Correct Answer: D. Xenopsylla cheopis
Buboes are the pathognomonic sign of plague, caused by Yersinia pestis. The question specifically asks for the vector responsible for this outbreak. Xenopsylla cheopis (the rat flea) is the primary vector of plague worldwide and in India. This flea parasitizes rats (especially Rattus rattus) and transmits the bacterium to humans through the bite wound or contaminated flea feces. In India, plague is endemic in certain regions (historically in parts of Maharashtra, Karnataka, and Himachal Pradesh), and rodent-flea transmission remains the epidemiologically significant route. The flea becomes infected when it takes a blood meal from a bacteremic rat, and the bacteria multiply in the flea's foregut, causing blockage. When the flea attempts to feed on a human, it regurgitates infected material, transmitting Y. pestis. Buboes (swollen, painful lymph nodes) develop 2–8 days after infection as the body's lymphatic response to the bacteremia. Understanding the vector is critical for outbreak control in India, where rodent surveillance and flea control are primary public health interventions under the National Vector Borne Disease Control Programme (NVBDCP).
Why the other options are wrong
A. Ixodes tick — Ixodes ticks are vectors of Lyme disease (Borrelia burgdorferi) and tick-borne encephalitis, not plague. While ticks are important vectors in temperate regions, they do not transmit Yersinia pestis. This is an NBE trap pairing a vector with a zoonotic disease, but the wrong vector-pathogen combination. B. Phlebotomus argentipes — Phlebotomus argentipes is the vector of visceral leishmaniasis (kala-azar) in India, not plague. This is a common NBE trap because both are vector-borne zoonotic diseases endemic in India. However, leishmaniasis presents with hepatosplenomegaly and fever, not buboes. C. Female Anopheles mosquito — Female Anopheles mosquitoes transmit malaria and Japanese encephalitis, not plague. This is another vector-borne disease trap. Anopheles does not transmit Yersinia pestis, and malaria presents with fever and hemolysis, not buboes.
High-Yield Facts
- Xenopsylla cheopis is the primary vector of plague (Yersinia pestis) globally and in India.
- Buboes are swollen, painful lymph nodes that appear 2–8 days after plague infection and are pathognomonic for the disease.
- Plague transmission occurs when an infected flea's foregut blockage causes regurgitation of bacteria into the bite wound.
- Rodent surveillance and flea control are the cornerstones of plague outbreak management under India's NVBDCP.
- Plague is endemic in certain Indian regions (historically Maharashtra, Karnataka, Himachal Pradesh) and requires mandatory reporting.
Mnemonics
PLAGUE vectors (India context) Xenopsylla cheopis = flea → plague (buboes). Remember: 'X marks the spot' for the rat flea that causes buboes. Vector-Disease pairs (NBE trap avoidance) Flea (Xenopsylla) → Plague; Tick (Ixodes) → Lyme; Phlebotomus → Kala-azar; Anopheles → Malaria. Each vector has ONE primary disease.
NBE Trap
NBE pairs plague with other vector-borne zoonotic diseases (leishmaniasis, Lyme disease, malaria) to test whether students can discriminate the specific vector-pathogen pair. The trap is strongest with Phlebotomus argentipes (also endemic in India, also causes buboes-like lymphadenopathy in some presentations) and Ixodes (also a classical vector in medical curricula).
Clinical Pearl
In India, plague outbreaks are investigated by identifying the flea vector in trapped rodents and implementing flea control (insecticides) and rodent management. A patient presenting with buboes + fever + history of rodent exposure in endemic areas (e.g., Himachal Pradesh) should trigger immediate plague suspicion and flea vector investigation.
_Reference: Park's Textbook of Preventive and Social Medicine (Vector-Borne Diseases section); NVBDCP Guidelines on Plague Surveillance and Control_