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Topic 8

Part of the FMGE study roadmap. Botany topic pathol-008 of Botany.

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Infectious Diseases

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Infectious Diseases — Key Facts for FMGE Core concept: Infectious diseases are caused by pathogens including bacteria, viruses, fungi, and parasites, each with characteristic transmission patterns and tissue involvement High-yield point: Recognizing the pattern of organ involvement and associated pathogens is critical for diagnosis ⚡ Exam tip: Know the specific infectious agents associated with granulomatous inflammation and the classic pathological findings

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Infectious Diseases — FMGE Study Guide

Bacterial Infections

Staphylococcus aureus

  • Gram-positive cocci in clusters
  • Diseases:
    • Skin infections: impetigo, cellulitis, abscesses, furuncles
    • Toxic shock syndrome (TSST-1 superantigen)
    • Food poisoning (preformed enterotoxin - 1-6 hours)
    • Osteomyelitis (most common in children)
    • Infective endocarditis (IV drug users)
    • Pneumonia (post-influenza, aspiration)
  • Virulence factors: Coagulase, protein A (binds Fc of IgG), hyaluronidase, DNases
  • MRSA: mecA gene encoding altered PBP2a; treatment with vancomycin, linezolid, daptomycin

Streptococcus species

  • Streptococcus pyogenes (Group A):

    • Pharyngitis: “Strep throat” - tonsillar exudate, fever, anterior cervical lymphadenopathy
    • Scarlet fever: Sandpaper rash, strawberry tongue, circumoral pallor
    • Skin infections: Impetigo (honey-crusted), cellulitis, erysipelas
    • Post-streptococcal sequelae: Rheumatic fever (carditis, arthritis, chorea), glomerulonephritis
    • Necrotizing fasciitis: Rapidly spreading, devastating
    • Virulence: M protein (anti-phagocytic), streptolysins O and S, hyaluronidase, streptokinase

  • Streptococcus pneumoniae:

    • Gram-positive lancet-shaped diplococci
    • Lobar pneumonia: Rusty sputum, consolidation
    • Meningitis, otitis media, sinusitis
    • Encapsulated (quellung reaction positive)
    • Most common cause of community-acquired pneumonia and bacterial meningitis in adults

  • Streptococcus agalactiae (Group B):

    • Newborn sepsis and meningitis
    • Colonizes vaginal canal

  • Enterococcus:

    • Part of gut flora
    • Hospital-acquired UTIs, endocarditis, wound infections
    • VRE (vancomycin-resistant) - important in hospital settings

Neisseria species

  • N. meningitidis:

    • Gram-negative diplococci (kidney bean shaped)
    • Meningococcal meningitis: Petechial rash, rapidly fatal
    • Waterhouse-Friderichsen syndrome: Bilateral adrenal hemorrhage
    • Encapsulated, transmitted via respiratory droplets

  • N. gonorrhoeae:

    • Gram-negative diplococci
    • Gonorrhea: Purulent discharge, cervicitis, urethritis
    • Disseminated: Arthritis-dermatitis syndrome, PID → infertility

Gram-Negative Bacilli

Enterobacteriaceae (lactose fermenters):

  • E. coli: UTIs (most common cause), neonatal meningitis, traveler’s diarrhea, HUS
  • Klebsiella: Pneumonia (currant jelly sputum), UTIs, nosocomial infections
  • Proteus: UTIs, struvite stones, “swarming” on culture media
  • Salmonella: Typhoid (rose spots, hepatosplenomegaly, relative bradycardia), gastroenteritis
  • Shigella: Dysentery (bloody diarrhea, tenesmus), bacillary dysentery

Non-fermenters:

  • Pseudomonas aeruginosa: Nosocomial pneumonia, burn infections, malignant otitis externa, hot tub folliculitis
  • Acinetobacter: Hospital-acquired infections, multi-drug resistant

Anaerobic Bacteria

  • Clostridium perfringens: Gas gangrene (crepitus, rapidly spreading), food poisoning
  • Clostridium tetani: Tetanus (spastic paralysis - lockjaw, opisthotonus)
  • Clostridium botulinum: Botulism (descending flaccid paralysis, dilated pupils)
  • Clostridioides difficile: Antibiotic-associated colitis (pseudomembranous colitis - associated with clindamycin, fluoroquinolones)
  • Bacteroides: Most common anaerobe in gut; intra-abdominal infections

