Study On Epidemiology And Etiology Of Congenital Ttremors Of Piglets

Congenital tremors (CT) of piglets, commonly called trembling disease, has the character of myelination retardation in central nervous system and clonic cramp in muscle. Up to now, there has no effective therapeutic measure for this disease and most of the diseased piglets are dead or wed out, which leads to much economic loss in pig industry. There are many etiological factors that can evoke CT directly or indirectly, including nutritional factors, genetic factor, viral infection, etc. More and more evidences have been proved that viral infection is the main causes.In order to study the epidemiology of CT of piglets and give a further research on the relation between infectious factor and CT of piglets, CT incidence was investigated in 21 intensive pig farms of Longyan city from February 2008 to December 2010. Moreover, serology and etiology of some related viruses such as classical swine fever virus (CSFV), porcine circovirus type 2 (PCV-2), pseudorabiesvirus (PRV) and porcineparvoviurs (PPV) were also investigated by ELISA, PCR, viral isolation and animal inoculation. The results were as follows:1. Piglets were infected with CT at any time of the year, showing periodic and sporadic, and the incidence rate is 1.4%-35.0%. The cases were found in piglets from primiparous sows and multiparous sows, and the cases were more common in piglets from multiparous sow. The pig farms, in which the newborn piglets were infected with CT and the sows belonged to multiparous sow, accounted for 71.4% (15/21) of survey farms. There was no correlation between CT of piglets and vaccination of classical swine fever vaccine and swine pseudorabies vaccine to sows, including the types of vaccines. The main pathological characteristics were dark brown to lymph nodes, renal cortex and periphery bleeding in the piglets. There has no effective therapeutic measure except adjuvant therapy for this disease.2. Two hundred and eighteen serum samples from sows in nineteen diseased intensive pig farms were detected for the antibody of CSFV, PCV-2, PRV-gE and PPV by ELISA. The results indicated that:①The immunizing effect of classical swine fever vaccine to sows was lower in diseased pig farms. 52.6 percent of the pig farms (10/19) were immune-unqualified. The antibody titers over the protective threshold in the sows whose newborn piglets were infected with CT, were obviously lower (54.0%, 34/63) than those (78.1%, 121/155) in the sows whose newborn piglets were normal.②The PR antibody positive rates were 42.1%(8/19)in diseased pig farms. The PRV-gE antibody positive rates in PR antigen positive pig farms, were 50.0%(28/56)for the sows whose newborn piglets were normal, 57.8%(22/38)for sows whose newborn piglets were infected with CT.③Sows were found to be infected with PVC-2 in diseased pig farms and the PCV-2 antibody positive rates were 75%-100%, which was not obviously different from those of sows (more than 94%) in PCV-2 antiboby negative pig farms.④The PPV antibody positive rates were 27.8% in diseased pig farms.3. Eleven tissue samples from diseased piglets were detected for virus antigen of CSFV, PCV-2 and PPV by PCR, showing that antigen positive rates of CSFV, PCV-2 and PPV were 81.8%, 87.5% and 37.5%, respectively. The mixed infection rate of CSFV, PCV-2 and PPV was 25.0%, while the mixed infection rate of CSFV and PCV-2 was 50.0%, and the mixed infection rate of CSFV and PPV was 12.5%.4. Sequence analysis showed that there was little difference in the nucleotide sequence of E2 gene of the seven CSFV strains gained in the study. E2 gene of the seven CSFV strains, whose nucleotide homogeneity was 99.0%～100%, shared 95.1%～96.2%9, 83.3%～84.7 and 83.3%～84.7 identity with the reported E2 gene of Alfort strain (Accession No. X87939), HCLV vaccine (Accession No. AF091507) and Shimen strains (Accession No. AF092448) in Genebank, respectively. Cap gene of the five PCV2 strains gained in the study, whose nucleotide homogeneity was more than 98.0%, shared 98.7%～99.4% identity with the reported standard sequence (Accession No. AF166528.1) in Genebank. VP2 gene of the three PPV strains gained in the study, whose nucleotide homogeneity was more than 99.4%～99.6%, shared 99.0%～99.8% identity with the reported standard sequence (Accession No. HM355807.1) in Genebank.5. The results of viral isolation and identification showed that cytopathic effect (CPE) was found in two cultures after the tissue suspension from four PCR-positive cases was co-cultured with PK-15 cells respectively. One culture was CSFV, PCV-2 and PPV antigen positive by PCT-based diagnosis, two cultures were CSFV and PCV-2 antigen positive, and one culture was CSFV and PPV antigen positive.6. Animal inoculation with PRV show the rabbits, which were injected with the tissue suspension from eleven cases and two cell cultures with CPE present, appeared normal and showed PRV antigen negative by PCR.7. The CT of piglets was controlled effectively after all the sows were vaccinated emergently with classical swine fever vaccine in fifteen diseased pig farms.In conclusion, the results of the present study showed that the incidence rate of CT was not high. Piglets were infected with CT periodically and sporadically and piglets from mutliparity sows were infected more susceptibly. There was no correlation between CT of piglets and vaccination of classical swine fever vaccine and swine pseudorabies vaccine to sows, including the types of vaccines. The main pathological characteristics were dark brown to lymph nodes, renal cortex and periphery bleeding in the piglets. The incidence of CT in piglets was not associated with PPV, PCV-2 and PRV infection in piglets, whereas there was an obvious correlation between the incidence of CT in piglets and CSFV infection in sows, especially the co-infection of CSFV and PCV-2 in sows. It is presumed that PCV-2 might play an important synergistic role in the infection of CT. Further study should be carried out to copy the case of CT in SPF pigs by animal regression experiment.