Communicable Diseases Overview

Definition: Communicable diseases are illnesses resulting from the infection and growth of pathogenic biologic agents in human or animal hosts.
Routes of Transmission:
- Aerosol: Inhaled droplets containing pathogens.
- Oral: Ingestion of contaminated food, water, or objects.
- Direct Contact: Contact with wounds, mucus, or reproductive fluids; includes bites and scratches.
- Fomite: Contact with contaminated objects or surfaces.
- Vector-borne: Transfer via insects such as fleas, ticks, or mosquitoes.
- Zoonoses: Diseases spread between animals and humans.
Control Principles:
1. Attacking the Source: Includes case treatment, contact tracing, isolation, surveillance, and reservoir control.
2. Interrupting Transmission: Focused on environmental hygiene, vector control, disinfection, and sterilization.
3. Protecting Susceptible Hosts: Utilizes immunization, chemoprophylaxis, personal protection, and improved nutrition.

Cholera: Caused by the bean-shaped bacterium $Vibrio\,cholerae$ .
- Common in areas with inadequate clean water (e.g., peri-urban slums).
- Clinical Presentation: Severe watery diarrhea and vomiting leading to rapid dehydration ( $1$ in $20$ cases).
- Management: Oral or intravenous hydration is primary; doxycycline is the first-line antibiotic.
Typhoid and Paratyphoid Fever: Caused by $Salmonella\,typhi$ or $Salmonella\,paratyphi$ .
- Presentation: Insidious onset with fever peaking at $102^{\circ}F-104^{\circ}F$ ( $38^{\circ}C-40^{\circ}C$ ), rose-colored spots, and hepatosplenomegaly.
- Management: Fluoroquinolones (ciprofloxacin) are the treatment of choice.
Bacillary Dysentery (Shigellosis): Caused by $Shigella$ species ( $S.\,sonnei$ , $S.\,flexneri$ , $S.\,dysenteriae$ ).
- Presentation: Acute bloody diarrhea, tenesmus, and abdominal pain.
- Management: Hydration and antibiotics; antimotility agents must be avoided to prevent toxic dilatation of the colon.
Giardiasis: Caused by the flagellate protozoan $Giardia\,intestinalis$ .
- Pathophysiology: Infection occurs upon ingestion of as few as $10$ cysts. Trophozoites adhere to the intestinal epithelium.
- Management: Metronidazole or tinidazole (single dose, $90\%$ efficacy).

Avian Flu (H1N1): Specifically swine-origin influenza H1N1 (2009 pandemic).
- Pathophysiology: Prefers the lower respiratory tract; deaths are primarily due to viral pneumonia.
- Management: Antiviral medication and supportive care (oxygen, fluids).
Leprosy: A chronic infection caused by the acid-fast bacillus $Mycobacterium\,leprae$ .
- Pathophysiology: Targets Schwann cells ( $SCs$ ), leading to demyelination and peripheral neuropathy.
- Signs: Painless skin patches, muscle wasting, clawed hands (ulnar/median nerve), and foot drop (common peroneal nerve).
- Management: WHO multidrug regimen including rifampicin, dapsone, and clofazimine.

Causative Agent: Neurotropic virus of the genus $Lyssavirus$ .
Pathophysiology: Spreads via nicotinic acetylcholine receptors to the CNS. Once in the CNS, it spreads via autonomic pathways to viscera.
Clinical Phases:
- Incubation: $20-90\,days$ (asymptomatic).
- Prodromal: $2-10\,days$ ; involves malaise and pathognomonic itching at the bite site ( $30\%$ of canine cases, $70\%$ of bat cases).
- Acute Neurologic: Duration of $2-7\,days$ ; "furious rabies" features agitation and hyperactivity.
- Coma: Respiratory arrest and death occur shortly after onset.
Management: Immediate wound flushing with soap and water for at least $10\,min$ . Active pre-exposure immunization is recommended for high-risk workers.