Mycobacteria

Mycobacterium tuberculosis:

  • Acid-fast bacilli (red rods against blue background in Ziehl-Neelsen stain)
  • Primary TB: Ghon complex (lower lobe involvement + ipsilateral hilar lymph node)
  • Secondary TB: Apical cavitary lesions (reactivation), fever, night sweats, weight loss
  • Caseating granulomas: Central caseous necrosis with Langhans giant cells
  • Extrapulmonary TB: Meningitis, spine (Pott disease), kidney, lymph nodes (scrofula)

Mycobacterium leprae:

  • Acid-fast, causes leprosy (Hansen’s disease)
  • Tuberculoid: Few granulomas, few bacilli, hypopigmented patches with sensory loss
  • Lepromatous: Many bacilli, diffuse involvement, leonine facies, Madarosis

Other Important Bacteria

  • Corynebacterium diphtheriae: Gray pseudomembrane in throat, diphtheria toxin (systemic effects on heart and nerves)
  • Bordetella pertussis: Whooping cough - paroxysmal coughing, inspiratory whoop
  • Treponema pallidum: Syphilis - primary (chancre), secondary (condylomata lata, rash), tertiary (tabes dorsalis, aortitis, gummas)
  • Chlamydia trachomatis: Inclusion conjunctivitis, urethritis, PID, trachoma (scarring)
  • Legionella pneumophila: Legionnaires’ disease - severe pneumonia with GI and neurological symptoms, transmitted via aerosols

Viral Infections

DNA Viruses

Herpesviruses:

  • HSV-1: Gingivostomatitis, herpes labialis (cold sores), encephalitis (temporal lobe)
  • HSV-2: Genital herpes
  • VZV: Chickenpox (vesicular rash “dew drops on rose petals”), zoster (shingles - dermatomal)
  • EBV: Infectious mononucleosis (fatigue, pharyngitis, lymphadenopathy, atypical lymphocytes, positive heterophile antibody/Monospot), Burkitt lymphoma, nasopharyngeal carcinoma
  • CMV: Congenital infections (INTRAUCEPHALIC calcifications, periventricular), mononucleosis, retinitis in AIDS

Hepatitis viruses:

  • Hepatitis B: Dane particles (complete virion), HBsAg, HBeAg, HBcAg; chronic infection → cirrhosis → hepatocellular carcinoma
  • Hepatitis C: Leading cause of chronic liver disease, cirrhosis; transmitted via blood

Others:

  • Adenovirus: Respiratory infections, conjunctivitis, viral conjunctivitis
  • Papillomavirus (HPV): Warts, cervical cancer (types 16, 18 - E6 inactivates p53, E7 inactivates Rb)

RNA Viruses

Paramyxoviruses:

  • Measles (Rubeola): Koplik spots (pathognomonic), maculopapular rash (cephalocaudal spread), Warthin-Finkeldey giant cells
  • Mumps: Parotitis, orchitis, meningitis
  • RSV: Bronchiolitis in infants

Orthomyxoviruses:

  • Influenza: Segmented genome, antigenic shift (major) and drift (minor), causes pandemics
  • Hemagglutinin (HA) and Neuraminidase (NA) - important antigens

Rhabdovirus:

  • Rabies: Negri bodies (intracytoplasmic eosinophilic inclusions in neurons), hydrophobia, aerophobia

Retroviruses:

  • HIV: CD4+ T cell depletion, opportunistic infections, AIDS-defining illnesses
  • HTLV-1: Adult T-cell leukemia/lymphoma

Enteroviruses:

  • Poliovirus: Anterior horn cell destruction → flaccid paralysis
  • Coxsackievirus: Hand-foot-mouth disease, myocarditis
  • Enterovirus 71: Hand-foot-mouth disease, encephalitis

Arboviruses:

  • Dengue: Breakbone fever, dengue hemorrhagic fever, plasma leakage
  • Chikungunya: Severe arthralgia
  • Japanese encephalitis: Mosquito-borne, high mortality

Prions

  • Creutzfeldt-Jakob Disease (CJD): Rapidly progressive dementia, myoclonus, EEG (periodic synchronous discharges), spongiform changes on histology
  • Kuru: Forebrain degeneration, associated with cannibalism rituals
  • No inflammatory response, long incubation period

