Zoonotic Infections

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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)

Zoonotic disease transmission cycle diagram

⭐ 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. Cutaneous anthrax eschar and Bacillus anthracis

⭐ 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).

Negri bodies and rabies virions

⭐ > 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.

Zoonotic infection transmission pathways

⭐ 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_

Practice Questions: Zoonotic Infections

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Which part of males are resistant to gonococcal infection?_____

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Which part of males are resistant to gonococcal infection?_____

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