Zoonotic Diseases — MCQs

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: 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.

Q: Which of the following is NOT a zoonotic disease?

Malaria. **Explanation:** The core concept in this question is the definition of a **zoonosis**: a disease or infection that is naturally transmissible from vertebrate animals to humans. **Why Malaria is the correct answer:** Malaria is an **anthroponotic** disease, not a zoonotic one. While it is transmitted by a vector (the female *Anopheles* mosquito), the natural reservoir for the human-pathogenic species of *Plasmodium* (*P. falciparum, P. vivax, P. ovale, P. malariae*) is humans. The cycle of transmission is human-to-mosquito-to-human. *(Note: While P. knowlesi is a zoonosis from macaques, in the context of standard medical exams, Malaria is classified as non-zoonotic).* **Analysis of Incorrect Options:** * **Brucellosis:** A classic zoonosis transmitted to humans via direct contact with infected livestock (cattle, goats, pigs) or consumption of unpasteurized dairy products. * **Rabies:** A viral zoonosis transmitted through the bite or scratch of an infected mammal (most commonly dogs in India, or bats/raccoons globally). * **Trichinosis:** A parasitic zoonosis caused by *Trichinella spiralis*, typically acquired by consuming undercooked meat (usually pork) containing encysted larvae. **High-Yield Clinical Pearls for NEET-PG:** * **Definition Check:** If a disease requires an animal reservoir to maintain the pathogen in nature, it is a zoonosis. * **Yellow Fever:** Usually exists in two cycles—**Sylvatic (Zoonotic)** involving monkeys and **Urban (Anthroponotic)** involving humans. * **Common Zoonoses to remember:** Anthrax, Plague, Leptospirosis, Kyasanur Forest Disease (KFD), and Hydatid disease. * **Key Distinction:** Vector-borne does not always mean zoonotic. Malaria and Dengue are vector-borne but primarily anthroponotic.

Q: Which of the following organisms can cause Rat bite fever?

Spirillum minus. **Explanation:** **Rat-bite fever (RBF)** is a systemic zoonotic illness caused by two distinct bacteria, depending on the geographic region. The correct answer is **Spirillum minus**, which is the primary causative agent of the **spirillary form** of the disease (also known as **Sodoku**), predominantly seen in Asia. 1. **Why Spirillum minus is correct:** It is a Gram-negative, thick, spiral-shaped bacterium with polar flagella. It is transmitted via the bite or scratch of an infected rat. Clinically, Sodoku is characterized by the initial wound healing, followed by a sudden onset of fever, lymphadenopathy, and a characteristic "re-ulceration" or chancre-like lesion at the bite site. 2. **Why the other options are incorrect:** * **Leptospira canicola:** Causes Leptospirosis (Canicola fever), typically transmitted through water contaminated with the urine of infected dogs, not rat bites. * **Streptobacillus moniliformis:** This is the *other* cause of Rat-bite fever (the **streptobacillary form**), common in North America. However, since *Spirillum minus* is the specific organism marked as correct in this context, it is important to distinguish them: *Streptobacillus* causes "Haverhill fever" (when transmitted via contaminated milk) and usually presents with prominent polyarthralgia, which is rare in *Spirillum* infections. * **Borrelia recurrentis:** The causative agent of Louse-borne Relapsing Fever (LBRF), transmitted by the human body louse. **NEET-PG High-Yield Pearls:** * **Sodoku (Spirillum minus):** Short incubation (1–4 weeks), relapsing fever, lymphadenopathy, and site inflammation. Diagnosed via **dark-ground microscopy** or animal inoculation (cannot be cultured on artificial media). * **Streptobacillary RBF:** Short incubation (<7 days), prominent joint pain (arthritis), and maculopapular rash on palms/soles. * **Treatment:** Penicillin G is the drug of choice for both forms.

Q: Prevention of human brucellosis depends primarily on?

