A 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.
Two 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.
Though 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. At New Hope, and many large producers, infected pigs are culled to prevent the spread, making the disease effectively fatal.
Although 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. Pork prices are still at record levels and China is under pressure to strengthen food security amid the COVID-19 pandemic.
“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.
Wayne Johnson, a Beijing-based veterinarian, said he diagnosed a chronic, or less-lethal, form of the disease in pigs last year. The 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.
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Research has shown that deleting some MGF360 genes from African swine fever creates immunity. But the modified virus was not developed into a vaccine because it tended to later mutate back to a harmful state.
“You can sequence these things, these double deletions, and if it’s exactly the same as described in the lab, it’s too much of a coincidence, because you would never get that exact deletion,” said Lucilla Steinaa, principal scientist at the International Livestock Research Institute (ILRI) in Nairobi.
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.
The new strains could proliferate globally through contaminated meat, infecting pigs that are fed on kitchen waste. The virus is known to survive for months in some pork products.
China’s Ministry of Agriculture and Rural Affairs did not respond to two requests for comment.
But it has issued at least three warnings against use of unauthorised African swine fever vaccines, cautioning that they could have severe side effects and that producers and users could be charged with a criminal offence.
In August, the ministry said it would test pigs for different strains of the virus as part of a nationwide investigation into illegal vaccine use.
Any strains with gene deletions could indicate a vaccine had been used, it said. No findings have been published so far on the issue, which is highly sensitive for Beijing. Reporting of the recent African swine fever outbreaks was extensively covered up.
After decades of research toward producing a vaccine against the huge, complex swine fever virus, researchers around the world are focussing on live-virus vaccines – the only type to have shown any promise.
But such vaccines carry higher risks because even after the virus is weakened so it does not cause serious illness, it can sometimes recover its virulence.
One such vaccine used in Spain in the 1960s caused a chronic disease with swollen joints, skin lesions and respiratory issues in pigs that complicated efforts to eradicate African swine fever over the next three decades. Since then, no nation has approved a vaccine for the disease.
A vaccine with both MGF360 and CD2v genes deleted is undergoing trials by China’s Harbin Veterinary Research Institute after showing promise.
Yan said he believes that people have replicated the sequences of virus strains being studied, which have been published in scientific literature, and that pigs injected with illicit vaccines based on them could be infecting others.
“It’s definitely man-made; this is not a natural strain,” he said.
Neither Johnson nor Yan have fully sequenced the new swine fever strains. Beijing strictly controls who is allowed to work with the virus, which can only be handled in laboratories with high biosecurity designations.
But several private companies have developed test kits that can check for specific genes.
GM Biotech, based in China’s central Hunan province, said in an online post last week it had developed a test that identifies whether the pathogen is a virulent strain, a single-gene deleted attenuated strain, or a double-gene deleted attenuated strain.
The test helps pig producers because the new strains are “very difficult to detect at the initial stage of infection and have a longer incubation period after infection,” the company said.
The government has not said how widely used illicit vaccines are or who has produced them. But a “vast amount” of pigs in China have nonetheless been vaccinated, Johnson said, a sentiment echoed by many other experts.
In 2004-5, when the H5 bird flu strains were spreading across Asia, Chinese laboratories produced several unauthorised live bird flu vaccines, said Mo Salman, a professor of veterinary medicine at Colorado State University, who has worked on animal health in Asia, raising fears that they could produce dangerous new variants.
“The current ASF unlawful vaccine(s) in China is repeating history,” Salman said.
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