1 / 9A new form of African swine fever identified in Chinese pig farms is most likely caused by illicit vaccines, industry insiders say, a fresh blow to the world's largest pork producer, still recovering from a devastating epidemic of the virus.2 / 9Two new strains of African swine fever have infected more than 1,000 sows on several farms owned by New Hope Liuhe, China's fourth-largest producer, as well as pigs being fattened for the firm by contract farmers, said Yan Zhichun, the company's chief science officer.3 / 9Though the strains, which are missing one or two key genes present in the wild African swine fever virus, don't kill pigs like the disease that ravaged China's farms in 2018 and 2019, they cause a chronic condition that reduces the number of healthy piglets born, Yan told Reuters. 4 / 9At New Hope, and many large producers, infected pigs are culled to prevent the spread, making the disease effectively fatal.5 / 9Although the known infections are limited now, if the strains spread widely, they could slash pork output in the world's top consumer and producer; two years ago, swine fever wiped out half of China's 400 million-head pig herd.6 / 9 Pork prices are still at record levels and China is under pressure to strengthen food security amid the COVID-19 pandemic.7 / 9'I don't know where they come from, but we find some mild field infections caused by some sort of gene-deleted viruses,' Yan said.8 / 9Wayne Johnson, a Beijing-based veterinarian, said he diagnosed a chronic, or less-lethal, form of the disease in pigs last year.9 / 9The virus lacked certain genetic components, known as the MGF360 genes. New Hope has found strains of the virus missing both the MGF360 genes and the CD2v genes, Yan said.There is no approved vaccine for African swine fever, which is not harmful to humans. But many Chinese farmers struggling to protect their pigs have resorted to unapproved products, industry insiders and experts said. They fear illicit vaccines have created accidental infections, which are now spreading.