| Gosling plague is an acute or sub-acute infectious disease of goslings and young Muscovy ducks, which is caused by goose parvovirus (Gosling Plague Virus, GPV). The disease is characterized by rapid transmission, high mortality, and large damage and predominantly affects 4 to 30-day-old goslings. Controlling large-scale epidemic of the disease mainly relies on the production of immune breeder geese to improve the level of gosling maternal antibodies, or the injection of hyperimmune serum to 1-day-old gosling. However, the disease still occurs clinically at an incidence of 20 % each year. It is mainly therapied by gosling plague anti-serum. Although this has a good effect, other infectious diseases spread easily and their cost of production is very high. There have been reports of the curative effect of Chinese herbal formula containing isatis root in the treatment of the disease, and they get good results. It has been reported that isatis root and its polysaccharide has effective anti-viral effect against more than 10 kinds of Virus such as the influenza virus, porcine parvovirus and Newcastle disease virus in the cells or embryos. In view of these, the research of isatis root polysaccharide on anti-GPV effect was carried out in this study.In the present study, the anti-GPV effects of isatis root polysaccharide (IRPS) and isatis root decoction were investigated on three levels: goose embryo, goose embryo fibroblasts and artificial infection in vivo. Experiments can be summarized as follows:Test 1, Antiviral effects of IRPS against GPV in goose embryoTo explore the direct antiviral effects of IRPS against GPV, IRPS was used directly to mix with GPV and then the mixing solution was injected into the goose embryos. In another treatment, the goose embryos were given IRPS in advance, after 24 h, the GPV were inoculated for investigating the infection interruption of IRPS against GPV. The contents of GPV in allantoic fluid from goose embryos, survival time of goose embryo, survival rate, lesion and immunohistochemical assay were conducted. The result indicated the embryos in infection interruption group were injected with 40μg IRPS, 72 h after inoculation by GPV, the highest positive dilution of allantoic fluid detected by ELISA was 23, the average survival time of goose embryos was 192 h with a survival rate of 6/6, and no visual lesion was observed. However, histopathological examination showed slight congestion and degeneration could be found in liver and kidney cells by microscopic observation, slight congestion and edema were also found in heart and brain tissues. The special GPV antigen in liver and kidney tissues from infection interruption group was weakly positive, and the antigen in heart and brain tissues was negative detected by immunohistochemical method. In contrast to that, the highest dilution of allantoic fluid from embryos in infection control group inoculated with GPV was 28, the average survival time was 138 h with no survival embryos. The dead embryos appeared serious atrophia, hemorrhage, edema and plasmatorrhexis, necrosis, GPV antigen in detected tissues of infection control group was all strong positive. So there were significant differences between the two groups. The results showed that IRPS can powerfully interrupt the infection and inhibit the proliferation of GPV and protect the embryos from injury by GPV.Test 2, Study of the Inhibitory Effects of IRPS on GPV In VitroThe test aimed at investigating the inhibitory effects of IRPS on GPV in goose embryo fibroblast (GEF). In this experiment, the GPV cell adapted strain was cultivated, and the effects of IRPS on proliferation of GEF were measured. On the foundation of above, the GPV cell adapted strain was used to investigate the infection interruption, proliferation inhibition and direct inactivation roles of IRPS on GPV in GEF by MTT assay. The results showed that the cell-adapted GPV could infect cytopathic effect of fibroblasts, that IRPS with the concentrations of 100~1000μg/mL significantly promoted GEF proliferation, and that IRPS with the concentrations of 100~750μg/mL could significantly block the infection of GPV or even inactivate it directly, while it had moderate inhibitorty effects on the proliferation of the virus at concentrations from 400μg/mL to 750μg/mL.Test 3, The study of the curative effect of Isatis Root and IRPS on GPThis test aimed at exploring the anti-GPV effects of Isatis Root water decoction and IRPS in experimentally GPV-infected goslings. On the basis of the determined polysaccharide content of Isatis Root water decoction and the LD50 of GPVyz strain, the effects of Isatis Root water decoction and IRPS on goslings experimentally infected with GPVyz were studied by observing the difference of survival time of goslings, survival rate, lesion and virus content. The results indicated that the average survival time of the group treated with IRPS(Group 1) and the group treated with water decoction(Group 2) was 130.7 h and 150.0 h respectively, while the infection control group(Group 3) was 118.8 h. The difference was statistically significant. The results showed that in experimentally infected goslings, treatment with isatis root and IRPS only slows down the speed of GPV replication and its dissemination in different tissues. It can not completely suppress viral replication or its pathogenicity.In summary, the results of this study shows that IRPS has a significant protective effect against GPVy06 in the goose embryos and fibroblasts by blocking GPV infection and proliferation. But in experimentally infected goslings, isatis root and IRPS only slows down the speed of GPV replication and its dissemination in different tissues. It can not completely suppress viral replication or its pathogenicity.
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