Case farm characteristics
Recently, a case of PEDV infection occurred in a pig farm in Jiangxi. There were 8082 sows on hand in the mating house of the pig farm, which were divided into 10 groups, with 800 pigs in each group. There were 902 sows in the farrowing house, with 80 farrowing beds as a unit and five units constituting a building. Before the onset of the disease, the farm recorded 25 weaned piglets per sow per year, and the loss of farrowing house was 8%. Loss from conservation and fattening was less than 6%. The pig farm regularly tested for enteric pathogens, and TGEV/PDCOV and PEDV had not occurred in the last two years. There had often been sporadic cases of porcine reproductive and respiratory syndrome (PRRS) on the pig farm.
Routinely used immunization regimens include vaccination of sows with live PRRS vaccine, swine fever vaccine, live pseudorabies vaccine, foot-and-mouth disease O + A vaccine, Japanese encephalitis vaccine and parvovirus vaccine. Sows were immunized with a live PED vaccine and an inactivated vaccine at 30 days before delivery and 14 days before delivery, respectively.
Clinical presentation and interventions
On October 28, 2020, at first, 80% of the sows in a farrowing unit developed depression, fatigue, and anorexia. Three days later, they developed mushy persistent diarrhea. The body temperature was normal or slightly higher, they showed anorexia and had fully recovered about 7 days after onset. Piglets presented with epidemic diseases with watery diarrhea and vomiting as the main clinical symptoms. Vomiting usually occurred after eating or feeding on breast milk. The severity of symptoms varied with age. The younger the age, the more severe the symptoms. In a period of 1 to 3 days after newborn piglets within 7 days of age developed diarrhea, they were severely dehydrated and died, with a mortality rate of 90–100%. Diarrhea occurred successively in sows in other units 5 days later. After on-site anatomy and laboratory testing of feces and tissues, PEDV was determined to be the pathogen underlying this epidemic. Therefore, on October 31, the staff closed the pigsty where diarrhea occurred, treated the sows and piglets with amoxicillin- or lincomycin-containing drinking water for health care, and disinfected the pigsty with Denaishuang dry powder and kept it dry; on November 7, intramuscular injection of autogenous vaccine was administered; on November 11, the farrowing house was disinfected, the pigsty was dried and decontaminated with hot water at 60℃, the sick pigs were given salt solution for replenishment, and the farrowing house was cleaned, disinfected, and dried; on November 13, weaned piglets with secondary infection were treated with antibiotics. On November 16th, the sow swinery of the mating house was restored, the personnel were released from lockdown, and the empty swinery units were cleaned and disinfected. After adopting a series of comprehensive prevention and control measures such as emergency vaccination with autogenous vaccine, the mortality rate of newborn piglets on the farrowing room of this farm dropped significantly at half a month after vaccination, and production gradually returned to stable (Fig. 1).
In general, four farrowing units showed clinical symptoms of PED, and the mortality rate of suckling piglets was 40%. A total of 85 sows died, and 187 litters had abortion, with an incidence rate of 30%. A total of 6,455 piglets died, with an incidence rate of 70%. At the current price, the net profit per pig was 300 US dollars, and the direct loss was nearly 2 million US dollars. Some weaned piglets also experienced diarrhea, weight loss, and growth retardation. The total loss from piglet feeding was 12%. In the fattening house, there was no sign of PED.
Epidemiological investigations revealed that at half a month before the outbreak of PED, PEDV broke out in another pig farm 800 meters away from the farm.
Laboratory findings
After preliminary confirmation of the PEDV pathogen by the pig farm laboratory, fecal samples, milk and serum of 20 sows, as well as the feces and intestines of three infected piglets were sent to South China Agriculture University for sequence analysis and serological testing. In order to reveal the molecular characteristics of the virus and determine the relationship between the current epidemic strains in China and other subtype strains, the Bootstrap method of MEGA6.0 software was used to analyze the evolutionary tree of domestic isolates and reference strains from different sources at different times, which indicated that the strain formed a new and unique branch in the PEDV genotype II (Fig. 2), which had the closest genetic distance to the genotype I strain. The S1 gene homology comparative analysis of this strain showed that it had 98.1%-100% homology with 59 epidemic strains uploaded by NCBI in the past year, 91.8% homology with CV777 strain, 97.8 homology with the AJ1102 vaccine strain, and 97.3%-98.1% homology with the 89 strains prevalent in 2014–2015 measured by our laboratory (Table 1), indicating that the PEDV of the current outbreak had mutated. Please see Supplementary Fig. 1 for specific similarities. The similarity with the 2020 strain was also poorer by 1–2% compared to the 2014–2015 strain, indicating that PEDV had continuously mutated.
