During a plague surveillance operation, 50 rats are trapped in a port town and examined for ectoparasites. A total of 150 Xenopsylla cheopis fleas are recovered. What does the calculated Cheopis index indicate?
- A 50, indicating that every rat carries fleas
- B 3, which is acceptable because the danger threshold is 10
- C 3, exceeding the critical level of 1, indicating a high risk of plague outbreak ✓
- D 0.33, indicating negligible risk since any value below 1 is safe
Correct answer: C. 3, exceeding the critical level of 1, indicating a high risk of plague outbreak
Explanation
The Cheopis index is defined as the average number of Xenopsylla cheopis fleas per rodent examined, here 150 divided by 50 equals 3. An index greater than 1 is regarded as dangerous because it indicates conditions favourable for a plague epizootic and subsequent human outbreak. Option D confuses the calculation, option A confuses the index with rat counts, and option B invents a threshold of 10. Flea density per rat is the standard early-warning indicator in plague surveillance.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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