Zoonoses — Rabies, Plague & Brucellosis
Community Medicine · Communicable Disease · lean revision notes
Zoonoses — Rabies, Plague & Brucellosis
Zoonoses are infections naturally transmitted between vertebrate animals and humans. For NEET PG, three are repeat favourites: rabies (the post-exposure prophylaxis algorithm is the single most-tested item in Community Medicine epidemiology), plague (the rat–flea–man cycle and Xenopsylla cheopis), and brucellosis (occupational, milk-borne, the great "undulant fever" mimic). This note builds each disease around reservoir → transmission → control, then drills the high-yield numbers.
Definitions & classification of zoonoses
A zoonosis (plural zoonoses) is "any disease or infection naturally transmissible from vertebrate animals to man" (WHO/FAO). Classification you can be asked to apply:
| Basis | Type | Examples |
|---|---|---|
| Direction | Anthropozoonosis (animal → man) | Rabies, plague, brucellosis |
| Zooanthroponosis (man → animal) | Human TB to cattle, amoebiasis | |
| Amphixenosis (both ways) | Staph aureus, Strep spp. | |
| Reservoir/cycle | Direct zoonosis (single host, direct contact) | Rabies, brucellosis |
| Cyclozoonosis (≥2 vertebrate hosts) | Echinococcosis, taeniasis | |
| Metazoonosis (vertebrate + invertebrate vector) | Plague, KFD, Japanese encephalitis | |
| Saprozoonosis (needs a non-animal site: soil/plants) | Histoplasmosis, larva migrans |
High-yield: Plague is the classic metazoonosis (needs the flea, an invertebrate). Rabies and brucellosis are direct zoonoses.
The unifying control principle for all three is reservoir + transmission interruption + exposed-host protection — and for rabies/brucellosis a strong One Health emphasis (control disease in the animal to protect humans).
RABIES
Etiology & pathophysiology
- Cause: Rabies virus, genus Lyssavirus, family Rhabdoviridae — a bullet-shaped, enveloped, single-stranded negative-sense RNA virus.
- Reservoir in India: the dog is responsible for ~96–97% of human rabies. Wildlife reservoirs elsewhere include foxes, raccoons, skunks; bats carry rabies-related lyssaviruses.
- Transmission: bite/scratch/lick on broken skin or mucosa by a rabid animal; virus is in saliva. Rare routes: aerosol (bat caves), corneal/organ transplantation, transplacental.
- Pathogenesis flow: Inoculation → local replication in muscle → entry at neuromuscular junction (binds nicotinic ACh receptor) → centripetal axonal travel to CNS → encephalitis → centrifugal spread to salivary glands, cornea, skin.
- Incubation: typically 20–90 days (range 4 days to >1 year). Shorter with bites on the face/head, highly innervated areas, multiple/deep bites, and in children (proximity to brain + rich nerve supply).
- Pathology: Negri bodies — eosinophilic cytoplasmic inclusions, classically in hippocampal pyramidal cells and cerebellar Purkinje cells.
High-yield: Negri bodies are pathognomonic but seen in only ~70–75% of cases; their absence does not exclude rabies.
Clinical features
- Prodrome: fever, malaise, and paraesthesia/pain/itching at the bite site (a near-diagnostic early clue).
- Furious rabies (~80%): hydrophobia, aerophobia, hyperexcitability, autonomic instability, lucid intervals.
- Paralytic ("dumb") rabies (~20%): ascending flaccid paralysis, mimics Guillain–Barré.
- Once symptomatic, rabies is almost 100% fatal. This is why prophylaxis is everything.
Diagnosis
- Ante-mortem: RT-PCR of saliva/CSF, direct fluorescent antibody (DFA) on skin biopsy from nape of neck (hair follicle nerves), corneal impression smear, serum/CSF antibodies (if unvaccinated).
- Post-mortem (gold standard): DFA on brain tissue (hippocampus, brainstem, cerebellum). Negri bodies on histology supportive.
