis, visit rural areas, stay longer

Risk Groups

• Children <5 years in endemic areas (lack acquired immunity)
• Non-immune adults (travelers, first-time exposures)
• Pregnant women (impaired immunity, risk of placental malaria with P. falciparum)
• HIV-positive individuals
• Asplenic individuals
• Visitors from non-endemic countries traveling to endemic areas

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Life Cycle of the Malaria Parasite

Human (Asexual) Phase:

• Infected female Anopheles injects sporozoites during blood meal
• Sporozoites travel via bloodstream to liver and invade hepatocytes
• Inside hepatocytes, parasites undergo asexual replication over 5-15 days, forming hepatic schizonts
• Schizonts rupture, releasing thousands of merozoites into bloodstream
• P. vivax and P. ovale ONLY: some sporozoites form dormant hypnozoites (latent liver stage) - the basis for true relapses

• Merozoites invade RBCs via specific surface receptors (Duffy antigen for P. vivax; glycophorins for P. falciparum)
• Inside RBC: ring trophozoite → mature trophozoite → schizont (containing 6-24 merozoites depending on species) → rupture → merozoites re-infect new RBCs
• Rupture causes the febrile paroxysm (see below)
• Some trophozoites differentiate into gametocytes (male microgametocytes, female macrogametocytes)

Mosquito (Sexual) Phase:

• Mosquito ingests gametocytes during blood meal
• Microgametes undergo exflagellation → fertilize macrogametes → ookinete → penetrates mosquito midgut → oocyst → ruptures, releasing thousands of sporozoites → migrate to salivary glands → ready to infect new host

Features of Asexual Phase by Species

Origin of Febrile Paroxysm

• Merozoites (antigenic stimulation)
• Hemozoin (malaria pigment - a byproduct of hemoglobin degradation; potent pro-inflammatory)
• Parasite metabolic byproducts and GPI anchors (glycosylphosphatidylinositol)

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Prevention of Malaria

• Insecticide-treated bed nets (ITNs/LLINs)
• Indoor residual spraying (IRS)
• Insect repellents (DEET, picaridin) on exposed skin
• Protective clothing (long sleeves, long pants)

• Chloroquine: only for chloroquine-sensitive areas (Central America west of Panama Canal, Haiti, Dominican Republic, parts of Middle East)
• Atovaquone-proguanil (Malarone): daily, start 1-2 days before, continue 7 days after
• Doxycycline: daily, start 1-2 days before, continue 4 weeks after
• Mefloquine: weekly, start ≥2 weeks before, continue 4 weeks after
• Primaquine: daily (requires G6PD testing); also covers P. vivax/ovale hypnozoites (terminal prophylaxis)

• RTS,S/AS01 (Mosquirix): first approved malaria vaccine; targets P. falciparum circumsporozoite protein; ~30-40% efficacy; recommended by WHO for children in endemic Africa
• R21/Matrix-M: newer vaccine with ~75% efficacy in trials; WHO prequalified 2023

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Pathogenesis of Relapses in Malaria

• The hypnozoite is a quiescent, metabolically inactive liver-stage parasite
• May remain dormant for weeks, months, or years
• Reactivated by unclear stimuli (possibly stress, febrile illness, immune changes)
• P. vivax: relapses occur 3 weeks to 8 months after primary infection (temperate strains may have longer dormancy of 6-12 months or more)
• P. ovale: typically 1-4 months
• Prevention requires primaquine (8-aminoquinoline) to eradicate hypnozoites; tafenoquine (newer 8-aminoquinoline, single-dose) is an alternative
• Both drugs require G6PD screening prior to use (risk of hemolysis in G6PD-deficient individuals)

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Complications of Malaria

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Diagnosis of Malaria

• Thick blood film: concentrated, more sensitive for detecting low parasitemia; allows detection and species identification; stained with Giemsa or Field's stain
• Thin blood film: monolayer for accurate species identification (morphology of infected RBCs, ring stage characteristics, gametocytes)
• Report: species, % parasitemia (parasites per 100 RBCs on thin film)
• Must be performed urgently (7 days/week, 24 hrs/day) - life-threatening disease

• P. falciparum: small delicate ring forms, double chromatin dots, "applique" (accolé) forms along RBC edge, Maurer's clefts, banana-shaped gametocytes, no RBC enlargement
• P. vivax: enlarged RBCs, ameboid trophozoites, Schüffner's dots (James' stippling)
• P. ovale: oval/fimbriated RBCs, Schüffner's dots, compact trophozoites
• P. malariae: normal-sized RBCs, band form trophozoites, rosette schizonts

