Zoonotic Diseases Practice Questions

Q: Rocky mountain spotted fever: What is the causative agent and its vector?

Rickettsia rickettsii and tick. ### Explanation **Correct Option: C (Rickettsia rickettsii and tick)** Rocky Mountain Spotted Fever (RMSF) is caused by ***Rickettsia rickettsii***, a member of the spotted fever group. It is transmitted to humans via the bite of infected **Ixodid (hard) ticks**, primarily *Dermacentor andersoni* (wood tick) and *Dermacentor variabilis* (dog tick). The pathogen targets vascular endothelial cells, leading to characteristic systemic vasculitis. **Analysis of Incorrect Options:** * **Option A:** *Rickettsia akari* causes **Rickettsialpox**, and its vector is the **mite** (*Liponyssoides sanguineus*). It typically presents with a characteristic eschar at the bite site. * **Option B:** *Rickettsia conorii* causes **Indian Tick Typhus** (or Mediterranean Spotted Fever). While the vector is a tick, the species is geographically and clinically distinct from RMSF. * **Option C:** *Rickettsia prowazekii* causes **Epidemic Typhus**, which is transmitted by the **human body louse**. It is known for causing large outbreaks in crowded conditions. **High-Yield Clinical Pearls for NEET-PG:** * **Triad of RMSF:** Fever, headache, and a characteristic rash. * **Rash Progression:** The rash typically begins on the **wrists and ankles** (centripetal spread) and characteristically involves the **palms and soles** before spreading to the trunk. * **Diagnosis:** The **Weil-Felix test** (heterophile agglutination) shows a positive reaction with **OX-19 and OX-2** for the spotted fever group. * **Drug of Choice:** **Doxycycline** is the gold standard treatment for all rickettsial diseases, regardless of the patient's age.

Q: The Neill-Mooser reaction is used to diagnose which of the following?

Rickettsiae. **Explanation:** The **Neill-Mooser reaction** (also known as the Tunica Reaction) is a diagnostic test used to differentiate between species of the genus **Rickettsia**. **Why Rickettsiae is Correct:** The reaction involves the intraperitoneal inoculation of a clinical sample (containing Rickettsiae) into a male guinea pig. A positive reaction is characterized by **marked scrotal swelling, inflammation, and tunica vaginalis involvement**. * **Rickettsia typhi (Endemic Typhus):** Produces a strongly positive Neill-Mooser reaction. * **Rickettsia prowazekii (Epidemic Typhus):** Produces a negative reaction (fever occurs, but no scrotal swelling). * **Rickettsia rickettsii (Rocky Mountain Spotted Fever):** Produces a positive reaction with potential scrotal necrosis. **Why Other Options are Incorrect:** * **Chlamydiae:** Diagnosed primarily via NAAT, Giemsa staining (inclusion bodies), or Frei’s test (for LGV), not animal inoculation. * **Mycoplasma:** Diagnosed via PPLO agar culture (fried-egg colonies) or serology (Cold agglutinin test). * **Herpes:** Diagnosed via Tzanck smear (multinucleated giant cells), PCR, or viral culture in cell lines (e.g., Vero cells). **High-Yield Clinical Pearls for NEET-PG:** 1. **Weil-Felix Reaction:** A heterophile agglutination test using *Proteus* antigens ($OX19, OX2, OXK$) used for Rickettsial diagnosis (Note: Negative in Q fever). 2. **Rickettsia prowazekii** is the only Rickettsia where humans are the primary reservoir. 3. **Drug of Choice:** Doxycycline is the gold standard treatment for all Rickettsial infections, regardless of age.

Q: A patient developed hemorrhagic blisters at the bite site following a dog bite. Which is the most likely causative organism?

Staphylococcus aureus. **Explanation:** The correct answer is **Staphylococcus aureus**. While several organisms are associated with dog bites, the development of **hemorrhagic blisters (bullae)** at the site of injury is a classic clinical presentation of *S. aureus* infection, often progressing to bullous impetigo or cellulitis. * **Staphylococcus aureus:** It is a common skin commensal and a frequent secondary invader in bite wounds. It produces exfoliative toxins and other virulence factors that lead to the formation of fluid-filled or hemorrhagic bullae. * **Pasteurella multocida:** This is the **most common** organism isolated from dog and cat bites. However, it typically presents as a rapidly progressing (within 24 hours) cellulitis with intense pain and swelling, but it is not characteristically associated with hemorrhagic blisters. * **Capnocytophaga canimorsus:** This is a fastidious Gram-negative rod found in dog saliva. While it can cause severe sepsis, disseminated intravascular coagulation (DIC), and symmetrical peripheral gangrene (purpura fulminans) in immunocompromised or asplenic patients, it does not typically present as localized hemorrhagic blisters at the bite site. * **Pseudomonas spp.:** These are more commonly associated with puncture wounds through sneakers or aquatic exposures, rather than primary dog bite infections. **High-Yield Clinical Pearls for NEET-PG:** * **Most common organism in dog/cat bites:** *Pasteurella multocida*. * **Drug of choice for dog bites:** Amoxicillin-Clavulanate (Co-amoxiclav). * **Capnocytophaga canimorsus:** High risk of overwhelming sepsis in **asplenic** patients. * **Eikenella corrodens:** Associated with **human** bites ("clenched fist" injuries).