Management — Post-Exposure Prophylaxis (PEP)
PEP has three pillars: wound management → vaccine → Rabies Immunoglobulin (RIG) for Category III.
Step 1 — Wound management (do this first, every category):
- Wash with soap and running water for ≥15 minutes, then apply a virucidal agent (povidone-iodine / 70% alcohol).
- Do NOT suture immediately; if suturing is unavoidable, infiltrate RIG first and delay/loose sutures.
- Avoid irritants (chillies, oil, dressing). Give tetanus prophylaxis and antibiotics as indicated.
Step 2 — WHO categorisation & action:
| Category | Type of contact | Action |
|---|---|---|
| I | Touching/feeding animals; licks on intact skin | No PEP — wash if contact; reassure |
| II | Nibbling of uncovered skin; minor scratches/abrasions without bleeding | Wound care + vaccine |
| III | Single/multiple transdermal bites or scratches, licks on broken skin, mucosal contact with saliva, all bat exposures | Wound care + vaccine + RIG |
High-yield: RIG is given ONLY in Category III (and in immunocompromised Category II). All exposures to bats are treated as Category III.
Step 3 — Vaccine schedules (modern cell-culture vaccines: HDCV, PVRV, PCECV):
Intramuscular (Essen regimen): 1 dose on days 0, 3, 7, 14, 28 (deltoid in adults, anterolateral thigh in infants — never gluteal, poor absorption). The updated WHO/IM "4-dose Zagreb-type" abbreviated regimens exist, but Essen (5-dose) remains the standard answer.
Intradermal (Updated Thai Red Cross, ID): 0.1 mL at 2 sites on days 0, 3, 7, and 28 (the "2-2-2-0-2" → now WHO 2024 promotes shorter 1-week ID regimens). For exams: TRC = 2-2-2-0-2 on days 0,3,7,28.
Re-exposure in a previously fully vaccinated person: only 2 booster doses (day 0 and 3); NO RIG.
Rabies Immunoglobulin (RIG):
| Type | Dose | Note |
|---|---|---|
| Human RIG (HRIG) | 20 IU/kg | No skin test; preferred |
| Equine RIG (ERIG) | 40 IU/kg | Skin test advised; cheaper |
- Infiltrate as much as possible into and around the wound; any remaining volume given IM at a site distant from the vaccine.
- Give with or within 7 days of the first vaccine dose (after day 7, endogenous antibody is forming and RIG may blunt it).
High-yield mnemonic — RIG dose: "Human 20, Equine 40." And "Wash 15, then vaccinate."
Pre-Exposure Prophylaxis (PrEP)
For high-risk groups: 3 doses on days 0, 7, and 21 or 28 (WHO 2018 onward allows a 2-dose, 1-week ID/IM regimen). Indicated for veterinarians, lab staff, animal handlers, dog catchers, and travellers to endemic areas.
Observation of the biting animal
- Healthy dog/cat that can be observed: "10-day observation rule" — if the animal remains healthy for 10 days, it was not infectious at the time of bite. Start PEP immediately and stop if the animal stays well for 10 days.
PLAGUE
Etiology & the vector cycle
- Cause: Yersinia pestis — a Gram-negative coccobacillus with bipolar ("safety-pin") staining (Wayson/Giemsa).
- Reservoir: wild and domestic rodents (rats). In India the classic urban cycle involves Rattus rattus and R. norvegicus.
- Vector: the rat flea — Xenopsylla cheopis (the principal Oriental rat flea).
- Classic transmission cycle:
Infected rat → rat flea (Xenopsylla cheopis) bites → flea gut blocked by clotted blood + bacteria ("blocked flea") → flea regurgitates Y. pestis into next host → man.
- "Blocking" is caused by the coagulase of Y. pestis; a blocked, starving flea bites repeatedly → efficient transmission.