Pasteurization of dairy products derived from goats, sheep, or cows. **Explanation:** **Brucellosis** (also known as Malta fever or Undulant fever) is a classic zoonotic infection. In humans, the primary mode of transmission is the consumption of **unpasteurized dairy products** (milk, cheese, butter) or direct contact with infected animal tissues/secretions. 1. **Why Option A is Correct:** The most effective way to prevent community-acquired brucellosis is the **pasteurization of milk**. Since *Brucella* species are heat-sensitive, pasteurization effectively kills the bacteria in dairy products, which are the most common vehicles for human infection. In endemic areas, this is the single most important public health measure. 2. **Why Other Options are Incorrect:** * **Option B:** Treatment of human cases prevents complications in the individual but does not prevent new infections, as human-to-human transmission is extremely rare. * **Option C:** *Brucella* is not transmitted by insects; it is transmitted via ingestion, inhalation, or skin contact. Therefore, vector control is irrelevant. * **Option D:** While animal vaccination (e.g., Strain 19, Rev 1) is a key control strategy, there is **no safe or effective vaccine available for humans**. The animal vaccines are live-attenuated and can actually cause disease in humans if accidentally injected. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Bone marrow culture (more sensitive than blood culture). * **Culture Media:** Castaneda’s medium (biphasic medium) is used for blood cultures. * **Serology:** Standard Agglutination Test (SAT) detects antibodies; a titer of >1:160 is significant. * **Treatment:** WHO recommends **Doxycycline + Rifampicin** for 6 weeks. * **Occupational Risk:** It is a significant laboratory-acquired infection (Bio-Safety Level 3 required).

Q: Which of the following are vectors for zoonotic disease transmission?

Ticks, rat fleas, sand flies, mosquitoes, and mites. **Explanation:** Zoonotic diseases are infections naturally transmitted between vertebrate animals and humans. Many of these diseases require **arthropod vectors** to bridge the gap between animal reservoirs and human hosts. **Why Option D is Correct:** Option D is the most comprehensive list. Each of these arthropods plays a critical role in transmitting major zoonotic pathogens: * **Ticks:** Transmit Kyasanur Forest Disease (KFD), Lyme disease, and Crimean-Congo hemorrhagic fever. * **Rat Fleas (*Xenopsylla cheopis*):** The classic vector for Bubonic Plague (*Yersinia pestis*) and Endemic typhus. * **Sand flies:** Transmit Leishmaniasis (Kala-azar), which has significant animal reservoirs (canines/rodents). * **Mosquitoes:** Transmit Japanese Encephalitis (pigs/herons as reservoirs) and Yellow Fever (monkeys). * **Mites:** Specifically, the Trombiculid mite transmits Scrub Typhus (*Orientia tsutsugamushi*), a major cause of acute febrile illness in India. **Why Other Options are Incorrect:** Options A, B, and C are **incomplete**. While the vectors listed in those options do transmit zoonotic diseases, they omit at least one essential vector included in the comprehensive list (Option D). In NEET-PG, when multiple options are factually "true," the most inclusive and complete option is the correct choice. **High-Yield Clinical Pearls for NEET-PG:** * **Scrub Typhus (Mites):** Look for the characteristic "Eschar" (painless ulcer with a black crust) in clinical vignettes. * **KFD (Ticks):** Also known as "Monkey Fever"; endemic to the Western Ghats of India. * **Plague (Rat Fleas):** Remember the "Blocked Flea" phenomenon caused by *Yersinia pestis* multiplication in the flea's proventriculus. * **Japanese Encephalitis:** Pigs are considered "amplifier hosts," while mosquitoes (*Culex tritaeniorhynchus*) are the vectors.

Q: What is the most likely causative agent of community-acquired pneumonia in a person rearing sheep?

Coxiella burnetii. **Explanation:** The correct answer is **Coxiella burnetii**, the causative agent of **Q fever**. **Why Coxiella burnetii is correct:** Q fever is a classic zoonotic infection associated with exposure to livestock, particularly **sheep, goats, and cattle**. The bacteria are excreted in high concentrations in the placenta, birth fluids, urine, and feces of infected animals. Transmission to humans occurs via **inhalation of contaminated aerosols** or dust. Clinically, acute Q fever most commonly presents as a self-limiting febrile illness, but it is a well-known cause of **culture-negative endocarditis** and **atypical community-acquired pneumonia** (often showing "ground-glass" opacities on imaging). **Why the other options are incorrect:** * **Histoplasma capsulatum:** While it causes pneumonia, it is associated with exposure to **bird or bat droppings** (guano), often in caves or old buildings, rather than livestock. * **Streptococcus pneumoniae:** This is the most common cause of typical community-acquired pneumonia in the general population, but it lacks a specific epidemiological link to sheep rearing. * **Bacillus anthracis:** While associated with herbivores (Woolsorter’s disease), inhalation anthrax typically presents as a rapidly progressive, fatal **hemorrhagic mediastinitis** (widened mediastinum) rather than a standard community-acquired pneumonia. **NEET-PG High-Yield Pearls:** * **Organism:** *Coxiella burnetii* is an obligate intracellular Gram-negative bacterium (formerly Rickettsial). * **Diagnosis:** Serology is the gold standard (Indirect Immunofluorescence Assay). * **Chronic Q Fever:** Characterized by **Phase I antibodies** (IgG ≥ 1:800); Acute Q fever shows higher **Phase II antibodies**. * **Treatment:** Doxycycline is the drug of choice for acute cases. * **Key Association:** If a question mentions a "pregnant sheep" or "veterinarian/farmer" with pneumonia or hepatitis, think Q fever.

