#19,057
We've a bit of a medical mystery today courtesy of an early release research article published this week in the EID Journal; one that touches on several topics we've discussed previously.
In a nutshell, researchers in Thailand recently tested two unvaccinated swine herds - hundreds of miles apart - which were showing signs of chronic infection.
The pigs tested positive for ASF, and sequencing of field samples from each herd revealed a unique genomic signature; one that indicates they originated from a live attenuated vaccine-derived virus.
The problem being, there are no globally approved and licensed ASF vaccines on the market (and these pigs were never vaccinated).
Vietnam is the only country permitting commercial use of an ASF vaccine (NAVETCO), and while based on a similar backbone ASF virus, it uses a different technique to attenuate the virus and contains an easily recognizable DIVA (Differentiating Infected from Vaccinated Animals) marker.
In 2024, however, we learned that a genetically modified live-attenuated African swine fever virus (ASFV-GUS-Vietnam) had been found circulating in Vietnamese swine.While the Vietnam strain's origin remains unknown, it was thought likely the result of unauthorized experimental use, a black-market vaccine, or a lab accident. And it is a much closer match to the strains recently detected in Thailand.
How it - or a close relative to it - ended up in Thailand is unknown, although possibilities include the use of illicit or illegal vaccines, cross border trade of pigs or pork products, and/or the horizontal transmission of the virus.
Volume 32, Number 2—February 2026
Research Letter
Vaccine-Like African Swine Fever Virus Strain in Domestic Pigs, Thailand, 2024
Trong Tung Nguyen, Dhithya Venkateswaran, Anwesha Prakash, Quynh Anh Nguyen, Roypim Suntisukwattana, Anan Jongkaewwattana, Theeradej Thaweerattanasinp, Janya Saenboonrueng, Van Phan Le, and Dachrit Nilubol
Abstract
African swine fever virus genotype II is endemic in Thailand, typically causing acute disease. We investigated a vaccine-like strain, characterized by 6 multigene family gene deletions, from nonvaccinated herds. We found this strain was associated with chronic disease in pigs.
African swine fever (ASF) is a fatal hemorrhagic disease of pigs, caused by African swine fever virus (ASFV), a complex DNA virus in the Asfarviridae family (1). Researchers first identified ASF in Kenya in 1921, and subsequent reports identified 24 genotypes in Africa on the basis of nucleotide variations within the partial B646L gen (2,3). Reports in the medical literature confirm incidence of only ASFV genotype I and genotype II outside Africa.
In 2018, researchers identified ASFV genotype II in China (4), and it rapidly spread across Asia within a few months. Since then, the situation in Asia has shifted from an epidemic to an endemic stage, with the highly virulent genotype II strain causing peracute, acute, and subacute disease. Recent research suggests the emergence of more genetically diverse ASFV variants, including chronic disease–associated genotype I, highly virulent recombinants of genotypes I and II, and naturally and artificially attenuated strains in domestic pigs in China and Vietnam (5–7).
Thailand health authorities officially reported ASFV in Thailand in 2022 (8), and the strain was genetically identical to the strain first reported in China and Vietnam. Currently, ASF cases in Thailand involve patients with chronic symptoms and low mortality rates, suggesting the emergence of low-virulent strains. We conducted a survey of ASFV from recent outbreaks in Thailand, employing whole-genome sequencing to investigate the underlying causes.
Veterinary clinicians reported suspected disease in pigs from 2 herds located in the western region of Thailand, ≈500 miles apart, all displaying clinical signs related to chronic forms of ASF: chronic respiratory disease, joint swelling, slow weight gain, and sporadic deaths. Both herds housed only finishing pigs, operating on an all-in/all-out basis, and pigs were not vaccinated with any types of ASF vaccines.
(SNIP)
We detected ASFV in 18 of 25 samples; cycle threshold (Ct) values ranged from 19.82 to 33.83. We performed whole-genome sequencing on samples with the lowest Ct from each herd,(Ct 19.82 for sample TH1_24/RB and Ct 21.25 for sample TH2_24/RB).
(SNIP)
Both genomes revealed the deletion of 6 genes in the multigene family (MGF) region (MGF 505-1R, MGF 360-12L, MGF 360-13L, MGF 360-14L, MGF 505-2R, and MGF 505-3R) and 2,348 bp of an Escherichia coli GusA gene (GUS) inserted at the deletion site (Figure 1).
This deletion pattern was like a live-attenuated vaccine strain (ASFV-G-ΔMGF) and a field-attenuated isolate (ASFV-GUS-Vietnam) described in previous studies (6,10). Phylogenetic analysis based on full-length genome indicated that the 2 isolates belonged to genotype II; however, the isolates were genetically distinct from the genotype II variant responsible for the first outbreak in Thailand (Figure 2). The 2 variants contained a total of 15 mutations throughout the genome, mostly silent and in noncoding regions, when compared with the Georgia 2007/1 strain. In addition, a 3-nucleotide insertion resulted in 1 additional amino acid in the MGF 110-10-L-MGF110-14L fusion protein.
In conclusion, we characterized a vaccine-like genotype II strain, similar to ASFV-G-ΔMGF, detected in finishing pigs unvaccinated against ASFV in Thailand. The spread of such vaccine-like strains with MGF deletions in this region is of concern, and the origin of the strains remains unknown. Further genomic surveillance and epidemiologic tracing would assist in clarifying the route of introduction. Possible explanations include the unauthorized use of live attenuated vaccines or cross-border movement of pigs and pork products.
Mr. Nguyen is a veterinarian and a graduate student at the Department of Veterinary Microbiology, Chulalongkorn University, Thailand. His research interests focus on molecular characterization and virology.
In the past, we've seen reports of vaccines being manufactured in squalid and primitive conditions (see YouTube video) with absolutely no testing for purity, contents, or strength.
The trade in illegal and often counterfeit vaccines, drugs, and medical devices fuel a global multi-billion-dollar industry, and we've seen repeated warnings (see The Lancet: WHO Estimates That 50% Of Drugs For Sale Online Are Fake) of the harm they can cause.
While exact numbers are impossible to come by, there are estimates that hundreds of thousands of people are killed each year from fake or adulterated drugs, and they can also help generate antibiotic and antiviral resistance.
While pandemics may be inevitable, self-inflicted wounds are often the hardest to deal with.