- Pneumonic plague spreads man-to-man by respiratory droplets (no vector needed) — the only directly contagious form.
High-yield: Vector = Xenopsylla cheopis. The "rat falls, flea jumps to man" phenomenon after a rat epizootic (rat fall) precedes human outbreaks. Cheopis index >1 signals epidemic risk.
Clinical forms
| Form | Route | Hallmark |
|---|---|---|
| Bubonic (most common) | Flea bite | Painful, matted bubo (usually inguinal); fever, prostration |
| Septicaemic | Spread from bubo / primary | Shock, DIC, acral gangrene → "Black Death" |
| Pneumonic | Inhalation / secondary | Cough, haemoptysis, fulminant pneumonia; highly contagious, near-100% fatal untreated |
Diagnosis
- Specimens: bubo aspirate, blood, sputum.
- Bipolar "safety-pin" bacilli on Giemsa/Wayson stain; culture on blood/MacConkey; F1 antigen detection; serology (passive haemagglutination, ≥4-fold rise).
Management — drug of choice
- Streptomycin is the classic drug of choice; Gentamicin is an equally effective alternative.
- Doxycycline or fluoroquinolones (ciprofloxacin) for treatment and post-exposure chemoprophylaxis of contacts.
- Notifiable disease under International Health Regulations (IHR) — internationally quarantinable.
Prevention & control
- Rodent + flea control sequence is order-sensitive: first kill the fleas (insecticides), THEN the rats. Killing rats first drives hungry fleas onto humans.
- Surveillance via cheopis flea index; ratproofing; doxycycline chemoprophylaxis of contacts; isolation of pneumonic cases; a vaccine exists (limited use, high-risk groups only).
BRUCELLOSIS
Etiology & transmission
- Cause: Brucella species — small Gram-negative, aerobic, facultative intracellular coccobacilli.
| Species | Animal reservoir | Virulence in man |
|---|---|---|
| B. melitensis | Goats, sheep | Most virulent / most common worldwide |
| B. abortus | Cattle | Milder |
| B. suis | Pigs | Suppurative, chronic |
| B. canis | Dogs | Mild |
- Transmission routes:
- Ingestion of unpasteurised milk / dairy (soft cheese) — the main route for the general public.
- Direct contact with infected tissues, placenta, aborted fetuses (occupational).
- Inhalation of aerosols (abattoirs, labs — a major laboratory-acquired infection).
- Occupational disease of farmers, shepherds, abattoir/slaughterhouse workers, veterinarians, and lab personnel.
High-yield: B. melitensis (goat/sheep) is the most pathogenic to humans; unpasteurised milk is the classic public-health source; pasteurisation is the key preventive step.
Clinical features
- Undulant fever (rising and falling temperature), drenching night sweats with a peculiar mouldy/hay odour, malaise, arthralgia, hepatosplenomegaly.
- Focal complications: sacroiliitis/spondylitis (most common osteoarticular site), orchitis/epididymitis, endocarditis (leading cause of death), neurobrucellosis.
Diagnosis — investigation of choice
- Blood culture (Castaneda biphasic medium; prolonged incubation) is the definitive/confirmatory test — gold standard but slow and low-yield in chronic cases.
- Serology — Standard Agglutination Test (SAT): titre ≥1:160 (with compatible illness) is significant; the most commonly used screening test.
- Beware the prozone phenomenon (false-negative SAT at low dilutions due to antibody excess).
- Bone marrow culture is more sensitive than blood culture in chronic/treated cases.
Management — drug of choice
- WHO regimen: Doxycycline (6 weeks) + Streptomycin (2–3 weeks) — most effective for relapse prevention.
- Alternative: Doxycycline + Rifampicin (both 6 weeks) (preferred in pregnancy as Rifampicin + co-trimoxazole, avoiding doxycycline/strepto).
- Single-drug therapy → high relapse, so always dual/triple therapy.
High-yield mnemonic — brucellosis treatment: "Doxy + Strepto, don't go solo."
