Zoonotic Diseases Practice Questions

Q: Rickettsia tsutsugamushi is a small gram-negative, obligate intracellular organism that causes which of the following diseases?

Scrub typhus. **Explanation:** **Orientia tsutsugamushi** (formerly *Rickettsia tsutsugamushi*) is the causative agent of **Scrub Typhus**. It is a small, Gram-negative, obligate intracellular bacterium. Unlike other Rickettsiae, it lacks a typical peptidoglycan layer and lipopolysaccharide, which is a high-yield distinction. 1. **Why Scrub Typhus is correct:** The disease is transmitted by the bite of the larval stage (chigger) of **Leptotrombidium mites**. A characteristic clinical feature is the **eschar**—a painless, black, crusty lesion at the site of the mite bite—followed by fever, lymphadenopathy, and a maculopapular rash. 2. **Why other options are incorrect:** * **Epidemic Typhus:** Caused by *Rickettsia prowazekii* and transmitted by the **human body louse**. It is associated with crowded, unsanitary conditions. * **Endemic (Murine) Typhus:** Caused by *Rickettsia typhi* and transmitted by **rat fleas** (*Xenopsylla cheopis*). * **Q Fever:** Caused by *Coxiella burnetii*. Unlike Rickettsiae, it does not cause a rash, is transmitted via inhalation of contaminated aerosols (not an arthropod vector), and is **Weil-Felix negative**. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** Larval mites (Chiggers). * **Reservoir:** Mites (via transovarial transmission). * **Diagnosis:** The **Weil-Felix test** shows agglutination with **OX-K** strains (Proteus antigen), while OX-19 and OX-2 are negative. * **Drug of Choice:** Doxycycline is the gold standard treatment for all Rickettsial diseases. * **Geography:** Part of the "Tsutsugamushi Triangle" (includes India).

Q: What is the specific test for the diagnosis of rickettsial infection?

Agglutination of rickettsial suspension. ### Explanation **1. Why "Agglutination of rickettsial suspension" is correct:** The diagnosis of Rickettsial infections can be categorized into non-specific and specific tests. The **Agglutination of rickettsial suspension** (Microagglutination test) is a **specific serological test** because it uses the actual Rickettsial antigens to detect homologous antibodies in the patient's serum. It is highly specific and can differentiate between various Rickettsial species (e.g., *R. prowazekii* vs. *R. typhi*), unlike heterophile tests. **2. Why the other options are incorrect:** * **A. Weil-Felix reaction:** While commonly used, this is a **non-specific heterophile agglutination test**. It relies on the cross-reactivity between Rickettsial antibodies and the alkali-stable polysaccharide antigens of certain *Proteus* strains (OX-19, OX-2, and OX-K). It is often negative in Q fever and Rickettsialpox. * **C. Paul-Bunnell test:** This is a heterophile antibody test used specifically for the diagnosis of **Infectious Mononucleosis** (Epstein-Barr Virus), where patient antibodies agglutinate sheep red blood cells. * **D. Rose-Waaler test:** This is an agglutination test used to detect **Rheumatoid Factor** (IgM against the Fc portion of IgG) in Rheumatoid Arthritis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** The Indirect Fluorescent Antibody (IFA) test is currently considered the gold standard for Rickettsial diagnosis. * **Weil-Felix Patterns:** * **Epidemic/Endemic Typhus:** OX-19 (+++) * **Scrub Typhus:** OX-K (+++) * **Rocky Mountain Spotted Fever:** OX-19 (+) and OX-2 (+) * **Culture:** Rickettsiae are obligate intracellular pathogens and are typically grown in the yolk sac of embryonated eggs or cell cultures (not routine). * **Drug of Choice:** Doxycycline is the treatment of choice for almost all Rickettsial infections.

