The periodicity of microfilariae

Abstract

A review is given of previous work on the periodicity of microfilariae. The evidence is now conclusive that Lane's hypothesis of a new brood of microfilariae produced afresh every 24 hours is untenable, and that the appearance and disappearance of microfilariae in and from the blood is due to migration backwards and forwards between different parts of the body (as proposed by Manson). Moreover, the increase of microfilariae in the blood at night is not a spurious effect due merely to congregation of the organisms in the capillaries of the skin but is a true increase affecting all the circulating blood. The periodic appearance and disappearance of microfilariae in the blood is due to their periodic liberation from and accumulation in the small vessels of the lungs. The mechanism by which this liberation and accumulation is brought about is unknown; apparently it involves an active response of the microfilariae to some unknown periodic change in the blood of the host which habitually precedes sleeping and waking. As a teleological explanation it is postulated that the lung forms the most favourable site in the body for the microfilariae to exist; but if they spent all the time there, they would never encounter the insect vector and so would never be transmitted to new hosts. The periodicity of microfilariae is a mechanism by which a compromise is arranged between the two requirements of the microfilariae (optimum survival and transmission). They circulate in the blood when the appropriate insect vector bites, and they spend the rest of the 24 hours enjoying the favourable conditions of the lungs.