ASFv mutation in China: What does it mean on-farm?

Most messages about China’s pig industry in 2020 and early 2021 are very positive. Even though the African Swine Fever (ASF) outbreaks of 2019 led to the estimated disappearance of at least 40% of the country’s sow herd, the shortage brought on record prices and profits. A country on the rebound, now swine facilities are built everywhere.

For instance, early December 2020 news agency Reuters sent out an impressive article covering a huge 21 multi-storey building swine farm by Muyuan in Henan province, eventually to be housing 84,000 sows. Yet, does that mean that ASF is something of the past? Officially, it does not seem to be a huge problem. In January 2021, the Chinese Ministry of Agriculture and Rural Affairs (MARA) reported 1 infected farm in Guangdong province with 1,015 pigs. The report before that dates from October 2020.

Variant ASF cases on farms

There are plenty of reasons to believe that the ASF epidemic is far from over in China, that is, if the virus will ever completely disappear. Mid-January, Reuters reported that various variant ASF viruses had been found in China and attributed that to illegal ASF vaccines (see Box). Dr E. Wayne Johnson is a US pig veterinary consultant in Beijing and he confirms coming across one of these variants.

In a 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 2 important things removed”, but can still eat and eat.

Effects of the new virus seen on-farm

According to Dr Johnson, the double gene deleted vaccine produces a PRRS-like viral reproductive syndrome in vaccinated sows, with stillbirths, mummifications, embryonic deaths, infertility, and abortion. He adds there is some apparent mortality as well as transient fever from the vaccine virus itself, but that vaccine-induced mortality is rather low.

Live-born pigs, he says, may be born weak with reduced viability. “Pigs that survive into the grow-finish period appear to shed the vaccine virus and pass it to other pigs. Chronic atypical 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.”

Is it a natural deletion mutation?

It is common in nature that viruses tend to mutate from time to time. Yet, there is no indication that the virus emerging in China is a natural deletion mutation, Dr Johnson says. “Rather, it is the result of deliberate introduction of the double-gene deletion vaccine virus into a substantial proportion of the pig population. “Since the vaccine is unapproved, illegal, and officially forbidden, there is no reliable information on just how much vaccine 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 vaccinate 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 consider that they have limited options. The introduction of the double-gene-deleted vaccine is an unwanted complication to a ghastly problem and an epidemiologic situation that is wildly out of control.”

Effect of illegal vaccine on pigs

Dr Johnson has observed the effect of the illegal vaccination on the pigs. “Vaccinated pigs generally have antibodies against the ASF virus but not all pigs have them. The negative effect of the illegal vaccine on sows and reproduction is frustrating and causes farms to stop using it, but the vaccine virus is contagious and appears to persist in the herd even after vaccination has been discontinued. Antibodies are detectable 2 to 3 weeks after vaccination. The 5-20% or so of pigs that survive a full-fledged outbreak of ASF generally have antibodies and the ASF wild type virus can be detected weeks and months later in their blood and tissue.”

Track the movement of African Swine Fever

For everything you need to know about ASF, from the latest outbreaks to controls stay up to-date…

Detection of the vaccine virus

Detecting the new vaccine virus by PCR can be rather difficult, Dr Johnson says. “Oral swabs are not reliable for ASF and are even less sensitive to the vaccine virus. The double-gene-deleted vaccine virus also may not be present in the blood but it is often possible to find it in lymph nodes and other tissues when the blood is negative.”

3 genes usually give away whether ASF virus is present in a pig. Dr Johnson says, “The ASF virus is detected by a PCR-test for the P72 gene. If P72 is detected, a second test for either CD2v, MGF360, or both will be conducted if it is desired to know if it is the field (wild type) virus or the vaccine virus. Wild type ASF virus should be positive for all 3 genes (P72, MGF360, and CD2v). The double deletion vaccine virus does not possess MGF360 or CD2v but does have P72 so the P72 will be PCR positive and the MGF360/CD2v will be negative.”

Official Chinese research into ASF vaccine

The Chinese authorities are working on an ASF vaccine as well but that development appears entirely unrelated to what is happening now with the illegal vaccines. After all, scientists at Harbin Veterinary Research Institute have openly published about their progress during 2020 and Pig Progress published about the progress of this vaccine in March and July. International experts expressed moderate optimism about their vaccine.

The Harbin live-attenuated virus has a deletion of 7 genes. In doing so, it builds on the approach taken by the Plum Island Animal Disease Center, part of the US Department of Agriculture’s Agricultural Research Service (ARS). Plum Island researchers deleted 6 genes but thought deleting a 7^th [CD4] didn’t improve the results.

Also, in Vietnam there are developments towards developing an ASF vaccine

The Harbin vaccine is still being researched, notes Dr Johnson, “As far as we know, the Harbin vaccine with 7 genes deleted has not been released onto the market. It is missing the 2 genes cut from the double deleted vaccine plus 5 other gene deletions as reported in their research publication. They would not likely release their vaccine into the market prematurely but should follow the procedures to the letter. “We know for sure that Harbin has been doing safety testing in the field on a limited and above-board basis as permitted by the applicable regulations.”

Origin of the illegal ASF vaccine

In other words, the origins of the illegal vaccine are a mystery. Dr Johnson says, “We do not know who is making and selling the bootleg vaccine. I have no verified or verifiable information on it. There is no clear information about how many illegal vaccines there are. There are legal tests going on in the field and under controlled conditions by reputable and decent scientists. The illegal vaccines are distributed by some well-organised hooligans and mountebanks. The farms are under extreme pressure and duress from the disease and are susceptible to the wiles of such predators.”

In his day-to-day practice, illegal vaccines are not found. “Our direct consultation clients do not use the vaccine and if they have been able to follow our instructions, they have avoided ASF entirely although they are under heavy pressure and much stress.”