Q: A 20-year-old male presented with left axillary pain, fever, malaise, and myalgias. On examination, there is a tender, boggy swelling in the left axilla with a hard core. A characteristic lesion was observed on the skin of the upper abdominal quadrant. Gram staining of the aspirate was performed. What is the drug of choice in the above condition?

Streptomycin. ***Streptomycin*** - The clinical presentation of **bubo** (tender, boggy swelling) in the axilla with fever and characteristic skin lesion suggests **bubonic plague** caused by *Yersinia pestis*. - **Streptomycin** is the **drug of choice** for plague, as it effectively penetrates tissues and rapidly kills the gram-negative coccobacilli with bipolar staining. *Tetracycline* - While **tetracycline** has activity against *Y. pestis*, it is **bacteriostatic** rather than bactericidal like streptomycin. - It is considered an **alternative therapy** but not first-line due to slower clinical response in severe infections. *Chloramphenicol* - **Chloramphenicol** is effective against *Y. pestis* and used as an alternative, especially in **plague meningitis**. - However, it is not the **first-line choice** due to potential **bone marrow toxicity** and slower onset of action compared to streptomycin. *Doxycycline* - **Doxycycline** is a tetracycline derivative that can be used as **alternative therapy** for plague. - Like tetracycline, it is **bacteriostatic** and not preferred over the **bactericidal** action of streptomycin in acute bubonic plague.

Q: Which of the following infections is caused by Rickettsia?

Rocky mountain spotted fever. **Explanation:** The correct answer is **Rocky Mountain Spotted Fever (RMSF)**. This infection is caused by ***Rickettsia rickettsii***, a member of the spotted fever group of the genus *Rickettsia*. These are obligate intracellular, Gram-negative coccobacilli that primarily infect vascular endothelial cells, leading to characteristic clinical features like fever, headache, and a centripetal petechial rash (starting on wrists/ankles and spreading to the trunk). **Analysis of Options:** * **A. Weil’s Disease:** This is a severe form of Leptospirosis caused by the spirochete ***Leptospira interrogans***. It is characterized by the triad of jaundice, renal failure, and hemorrhage. * **C. Scrub Typhus:** While clinically similar to Rickettsial diseases, it is caused by ***Orientia tsutsugamushi*** (formerly *Rickettsia tsutsugamushi*). It is transmitted by larval mites (chiggers) and is distinguished by the presence of a necrotic **eschar** at the bite site. * **D. Lyme Disease:** This is caused by the spirochete ***Borrelia burgdorferi*** and is transmitted by *Ixodes* ticks. It typically presents with a "bull’s eye" rash (*Erythema migrans*). **High-Yield Clinical Pearls for NEET-PG:** 1. **Vector:** RMSF is transmitted by the **Dermacentor tick** (Dog tick/Wood tick). 2. **Diagnostic Test:** The **Weil-Felix reaction** is a heterophile agglutination test used for Rickettsial infections (though being replaced by IFA). In RMSF, OX-19 and OX-2 are positive. 3. **Drug of Choice:** **Doxycycline** is the gold standard treatment for all Rickettsial infections, including in children. 4. **Rash Pattern:** In Rickettsial infections (except Q fever), the rash is typically **centripetal** (extremities to trunk), whereas in Typhus, it is usually centrifugal (trunk to extremities).

Q: A veterinary doctor had pyrexia of unknown origin. Blood culture in special laboratory media was positive for Gram-negative, oxidase-positive short bacilli. Which one of the following is the likely organism grown in culture?