Q: The milk ring test is a diagnostic method for which of the following conditions?

Brucellosis. **Explanation:** **1. Why Brucellosis is the Correct Answer:** The **Milk Ring Test (MRT)**, also known as the Abortus Bang Ring Test, is a rapid screening method used to detect **Brucellosis** in dairy cattle. The test identifies antibodies (IgA and IgM) against *Brucella* species present in the milk. * **Mechanism:** Hematoxylin-stained *Brucella* antigen is added to a sample of whole milk. If antibodies are present, they bind to the antigen and form clusters. These clusters adhere to fat globules, which rise to the surface as the cream separates, forming a **deep blue/violet ring** at the top. If no antibodies are present, the blue color remains dispersed throughout the milk column. **2. Why the Other Options are Incorrect:** * **Bacteroides infection:** These are anaerobic bacteria typically diagnosed via Gram stain (pleomorphic bacilli) and anaerobic culture on specialized media like BBE agar. * **Tuberculosis:** While *Mycobacterium bovis* can be transmitted via milk, the standard screening in cattle is the Tuberculin skin test (Mantoux). Milk is tested using PCR or culture, not a ring test. * **Salmonellosis:** Diagnosis primarily relies on stool/blood cultures and serological tests like the Widal test (for enteric fever). **3. High-Yield Clinical Pearls for NEET-PG:** * **Rose Bengal Plate Test (RBPT):** The most common rapid screening test for Brucellosis in human/animal serum. * **Standard Agglutination Test (SAT):** The gold standard serological test; a titer of **1:160** or more is significant. * **Culture:** *Brucella* is a fastidious organism requiring **Castaneda’s medium** (biphasic medium) for blood culture. * **Clinical Presentation:** Characterized by **undulant fever**, profuse sweating (moldy smell), and hepatosplenomegaly. It is an occupational hazard for veterinarians, butchers, and dairy workers.

Q: Brucella abortus primarily affects which species?

Cattle. **Explanation:** Brucellosis is a classic zoonotic infection caused by small, Gram-negative coccobacilli. The species of *Brucella* are highly host-specific, which is a high-yield concept for NEET-PG. **1. Why Cattle is Correct:** *Brucella abortus* primarily infects **cattle**. In these animals, the bacteria have a predilection for the reproductive tract due to the presence of **erythritol**, a sugar alcohol produced in the bovine placenta that stimulates the growth of *B. abortus*. This leads to "contagious abortion" in cattle. Humans usually acquire the infection through direct contact with infected carcasses or by consuming unpasteurized milk (leading to Undulant fever). **2. Analysis of Incorrect Options:** * **Option A (Dog):** Primarily affected by ***Brucella canis***. While human infection is possible, it is rare compared to other species. * **Option C (Swine):** Primarily affected by ***Brucella suis***. This species is known for being more virulent in humans, often causing suppurative destructive lesions. * **Option D (Goat/Sheep):** Primarily affected by ***Brucella melitensis***. This is the most common cause of human brucellosis worldwide and is generally the most pathogenic species for humans. **Clinical Pearls for NEET-PG:** * **Most common species worldwide:** *B. melitensis* (Goats/Sheep). * **Most common species in India:** *B. melitensis*. * **Culture Media:** Castaneda’s medium (biphasic medium) is the traditional method; automated systems like BACT/ALERT are now preferred. * **Serology:** Standard Agglutination Test (SAT) detects antibodies; a titer of >1:160 is significant. * **Rose Bengal Plate Test:** Used as a rapid screening test. * **Drug of Choice:** Doxycycline + Rifampicin (for 6 weeks).