Fungal Infections

Superficial Mycoses

  • Dermatophytes: Tinea (capitis, corporis, cruris, pedis - athlete’s foot, unguium - nail infection)
  • Pityriasis versicolor: Malassezia furfur - hypo/hyperpigmented patches

Systemic Mycoses

Histoplasma capsulatum:

  • Ohio/Mississippi River valleys
  • Intracellular within macrophages
  • Granulomatous inflammation

Blastomyces dermatitidis:

  • Broad-based budding yeast
  • Skin and pulmonary involvement

Cryptococcus neoformans:

  • India ink preparation shows thick capsule
  • Latex agglutination test for antigen detection
  • Causes meningitis in immunocompromised (AIDS)
  • Found in pigeon droppings

Candida albicans:

  • Pseudohyphae with budding yeast
  • Thrush (oral), vulvovaginitis, esophageal candidiasis in HIV
  • Disseminated candidiasis in immunocompromised

Aspergillus:

  • Acute angles (45°) branching septate hyphae
  • Allergic bronchopulmonary aspergillosis (ABPA), aspergilloma (fungus ball), invasive aspergillosis

Mucor/Rhizopus:

  • Broad-angle (90°) nonseptate hyphae
  • Rhinocerebral mucormycosis in diabetics (Kussmaul breathing pattern, orbital involvement)

Parasitic Infections

Protozoa

Entamoeba histolytica:

  • Intestinal amebiasis: Bloody diarrhea (amebic dysentery), flask-shaped ulcers
  • Liver abscess: Right lobe, anchovy paste material
  • Trophozoites with ingested RBCs; cysts with 4 nuclei

Giardia lamblia:

  • Trophozoite with falling leaf appearance (bilateral flagella)
  • Malabsorption, foul-smelling fatty stools
  • Found in contaminated water

Plasmodium (Malaria):

  • P. vivax/ovale: Tertian fever (48 hours), dormant liver stages (hypnozoites)
  • P. falciparum: Malignant tertian, severe malaria (cerebral, renal), irregular fever, high parasitemia
  • P. malariae: Quartan fever (72 hours)
  • Bx: Ring forms, banana-shaped gametocytes (P. falciparum)
  • Liver stage: Schizonts, hepatocyte enlargement

Trypanosoma:

  • African sleeping sickness (T. brucei): Tsetse fly, CNS involvement (somnolence), chancre
  • Chagas disease (T. cruzi): Reduviid bug, cardiomyopathy, megacolon

Leishmania:

  • Sandfly transmission
  • Visceral (L. donovani - Kala-azar): Hepatosplenomegaly, pancytopenia, hypergammaglobulinemia
  • Cutaneous: Skin ulcers

Toxoplasma gondii:

  • Cat feces (oocysts), undercooked meat
  • Congenital: Hydrocephalus, intracranial calcifications, chorioretinitis
  • Adult: Usually asymptomatic, lymphadenopathy
  • AIDS: Toxoplasma encephalitis (ring-enhancing brain lesions)

Helminths

Nematodes (Roundworms):

  • Ascaris lumbricoides: Roundworms, migration through lungs (eosinophilic pneumonia - Löffler syndrome), intestinal obstruction
  • Enterobius vermicularis (Pinworm): Perianal itching, scotch tape test
  • Ancylostoma duodenale/Necator americanus (Hookworms): Iron deficiency anemia, ground itch
  • Wuchereria bancrofti: Filariasis - elephantiasis (lymphatic obstruction), hydrocele, lymphangitis
  • Strongyloides stercoralis: Hyperinfection in immunocompromised (auto-infection)
  • Trichinella spiralis: Undercooked pork, muscle invasion (eosinophilic myositis)

Cestodes (Tapeworms):

  • Taenia saginata (beef): Proglottids motile in stool
  • Taenia solium (pork): Cysticercosis (neurocysticercosis - seizures, calcifications)
  • Echinococcus granulosus: Hydatid cyst (liver, lung), anaphylactic risk if ruptured

Trematodes (Flukes):

  • Schistosoma: Snail transmission, mansoni (intestinal and hepatic), haematobium (bladder), japonicum (portal hypertension)
  • Liver flukes: Clonorchis sinensis, Fasciola hepatica - cholangitis, cholangiocarcinoma

Tissue Nematodes:

  • Dracunculus medinensis (Guinea worm): Water-borne, serpentine skin lesions
  • Onchocerca volvulus (River blindness): Black fly, skin nodules, blindness

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