Brucella spp.. ### Explanation The clinical presentation of **Pyrexia of Unknown Origin (PUO)** in a high-risk professional (veterinary doctor) strongly suggests **Brucellosis**, a classic zoonotic infection. **1. Why Brucella spp. is correct:** * **Occupational Risk:** Veterinarians, farmers, and slaughterhouse workers are at high risk through contact with infected animals or unpasteurized dairy. * **Microbiology:** *Brucella* are small, Gram-negative coccobacilli (short bacilli) that are **oxidase-positive** and catalase-positive. * **Culture:** They are fastidious and require **special media** (e.g., Castaneda’s biphasic medium) for growth. They are notorious for causing "culture-negative" endocarditis if standard media are used or if incubation is too short. **2. Why the other options are incorrect:** * **Pasteurella spp.:** While Gram-negative and oxidase-positive, it is typically associated with **animal bites** (cats/dogs) causing cellulitis, not prolonged PUO. * **Francisella spp.:** Causes Tularemia. While it is a Gram-negative coccobacillus, it is typically **oxidase-negative** and requires cysteine-enriched media (BCYE agar). * **Bartonella spp.:** Causes Cat Scratch Disease or Trench Fever. It is a fastidious Gram-negative rod but is **oxidase-negative**. **3. High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Bone marrow culture (higher yield than blood culture). * **Standard Serology:** Standard Agglutination Test (SAT); titers >1:160 are significant. * **Rose Bengal Plate Test:** Used as a rapid screening tool. * **Treatment:** WHO recommends **Rifampicin + Doxycycline** for 6 weeks. * **Complication:** Osteoarticular involvement (sacroiliitis) is the most common complication.

Q: Which of the following is transmitted by rat urine?

Leptospira. **Explanation:** **Leptospira interrogans** is the correct answer. It is a spirochete that causes **Leptospirosis**, a classic zoonotic disease. The bacteria colonize the renal tubules of reservoir animals, most commonly **rats**, and are excreted in their **urine**. Humans typically acquire the infection through direct contact with infected urine or indirectly via contaminated water or soil (e.g., during floods or farming) through skin abrasions or mucous membranes. **Why other options are incorrect:** * **Listeria monocytogenes:** Primarily transmitted through the ingestion of contaminated **food** (unpasteurized milk, soft cheeses, deli meats). It is known for its ability to grow at refrigeration temperatures. * **Legionella pneumophila:** Transmitted via the **inhalation of aerosols** from contaminated water sources like air conditioning cooling towers, showers, or humidifiers. There is no person-to-person or animal-to-human transmission. * **Mycoplasma pneumoniae:** Transmitted via **respiratory droplets** from person to person. It is a common cause of "walking pneumonia." **High-Yield Clinical Pearls for NEET-PG:** * **Weil’s Disease:** The severe form of Leptospirosis characterized by the triad of **jaundice, renal failure, and hemorrhage**. * **Occupational Hazard:** Common in sewage workers, rice field farmers, and veterinarians. * **Diagnosis:** Gold standard is the **Microscopic Agglutination Test (MAT)**. Culture requires specialized media like **EMJH** or **Fletcher’s medium**. * **Biphasic illness:** Starts with a septicemic phase (fever, conjunctival suffusion) followed by an immune phase (meningitis, uveitis).

Q: Which vector transmits diseases caused by both Rickettsia and Orientia?

Trombiculid mite. **Explanation:** The correct answer is **Trombiculid mite** (specifically the larval stage, known as the **chigger**). **Why it is correct:** The family *Rickettsiaceae* is divided into two main genera: *Rickettsia* and *Orientia*. While most *Rickettsia* species are transmitted by ticks, fleas, or lice, **Scrub Typhus** (caused by ***Orientia tsutsugamushi***) is uniquely transmitted by the bite of the larval Trombiculid mite. Additionally, certain species of the genus *Rickettsia*, such as ***Rickettsia akari*** (the causative agent of Rickettsialpox), are also transmitted by mites (specifically the house mouse mite). Therefore, the mite is the common vector for both genera. **Why other options are incorrect:** * **Rat flea (*Xenopsylla cheopis*):** Transmits *Rickettsia typhi* (Endemic/Murine Typhus) and *Yersinia pestis* (Plague), but does not transmit *Orientia*. * **Tick:** Transmits the Spotted Fever Group (e.g., *R. rickettsii* - RMSF, *R. conorii* - Indian Tick Typhus), but not *Orientia*. * **Louse (*Pediculus humanus corporis*):** Transmits *Rickettsia prowazekii* (Epidemic Typhus), but not *Orientia*. **High-Yield Clinical Pearls for NEET-PG:** * **Scrub Typhus Triad:** Fever, headache, and a characteristic **Eschar** (a black, necrotic scab at the site of the mite bite). * **Weil-Felix Test:** Scrub Typhus shows a positive reaction with **OX-K** (Proteus antigen), while the Typhus and Spotted Fever groups react with OX-19 and OX-2. * **Drug of Choice:** Doxycycline is the gold standard treatment for all rickettsial and oriential infections. * **Transovarial Transmission:** In mites, the pathogen is passed from the adult to the egg, making the mite both a vector and a reservoir.

