Antibiotics and Wolbachia in filarial nematodes: antifilarial activity of rifampicin, oxytetracycline and chloramphenicol against Onchocerca gutturosa, Onchocerca lienalis and Brugia pahangi

The activity against filarial parasites of the antibiotics rifampicin, oxyt etracycline and chloramphenicol was examined. In addition, transmission ele ctron microscopy was used to study the effects of rifampicin and oxytetracy cline on filarial tissues and on the endosymbiont bacterium, Wolbachia. When tested in vitro at a concentration of 50.0 muM, each of the three anti biotics significantly reduced the motility levels of male Onchocerca guttur osa. Rifampicin, however, was the most active, virtually immobilizing the p arasite by the end of the 40-day trial and producing an 84% reduction in vi ability (as measured by formazan-based colorimetry). In tests against O. li enalis microfilariae (mff) in CBA mice, the numbers of mff recovered after treatment with oxytetracycline at 100, 25 or 6.5 mg/kg daily, for 15 days, were 56% (P less than or equal to0.03), 38% (P>0.05) and 45% (P = 0.05) les s than that recovered from the untreated controls, respectively. In another trial in mice, rifampicin (100 mg/kg daily for 15 days) was found to be th e most active (causing a 74% reduction in the number of mff recovered-appro ximately equal to that achieved with the positive control of a single dose of ivermectin at 2 mug/kg), with chloramphenicol also showing significant a ctivity (39% reduction). In further, in-vivo trials, at three dose levels ( 100, 25 or 6.25 mg/kg daily, for 15 days), all three antibiotics were teste d against adult Brugia pahangi in the peritoneal cavities of jirds. None of the antibiotics produced a significant reduction in the numbers of live wo rms recovered, although a marginal effect was observed in eight of the nine antibiotic-treated groups. A further extended trial with rifampicin and ox ytetracycline resulted in 43% and 38% reductions in worm recoveries, respec tively (not statistically significant but consistent with a marginal effect ); some of these worms appeared less motile and qualitatively in poor condi tion compared with those recovered from untreated jirds. Ultrastructural st udies of these treated worms revealed that virtually all of the endosymbion t bacteria had been cleared from the parasite tissues. The tissues of the a dult worms appeared to be largely intact but with a granulomatous response of host cells adhering to some specimens. However, developing uterine forms appeared to be abnormal and extensively damaged, showing an abrogation of embryogenesis. In contrast, worms recovered from control animals contained large numbers of Wolbachia, had no adherent host cells, and showed normal u ltrastructure; the female worms exhibited a full range of intra-uterine dev eloping stages from eggs to stretched mff. It is likely that the activity o f these antibiotics against the endosymbiont Wolbachia causes the observed antifilarial activity, although some direct effect of each drug on filarial viability cannot be ruled out.