| Duck hepatitis virus type 3 is one of the major pathogens which is harmful to the health of duckling and it caused huge economic losses to the duck industry each year. At present, vaccine has not been applied to the prevention of DHAV-3, so the prevention of this disease mainly rely on the yolk antibody. However, the yolk antibody could cause secondry infection by E.coli and serositis.If we can breed a specific strain resistant to DHAV-3, it is undoubtedly a more effective way for the prevention of the disease. However livestock breeding is still in its infancy stage and the molecular mechanism of disease resistance is little understood, so as ducks and other waterfowls. This paper takes Peking ducks as experimental animal and select resistant families and individuals as parents through two generations of challenging them with DHAV-3 virus.In the process of the study, the differences in serum biochemical parameters and innate immune gene expression between the resistant and susceptible ducks were studied to explore the mechanism of some ducks resistant to DHAV-3.The results were as follows:Experiment 1 was conducted with Z8 Peking ducks as the research object. The half death concentration of embroy leathal death of DHAV-3 virus was 10-5.7/0.2ml with 50 embroy in the first step. The family of F0 generation was set up as the principle of female:male=4:1. 2 batches of eggs were collected,the first batche was collected for 14 days, with 1132 ducklings; the second batch was collected for 7 days with 720 ducklings. The first group of ducklings were artificially infected with DHAV-3 at seven days age; the second group of ducklings were put into the first group at its three days old. The results were as follows:(1) There were 44 families in which mortality rate was above 90% and 26 families whose mortality rate was less than 10% for the artifically infected group;(2)There were 57 families in which mortality rate was above 90% and 31 families whose mortality rate was less than 10% for the natrually infected group;(3) the death peak for the artificial group was 24 h after the infection, while the death peak for the naturally infected group was 40 h after the infection. It showed that there was significant difference between different families resistant to DHAV-3 no matter of the way of infection.Experiment two was to measure the disease resistance of the offsprings of resistant ducks. This experiment takes strong resistant families as the parents of F2 strong resistant group; the general resistant families as the parents of general resistant group; the compatriots of strong resistant families as the parents of compatriots of strong resistant group; the compatriots of the strong susceptible families as the parents of F2 control group; the natural families as the parents of natural group.The mortality of F2 generation and DHAV-3 detection rate in these groups were recorded.The results show that:(1)In F2 generation, the mortality rate of strong resistance group was 2.22% and that of the general resistance group was 6.79%, which was significantly lower than that of the strong susceptible full sibling group(P < 0.05).(2) the detection rate of DHAV-3 in strong susceptibility full siblings and strong resistance full siblings was 100%, which were higher than that of the strong resistance and the general resistance group.(3) the mortality rate of 22 families in the strong resistance group was 0, and the mortality rate of 8 families in the strong susceptible group was greater than 56%. The results showed that resistant duck offsprings of F1 generation had the characteristics of anti-DHAV-3.In the third experiment, the differences of serum biochemical indexes and innate immune genes in the resistant ducks and susceptible ducks infected with DHAV-3 were studied. The experiment took the F2 generation of Z8 strain in natural populations as subjects and 90 ducklings were randomly divided into two groups: 60 rats in the experimental group and 30 rats in the control group. The experiment group was injected with DHAV-3 of 0.2ml virus, and the control group was injected with the same amount of normal saline. The incidence was observed in the experiment group. After 22 h, 28 h, 31 h, 34 h and 37 h of inoculation, the experiment took 6 dying ducks in the susceptible group 、6 normal ducks in resistant group and 6 ducks in control group.The levels of ALT and AST in susceptible group were higher than the control group in the whole experiment process, while the level of ALP in susceptible group was lower than the control group. The level of these two indexes in resistance group fluctuated and was lower than that of susceptible group. The level of IgA 、IgM and IgY in the susceptible group were less than those in the control group. While these three kinds of immunoglobulin in the resistant group were higher than the control group from 22 h to 28h(P<0.05),while they decreased below that of the control group from 31 h to 37 h. The expression of MDA-5 gene in the liver of susceptible group was significantly higher than that in the control group(P<0.05). However, there was no significant difference for the MDA5 gene expression in liver between the control group and resistant group(P>0.05). The expression levels of IFN-α and TLR7 genes in the liver were significantly higher than those in the control group between 22 h to 31h(P<0.05), and the expression of these two genes was not significantly different between the control group(P>0.05) and the control group between 34 h to 37 h. The results showed that the liver function of susceptible duckling was damaged badly after DHAV-3 infection, while the resistant duckling could induce the immune responses.Experiment four studied the effects of DHAV-3 on the production performance of two generation resistant Peking ducks.The weight and immune organ indexs of 6 ducks from resistant group of F1 and F2 generation and control group were determined after 5 days,12 days, 21 days, 28 days, 35 days of inoculation. The slaughter performances of 10 females and 10 males from these three groups were measured at its seven weeks of age. The results showed that DHAV- 3 could infect the immune organs of Peking ducks and resistance ducks of two generations weight less than the control group after 5 days to 34 days. Slaughter results at seven weeks of age showed that keel length of resistant female ducks of two generations was significantly lower than that of control group(P < 0.05) and other performances were not significant different. Chest width of F1 resistant female ducks was significantly lower than the control group while other slaughter performance were of no significant differences compared with control group. Chest width, carcass weight, breast muscle weight, skeleton weight, breast muscle weight and sebum weight of resistant male ducks of F2 generation were significantly lower than that of control group(P < 0.05), however other slaughter performances were of no significant differences(P > 0.05). The slaughter performances were of no significant differences for male ducks of F1 generation and F2 generation(P>0.05). The results showed that the effects of the virus on the production performance of the male were more than female.In summary: the results showed that different families had different mortalities when inoculated with DHAV-3. When the offsprings of resistant ducks were challenged with DHAV-3, the mortality declined significantly. The results showed the feasibility of breeding of anti –DHAV-3 strains. |
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