Q: Scrub typhus is caused by which organism?

Rickettsia tsutsugamushi. **Explanation:** Scrub typhus is a zoonotic rickettsial infection caused by **Orientia tsutsugamushi** (formerly known as **Rickettsia tsutsugamushi**). It is transmitted to humans through the bite of infected larval mites (chiggers) of the genus *Leptotrombidium*. **Analysis of Options:** * **Option A (Correct):** *Rickettsia tsutsugamushi* is the causative agent. It is an obligate intracellular bacterium. The hallmark clinical sign is an **eschar** (a painless, black crusty sore) at the site of the mite bite. * **Option B (Incorrect):** *Rickettsia typhi* causes **Endemic (Murine) Typhus**, which is transmitted by the rat flea (*Xenopsylla cheopis*). * **Option C (Incorrect):** *Rickettsia akari* causes **Rickettsialpox**, transmitted by the house mouse mite (*Liponyssoides sanguineus*). * **Option D (Incorrect):** *Rickettsia conorii* causes **Indian Tick Typhus** (Boutonneuse fever), transmitted by the dog tick (*Rhipicephalus sanguineus*). **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** Larval mite (Chigger). * **Reservoir:** Trombiculid mites (undergo transovarial transmission). * **Diagnosis:** The **Weil-Felix test** shows agglutination with **OX-K** strain (negative for OX-19 and OX-2). However, the gold standard is the Indirect Immunofluorescence Assay (IFA). * **Drug of Choice:** **Doxycycline** is the first-line treatment for all age groups. Azithromycin is an alternative, especially in pregnancy. * **Geography:** Part of the "Tsutsugamushi Triangle" (extending from Japan to Australia to India/Pakistan). In India, it is a significant cause of "Acute Undifferentiated Febrile Illness."

Q: A patient presented with pyrexia of unknown origin. Blood culture in special laboratory media was positive for gram-negative short bacilli that were oxidase-positive. Which one of the following is the likely organism grown in culture?

Brucella spp.. ### Explanation **Correct Answer: D. Brucella spp.** **Why Brucella is the correct answer:** Brucellosis is a classic cause of **Pyrexia of Unknown Origin (PUO)**, often presenting as "undulant fever." The organism is a small, **Gram-negative coccobacillus** (short bacilli) that is **oxidase-positive** and catalase-positive. A key diagnostic feature is its growth characteristics: it is a fastidious aerobe that requires **special laboratory media** (e.g., Castaneda’s biphasic medium) and often takes prolonged incubation (up to 2-4 weeks) to grow. **Analysis of Incorrect Options:** * **A. Pasteurella spp.:** While these are Gram-negative, oxidase-positive bacilli, they are typically associated with **animal bites** (cats/dogs) and cause acute cellulitis or abscesses rather than chronic PUO. * **B. Francisella spp.:** *Francisella tularensis* (Tularemia) is a Gram-negative coccobacillus, but it is characteristically **oxidase-negative**. It also requires cysteine-enriched media (like BCYE) for growth. * **C. Bartonella spp.:** These are Gram-negative bacilli but are typically **oxidase-negative**. They cause Cat Scratch Disease or Bacillary Angiomatosis and are extremely difficult to culture on standard blood media. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Bone marrow culture (higher yield than blood culture). * **Culture Media:** **Castaneda’s Medium** is the traditional biphasic medium used to reduce the risk of laboratory-acquired infections. * **Serology:** The **Standard Agglutination Test (SAT)** detects antibodies; a titer >1:160 is significant. * **Treatment:** The WHO recommends **Rifampicin + Doxycycline** for 6 weeks. * **Occupational Hazard:** It is the most common laboratory-acquired infection; samples must be handled in Biosafety Level 3 (BSL-3) cabinets.

