Study On The Pathogenicity Of Low Pathogenic And Highly Pathogenic Avian Influenza Viruses Isolated From Wild Birds To Domestic And Wild Ducks And Evaluation On Their Transmission Patterns

Wild birds and waterfowl (Anseriformes) are primary natural hosts and reservoirs of avian influenza virus (AIV). Almost all subtypes of AIV have been isolated from waterfowl and other wild birds, including the highly pathogenic avian influenza (HPAI) and lowly pathogenic avian influenza (LPAI). The highly pathogenic avian influenza virus (HPAIV) H5N1attracts a lot of attention recently not only because it causes enormous economic damage to aviculture around the world, but also it has become a serious problem to the public health. At the same time,the lowly pathogenic avian influenza viruses (LPAIVs) should not be ignored, they are widely distributed in wild birds and most of them possess mild pathogenecities that help them to escape from host immune system and exist in waterfowl for a long time without causing severe problem. The LPAIVs have potential to become HPAIVs, because infected wild birds provide an ideal incubator for genetic exchange, reassortment and mutant during infection. Waterfowl (including ducks) play a key role during the transmission of the avian influenza virus. The ability of long-period hosting virus facilitates continuously virus releasing to environment and greatly promote the chance of transmitting viruses to other hosts, and AIV pathogenicity to waterfowl has obvious enhanced. Therefore, researches on the pathogenicity of AIV to waterfowl, the role waterfowl plays during the transmission of the avian influenza virus and spreading pattern of AIV are urgently needed.To define the role ducks and wild ducks plays during the transmission of the HPAIV and LPAIV, determine spreading pattern of HPAIV or LPAIV, and evaluate the possibilities of wild ducks contribute to the hypothesis of bird to bird transmission of HPAIV in the nature, two LPAIV, A/Garganey/SanJiang/160/2006(H5N2)(H5N2SJ160) and A/Mallard/SanJiang/162/2006(H6N2)(H6N2SJ162) isolated from wild ducks in SanJiang natural reserve of Heilongjiang Province were applied to the study. To tested the pathogenicity of the LPAIV and the spreading pattern of LPAIV. the juvenile (2-week-old) SPF domestic ducks (Shaoxing sheldrake) and wild ducks (mallards) as natural infection model were infected by two isolates of LPAIV, with dosage of106EID50. Clinical symptoms, repiratory and intestinal virus, virus replication titers in tissues, HI were recorded daily This study show that pre-existing immunity induced by infection with homo-or heterosubtypic LPAIV modifies the course of an experimental infection with HPAIV A/Bar Headed Goose/Qinghai/3/05(H5N1)(H5N1BHG05)in juvenile (5-week-old)ducks, as a natural infection model, with dosage of105EID50.We evaluated the pathogenicity of the HPAIV, and found homo-or heterosubtypic cross reactive immunity can derogate clinical symptoms of an HPAIV H5N1infection, reduce the amount and duration of viral shedding from the respiratory tract and prevent viral shedding from the intestinal tract. Still, The juvenile(3-week-old) and subadult (13-week-old)ducks were infected by isolate of HPAIV H5N1BHG05as natural infection model, with dosage of105EID50and106EID50to study the pathogenicity of the HPAIV and the spread pattern of HPAIV.This study can be concluded as following1. The LPAIV H5N2SJ160presents low pathogenicity to juvenile(1-day-old and2-week-old)SPF domestic ducks(Shaoxing sheldrake) and wild ducks(mallards). All the infected ducks survived and showed no obvious clinical symptoms. There is a certain correspondence relation between the pathogenicity of the LPAIV H5N2SJ160and the host species or waterfowl age. It showed that2-week-old SPF juvenile domestic ducks and wild ducks are more sensitive to the LPAIV H5N2SJ160than1-day-old juvenile ducks, and SPF domestic ducks are more sensitive to the LPAIV H5N2SJ160than wild ducks.2. The pathogenicity of the LPAIV H5N2SJ160and H6N2SJ162are low to2-week-old SPF juvenile domestic ducks(Shaoxing sheldrake). No obvious clinical symptoms were observed on infected ducks and100%survived. The results showed that virus can propagate more effectively in intestinal