Zoonotic Infections

Zoonoses: Overview - Critter Culprits

Zoonoses: Infections naturally transmitted from vertebrate animals to humans. Modes include direct/indirect contact, foodborne, vector-borne, or airborne.

• Major Animal Groups & Associated Diseases:
  • Mammals:
    • Dogs: Rabies, Leishmaniasis, Echinococcosis
    • Cats: Toxoplasmosis, Cat Scratch Disease
    • Rodents: Plague, Leptospirosis, Hantavirus
    • Livestock (cattle, pigs, sheep): Brucellosis, Anthrax, Cysticercosis/Taeniasis, Q Fever
    • Bats: Rabies, Nipah, Coronaviruses (e.g., SARS, MERS)
  • Birds: Avian Influenza, Psittacosis
  • Arthropods (as vectors, transmitting from animal reservoirs):
    • Ticks: Kyasanur Forest Disease (KFD), Lyme Disease
    • Mosquitoes: Japanese Encephalitis (pigs/birds are reservoirs)

⭐ India is a global hotspot for rabies, primarily transmitted by dog bites (>95% of human cases).

Bacterial Zoonoses - Fever & Frights

• Plague (Yersinia pestis): Rodents, fleas.
  • Forms: Bubonic (painful buboes), Septicemic, Pneumonic (highly contagious).
  • Microscopy: Bipolar "safety pin" stain.
• Brucellosis (Brucella spp.): Cattle, goats, sheep, pigs.
  • Transmission: Unpasteurized dairy, contact.
  • Symptoms: Undulant fever, arthralgia, hepatosplenomegaly.
• Anthrax (Bacillus anthracis): Herbivores (spores in soil).
  • Forms: Cutaneous (painless black eschar), Inhalational (widened mediastinum), GI.
• Leptospirosis (Leptospira interrogans): Rodents (urine).
  • Transmission: Contaminated water/soil.
  • Weil’s Disease: Jaundice, renal failure, hemorrhage.

⭐ Widened mediastinum on chest X-ray is a hallmark of inhalational anthrax.

Viral Zoonoses - Neuro & Hemorrhage

• Rabies (Neurotropic)
  • Lyssavirus (RNA). Transmission: animal bite (e.g., dog).
  • Pathogenesis: Retrograde axonal transport to CNS → fatal encephalitis.
  • Key features: Hydrophobia, aerophobia. Diagnosis: Negri bodies (intracytoplasmic inclusions).
  • Post-Exposure Prophylaxis (PEP): Crucial; wound care, vaccine, Rabies Immunoglobulin (RIG).
• Kyasanur Forest Disease (KFD) (Hemorrhagic/Neuro)
  • Flavivirus. "Monkey Fever". Vector: Haemaphysalis tick.
  • Endemic: Karnataka (Western Ghats). Reservoir: monkeys, rodents.
  • Clinical: Biphasic illness - initial high fever, headache, myalgia → can progress to hemorrhagic manifestations (epistaxis, GI bleed) & neurological (meningoencephalitis).
  • Vaccine available (inactivated, for endemic areas).

⭐ > Rabies: Intracytoplasmic eosinophilic Negri bodies in neurons (especially hippocampus pyramidal cells, cerebellar Purkinje cells) are pathognomonic, though not found in all cases (~70-80%).

Parasitic & Rickettsial Zoonoses - Insidious Invaders

• Parasitic Zoonoses:
  • Toxoplasmosis (Toxoplasma gondii): Cats, undercooked meat. Risk in pregnancy; chorioretinitis, hydrocephalus.
  • Cysticercosis (Taenia solium): Pork tapeworm (fecal-oral). Neurocysticercosis (seizures, hydrocephalus).
  • Hydatid Disease (Echinococcus granulosus): Dog tapeworm (fecal-oral). Liver/lung cysts; anaphylaxis on rupture.
  • Leishmaniasis (Leishmania spp.): Sandfly bite. Visceral (Kala-azar: fever, hepatosplenomegaly, pancytopenia), Cutaneous (ulcers).
  • Strongyloidiasis (Strongyloides stercoralis): Soil (larval penetration). Hyperinfection in immunocompromised.
• Rickettsial Zoonoses: (Fever, rash, headache common)
  • Scrub Typhus (Orientia tsutsugamushi): Chigger mite. Eschar common. IgM ELISA diagnostic.
  • Indian Tick Typhus (Rickettsia conorii): Tick bite. Eschar (tache noire), maculopapular rash.
  • Murine Typhus (Rickettsia typhi): Rat flea. Rash spares palms/soles.
  • Q Fever (Coxiella burnetii): Aerosol from livestock. Atypical pneumonia, hepatitis. No rash/vector usually.

⭐ Scrub typhus, caused by Orientia tsutsugamushi, is a major cause of acute febrile illness in India, often presenting with a characteristic eschar at the chigger bite site; doxycycline is the treatment of choice.

High‑Yield Points - ⚡ Biggest Takeaways

• Brucellosis: Undulant fever, arthralgia; unpasteurized milk; Rose Bengal test.
• Leptospirosis: Weil's disease (jaundice, renal failure); rodent urine exposure; MAT gold standard.
• Plague: Yersinia pestis; rat flea vector; bubonic (buboes), pneumonic forms.
• Anthrax: Bacillus anthracis; malignant pustule (cutaneous), wool-sorter's disease (inhalational).
• Rabies: Viral encephalitis; Negri bodies pathognomonic; crucial post-exposure prophylaxis.
• KFD: Tick-borne flavivirus; hemorrhagic fever; Karnataka focus.
• Scrub Typhus: Orientia tsutsugamushi; eschar at chigger bite; Weil-Felix OX-K positive_markdown_