Zoonotic Diseases Indian Medical PG Practice Questions and MCQs

Question 1: What is the most effective natural barrier to rabies?

A. Heat
B. Humidity
C. Water (Correct Answer)
D. None of the above

Explanation: **Explanation:** The correct answer is **Water**. This question refers to the ecological and behavioral transmission of rabies rather than a physiological barrier within the human body. **Why Water is the Correct Answer:** Rabies is primarily transmitted through the bites of terrestrial mammals (dogs, foxes, raccoons). Large bodies of water, such as wide rivers, lakes, and oceans, act as the most effective **natural geographic barriers** to the spread of the virus. These barriers limit the migration and interaction of infected animal populations, effectively "quarantining" specific regions. For example, island nations like the UK, Japan, and Australia have historically maintained a rabies-free status largely due to the surrounding water acting as a physical deterrent to infected terrestrial vectors. **Analysis of Incorrect Options:** * **Heat & Humidity:** While the Rabies virus (*Lyssavirus*) is thermolabile (easily destroyed by heat and desiccation outside the host), environmental temperature and humidity do not stop the spread of the disease. Rabies thrives in both tropical (India) and arctic (Arctic fox) climates. These factors affect the survival of the virus on surfaces but do not act as a barrier to the movement of the vectors. **NEET-PG High-Yield Pearls:** * **Virus Type:** Negative-sense, single-stranded RNA virus; Bullet-shaped (Rhabdoviridae family). * **Pathognomonic Feature:** **Negri bodies** (intracytoplasmic eosinophilic inclusions) found most commonly in the Hippocampus (Ammon’s horn) and Cerebellum (Purkinje cells). * **Centripetal Spread:** The virus travels from the site of the bite to the CNS via **retrograde axonal transport** (dynein motors). * **Hydrophobia:** A clinical hallmark caused by forceful spasms of the accessory respiratory muscles and diaphragm when attempting to swallow water. * **Post-Exposure Prophylaxis (PEP):** Includes wound washing (most important initial step), Rabies vaccine (Days 0, 3, 7, 14, 28), and Human Rabies Immunoglobulin (HRIG).

Question 2: 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?

A. Pasteurella spp.
B. Francisella spp.
C. Bartonella spp.
D. Brucella spp. (Correct Answer)

Explanation: ### 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.

Question 3: 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?

A. Pasteurella spp.
B. Francisella spp.
C. Bartonella spp.
D. Brucella spp. (Correct Answer)

Explanation: ### 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 4: Which of the following is NOT a zoonotic disease?

A. Malaria (Correct Answer)
B. Brucellosis
C. Rabies
D. Trichinosis

Explanation: **Explanation:** The core concept in this question is the definition of a **zoonosis**: a disease or infection that is naturally transmissible from vertebrate animals to humans. **Why Malaria is the correct answer:** Malaria is an **anthroponotic** disease, not a zoonotic one. While it is transmitted by a vector (the female *Anopheles* mosquito), the natural reservoir for the human-pathogenic species of *Plasmodium* (*P. falciparum, P. vivax, P. ovale, P. malariae*) is humans. The cycle of transmission is human-to-mosquito-to-human. *(Note: While P. knowlesi is a zoonosis from macaques, in the context of standard medical exams, Malaria is classified as non-zoonotic).* **Analysis of Incorrect Options:** * **Brucellosis:** A classic zoonosis transmitted to humans via direct contact with infected livestock (cattle, goats, pigs) or consumption of unpasteurized dairy products. * **Rabies:** A viral zoonosis transmitted through the bite or scratch of an infected mammal (most commonly dogs in India, or bats/raccoons globally). * **Trichinosis:** A parasitic zoonosis caused by *Trichinella spiralis*, typically acquired by consuming undercooked meat (usually pork) containing encysted larvae. **High-Yield Clinical Pearls for NEET-PG:** * **Definition Check:** If a disease requires an animal reservoir to maintain the pathogen in nature, it is a zoonosis. * **Yellow Fever:** Usually exists in two cycles—**Sylvatic (Zoonotic)** involving monkeys and **Urban (Anthroponotic)** involving humans. * **Common Zoonoses to remember:** Anthrax, Plague, Leptospirosis, Kyasanur Forest Disease (KFD), and Hydatid disease. * **Key Distinction:** Vector-borne does not always mean zoonotic. Malaria and Dengue are vector-borne but primarily anthroponotic.