Table 1
Comparison of S1 gene homology between PEDV epidemic strain and vaccine strain in 2020
Strain comparison
|
S1 homology
|
59 epidemic strains in 2020
|
98.1%-100%
|
CV777 strain
|
90.3%-92.1%
|
AJ1102 vaccine strain
|
96.8%-97.9%
|
89 strains from 2014 to 2015
|
97.3%-98.1%
|
To evaluate the immunizing effect of the PED vaccine in this farm, 20 diseased sows in farrowing season were randomly selected before the immunization with autogenous inactivated vaccines and numbered 1–20. Colostrum, feces, and serum were collected respectively to test the levels of PED antigen and IgA antibody in colostrum, as well as the levels of PED antigen in feces and specific IgG antibody in serum. The results showed that the pigs had a good immune response after immunization with the PED vaccine, with high levels of IgA and IgG antibodies. However, for the three sows numbered 8, 11, and 20, despite the high levels of IgA in milk and serum IgG antibodies, there were still detectable PED antigens (Table 2). PED antigens were detected in feces of more sows, indicating that the antibody levels produced by the existing commercial vaccines cannot completely neutralize the new epidemic strain of PEDV.
Table 2
Analysis of antibody and virus shedding of infected sows
No.
|
Antigen
|
Antibody
|
Colostrum
|
Feces
|
Milk IgA
|
Serum IgG
|
1
|
|
36.77
|
1.225
|
1.532
|
2
|
|
––
|
2.013
|
1.428
|
3
|
|
––
|
1.934
|
1.62
|
4
|
|
33.8
|
0.463
|
1.219
|
5
|
|
34.71
|
0.488
|
0.893
|
6
|
|
––
|
0.887
|
1.117
|
7
|
|
34.27
|
0.382
|
0.681
|
8
|
37.11
|
––
|
0.585
|
0.788
|
9
|
|
––
|
1.542
|
0.889
|
10
|
|
34.5
|
1.368
|
1.213
|
11
|
34.49
|
––
|
0.906
|
0.982
|
12
|
|
35.65
|
0.627
|
0.4136
|
13
|
|
––
|
1.327
|
1.348
|
14
|
|
––
|
2.323
|
1.787
|
15
|
|
––
|
0.185
|
0.685
|
16
|
|
––
|
2.086
|
1.782
|
17
|
|
––
|
0.041
|
0.182
|
18
|
|
32.12
|
0.862
|
0.464
|
19
|
|
––
|
0.808
|
0.686
|
20
|
38.21
|
35.32
|
0.332
|
0.514
|
To evaluate the fecal virus shedding of sows during the disease course, we conducted a two-week monitoring of virus shedding on 7 diseased sows. The results showed that the affected sows had intermittent virus shedding within two weeks after delivery. Some of the sows shed a great load of virus due to individual differences (Table 3).
Table 3
Monitoring results of continuous virus shedding of diseased sows
No.
|
11.3
|
11.4
|
11.5
|
11.6
|
11.7
|
11.8
|
11.9
|
11.10
|
11.11
|
11.12
|
11.13
|
1
|
-
|
-
|
37.74
|
-
|
36.26
|
36.44
|
-
|
36.12
|
-
|
38.34
|
38.06
|
2
|
-
|
35.97
|
-
|
34.49
|
36.03
|
-
|
36.17
|
37.37
|
30.66
|
35.58
|
-
|
3
|
-
|
36.99
|
-
|
-
|
-
|
33.78
|
-
|
39.64
|
36.41
|
36.94
|
-
|
4
|
-
|
-
|
-
|
-
|
-
|
39.16
|
35.03
|
34.37
|
36.68
|
-
|
-
|
5
|
-
|
35.24
|
36.72
|
-
|
-
|
-
|
36.93
|
33.49
|
33.8
|
-
|
-
|
6
|
-
|
36.13
|
35.71
|
30.21
|
36.03
|
-
|
-
|
32.08
|
-
|
-
|
-
|
7
|
-
|
-
|
-
|
29.87
|
-
|
-
|
-
|
37.43
|
37.1
|
38.25
|
-
|