MOSCOW—Imprisoned Russian opposition leader Alexei Navalny said Wednesday he has started a hunger strike to protest authorities’ failure to provide proper treatment for his back and leg pains. In a statement posted on Instagram, Navalny complained about prison officials’ refusal to give him the right medicines and to allow his doctor to visit him behind bars. He also protested the hourly checks a guard makes on him at night, saying they amount to sleep deprivation torture. The 44-year-old Navalny, who is an outspoken opponent of President Vladimir Putin, was arrested in January upon his return from Germany, where he spent five months recovering from a nerve-agent poisoning that he blames on the Kremlin. Russian authorities have rejected the accusation. Navalny’s poisoning and conviction have further strained Russia’s ties with the United States and the European Union, which sank to post-Cold War lows after Moscow’s 2014 annexation of Ukraine’s Crimea, its …