Question 5: Which of the following organisms can cause Rat bite fever?

A. Leptospira canicola
B. Streptobacillus moniliformis
C. Borrelia recurrentis
D. Spirillum minus (Correct Answer)

Explanation: **Explanation:** **Rat-bite fever (RBF)** is a systemic zoonotic illness caused by two distinct bacteria, depending on the geographic region. The correct answer is **Spirillum minus**, which is the primary causative agent of the **spirillary form** of the disease (also known as **Sodoku**), predominantly seen in Asia. 1. **Why Spirillum minus is correct:** It is a Gram-negative, thick, spiral-shaped bacterium with polar flagella. It is transmitted via the bite or scratch of an infected rat. Clinically, Sodoku is characterized by the initial wound healing, followed by a sudden onset of fever, lymphadenopathy, and a characteristic "re-ulceration" or chancre-like lesion at the bite site. 2. **Why the other options are incorrect:** * **Leptospira canicola:** Causes Leptospirosis (Canicola fever), typically transmitted through water contaminated with the urine of infected dogs, not rat bites. * **Streptobacillus moniliformis:** This is the *other* cause of Rat-bite fever (the **streptobacillary form**), common in North America. However, since *Spirillum minus* is the specific organism marked as correct in this context, it is important to distinguish them: *Streptobacillus* causes "Haverhill fever" (when transmitted via contaminated milk) and usually presents with prominent polyarthralgia, which is rare in *Spirillum* infections. * **Borrelia recurrentis:** The causative agent of Louse-borne Relapsing Fever (LBRF), transmitted by the human body louse. **NEET-PG High-Yield Pearls:** * **Sodoku (Spirillum minus):** Short incubation (1–4 weeks), relapsing fever, lymphadenopathy, and site inflammation. Diagnosed via **dark-ground microscopy** or animal inoculation (cannot be cultured on artificial media). * **Streptobacillary RBF:** Short incubation (<7 days), prominent joint pain (arthritis), and maculopapular rash on palms/soles. * **Treatment:** Penicillin G is the drug of choice for both forms.

Question 6: Prevention of human brucellosis depends primarily on?

A. Pasteurization of dairy products derived from goats, sheep, or cows (Correct Answer)
B. Treatment of human cases
C. Control of the insect vector
D. Immunization of farmers and slaughterhouse workers

Explanation: **Explanation:** **Brucellosis** (also known as Malta fever or Undulant fever) is a classic zoonotic infection. In humans, the primary mode of transmission is the consumption of **unpasteurized dairy products** (milk, cheese, butter) or direct contact with infected animal tissues/secretions. 1. **Why Option A is Correct:** The most effective way to prevent community-acquired brucellosis is the **pasteurization of milk**. Since *Brucella* species are heat-sensitive, pasteurization effectively kills the bacteria in dairy products, which are the most common vehicles for human infection. In endemic areas, this is the single most important public health measure. 2. **Why Other Options are Incorrect:** * **Option B:** Treatment of human cases prevents complications in the individual but does not prevent new infections, as human-to-human transmission is extremely rare. * **Option C:** *Brucella* is not transmitted by insects; it is transmitted via ingestion, inhalation, or skin contact. Therefore, vector control is irrelevant. * **Option D:** While animal vaccination (e.g., Strain 19, Rev 1) is a key control strategy, there is **no safe or effective vaccine available for humans**. The animal vaccines are live-attenuated and can actually cause disease in humans if accidentally injected. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Bone marrow culture (more sensitive than blood culture). * **Culture Media:** Castaneda’s medium (biphasic medium) is used for blood cultures. * **Serology:** Standard Agglutination Test (SAT) detects antibodies; a titer of >1:160 is significant. * **Treatment:** WHO recommends **Doxycycline + Rifampicin** for 6 weeks. * **Occupational Risk:** It is a significant laboratory-acquired infection (Bio-Safety Level 3 required).

Question 7: Which of the following are vectors for zoonotic disease transmission?

