Re-emergence of porcine epidemic diarrhea virus in a piglet-producing farm in Jiangxi province in China in 2020
Qi Gao1, Zezhong Zheng1,4, Heng Wang1,2,3, Guihong Zhang1,2,3,4*, Lang Gong1,2*
1College of Veterinary Medicine, South China Agricultural University, Guangzhou 510462, China
2Key Laboratory of Zoonosis Prevention and Control of Guangdong Province, South China Agricultural University, Guangzhou 510462, China
3National Engineering Research Center for Breeding Swine Industry, South China Agricultural University, Guangzhou 510462, China
4Guangdong Laboratory for Lingnan Modern Agriculture,Guangzhou, 510642, China
* Correspondence: gonglang@scau.edu.cn
Summary: On October 30, 2020, piglets and sows in the farrowing house of a pig farm in Jiangxi showed clinical symptoms such as anorexia, watery diarrhea, and vomiting. Epidemiological test, clinical necropsy, and RT-PCR test were carried out on the pig farm for diagnosis. After comprehensive considerations, the disease was judged as porcine epidemic diarrhea virus infection. Thereafter, a series of comprehensive prevention and control measures such as emergency vaccination with autogenous vaccines were adopted. Half a month after inoculation with autogenous vaccines for the farm, the mortality rate of newborn piglets in the farrowing house began to decline, and production gradually returned to being stable. The second-generation sequencing analysis and phylogenetic analysis showed that the porcine epidemic diarrhea virus (PEDV) sequence obtained from the stool and small intestine samples of the diseased pigs on the farm was 97.8% homologous to the vaccine strain. At the same time, antibody testing found that the vaccinated pigs on the pig farm had satisfactory immune response, indicating that the PEDV outbreak on the pig farm might aggravate owing to the strain being mutated and could escape the immune protection of the existing vaccine. This case has accumulated technical data for the clinical prevention and control of porcine epidemic diarrhea.
Keywords: cross protection, phylogenetic analysis, porcine epidemic diarrhea virus, strain variation
Background
Porcine epidemic diarrhea is an acute and highly contagious intestinal infectious disease caused by porcine epidemic diarrhea virus infection. The disease is prevalent in the cold season and is transmitted through fecal-oral transmission. Its clinical symptoms include elevated body temperature, anorexia, vomiting, diarrhea, dehydration, etc (Lee, C., 2015) Porcine epidemic diarrhea virus (PEDV) is a single-stranded RNA virus belonging to genus Coronavirus, family Coronaviridae, with a genome length of 27-31 kb (Jung, K et al., 2015). The disease was first discovered in the UK in 1971 (Debouck, P. et al., 1980) and successively discovered in China in the early 1980s (Xuanhu, 1984). Since October 2010, a new variant strain of porcine epidemic diarrhea has appeared, which has been widespread in China and other major pig-raising countries in the world, and its prevalence in East Asia and North America is severe, causing large economic losses to the farms (Wan Liang et al., 2020, Huang, Y.W. et al., 2013). PEDV infection has become one of the main reasons for the high mortality of suckling piglets.
Vaccination is an important way to control infectious diseases. In 1994, Ma et al. used the attenuated CV777 strain to prepare an inactivated vaccine with aluminum gel adjuvant. After immunizing piglets, the active and passive protection efficiency exceeded 85% (Ma Siqi, 1994). In 1995, the study team successfully developed a commercial TGEV+PEDV bivalent vaccine. In 1998, Tong et al. confirmed that the CV777 strain obtained by continuous in vitro passage for 90 generations is suitable for the preparation of attenuated vaccines. In 1999, they successfully developed a PEDV+TGEV bivalent vaccine. The ratio of virion counts of TGEV to PEDV was 1:1. After immunization, the active and passive protection rates against PEDV were as high as 97.7% and 98%, respectively (Tong Youen, 1999, Tong Youen, 1998). The PEDV vaccine strains in these two commercial vaccines were both classic CV777 strains. Before 2010, these two bivalent vaccines were widely used in China, effectively controlling the spread of PEDV and TGEV.
After the outbreak of PED in the United States in 2013, Collin et al. prepared an inactivated vaccine of the American isolate (G2 genotype) and proved that inactivated PEDV can induce sufficient humoral immunity level in immunized 4-week-old piglets (Collin, E.A., 2015). The PEDV strain used in the PEDV-TGEV and PoRV trivalent attenuated live vaccine produced by the Harbin Veterinary Research Institute subordinated to the Chinese Academy of Agricultural Sciences in early 2015 was still CV777. The PEDV strain in the bivalent vaccine marketed in November of the same year was ZJ08, which belongs to the G1b subtype (Liu Hao, 2018). In December 2017, the PEDV bivalent inactivated vaccine and attenuated vaccine prepared by Prof. Xiao of Huazhong Agricultural University were successfully marketed. The vaccine strain was AJ1102, which was isolated in 2011, and belongs to the G2b type (Jing Bi, 2012).
In the winter of 2020, a new wave of PEDV epidemic swept across China. The question is, why was there a large-scale outbreak of PEDV under the conditions that biosafety control was so strict under a non-plague background, and whether new strains have emerged. Here, we report a clinical case of PEDV in Jiangxi, which indicates that China’s existing PED variant vaccine may not fully protect against the PEDV epidemic strain.