Question 1

Yersinia enterocolitica, formerly a Pasteurella, has more than 50 serotypes that can be isolated from rodents, sheep, cattle, swine, dogs, and cats, and from water contaminated by them. Which of the following best describes this organism?

Accepted Answer

Gram-negative bipolar stained bacilli that cause diarrhea by means of a heat-stable enterotoxin, with abdominal pain that may be mistaken for appendicitis

Answer

Commonly inhabits the canine respiratory tract and is an occasional pathogen for humans; strongly urease-positive

Answer

Pits agar, grows both in carbon dioxide and under anaerobic conditions, and is part of the normal oral cavity flora

Answer

Typically infects cattle, requires 5% to 10% carbon dioxide for growth, and is inhibited by the dye thionine

Question 2

Rickettsia tsutsugamushi is a small gram-negative, obligate intracellular organism that causes which of the following diseases?

Accepted Answer

Scrub typhus

Answer

Epidemic typhus

Answer

Endemic typhus

Answer

Q fever

Question 3

What is the specific test for the diagnosis of rickettsial infection?

Accepted Answer

Agglutination of rickettsial suspension

Answer

Weil-Felix reaction

Answer

Paul-Bunnell test

Answer

Rose-Waaler test

Question 4

A 20-year-old male presented with left axillary pain, fever, malaise, and myalgias. On examination, there is a tender, boggy swelling in the left axilla with a hard core. A characteristic lesion was observed on the skin of the upper abdominal quadrant. Gram staining of the aspirate was performed. What is the drug of choice in the above condition?

Accepted Answer

Streptomycin

Answer

Tetracycline

Answer

Chloramphenicol

Answer

Doxycycline

Question 5

Which of the following infections is caused by Rickettsia?

Accepted Answer

Rocky mountain spotted fever

Answer

Weil's disease

Answer

Scrub typhus

Answer

Lyme disease

Question 6

A veterinary doctor had pyrexia of unknown origin. Blood culture in special laboratory media was positive for Gram-negative, oxidase-positive short bacilli. Which one of the following is the likely organism grown in culture?

Accepted Answer

Brucella spp.

Answer

Pasteurella spp.

Answer

Francisella spp.

Answer

Bartonella spp.

Question 7

Which of the following is transmitted by rat urine?

Accepted Answer

Leptospira

Answer

Listeria

Answer

Legionella

Answer

Mycoplasma

Question 8

Which vector transmits diseases caused by both Rickettsia and Orientia?

Accepted Answer

Trombiculid mite

Answer

Rat flea

Answer

Tick

Answer

Louse

Question 9

Scrub typhus is caused by which organism?

Accepted Answer

Rickettsia tsutsugamushi

Answer

Rickettsia typhi

Answer

Rickettsia akari

Answer

Rickettsia conorii

Question 10

A patient presented with pyrexia of unknown origin. Blood culture in special laboratory media was positive for gram-negative short bacilli that were oxidase-positive. Which one of the following is the likely organism grown in culture?

Accepted Answer

Brucella spp.

Answer

Pasteurella spp.

Answer

Francisella spp.

Answer

Bartonella spp.

Zoonotic Diseases — MCQs

Zoonotic Diseases — MCQs

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