A. Ticks, rat fleas, sand flies, and mosquitoes
B. Ticks, sand flies, mosquitoes, and mites
C. Rat fleas, sand flies, mosquitoes, and mites
D. Ticks, rat fleas, sand flies, mosquitoes, and mites (Correct Answer)

Explanation: **Explanation:** Zoonotic diseases are infections naturally transmitted between vertebrate animals and humans. Many of these diseases require **arthropod vectors** to bridge the gap between animal reservoirs and human hosts. **Why Option D is Correct:** Option D is the most comprehensive list. Each of these arthropods plays a critical role in transmitting major zoonotic pathogens: * **Ticks:** Transmit Kyasanur Forest Disease (KFD), Lyme disease, and Crimean-Congo hemorrhagic fever. * **Rat Fleas (*Xenopsylla cheopis*):** The classic vector for Bubonic Plague (*Yersinia pestis*) and Endemic typhus. * **Sand flies:** Transmit Leishmaniasis (Kala-azar), which has significant animal reservoirs (canines/rodents). * **Mosquitoes:** Transmit Japanese Encephalitis (pigs/herons as reservoirs) and Yellow Fever (monkeys). * **Mites:** Specifically, the Trombiculid mite transmits Scrub Typhus (*Orientia tsutsugamushi*), a major cause of acute febrile illness in India. **Why Other Options are Incorrect:** Options A, B, and C are **incomplete**. While the vectors listed in those options do transmit zoonotic diseases, they omit at least one essential vector included in the comprehensive list (Option D). In NEET-PG, when multiple options are factually "true," the most inclusive and complete option is the correct choice. **High-Yield Clinical Pearls for NEET-PG:** * **Scrub Typhus (Mites):** Look for the characteristic "Eschar" (painless ulcer with a black crust) in clinical vignettes. * **KFD (Ticks):** Also known as "Monkey Fever"; endemic to the Western Ghats of India. * **Plague (Rat Fleas):** Remember the "Blocked Flea" phenomenon caused by *Yersinia pestis* multiplication in the flea's proventriculus. * **Japanese Encephalitis:** Pigs are considered "amplifier hosts," while mosquitoes (*Culex tritaeniorhynchus*) are the vectors.

Question 8: What is the most likely causative agent of community-acquired pneumonia in a person rearing sheep?

A. Coxiella burnetii (Correct Answer)
B. Histoplasma capsulatum
C. Streptococcus pneumoniae
D. Bacillus anthracis

Explanation: **Explanation:** The correct answer is **Coxiella burnetii**, the causative agent of **Q fever**. **Why Coxiella burnetii is correct:** Q fever is a classic zoonotic infection associated with exposure to livestock, particularly **sheep, goats, and cattle**. The bacteria are excreted in high concentrations in the placenta, birth fluids, urine, and feces of infected animals. Transmission to humans occurs via **inhalation of contaminated aerosols** or dust. Clinically, acute Q fever most commonly presents as a self-limiting febrile illness, but it is a well-known cause of **culture-negative endocarditis** and **atypical community-acquired pneumonia** (often showing "ground-glass" opacities on imaging). **Why the other options are incorrect:** * **Histoplasma capsulatum:** While it causes pneumonia, it is associated with exposure to **bird or bat droppings** (guano), often in caves or old buildings, rather than livestock. * **Streptococcus pneumoniae:** This is the most common cause of typical community-acquired pneumonia in the general population, but it lacks a specific epidemiological link to sheep rearing. * **Bacillus anthracis:** While associated with herbivores (Woolsorter’s disease), inhalation anthrax typically presents as a rapidly progressive, fatal **hemorrhagic mediastinitis** (widened mediastinum) rather than a standard community-acquired pneumonia. **NEET-PG High-Yield Pearls:** * **Organism:** *Coxiella burnetii* is an obligate intracellular Gram-negative bacterium (formerly Rickettsial). * **Diagnosis:** Serology is the gold standard (Indirect Immunofluorescence Assay). * **Chronic Q Fever:** Characterized by **Phase I antibodies** (IgG ≥ 1:800); Acute Q fever shows higher **Phase II antibodies**. * **Treatment:** Doxycycline is the drug of choice for acute cases. * **Key Association:** If a question mentions a "pregnant sheep" or "veterinarian/farmer" with pneumonia or hepatitis, think Q fever.

Question 9: The milk ring test is a diagnostic method for which of the following conditions?

