Enhanced treatment of Lassa fever by immune plasma combined with ribavirin in cynomolgus monkeys.

Lassa virus-infected cynomolgus monkeys were treated with Lassa virus-immune monkey serum containing a high concentration of neutralizing antibody, the antiviral drug ribavirin, or a combination of ribavirin plus immune serum at various times after infection. When treatment was initiated early (day 0 or 4), either ribavirin (30 mg/kg of body weight per day) or immune serum alone protected monkeys. However, when the initial treatment was delayed until day 7, only four of eight ribavirin-treated and only one of six serum-treated monkeys survived. Treatment with ribavirin combined with immune serum was more successful; all infected monkeys given combination treatment survived, including six treated initially on day 10. Increased doses of ribavirin given alone were toxic. The empirical observation that combined treatment with ribavirin plus serum results in enhanced survival in experimentally infected monkeys suggests that combined treatment might benefit human patients with Lassa fever as well.