Culex quinquefasciatus is the primary vector for Bancroftian filariasis in India. Which of the following correctly describes the microfilarial periodicity and its relationship to the vector's biting behavior?

• A Nocturnal periodicity; Culex is a night-biting mosquito, so microfilariae accumulate in peripheral blood during nighttime ✓
• B Diurnal periodicity; Culex bites during the day
• C Diurnal subperiodicity; Culex feeds primarily in the morning hours
• D Aperiodic; microfilariae are equally distributed in peripheral blood throughout the day

Explanation

Wuchereria bancrofti (Bancroftian filariasis) transmitted by Culex quinquefasciatus shows nocturnal periodicity: microfilariae concentrate in peripheral capillaries of lungs during the day and migrate to peripheral blood at night (10 PM–2 AM peak). This coincides precisely with the nocturnal biting peak of Culex, a co-evolutionary adaptation. Blood smear for filariasis diagnosis must therefore be taken between 9 PM and midnight. Pacific island strains show diurnal subperiodicity, transmitted by daytime-biting Aedes mosquitoes.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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