Infrared light photobiostimulation mediates periodicity in Dirofilaria immitis microfilariae.

Microfilariae (Mfs) of filarial nematode parasites exhibit nocturnal periodicity, with their numbers in peripheral blood peaking at night and decreasing during the day. However, the reason for their appearance at night remains unknown. In this study, in vitro photobiostimulation experiments showed that Mfs exhibited positive phototaxis toward infrared light with lower photon flux densities of infrared light at wavelengths of 890 and 700 nm, in particular, mediating paradoxically higher velocity than intense ones. Microarray analysis revealed that infrared light stimulation influenced gene expression in Mfs and induced significant upregulation of genes, with phosphorylation- and neurogenesis-related genes being highly enriched. Weaker natural infrared beams from the atmosphere only at midnight may induce microfilaria periodicity, and the nature of the periodic pattern is innate and plastic, as demonstrated by artificially changing the light-dark cycle. This is the first report of positive phototaxis toward infrared light in Dirofilaria immitis Mfs. The notable finding is that they moved in union despite the lack of a fluid current inside the container, indicating that infrared light appears to control nocturnal periodicity in D. immitis Mfs. The newly developed culture medium and the adoption of charge-coupled device (CCD) camera and time-lapse VHS videocassette recorder used in this study made possible to be a long observation.

Brugia malayi: effects of gamma radiation on adult worms and their intracellular Wolbachia bacteria.

Prior studies have shown that intracellular Wolbachia endobacteria are necessary for the normal development, reproduction, and survival of filarial nematodes. The purpose of this study was to examine effects of gamma radiation on Wolbachia and reproduction in Brugia malayi adult worms. Worms were exposed to 0, 10, 25, 45, 75, and 105 krad of gamma radiation from a 137cesium source and cultured in vitro for 10 days. Irradiation reduced production of microfilariae in a dose-dependent manner. Embryograms of irradiated female worms showed dose-related abnormalities with arrested development at the early embryo stage. Irradiation reduced the viability of adult worms in a dose-dependent manner, but no lethal effect was observed. Electron microscopy studies showed that irradiation cleared Wolbachia from worm tissues. Real-time polymerase chain reaction studies demonstrated greatly reduced Wolbachia DNA in irradiated worms. These effects are essentially the same as those observed in adult worms treated with doxycycline. These studies suggest that effects of irradiation on reproduction in Brugia malayi may be caused by effects of irradiation on Wolbachia.