Molecular Characterization of African Swine Fever Virus, China, 2018

On August 3, 2018, an outbreak of African swine fever in pigs was reported in China. We subjected a virus from an African swine fever–positive pig sample to phylogenetic analysis. This analysis showed that the causative strain belonged to the p72 genotype II and CD2v serogroup 8.

sample for conventional PCR amplification with 3 pairs of primers. We amplified 3 gene fragments: a partial gene fragment of the B646L gene encoding the p72 capsid protein (3), a fragment of the EP402R gene encoding the CD2v protein (4), and a tandem repeat sequence (TRS) located between the I73R and I329L genes (5).
We subjected 3 amplified products to nucleotide sequencing and deposited the resulting sequences in GenBank (accession nos. MH722357, MH735142, and MH735144). We used the p72 fragment sequence for phylogenetic analysis of the genotype, and the CD2v fragment sequence for phylogenetic analysis of the serogroup (6). We constructed 2 phylogenetic trees by using MEGA 5.0 software (https://www.megasoftware.net/). These trees showed that the causative strain (China 2018/1) in this study belonged to p72 genotype II ( Figure,  Genotype identification of ASFV often depends on partial p72 gene characterization (3). During ASF outbreaks, this genotyping approach can be used to identify possible origins of viruses and differentiate them from closely related strains (7). In this study, we classified China 2018/1 as genotype II ( Figure, panel A), the sequences we obtained had extremely high homology with those of other genotype II strains, therefore suggesting the origin of China 2018/1 from a homogenotypic strain.
In addition to conventional genetic typing, serologic typing is another method for classifying ASFVs on the basis of hemadsorption inhibition (HAI). Eight ASFV serogroups have been identified (6). Moreover, HAI typing places ASFV into discrete serogroups not necessarily resolved by the p72based genetic typing. For example, serogroup 1, 2, and 4 strains can be simultaneously classified as having the P72 genotype I (7). In this study, we found that China 2018/1 belonged to serogroup 8 as determined by phylogenetic analysis, suggesting the same HAI characteristics as those for other strains in the homoserogroup ( Figure, panel B).
ASF causes devastating socioeconomic consequences in the global pig industry, especially for countries with large-scale pig production and pork consumption. After the confirmation of ASF outbreak in China in August 2018, we characterized the causative strain, China 2018/1, by phylogenetic comparison with previous strains. We classified this new strain as having the p72 genotype II and 100% p72 sequence identity with several strains from eastern Europe and Africa, such as Bel13/Grodno, Voronezh 2016, Mal 2011/01, and ZIM/2015/01.
On the basis of serologic typing, we found that China 2018/1 belonged to the CD2v serogroup 8. In addition, this strain had a 10-bp additional fragment (5′-GGAATATATA-3′) in its TRS when compared with others in the homogenotype.
Because ASFV is a large DNA virus with a stable genome, it is hardly unexpected that the sequence of China 2018/1 had extremely high homology with sequences in genotype II over a wide geographic area. However, it remains to be elucidated from where China 2018/1 was introduced into China.  (1). The report illustrates the increased risk of infection for persons living in a household with someone with iGAS infection. We write to draw readers' attention to our recent study, which adds to the body of evidence on the risk of household transmission of iGAS (2).
Population-based studies from Australia, Canada, the United Kingdom, and the United States, based on 13 household clusters, assessed the risk of transmitting iGAS infection through household contact (3). We identified an additional 24 household clusters in England using addresses captured through national surveillance in 2009 and 2011-2013. For all 12 clusters in which emm typing was performed on both patients, results were the same for both. All secondary cases occurred within 1 month of the index case (median 2 days). Among contacts, the 30day incidence rate was 4,520/100,000 person-years, 1,940 times higher than the background incidence (2.34/100,000 person-years). Spouses and partners ≥75 years of age (6 pairs) were at particularly high risk for developing infection; incidence was estimated at 15,000 (95% CI 5,510-32,650)/100,000 person-years, 1,650 times higher than the background risk in this age group (9.09/100,000, 95% CI 5,510-32,650). These data resulted in an estimated number needed to treat of 82 (46-417).
Duployez's article also highlights differences between countries in policies for antimicrobial chemoprophylaxis. National guidance for public health management of community iGAS infection is being revised in the United Kingdom; oral penicillin V is currently recommended as the first choice for chemoprophylaxis (4). However, questions remain about the efficacy of chemoprophylaxis and the practicalities of timely administration to benefit others in a household, given that 38% of pairs were co-primary cases or had only 1 day between initial and subsequent infections.