Microbiology Cheat Sheet
Zoonoses: Exposure Is the Clue
Zoonoses for NEET-PG and FMGE: anthrax eschar, brucellosis undulant fever, leptospirosis Weil's disease, plague buboes, scrub typhus, Q fever, and dengue NS1.
MedNext Academy | 3 min read
Zoonoses: Exposure Is the Clue
Zoonoses for NEET-PG and FMGE: anthrax eschar, brucellosis undulant fever, leptospirosis Weil's disease, plague buboes, scrub typhus, Q fever, and dengue NS1.
This topic covers the microbial agents, vectors, transmission, and laboratory diagnosis of zoonotic diseases, where exposure history is the key diagnostic clue.
High-yield lines
- Exposure history, including animal contact, occupation, raw milk, and water exposure, is the single most important diagnostic clue in zoonoses.
- Anthrax (Bacillus anthracis) has a unique poly-D-glutamic acid polypeptide capsule and a tripartite toxin of protective antigen, lethal factor, and oedema factor.
- Cutaneous anthrax causes a painless black eschar, while inhalational anthrax causes a widened mediastinum; the organism is non-motile and non-haemolytic.
- Brucellosis presents with undulant fever and is a facultative intracellular pathogen diagnosed by blood or bone marrow culture and the standard agglutination test.
- Leptospirosis is the most common zoonosis worldwide, presenting with conjunctival suffusion and calf myalgia, with Weil's disease showing jaundice, renal failure, and haemorrhage.
- The microscopic agglutination test is the serological gold standard for leptospirosis, while IgM ELISA and PCR allow earlier diagnosis.
- Plague (Yersinia pestis) shows safety-pin bipolar staining, causes buboes, and pneumonic plague is the only person-to-person transmissible form.
- Xenopsylla cheopis (the oriental rat flea) is the principal vector of plague, and the disease is a Category A bioterrorism agent requiring BSL-3 handling.
- Scrub typhus (Orientia tsutsugamushi) is the most common rickettsiosis in India, transmitted by chigger mites, presenting with an eschar and lymphadenopathy.
- In the Weil-Felix test, scrub typhus agglutinates only Proteus OX-K, while Q fever and rickettsialpox are Weil-Felix negative.
- Doxycycline is the drug of choice for all rickettsial infections, including in children for short courses.
- Q fever (Coxiella burnetii) is transmitted by inhalation without an arthropod vector, causes no rash, and produces doughnut granulomas in the liver.
- Dengue NS1 antigen is the earliest diagnostic marker (days 1 to 7), and antibody-dependent enhancement explains severe secondary heterotypic infection.
- Brucella, Francisella, and Yersinia pestis are the three classic BSL-3 bacterial zoonoses causing the highest rates of laboratory-acquired infection.
Mapped competency codes
- MI8.1
Continue into the full chapter
This summary maps to MI8-zoonotic-diseases-and-miscellaneous.
Frequently Asked Questions
What is the most important diagnostic clue in zoonotic infections?
The exposure history, including animal contact, occupation, insect bites, raw milk, and recreational water exposure.
Which form of plague is transmissible person-to-person?
Pneumonic plague, spread by respiratory droplets, requiring droplet isolation and post-exposure chemoprophylaxis for contacts.
What is the classic clinical presentation of leptospirosis?
Conjunctival suffusion with severe calf myalgia, high fever, and water or flood exposure, with Weil's disease adding jaundice, renal failure, and haemorrhage.
Which rickettsiosis is Weil-Felix OX-K positive?
Scrub typhus caused by Orientia tsutsugamushi, the most common rickettsiosis in India, transmitted by chigger mites.
Continue reading
Cheat SheetsAll Microbiology cheat sheets
Browse the complete authored set for this subject.
Open the complete revision pathway
Explore clinician-written learning resources, structured revision and practice across the MedNext platform.
Open in the MedNext appSee plans