tract, as well as higher titers of virus and longer period of virus shedding(10dpi). Moreover, the virus titers were higher in caecum tonsil and bursa of fabricius than the other organs, while the tissue damage occurred in few organs of duck at different level.3.2-week-old SPF juvenile domestic ducks are susceptible to the LPAIV H5N2SJ160and H6N2SJ162with a longer period(10dpi) of virus shedding and had stronger ability of contaction transmission. The2-week-old SPF juvenile domestic ducks play a key role during LPAIV spread.The results showed that virus can propagate effectively in intestinal tract compared to other organs. It indicated the transmission pattern of the virus is very likely to be oral-to-cloacal spread way.4.2-week-old SPF juvenile domestic ducks with prior exposure to homosubtypic or heterosubtypic LPAIV might can ease clinical symptoms of the HPAIV H5N1infection, reduce the amount of infected ducks and shorten the duration of viral shedding from the respiratory tract. The tissue damage and HI antibody of ducks were various in defferent groups. Ducks with prior exposure to homosubtypic LPAIV may remain healthy with no virus shedding. Ducks with prior exposure to heterosubtypic LPAIV may remain healthy and might be suitable for long-distance transmission of HPAIV, with low titers of virus shedding. The high titers of virus shedding and no clinical symptoms were observed in the ducks with prior exposure to heterosubtypic LPAI viruses, resulting in high risk for transmission and spread of HPAIV. Our study, for the first time in China, indiacted that migrating Anatidae may play important role in long distance transposition of HPAIV during poultry inter-district trade.5. The HPAIV H5N1BHG05exhibited various pathogenicity in different host (SPF domestic ducks and wild ducks) or age, while the virus replication and the tissue damage occurred in tissus at different level.. It showed that SPF domestic ducks (Shaoxing sheldrake) are more sensitive to HPAIVH5N1BHG05than to wild ducks(mallards) in the same old. Furthermore, juvenile ducks (Shaoxing sheldrake) are more sensitive to the HPAIVH5N1BHG05than subadult ducks are. The mortality of the HPAIV H5N1BHG053-week-old SPF juvenile ducks was100%. The pathogenicity of the HPAIV H5N1BHG05to13-week-old subadult mallards is lowest. The shedding of HPAIV from respiratory and intestinal tracts of6-week-old SPF ducks are of highest virus titers(mean102.03EID50/0.1mL), and13-week-old subadult mallards shedding of HPAIV from respiratory and intestinal tracts was same time compared to the13-week-old SPF subadult ducks(-day duration). It concluded that the transmission pattern is different between HPAIV and LPAIV, and the LPAIV has a higher transmission ability in the host than the HPAIV.6. The1-week-old and2-week-old SPF juvenile domestic ducks were resistant to the HPAIV H5N1BHG05challenge and had no virus shedding or transmission. The3～6-week-old SPF juvenile domestic ducks infected with the HPAIV H5N1BHG05shed virus(virus titers mean102.03EID50/0.1mL) and virus transmission occured among the individuals(contact ducks can be infected without inoculated). It inferred that the3～6-week-old SPF juvenile domestic ducks can spread virus.13-week-old subadult ducks showed mild clinical symptoms with HPAIV infection, but viral shedding from the respiratory tract and cloacal tract are in lower titers(virus titers of13-week-old SPF subadult ducks mean101.5EID50/0.1mL), and viral shedding from feather in lower titers(virus titers of13-week-old mallards mean101.25EID50/0.1mL), It inferred that the13-week-old SPF subadult ducks can spread virus.7. Viral shedding from the respiratory tract was most evident in infected juvenile and subadult ducks with HPAIV by H5N1BHG05infection, while oropharynxviral viral shedding was evident only in infected ducks, with high titers and longer virus-shedding period. The results showed that virus can propagate more effectively in respiratory tract than in intestinal tract. It indicated the transmission pattern of the virus might be of oral-to-oral way.8. Our study construct a laboratory mode which can simulate HPAIV and LPAIV nature infection in domestic and wild ducks, contributing the study of real natural infection.