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ASFv mutation in China: What happens on-farm?
How is China’s pig industry dealing with the aftermath of African Swine Fever (ASF)? Although most messages appear to be positive, there is also reason to remain vigilant. ASF virus (ASFv) appears to be popping up in a mutated form. Dr E. Wayne Johnson, veterinarian in Beijing, describes what he sees happening on farms.
BY VINCENT TER BEEK, EDITOR, PIG PROGRESS M
ost messages about China’s pig industry in 2020 and early 2021 are very positive. Even though the ASF outbreaks of 2019 led to the estimated loss of at least 40% of the country’s
sow herd, the shortage led to record prices and profits. A country on the rebound, China now has swine facilities being built everywhere. For instance, early in December 2020 news agency Reuters published an impressive article about a huge complex being built by Muyuan in Henan province of 21 multistorey build- ings that will become a swine farm, eventually housing 84,000 sows. Does that mean that ASFv is a thing of the past? Officially it does not seem to be a huge problem. In January 2021, the Chinese Ministry of Agriculture and Rural Affairs reported that one farm was found to be infected in Guangdong prov- ince with 1,015 pigs; the last report before that was from October 2020.
Variant ASF cases There is plenty of reason to believe that the ASF epidemic is far from over in China – if the virus will ever completely disap- pear. In mid-January, Reuters reported that a number of vari- ant ASF viruses had been found in China and attributed that to illegal ASF vaccines (see Box). US veterinarian Dr E. Wayne Johnson is a pig veterinary consultant in Beijing, and he con- firms having come across one of these variants. In correspondence with Pig Progress, he writes: “The ‘new virus’ circulating in China is the double-gene-deleted vaccine virus. It lacks safety and its efficacy is not clear. This vaccine has deletions in the MGF360 locus and the CD2v locus, which weakens the virus but does not render it innocuous. It just produces a milder form of the disease.” Dr Johnson compares the vaccine to a barrow “which has had
6 ▶ PIG PROGRESS | Volume 37, No. 1, 2021 two important things removed”, but can still eat and eat.
Effects seen on-farm According to Dr Johnson, the double-gene-deleted vaccine produces a Porcine Reproductive and Respiratory Syndrome– like viral syndrome in vaccinated sows, with stillbirths, mum- mifications, embryonic deaths, infertility and abortion. He adds there is some apparent mortality as well as transient fe- ver from the vaccine virus itself but that vaccine-induced mortality is rather low. Live-born pigs, he says, may be born weak with reduced via- bility. “Pigs that survive into the grow-finish period appear to shed the vaccine virus and pass it to other pigs. Chronic atypi- cal ASF is the result with chronic blood vessel and immune system disease. Infarcts and thrombi with active zones of necrosis and haemorrhage and chronic lesions with fibrosis are seen in multiple organs microscopically and visibly, particularly in the lymph nodes and the kidneys.”
Natural deletion mutation Viruses commonly mutate in nature. Yet there is no indication that the virus emerging in China is a natural deletion muta- tion, Dr Johnson says. “Rather, it is the result of deliberate in- troduction of the two-gene-deletion vaccine virus into a substantial proportion of the pig population. “Since the vaccine is unapproved, illegal and officially forbid- den, there is no reliable information on just how much vac- cine was used and how much of the presence of the vaccine virus is due to secondary spread from pigs shedding the virus and transmission by fomites.” Dr Johnson goes on to say: “It is officially forbidden to vacci- nate with the bootleg ASF vaccine, but there are indications that vaccination is still continuing, as ASF is widespread in many regions of China and farmers feel desperate and con- sider that they have limited options. “The introduction of the two-gene-deleted vaccine is an unwanted complication to a ghastly problem and an epidemiologic situation that is wildly out of control.”
The illegal vaccine Dr Johnson has observed the effect of the illegal vaccination on the pigs. He says, “Vaccinated pigs generally do have anti- bodies to ASFv, but not all pigs have antibodies. The negative effect of the illegal vaccine on sows and reproduction is frus- trating and causes farms to stop using it, but the vaccine virus
PHOTOS: VINCENT TER BEEK
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