• Detect parasite antigens (HRP-2 for P. falciparum; LDH/aldolase for all species)
• Point-of-care, no microscopy needed
• Limitation: cannot quantify parasitemia; false negatives with pfhrp2/3 gene deletions (P. falciparum)

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Treatment of Malaria

P. falciparum (and unknown species)

• Chloroquine 25 mg base/kg over 3 days (rare areas: Central America, Caribbean)

• Artemisinin-based Combination Therapies (ACTs) - first-line:
  • Artemether-lumefantrine (Coartem): 3-day course; most widely available ACT
  • Artesunate-amodiaquine
  • Artesunate-mefloquine (Southeast Asia)
  • Dihydroartemisinin-piperaquine
• Alternative: Atovaquone-proguanil (Malarone), quinine + doxycycline, quinine + clindamycin (in pregnancy)

• IV/IM artesunate - drug of choice (superior to quinine in trials); available via CDC IND protocol in USA
• IV quinine + doxycycline or clindamycin (alternative if artesunate unavailable)
• Supportive: manage hypoglycemia (dextrose), seizures (benzodiazepines), renal failure (dialysis), respiratory failure (mechanical ventilation), severe anemia (transfusion)
• Exchange transfusion for parasitemia >10% (per ASFA guidelines)

P. vivax and P. ovale

• Chloroquine 25 mg base/kg over 3 days (if chloroquine-sensitive) + primaquine (0.5 mg base/kg/day × 14 days or tafenoquine 300 mg single dose) to eradicate hypnozoites
• In chloroquine-resistant P. vivax (Indonesia, parts of Oceania): ACT or mefloquine
• Always screen for G6PD deficiency before primaquine/tafenoquine

P. malariae

• Chloroquine alone (no hypnozoites; no radical cure needed)

Drug groups summary:

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3. VIRAL HEPATITIS A AND E

Etiology

Hepatitis A Virus (HAV)

• Family Picornaviridae, genus Hepatovirus
• 27-nm nonenveloped icosahedral nucleocapsid
• Positive-sense single-stranded RNA genome, ~7.5 kb
• Single open reading frame encoding structural and nonstructural proteins
• Three genotypes in humans (I, II, III), with genotype I predominating worldwide; further divided into subtypes A and B
• Highly stable in environment; resistant to acid, detergents, heat (destroyed by boiling for 1 min or autoclaving)

Hepatitis E Virus (HEV)

• Family Hepeviridae, genus Orthohepevirus
• Non-enveloped, positive-sense ssRNA virus (~7.2 kb)
• 5 genotypes:
  • Genotypes 1 and 2: exclusively human; cause large waterborne epidemics in developing countries
  • Genotypes 3 and 4: zoonotic (humans and swine/boar/deer); responsible for sporadic cases in developed countries
  • Genotype 5: avian HEV (chickens)
• Genotypes 1-2 affect younger populations; genotypes 3-4 affect older/immunocompromised

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Epidemiology

Hepatitis A

• Worldwide distribution; seroprevalence reflects sanitation standards
• In developing countries: near-universal childhood infection (seroprevalence approaches 100% in adults)
• In developed countries: incidence has fallen dramatically (US: from 12/100,000 in 1995 to 0.5/100,000 in 2012; recently rising to 3.8/100,000 in 2018 due to outbreaks in drug users and homeless persons)
• Route of transmission: primarily fecal-oral (person-to-person, contaminated water/food)
• Less commonly: blood transfusion, perinatal transmission

• Travelers to developing countries
• Children in day-care centers and their parents
• Men who have sex with men (MSM)
• Injection drug users
• Homeless persons
• Patients receiving plasma products for hemophilia
• Persons in institutions (prisons, military camps)

Hepatitis E

• Common in parts of Asia (India, Central Asia), Africa, and Mexico
• Rare in the US except in travelers to endemic areas and, increasingly, via zoonotic routes (genotypes 3/4)
• Route of transmission: primarily fecal-oral (contaminated water); waterborne epidemic pattern
• Person-to-person transmission is uncommon (unlike HAV)
• Zoonotic route: uncooked boar, deer, or pork meat (genotypes 3/4)
• Special risk: Pregnant women - particularly high mortality (~20%) in genotype 1 infection, especially in 3rd trimester; mechanism involves impaired immune regulation in pregnancy

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Clinical Classification

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Clinical Presentation

Typical Acute Icteric Hepatitis (HAV and HEV)

• HAV: 15-45 days (mean ~28-30 days)
• HEV: 15-60 days (mean ~40 days)

• Fatigue, malaise, anorexia
• Nausea, vomiting
• Low-grade fever (more prominent in HAV)
• Right upper quadrant discomfort/pain (liver tenderness)
• Myalgia, arthralgia
• Headache

