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.

Q: A patient presented with interstitial pneumonia and denied being bitten by insects. Of the following rickettsial diseases, which could have been caused without an insect bite?

Q fever. **Explanation:** The correct answer is **Q fever** (caused by *Coxiella burnetii*). Unlike other members of the Rickettsiales order, *Coxiella burnetii* is unique because it does not require an arthropod vector for transmission to humans. **Why Q fever is correct:** * **Transmission:** It is primarily a zoonosis transmitted via **inhalation** of contaminated aerosols or dust from the birth products, feces, or urine of infected livestock (cattle, sheep, goats). It can also be transmitted through the consumption of unpasteurized milk. * **Clinical Presentation:** It characteristically presents as an undifferentiated febrile illness, **interstitial pneumonia**, or hepatitis. * **Biological Feature:** *C. burnetii* forms spore-like structures that allow it to survive harsh environmental conditions, facilitating airborne spread. **Why other options are incorrect:** * **Scrub typhus:** Caused by *Orientia tsutsugamushi*, it is transmitted by the bite of a **larval mite (chigger)**. It typically presents with an eschar at the bite site. * **Rickettsial pox:** Caused by *Rickettsia akari*, it is transmitted by the **mite** *Liponyssoides sanguineus*. * **Brill-Zinsser disease:** This is a recrudescent form of Epidemic typhus (*Rickettsia prowazekii*). While the initial infection is transmitted by the **human body louse**, the reactivation occurs years later without a new bite; however, the primary disease is strictly vector-borne. **High-Yield Clinical Pearls for NEET-PG:** * **Q fever** is the only rickettsial disease that **does not present with a rash** and is **Weil-Felix reaction negative**. * It is a highly infectious agent (Category B bioterrorism agent); a single organism can cause disease. * **Chronic Q fever** most commonly manifests as culture-negative endocarditis. * **Drug of choice:** Doxycycline.

Q: A 15-year-old boy presents 1 week after being bitten by an Ixodes tick with fatigue, fever, headache, and a reddish rash over his trunk and extremities. A positive IgM antibody titer (1:200) to Borrelia burgdorferi is associated with which of the following?

Acute Lyme disease. **Explanation:** The clinical presentation of a tick bite followed by fever, headache, and a characteristic rash (Erythema migrans) is classic for **Lyme disease**, caused by the spirochete *Borrelia burgdorferi*. In immunology, **IgM** is the first antibody isotype produced during a primary immune response. A high IgM titer (1:200) within one week of symptom onset confirms an **acute infection**. **Why the other options are incorrect:** * **Fifth Disease (Erythema Infectiosum):** Caused by Parvovirus B19, it typically presents with a "slapped-cheek" rash in children and is not transmitted by ticks. * **Hepatitis B Infection:** Diagnosis relies on HBsAg and anti-HBc IgM markers. While IgM is present in acute HBV, the clinical history of a tick bite and skin rash is inconsistent with hepatitis. * **Subacute Sclerosing Panencephalitis (SSPE):** This is a late, progressive neurological complication of a Measles virus infection. It is characterized by extremely high titers of **IgG** (not IgM) in both serum and CSF, occurring years after the initial infection. **High-Yield Clinical Pearls for NEET-PG:** * **Vector:** *Ixodes* tick (also transmits Babesia and Anaplasma). * **Early Localized Stage:** Erythema migrans (target-like/bull’s eye rash). * **Serology:** Two-tier testing is recommended (ELISA followed by Western Blot). IgM appears in 1–2 weeks; IgG appears in 4–6 weeks. * **Treatment:** Doxycycline is the drug of choice (Amoxicillin in children <8 years or pregnant women).

Q: Which of the following statements is TRUE about endemic typhus?

Is transmitted by the bite of fleas. **Explanation:** **Endemic Typhus** (also known as Murine Typhus) is caused by the bacterium **_Rickettsia typhi_**. 1. **Why Option B is Correct:** The primary vector for endemic typhus is the **oriental rat flea (_Xenopsylla cheopis_)**. The infection is transmitted to humans when flea feces (containing the bacteria) are rubbed into the bite wound or other abrasions on the skin. This distinguishes it from Epidemic Typhus, which is transmitted by the human body louse. 2. **Why the other options are Incorrect:** * **Option A:** _Rickettsia rickettsii_ is the causative agent of **Rocky Mountain Spotted Fever**, not endemic typhus. * **Option C:** Endemic typhus has a significant mammalian reservoir, primarily **rats** (_Rattus rattus_ and _Rattus norvegicus_). The cycle persists in nature between rats and fleas. * **Option D:** _Coxiella burnetii_ is the causative agent of **Q Fever**, which is unique among rickettsial diseases because it does not cause a rash and is typically transmitted via inhalation of contaminated aerosols rather than an arthropod vector. **High-Yield Clinical Pearls for NEET-PG:** * **The "Typhus" Distinction:** * **Epidemic Typhus:** _R. prowazekii_ (Vector: Louse). * **Endemic Typhus:** _R. typhi_ (Vector: Flea). * **Scrub Typhus:** _Orientia tsutsugamushi_ (Vector: Trombiculid mite/Chigger). * **Weil-Felix Test:** In Endemic Typhus, the serum shows a positive reaction with **OX19** (similar to Epidemic Typhus), but the clinical course is generally milder. * **Drug of Choice:** **Doxycycline** remains the first-line treatment for almost all rickettsial infections.

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

Question 6

A patient presented with interstitial pneumonia and denied being bitten by insects. Of the following rickettsial diseases, which could have been caused without an insect bite?

Accepted Answer

Q fever

Answer

Scrub typhus

Answer

Rickettsial pox

Answer

Brill-Zinsser disease

Question 7

Brill-Zinsser disease is caused by which of the following organisms?

Accepted Answer

Rickettsia prowazekii

Answer

Coxiella burnetii

Answer

Rickettsia typhi

Answer

Rickettsia slovaca

Question 8

A 15-year-old boy presents 1 week after being bitten by an Ixodes tick with fatigue, fever, headache, and a reddish rash over his trunk and extremities. A positive IgM antibody titer (1:200) to Borrelia burgdorferi is associated with which of the following?

Accepted Answer

Acute Lyme disease

Answer

Fifth disease

Answer

Possible hepatitis B infection

Answer

Possible subacute sclerosing panencephalitis (SSPE)

Question 9

Which of the following statements is TRUE about endemic typhus?

Accepted Answer

Is transmitted by the bite of fleas

Answer

Is caused by R. rickettsii

Answer

Has no mammalian reservoir

Answer

Is caused by Coxiella burnetii

Question 10

A 43-year-old executive presents with chronic symmetric polyarthritis involving the knees. He has a history of an extensive rash after a deer-hunting trip several years earlier, accompanied by 'sick' feeling and knee pain that partially resolved after several months. Which of the following organisms is most likely etiologically related to the patient's arthritis?

Accepted Answer

Spirochete

Answer

Fungus

Answer

Gram-negative cocci

Answer

Gram-positive cocci

Zoonotic Diseases — MCQs

Zoonotic Diseases — MCQs

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