A night blood suyrvey is done in -

<p> night blood survey:- A considerable amount of people affected with filariasis are asymptomatic,although the blood is positive for microfilaria. They may remain asymptomatic for years. They are an impoant source of infection in the community. These carriers are usually detected by night blood survey examination since the microfilaria is best seen in blood from around 10 pm-2 am. {Reference: park&;s textbook of preventive and social medicine, 23rd edition, pg no.272}