• Jaundice (scleral icterus preceding skin jaundice)
• Dark urine (bilirubinuria - conjugated bilirubin; appears before clinical jaundice)
• Pale/clay-colored stools (acholic stools from decreased bile in intestine)
• Pruritus (especially in cholestatic forms)
• Hepatomegaly (tender), sometimes splenomegaly
• With onset of jaundice, constitutional symptoms often improve
• Elevated ALT/AST (ALT > AST typically; values often >1000 IU/L)

• Gradual resolution of jaundice (2-8 weeks)
• Residual fatigue may persist months
• Full recovery is the rule in immunocompetent patients

• HEV in pregnancy: more severe, higher risk of fulminant failure
• HAV more likely in children, often mild/asymptomatic; symptomatic icteric cases more frequent in adults
• In adults, HAV may require hospitalization in up to 13% of cases
• Prolonged courses (6-9 months) in 10% of adults with HAV

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Diagnosis

Hepatitis A

• Only symptomatic individuals should be screened with IgM (some false positives exist)
• Following vaccination: detectable IgG develops in 2-4 weeks; persists ≥5 years in 99% of responders

• ALT and AST: markedly elevated (often 1000-5000 IU/L; peaks before or at onset of jaundice)
• Serum bilirubin: elevated (both direct and indirect)
• ALP/GGT: elevated, especially in cholestatic variant
• PT/INR: elevated in severe/fulminant cases (marker of liver synthetic function)

Hepatitis E

• Definitive indicator of acute infection
• Detection window: 2-7 weeks after infection
• Gold standard for confirmation

• Newer test; detects viral antigen with higher sensitivity than IgM or HEV RNA
• Antigen detectable for ~3 weeks longer than previously thought

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Treatment

Both HAV and HEV (Acute, Uncomplicated)

1. Rest: activity restriction proportional to symptoms; avoid exhausting physical work
2. Diet: low-fat, high-carbohydrate diet (reduces nausea); adequate caloric intake; no fasting
3. Avoid hepatotoxins: strictly avoid alcohol, hepatotoxic medications (paracetamol in large doses, NSAIDs)
4. Antiemetics: metoclopramide or ondansetron for severe nausea/vomiting
5. Antipruritics: antihistamines or cholestyramine for cholestatic pruritus
6. IV glucose: for severe anorexia/vomiting with risk of hypoglycemia
7. Hospitalization indications: inability to maintain oral intake, INR >1.5, encephalopathy, hypoglycemia, severe coagulopathy

Fulminant Hepatic Failure

• Transfer to liver transplant center
• Management: lactulose/rifaximin for encephalopathy, fresh frozen plasma for coagulopathy, N-acetylcysteine (NAC), liver transplant evaluation
• HEV in immunocompromised: ribavirin has been used with some success for chronic genotype 3 infection (off-label)

Prevention

• Vaccine (inactivated): 2-dose schedule; highly effective (>95%); recommended for all children ≥1 year, travelers to endemic areas, MSM, IDU, homeless persons, chronic liver disease
• Post-exposure prophylaxis: HAV vaccine (preferred) or immunoglobulin (within 2 weeks of exposure)
• Sanitation: safe water, proper sewage disposal, handwashing, food hygiene

• HEV 239 vaccine (Hecolin): approved in China; not yet globally available
• Sanitation: safe water supply, especially during floods in endemic areas
• Pregnant women: avoid travel to endemic areas; careful food/water hygiene
• No post-exposure prophylaxis of proven efficacy

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• Section 1 – Cholera: Etiology, epidemiology, pathogenesis (CT mechanism, ADP-ribosylation, cAMP cascade), clinical presentation, dehydration degree table, emergency IV/ORS therapy tables, diagnosis, differential diagnosis, and treatment principles
• Section 2 – Malaria: Etiology (5 species table), epidemiology, risk groups, full life cycle (exo-erythrocytic + erythrocytic + mosquito sexual phase), species comparison table (fever cycle, hypnozoites, smear features), origin of febrile paroxysm, prevention/chemoprophylaxis, relapse pathogenesis, complications table, diagnosis (microscopy, RDT, PCR), and treatment by species with full drug groups table
• Section 3 – Viral Hepatitis A & E: Etiology (HAV vs HEV genotypes), epidemiology, clinical classification table (all forms including fulminant and chronic), clinical presentation (3 phases), diagnosis tables (serology markers for HAV and HEV), treatment (supportive care, indications for hospitalization, fulminant management, ribavirin for chronic HEV), and prevention table