Thousands of kilometres from the cities of China’s eastern seaboard, millions of Tibetans, Kazakhs, Mongols and other ethnic groups live as nomadic herders on vast grassy uplands in the country’s western reaches, dependent on yak, cows and sheep for milk and meat through sub-zero winters.

These regions, with their natural beauty and cultural diversity, are also home to rare tapeworms less than 3mm long, which spread from wild rodents to livestock and, through contact with dogs, to humans. They cause cysts that slowly enlarge in people’s internal organs. Though symptoms can go unnoticed for years, the result of infection with the disease, echinococcosis, is generally years of suffering followed by a premature death.

A sip of water polluted with parasites from dog faeces in a river near his home in the countryside of China’s Xinjiang region was probably all it took for Tang Zhu, 32, to contract the disease. It went undetected for years before spreading from his liver to his brain, causing him to go blind, suffer seizures and have difficulty walking.