This study was carried out to determine the periodicity of micrc1ilaria of Dirofilaria immitis in a dog naturally infectected with branol in every hours of 3 consecutive days (72 times). The blood samples were examined by using modified knott method. Although microfilaraemia was seen in 24 hours but a nocturnal trend of periodicity was observed for D. immitis. The highest number of microfilaria was seen at I AM. (12798) and the lowest at 11 AM. (6800). The mean number of microfilaria in day time (between 5 AM. to 20 PM in September) was 9027 comparing with 10825 at dark time of a day (20 PM. to 5 AM. in September) The latter results also confirmed the existance of a nocturnal periodicity.
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