A. Brucellosis (Correct Answer)
B. Bacteroides infection
C. Tuberculosis
D. Salmonellosis

Explanation: **Explanation:** **1. Why Brucellosis is the Correct Answer:** The **Milk Ring Test (MRT)**, also known as the Abortus Bang Ring Test, is a rapid screening method used to detect **Brucellosis** in dairy cattle. The test identifies antibodies (IgA and IgM) against *Brucella* species present in the milk. * **Mechanism:** Hematoxylin-stained *Brucella* antigen is added to a sample of whole milk. If antibodies are present, they bind to the antigen and form clusters. These clusters adhere to fat globules, which rise to the surface as the cream separates, forming a **deep blue/violet ring** at the top. If no antibodies are present, the blue color remains dispersed throughout the milk column. **2. Why the Other Options are Incorrect:** * **Bacteroides infection:** These are anaerobic bacteria typically diagnosed via Gram stain (pleomorphic bacilli) and anaerobic culture on specialized media like BBE agar. * **Tuberculosis:** While *Mycobacterium bovis* can be transmitted via milk, the standard screening in cattle is the Tuberculin skin test (Mantoux). Milk is tested using PCR or culture, not a ring test. * **Salmonellosis:** Diagnosis primarily relies on stool/blood cultures and serological tests like the Widal test (for enteric fever). **3. High-Yield Clinical Pearls for NEET-PG:** * **Rose Bengal Plate Test (RBPT):** The most common rapid screening test for Brucellosis in human/animal serum. * **Standard Agglutination Test (SAT):** The gold standard serological test; a titer of **1:160** or more is significant. * **Culture:** *Brucella* is a fastidious organism requiring **Castaneda’s medium** (biphasic medium) for blood culture. * **Clinical Presentation:** Characterized by **undulant fever**, profuse sweating (moldy smell), and hepatosplenomegaly. It is an occupational hazard for veterinarians, butchers, and dairy workers.

Question 10: Brucella abortus primarily affects which species?

A. Dog
B. Cattle (Correct Answer)
C. Swine
D. Goat

Explanation: **Explanation:** Brucellosis is a classic zoonotic infection caused by small, Gram-negative coccobacilli. The species of *Brucella* are highly host-specific, which is a high-yield concept for NEET-PG. **1. Why Cattle is Correct:** *Brucella abortus* primarily infects **cattle**. In these animals, the bacteria have a predilection for the reproductive tract due to the presence of **erythritol**, a sugar alcohol produced in the bovine placenta that stimulates the growth of *B. abortus*. This leads to "contagious abortion" in cattle. Humans usually acquire the infection through direct contact with infected carcasses or by consuming unpasteurized milk (leading to Undulant fever). **2. Analysis of Incorrect Options:** * **Option A (Dog):** Primarily affected by ***Brucella canis***. While human infection is possible, it is rare compared to other species. * **Option C (Swine):** Primarily affected by ***Brucella suis***. This species is known for being more virulent in humans, often causing suppurative destructive lesions. * **Option D (Goat/Sheep):** Primarily affected by ***Brucella melitensis***. This is the most common cause of human brucellosis worldwide and is generally the most pathogenic species for humans. **Clinical Pearls for NEET-PG:** * **Most common species worldwide:** *B. melitensis* (Goats/Sheep). * **Most common species in India:** *B. melitensis*. * **Culture Media:** Castaneda’s medium (biphasic medium) is the traditional method; automated systems like BACT/ALERT are now preferred. * **Serology:** Standard Agglutination Test (SAT) detects antibodies; a titer of >1:160 is significant. * **Rose Bengal Plate Test:** Used as a rapid screening test. * **Drug of Choice:** Doxycycline + Rifampicin (for 6 weeks).

Practice by Chapter

Principles of Zoonotic Transmission

Practice Questions

Viral Zoonoses

Practice Questions

Bacterial Zoonoses

Practice Questions

Parasitic Zoonoses

Practice Questions

Vector-Borne Zoonoses

Practice Questions

Foodborne Zoonoses

Practice Questions

Emerging Zoonotic Diseases

Practice Questions

One Health Approach to Zoonoses

Practice Questions

Prevention and Control of Zoonoses

Practice Questions

Occupational Exposure to Zoonoses

Practice Questions

Wildlife Reservoirs of Zoonotic Pathogens

Practice Questions

Global Impact of Zoonotic Diseases

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free