Q: Which of the following bacteria does not have a known animal reservoir?

Bordetella pertussis. **Explanation:** The correct answer is **Bordetella pertussis**. The fundamental concept here is the distinction between **zoonotic pathogens** and **obligate human pathogens**. 1. **Bordetella pertussis (Correct Answer):** This bacterium is the causative agent of Whooping Cough. It is an **obligate human pathogen**, meaning humans are the only known natural reservoir. It is transmitted via respiratory droplets, and there is no animal or environmental source of infection. This makes it a prime candidate for vaccine-controlled eradication. 2. **Brucella melitensis (Incorrect):** This is a classic zoonotic pathogen. Its primary reservoirs are **goats and sheep**. Humans usually acquire the infection through direct contact with infected animals or consumption of unpasteurized dairy products. 3. **Pasteurella multocida (Incorrect):** This organism is part of the normal oral flora of many animals, most notably **cats and dogs**. Human infection typically occurs following animal bites or scratches, leading to rapidly progressing cellulitis. 4. **Francisella tularensis (Incorrect):** The causative agent of Tularemia, it has a very broad animal reservoir including **rabbits, hares, and rodents** (hence the name "Rabbit Fever"). It can also be transmitted via tick or deer fly bites. **NEET-PG High-Yield Pearls:** * **Obligate Human Pathogens (No Animal Reservoir):** *B. pertussis, N. meningitidis, S. typhi, Shigella* (except *S. dysenteriae* in primates), and *Treponema pallidum*. * **Bordetella pertussis** is a fastidious, gram-negative coccobacillus. * **Culture Media:** Regan-Lowe medium or Bordet-Gengou (potato-blood-glycerol) agar are the gold standards for isolation. * **Clinical Stage:** The **paroxysmal stage** is characterized by the classic "whoop," but the **catarrhal stage** is the most infectious period.

Q: Both OX 2 and OX 19 are seen as positive tests in case of which of the following conditions?

Rocky mountain spotted fever. ### Explanation The question refers to the **Weil-Felix Reaction**, a heterophile agglutination test used for the presumptive diagnosis of Rickettsial infections. It utilizes the cross-reactivity between antibodies produced against Rickettsial antigens and the somatic (O) antigens of certain *Proteus* strains (*P. vulgaris* OX-19, OX-2, and *P. mirabilis* OX-K). **Why Rocky Mountain Spotted Fever (RMSF) is correct:** RMSF is caused by *Rickettsia rickettsii* (Spotted Fever Group). In this group, antibodies typically react with **both OX-19 and OX-2** strains of *Proteus vulgaris*. While OX-19 usually shows a stronger reaction, the positivity of both is a characteristic diagnostic pattern for the Spotted Fever group. **Analysis of Incorrect Options:** * **Scrub Typhus (A):** Caused by *Orientia tsutsugamushi*. It characteristically reacts **only with OX-K** (Kauffman strain). OX-19 and OX-2 are negative. * **Epidemic Typhus (C):** Caused by *Rickettsia prowazekii* (Typhus Group). This group shows a strong positive reaction with **OX-19 only**, with OX-2 being negative or weakly positive. * **Trench Fever (D):** Caused by *Bartonella quintana*. The Weil-Felix test is **negative** in Trench fever, Q fever (*Coxiella burnetii*), and Rickettsialpox. **High-Yield Clinical Pearls for NEET-PG:** * **Weil-Felix Pattern Summary:** * **OX-19 (+), OX-2 (+):** Spotted Fever Group (e.g., RMSF). * **OX-19 (+), OX-2 (-):** Typhus Group (Epidemic & Endemic Typhus). * **OX-K (+):** Scrub Typhus. * **Negative Weil-Felix:** Always remember that **Q Fever** and **Rickettsialpox** do not show agglutination (Negative test). * **Gold Standard:** While Weil-Felix is high-yield for exams due to its historical importance, the **Indirect Fluorescent Antibody (IFA)** test is the current gold standard for Rickettsial diagnosis.

Question 1

Rocky mountain spotted fever: What is the causative agent and its vector?

Accepted Answer

Rickettsia rickettsii and tick

Answer

Rickettsia akari and mite

Answer

Rickettsia conorii and tick

Answer

Rickettsia prowazekii and louse

Question 2

The Neill-Mooser reaction is used to diagnose which of the following?

Accepted Answer

Rickettsiae

Answer

Chlamydiae

Answer

Mycoplasma

Answer

Herpes

Question 3

A patient developed hemorrhagic blisters at the bite site following a dog bite. Which is the most likely causative organism?

Accepted Answer

Staphylococcus aureus

Answer

Pseudomonas spp.

Answer

Capnocytophaga canimorsus

Answer

Pasteurella spp.

Question 4

Which of the following bacteria does not have a known animal reservoir?

Accepted Answer

Bordetella pertussis

Answer

Brucella melitensis

Answer

Pasteurella multocida

Answer

Francisella tularensis

Question 5

Both OX 2 and OX 19 are seen as positive tests in case of which of the following conditions?

Accepted Answer

Rocky mountain spotted fever

Answer

Scrub typhus

Answer

Epidemic typhus

Answer

Trench fever

Zoonotic Diseases — MCQs

Zoonotic Diseases — MCQs

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