Prevention
- Pasteurisation of milk; animal vaccination (S19 for cattle, Rev-1 for sheep/goats); test-and-slaughter of infected herds; protective gear for occupational groups. No licensed human vaccine in routine use.
Occupational & exposure risk groups (commonly tested)
| Zoonosis | Highest-risk groups | Key prevention |
|---|---|---|
| Rabies | Veterinarians, dog catchers, lab staff, animal handlers | PrEP + prompt PEP + dog vaccination |
| Plague | People in rodent-infested areas, field workers, lab staff | Flea-then-rat control; doxycycline prophylaxis |
| Brucellosis | Shepherds, abattoir workers, vets, dairy & lab workers | Milk pasteurisation; PPE; animal vaccination |
Key differentials
- Rabies (paralytic) vs Guillain–Barré syndrome: fever, bite history, autonomic storm, and sphincter involvement favour rabies.
- Rabies vs tetanus: tetanus has trismus + sustained rigidity between spasms; rabies has hydrophobia and lucid intervals.
- Bubonic plague vs other causes of regional lymphadenopathy: tularemia, cat-scratch disease, lymphogranuloma venereum, suppurative adenitis.
- Brucellosis vs typhoid, TB, lymphoma, infective endocarditis: all cause PUO with hepatosplenomegaly — culture + serology + travel/occupational/milk history distinguish.
Recently asked / exam angle
- PEP categorisation is the perennial favourite: "Patient with lick on broken skin / mucosal contact / bat exposure → Category III → vaccine + RIG." A scenario of a lick on intact skin = Category I = no PEP.
- RIG dose numbers: HRIG 20 IU/kg, ERIG 40 IU/kg; infiltrate into the wound; give within 7 days of the first vaccine dose.
- Wound wash time = ≥15 minutes with soap and water; no immediate suturing.
- Vector of plague = Xenopsylla cheopis; safety-pin/bipolar staining of Y. pestis; drug of choice = streptomycin.
- Control sequence in plague = fleas first, then rats (single-best-answer trap).
- Brucellosis: most virulent species = B. melitensis; source = unpasteurised milk; treatment = doxycycline + streptomycin/rifampicin; investigation of choice = blood culture (confirmatory), SAT ≥1:160 (screening).
- Negri bodies site (hippocampus / Purkinje cells); rabies virus = bullet-shaped Rhabdovirus.
- Classification one-liners: plague = metazoonosis; rabies & brucellosis = direct zoonoses.
Rapid revision
- Rabies virus = bullet-shaped, negative-sense RNA Rhabdovirus; Negri bodies in hippocampus/Purkinje cells.
- Wash bite for ≥15 min with soap + water + virucidal; never gluteal, never immediate sutures.
- WHO PEP: Cat I → none; Cat II → vaccine; Cat III → vaccine + RIG; all bat exposures = Cat III.
- HRIG 20 IU/kg, ERIG 40 IU/kg, infiltrate the wound, give within 7 days of first dose.
- Essen IM schedule = days 0, 3, 7, 14, 28; re-exposure (vaccinated) = only days 0 & 3, no RIG.
- "10-day observation rule" for a healthy biting dog/cat.
- Plague vector = Xenopsylla cheopis; cycle = rat → blocked flea → man; pneumonic spreads man-to-man.
- Y. pestis = bipolar "safety-pin" Gram-negative; DOC = streptomycin (or gentamicin); IHR-notifiable.
- Plague control: kill fleas first, then rats; monitor the cheopis flea index.
- Brucellosis: B. melitensis most virulent; unpasteurised milk + occupational contact; undulant fever, sacroiliitis, endocarditis.
- Brucellosis Ix: blood culture (Castaneda, confirmatory); SAT ≥1:160 screening; beware prozone.
- Brucellosis Rx: doxycycline + streptomycin (or + rifampicin); pasteurisation is the key prevention.