Efficacy of three-week oxytetracycline or rifampin monotherapy compared with a combination regimen against the filarial nematode Onchocerca ochengi

Abstract

Onchocerciasis (river blindness), caused by the filarial nematode Onchocerca volvulus, is a major cause of visual impairment and dermatitis in sub-Saharan Africa. As O. volvulus contains an obligatory bacterial symbiont (Wolbachia), it is susceptible to antibiotic chemotherapy, although current regimens are considered too prolonged for community-level control programs. The aim of this study was to compare the efficacies of oxytetracycline and rifampin, administered separately or in combination, against a close relative of O. volvulus (Onchocerca ochengi) in cattle. Six animals per group were treated with continuous or intermittent oxytetracycline regimens, and effects on adult worm viability, dermal microfilarial loads, and Wolbachia density in worm tissues were assessed. Subsequently, the efficacies of 3-week regimens of oxytetracycline and rifampin alone and a combination regimen were compared, and rifampin levels in plasma and skin were quantified. A 6-month regimen of oxytetracycline with monthly dosing was strongly adulticidal, while 3-week and 6-week regimens exhibited weaker adulticidal effects. However, all three regimens achieved >2-log reductions in microfilarial load. In contrast, rifampin monotherapy and oxytetracycline-rifampin duotherapy failed to induce substantive reductions in either adult worm burden or microfilarial load, although a borderline effect on Wolbachia density was observed following duotherapy. Dermal rifampin levels were maintained above the MIC for >24 h after a single intravenous dose. We conclude that oxytetracycline-rifampin duotherapy is less efficacious against O. ochengi than oxytetracycline alone. Further studies will be required to determine whether rifampin reduces oxytetracycline bioavailability in this system, as suggested by human studies using other tetracycline-rifampin combinations.

FIG 1

Nodule diameters (medians and ranges) in Onchocerca ochengi-infected cattle treated with oxytetracycline delivered once monthly for 6 months (PIR; n = 5), twice weekly for 3 weeks (OXY3-1; n = 6), or twice weekly for 6 weeks (OXY6; n = 4) (a) and with rifampin alone twice weekly for 3 weeks (RIF3; n = 6), rifampin and oxytetracycline twice weekly for 3 weeks (COM; n = 6), or oxytetracycline alone twice weekly for 3 weeks (OXY3-2; n = 6) (b). Within groups, medians at time points annotated with different letters are significantly different at a P of <0.05 (lowercase letters) or a P of <0.01 (uppercase letters) by Friedman's two-way ANOVA by ranks. Between groups, medians at time points annotated with an asterisk are significantly different from medians for the control group (P < 0.05 by the Mann-Whitney U test).

FIG 2

Microfilarial densities per 100 mg skin (medians and ranges, plotted on a log x + 1-transformed scale) in Onchocerca ochengi-infected cattle treated with oxytetracycline delivered once monthly for 6 months (PIR; n = 5), twice weekly for 3 weeks (OXY3-1; n = 6), or twice weekly for 6 weeks (OXY6; n = 4) (a) and with rifampin alone twice weekly for 3 weeks (RIF3; n = 6), rifampin and oxytetracycline twice weekly for 3 weeks (COM; n = 6), or oxytetracycline alone twice weekly for 3 weeks (OXY3-2; n = 6) (b). (a) Control animals (CON-1; n = 5) received no treatment; (b) control animals (CON-2; n = 5) received DMSO solvent at a dose equivalent to that of the RIF3 and COM groups. Within groups, medians at time points annotated with different letters are significantly different at a P of < 0.05 by Friedman's two-way ANOVA by ranks. Between groups, medians at time points annotated with asterisks are significantly different from those of the control group (P < 0.05 [*] or P < 0.01 [**] by the Mann-Whitney U test).

FIG 3

Localization of Wolbachia endobacteria in nodule sections from Onchocerca ochengi-infected cattle treated with DMSO solvent only (a, b, c), rifampin alone twice weekly for 3 weeks (d, e, f), oxytetracycline alone twice weekly for 3 weeks (g, h, i), or rifampin and oxytetracycline twice weekly for 3 weeks (j, k, l) at 0 (a, d, g, j), 24 (b, e, h, k), and 36 (c, f, i, l) weeks posttreatment start. Yellow arrows, intrauterine microfilariae; filled black arrowheads, Wolbachia organisms in the hypodermal cords; white arrows, morulae; open black arrowheads, Wolbachia organisms in the female reproductive tract; black arrow, migrating microfilaria; open yellow arrowhead, partially resorbed worm section. Sections were probed with an anti-Wolbachia surface protein polyclonal antibody, which was visualized using 3,3′-diaminobenzidine precipitate, and counterstained with Giemsa stain. Bars, 50 μm.

FIG 4

Endobacterial density scores in Onchocerca ochengi hypodermal cords (a) or O. ochengi female reproductive tracts (b) in nodular histological sections from infected cattle treated with DMSO solvent only (CON-2), rifampin alone twice weekly for 3 weeks (RIF3), rifampin and oxytetracycline twice weekly for 3 weeks (COM), or oxytetracycline alone twice weekly for 3 weeks (OXY3-2) at 0, 24, and 36 weeks posttreatment start. Wolbachia staining in worm sections across the entirety of each slide was semiquantified on a 3-point scale, as follows: 0, no visible bacteria; 1, sparse bacteria (<50% of worm sections were positive); and 2, profuse bacterial (≥50% of worm sections were positive). Note that data were not available for two OXY3-2 nodules, three COM nodules, and two RIF3 nodules due to complete resorption of the worms (n = 5) or misidentification of nodules caused by Demodex bovis (n = 2).

FIG 5

Rifampin concentration-time profiles (means ± standard deviations; n = 4) in bovine plasma (a) and bovine skin (b) following a single intravenous injection at 10 mg/kg.