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Tendency to recurrence at regular intervals.

- diurnal p. a circadian rhythm with primary expression of the p. during daylight hours, as in the release of microfilariae of Loa loa into the peripheral blood during the day, with far fewer released at night; associated with the day-biting habits of the vector, Chrysops species.

- filarial p. the circadian rhythm observed in the appearance of filarial microfilariae in the peripheral blood. SEE ALSO: diurnal p., nocturnal p..

- lunar p. any rhythmic phenomenon that follows a lunar or monthly cycle.

- malarial p. a clinical rhythmicity reflected in periodic fevers and chills recurring at approximately 48-hour intervals in tertian malaria (Plasmodium vivax or P. ovale) or at 72-hour intervals in quartan malaria (P. malariae); the rhythm of tertian or 48-hour cycles is frequently modified in malignant tertian or falciparum malaria (P. falciparum); associated with release of merozoites from red cells during erythrocytic schizogony, although the controlling mechanism for the synchronous release is unknown.

- nocturnal p. a circadian rhythm with the p. expressed during nighttime hours, as in the night release of microfilariae of the human filaria Wuchereria bancrofti into the peripheral blood; this type of p. is found in regions where the vector mosquito is a night-biting species.

- subperiodic p. a modified circadian rhythm in which the p. is not clearcut, as in certain zoonotic strains of Malayan filariasis caused by Brugia malayi; as in examples of strict filarial p., this response is correlated with the biting habits of the vector insect (mosquito), although the precise mechanism inducing this microfilarial response is not clearly established.

pe·ri·od·ic·i·ty .pir-ē-ə-'dis-ət-ē n, pl -ties the quality, state, or fact of being regularly recurrent or having periods <the \periodicity of peptic ulcer is...characteristic of the disease (Jour. Amer. Med. Assoc.)>