Epidemiological Investigation Of Circovirus And Optimization Of Immune Program In Large-scale Pig Farms In Shandong Province

As the main province of pig farming in China,porcine circovirus disease is one of the most important diseases harmful to the pig industry in Shandong.Although pig farms are generally immunized with the disease,the immune effect of many pig farms is not ideal and can not significantly reduce the incidence of the disease.Based on the detection results of clinical samples from large-scale pig farms and experimental farms in Shandong Province from 2018 to 2019,this study conducted an epidemiological investigation and analysis of porcine circovirus disease in large-scale pig farms in Shandong Province.evaluate the immune effect of the disease and adjust and optimize the immune program.The results of etiological investigation showed that in 2018,the positive rate of PCV2 in the tested samples was 27.11%(125/461),of which the positive rates in the first,second,third and fourth quarters were 25.93%,25.71%,12.00% and 40.54%,respectively;in 2019,the positive rate was 22.03%(87/395),and the positive rates in the first,second,third and fourth quarters were26.67%,19.13%,15.32% and 30.63%,respectively.The ORF2 genes of 37 PCV2 isolates from Shandong were cloned and sequenced.the results showed that the prevalent strains in Shandong were mainly 2.1d subtype,followed by 2.1b and 2.1a.The nucleotide and amino acid homology among the isolates were 89.6% ~ 100% and 87.2% ~ 100%,respectively,and the nucleotide and amino acid homology with other reference strains at home and abroad were81.1% ~ 100% and 81.2% ~ 100%,respectively.The results of serological investigation showed that the overall antibody positive rate of PCV2 in 2018 was 89.03%,the average S / P value was 1.47,and the coefficient of variation was 37.86%.In 2019,the overall antibody positive rate was 94.21%,the average S / P value was 1.66,and the coefficient of variation was 38.45%.In many pig farms,the Sripp value of sows,boars and reserve pigs is too high.Based on the fact that the breeding herds are less immunized,it is speculated that there is a higher proportion of wild virus infection and the vertical transmission pressure of PCV2 is higher.The antibody positive rate and average S / P value of commercial pigs decreased from2 weeks old,increased gradually at 10 or 13 weeks old,and the coefficient of variationincreased from 2 weeks old,and decreased gradually at 10 weeks old or 13 weeks old.According to the detection and analysis of PCV2 antibody levels in different immunization periods,it was found that the maternal antibody levels of 14-day-old piglets in most pig farms were higher,had better protective power and could interfere with the early vaccine immunity of piglets.After 14-day-old immunization,the antibody level only increased slightly within 1month,and gradually decreased to a lower level 2 months after immunization,losing the protection of the vaccine,which led to wild virus infection within 2 ~ 3 months after immunization.In the experiment of exploring the detoxification of PCV2 in infected pigs,it was found that the amount of virus excretion increased from 3 weeks to 5 weeks of age.It was speculated that due to the early infection of vertical transmission,the amount of virus excretion decreased gradually at 7 weeks of age,increased again after 12 weeks of age,and reached the highest level at 20 weeks of age,suggesting that wild virus infection was taking place in this period.The results of pathogen and antibody detection showed that the immune effect of PCV2 vaccine in most large-scale pig farms was not reasonable,and it was urgent to establish a scientific and reasonable immunization procedure.The results of immune program optimization showed that in this study,the immune program of a proving ground was adjusted from 14-day-old once to 14-day-old piglets,35-day-old second immunity,reserve sows were immunized once at the age of 5 months,and then enhanced before mating.Breeding boars and parturient sows are not immune.The overall positive rate of PCV2 antibody in pigs in each stage increased from 65.49% before adjustment to 94.20%.After the first immunization and the second enhanced immunization,the positive rate increased to 100%,and there was no virus-carrying and detoxification in pigs at all stages,which achieved a good protective effect and reduced the pressure of infection.However,there are some defects such as low antibody level in high gestational age sows and low maternal antibody level in 2-week-old piglets.Therefore,the conclusions of this study are as follows:(1)the infection rate of PCV2 in large-scale pig farms in Shandong is relatively high,and there are generally clinical cases of PCVAD.The disease is prevalent in four seasons,and the infection and incidence rate is high in winter and spring.The prevalent strains are mainly 2.1d subtype,followed by 2.1b and 2.1a subtypes.(2)the immune effect of PCV2 in Shandong large-scale pig farms in 2019 is betterthan that in 2018,but the PCV2 immune program in most pig farms needs to be adjusted and optimized.There is still an immune risk period of about 11 weeks of age for piglets.Piglets at this stage are easy to be infected by wild virus,resulting in a higher amount of detoxification in 20-week-old pigs.(3)based on epidemiological investigation and immune detection and evaluation,the immune procedure of the test site was adjusted as follows: 14-day-old piglets were first immunized,35-day-old second immunized,reserve sows were immunized once at the age of 5 months,and then enhanced before mating;the sows were immunized with the fetus at 3-5 weeks before delivery,and the breeding boars were immunized twice a year,and a good immune effect was achieved.