Animal research was conducted in compliance with the Animal Welfare Act and other federal statutes and regulations relating to animals and experiments involving animals and adhered to the principles stated in the Guide for the Care and Use of Laboratory Animals, National Research Council, 1996. Fifteen healthy, filovirus-seronegative rhesus macaques (each weighing 4 kg–7 kg) were randomized into 2 experimental groups of 6 monkeys per group and 3 control groups of 1 animal per group. All 15 animals were challenged by intramuscular (IM) injection with 1,000 PFU of MBGV (Musoke strain). Approximately 24 h after MBGV challenge, animals in experimental group 1 received a single IM injection of VSVΔG MBGV GP (≈2 × 10⁷ PFU) (8), and the animal in control group 1 received an equal dose of a VSV vector encoding a nonrelated GP (VSVΔG/LassaGPC). Approximately 48 h after MBGV challenge, animals in experimental group 2 received a single IM injection of VSVΔG MBGV GP (≈2 × 10⁷ PFU), and the animal in control group 2 received an equal dose of VSVΔG/LassaGPC. The animal in control group 3 was not treated. Blood samples for viral infectivity titration, reverse transcription–PCR (RT-PCR), hematologic analysis, serum biochemical analysis, and immunoglobulin (Ig) G were collected before MBGV challenge and on days 3, 6, 10, 14, and 31–35 after MBGV challenge.

Five of the 6 animals treated with VSVΔG/MBGV GP 24 h after MBGV challenge (animals 1, 2, 4–6) and 2 of the 6 animals treated with VSVΔG/MBGV GP 48 h after MBGV challenge (animals 7 and 10) survived (Figure; Table A1). In contrast, symptoms consistent with MBGV HF developed in 1 of the 6 macaques treated with VSVΔG/MBGV GP at 24 h (animal 3) and in 4 of the 6 animals treated with VSVΔG/MBGV GP at 48 h (animals 8, 9, 11, and 12); these included anorexia and a macular rash (Table 1). The 5 animals in which macular rash developed (animals 3, 8, 9, 11, and 12) also had plasma viremias >6.0 log₁₀ PFU/mL by day 10; all 5 animals died during days 10–12 (Figure; Table 1; Table A1). Symptoms developed in control animals 1–3 consistent with MBGV HF; each had plasma viremia levels >7.0 log₁₀ PFU/mL by day 10 and died on days 12, 12, and 11, respectively (Table A1).

Two of the 6 animals that survived MBGV challenge (animals 1 and 6) showed no change in appearance or behavior that indicated overt illness. Changes in hematologic results and/or blood parameters were observed in 5 of the surviving animals (2, 4, 5, 7, and 10) during the course of the study (Table A1). Plaque assay and RT-PCR were unable to detect any evidence of MBGV in the plasma of 6 of the 7 surviving animals (1, 2, 4–6, and 10). However, RT-PCR showed evidence of MBGV in peripheral blood mononuclear cells of 2 of these surviving animals (1 and 2) at day 10 (Table 2). Viremia of 4.2 log₁₀ PFU/mL developed on day 10 in 1 surviving animal (7) treated 48 h after infection, and RT-PCR showed evidence of MBGV in peripheral blood mononuclear cells of this animal on days 6 and 10. Viremia in plasma was cleared, and the animal showed little evidence of illness by day 14. The serologic response profile of MBGV infection after treatment was evaluated by IgG ELISA. All 7 animals that were treated with VSV∆G/MBGV GP and survived infection showed moderate to high levels of IgG by day 14 (320–1,000); humoral response against MBGV was not detectable in the treated animals that died or in the control animals (Table 2).

This rhesus macaque model represents a worse-case scenario such as an accidental needle-stick exposure of a laboratory worker or first responder to a high infectious dose of a filovirus. Accidents such as these have occurred several times over the past 5 years (11–13). Of direct relevance to our study was a recent laboratory accident in which an rVSV vector expressing the ZEBOV GP, which had been used successfully in postexposure treatment of experimentally infected nonhuman primates (9), was administered to a human ≈40 h after a ZEBOV needle-stick exposure (13). The patient received a dose of ≈5 × 10⁷ PFU of the VSV ZEBOV GP vaccine, which is consistent with doses used in nonhuman primate studies (7,9,10). Fever, headache, and myalgia developed in the patient hours after injection but were successfully controlled with analgesics and antipyretics. Other adverse effects were not reported, but whether treatment was effective or whether the patient never became infected remains uncertain.

MBGV infection of humans normally progresses at a slower rate than does MBGV infection of macaques, with case-fatality rates in humans of 23%–90% (1) suggesting that the therapeutic window may be larger for humans than for infected macaques. In addition, the challenge dose that we employed in the rhesus monkey model of MBGV HF of 1,000 PFU represents >10,000 LD₅₀ doses (14), again showing that this is a robust challenge model. In the current study, we achieved near complete protection from death when treatment with a single-dose regimen was delayed 24 h and 33% protection when treatment was delayed 48 h postexposure. Because no approved treatments exist for exposure to infectious filoviruses, the rVSV vectors described in the current study merit consideration for treating potential exposures and for further development for human use.