Virulence of microbes

1. Adhesion: Microbe attaches to body surface

2. Invasiveness: Microbe spreads to host tissues after establishing infection

3. Toxigenicity: Endotoxins and exotoxins

4. Communicability: Spread from one host to another

5. Evade immune defense:

• Inaccessibility to immune response
• Resist complement mediated lysis and phagocytosis
• Variation or shedding of antigens
• Specific/ non specific immune suppression

6. Induce damage to host tissue defense: Scarring, hypersensitivity reaction

7. Cell lysis: Due to rapid replication in host ells, therefore burst

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Rheumatic heart disease

Autoimmune disease

Etiology:

• β hemolytic streptococcal pharyngitis
• 2-3 weeks after strep pharyngitis in patients 5-15 years old
• Immune destruction of cardiac myosin by cross reactivity – antistrep M proteins react with cardiac myosin

Diagnosis:

• Evidence of preceding strep infection
• 2 major or 1 major + 2 minor manifestations

NB: Orofacial symptoms: Tonsillar exudates, strawberry tongue

Complications:

• Pancarditis
• RHD which involves vascular fibrosis (vegetations) and subsequent insufficiency – Infective endocarditis

Histology:

Focal inflammatory lesions (Aschoff bodies) + Plump macrophages (Anitschkow cells)

Fatal due to:

1. Cardiac failure:
   • Valvular deformity
   • Aortic stenosis
   • Cardiac thrombi
2. Bacterial endocarditis: Acute, subacute
3. Emboli: in brain, lungs, spleen, kidney

Infective endocarditis

Pathogenesis:

• Damage to endothelial lining of valves – expose underlying collagen and tissue factor – platelets and fibrin adhere to it and form blood clot – sterile thrombus
• Bacteremia – bacteria attach to thrombus using adhesins – more bacteria adhere to thrombi – form bacterial colonies on the valve vegetation
• Vegetations are friable

Diagnosis:

Difference between RHD and IE

	IE	RHD
Etiology	– Strep viridans: 50-60% – Staph aureus: 10-20% – Culture negative cases: 10% HACEK: Normal flora in mouth – Haemophilus species – Aggregatibacter actinomycetemcomitans – Cardiobacterium hominis – Eikenella corrodens – Kingella kingae	Hypersensitivity reaction to group A β hemolytic streptococci
Risk factors	Mitral & aortic valve: – Valve disease – Prosthetic valve – Vascular grafts – Vascular catheters Tricuspid valve: – IV drug use	Repeated and untreated pharyngeal infections
Diagnostics	– Positive blood cultures in 90% of cases – Septic infarcts: CVS, CNS, renal, therefore organ failure	– M protein – antibodies cross react with cardiac glycoproteins – ASOT – Anti-DNAase B – Aschoff bodies, Anitschkow cells

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Paramyxoviridae

Read pathogenesis in virology

Measles:

• Pharyngitis
• Contagious
• Incubation 7-10 days
• Prodromal – fever, malaise, photophobia, cough
• Koplik’s spots (Picture) on buccal mucosa
• Symptomatic treatment – bed rest, hydration, quarantine, treat fever
• Prevention MMR vaccine

Mumps:

• Droplet infection
• Causes viral sialadenitis
• Trismus, fever, headache, malaise, preauricular pain
• Systemic – oophoritis, orchitis, affect kidney, liver, pancreas
• Subside in 10 days
• Prevention MMR vaccine

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Common surgical infections

1. Abscess

• Collection of pus
• Complication: if partly sterilized by antibiotics – forms antibioma

2. Cellulitis

• Non suppurative and poorly localized
• Toxaemia common

3. Lymphangitis

• Painful red streaks in affected lymphatic vessels + painful lymph nodes in affected areas

4. Gas gangrene

• Etiology: C. perfringens and β hemolytic strep
• At risk:
  • Military surgery
  • Traumatic surgery
  • Colorectal ops
  • Immunocompromised patient
  • DIabetes
  • Malignancy
• Clinical:
  • Severe wound pain
  • Crepitus
• Management:
  • Aggressive debridement
  • Antibiotics

5. Necrotizing fasciitis

• Rapid progressing fasciitis of subcutaneous tissue and fascia
• Does not damage muscles like NOMA
• Synergistic spread of gangrene caused by mixed infection
• Etiology: Group A streptococcus
  • Dental infection, salivary gland infection, epiglottic infection, otological/dermal infection
  • Peritonsillar abscess
  • Cervical adenitis
  • Post surgical
  • Trauma
• Always immunosuppressive illness present:
  • Diabetes
  • Renal failure
  • HIV
  • Liver cirrhosis
  • Lymphoma
  • Leukemia
  • Elderly
  • Obese
• Variants:
  • Meleney’s synergistic gangrene
  • Fournier’s gangrene (scrotal area)
  • Lemierre’s syndrome
• Classification:
  • Type I – Polymicrobial infection – eg. strep pyogenes, staph aureus, clostridium perfringens, bacteroides fragilis
  • Type II – Monomicrobial – MRSA (Methicillin-resistant Staphylococcus aureus)
• Patholgy:
  • Small innocuous wound – pain, erythema, increased temperature, like cellulitis – paresthesia, blistering – skin necrosis – septic shock, hypotension, fever, malaise – multiple organ failure and death
  • NB: Thrombosis of blood vessel in skin lead to skin necrosis
• Clinical:
  • High fever + toxicity
  • Severe wound pain
  • Crepitus
  • Spreading inflammation
  • Smell
• Management:
  • Debridement
  • Broad spectrum antibiotic
  • Allow granulation
  • Skin grafting
  • Address immunosuppressive disease
  • Good nutrition for wound healing

6. Alveolar osteitis (dry socket)

• Etiology:
  • Excessive trauma during extraction
  • Misuse of LA
  • Limited local blood supply
  • Oral contraceptives
  • Radiotherapy
  • Dislodged clot from socket by patient (by finger, gargling, spitting)
  • Osteosclerotic disease
• Clinical:
  • Pain a few days after extraction – deep seated, severe, aching, throbbing
  • Mucosa red and tender
  • Clot absent – replaced with saliva and food debris
  • White dead bone
• Pathology:
  • Destruction of blood clot in socket – open socket – food and debris in direct contact with bone – dense bone dies
  • Necrotic bone lodges bacteria – proliferate
  • Inflammation occurs to localize infection in socket wall
  • Osteoclasts seperate dead bone
  • Healing by granulation tissue
• Management:
  • Anaesthetize patient
  • Irrigate the socket: Normal saline, chlorhexidine
  • Suction
  • Curettage of necrotic white bone till it bleeds
  • Place alvogyl dressing in the socket

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Infection pathways