Epizootiology of avian influenza: effect of season on incidence in sentinel ducks and domestic turkeys in Minnesota.

Sentinel ducks and domestic turkey flocks were monitored for influenza infection during a 4-year period. The onset of infection among ducks was similar each year, occurring in late July or early August. Influenza in turkeys was also shown to be seasonal, but the usual onset was 6 to 8 weeks after the detection of influenza in sentinel ducks. Possible explanations for the delayed infection in turkeys are (i) increased waterfowl activity associated with fledging and congregating in late summer and early fall; (ii) vectors transmitting virus from the waterfowl habitat to poultry farms; (iii) cooler environmental temperature, allowing prolonged virus viability; (iv) cooler surface water temperature, allowing prolonged virus viability; (v) groundwater contamination from contaminated surface water; and (vi) virus adaptation in domestic turkeys before infection is detected. We conclude that ducks are not only a natural reservoir of influenza but also have a seasonal infection that appears to be related to seasonal influenza outbreaks in domestic turkeys in Minnesota. However, only some influenza A virus isolates circulating among waterfowl at any given time appear capable of causing detectable infection in turkeys. It is speculated that the seasonal infection in migratory waterfowl may also be related to seasonal influenza infections in other species including humans.     

Epizootiology of Plasmodium hermani in Florida: chronicity of experimental infections in domestic turkeys and northern bobwhites

A pen-reared northern bobwhite and a domestic turkey were infected with a strain of Plasmodium hermani obtained originally from a wild turkey in southern Florida. Blood films from these 2 birds were positive microscopically for 188 and 370 days postinfection (PI), respectively. Culicine mosquitoes (Culex nigripalpus and C. salinarius) were blood fed on the bobwhite and the turkey at different times during the infection and used to transmit the malaria to other bobwhites and turkeys up to days 298 and 473 PI, respectively. It was concluded that in nature, P. hermani could remain in a chronic phase in avian hosts for a year, or longer, allowing survival of the parasite between seasons of mosquito transmission.     

Comparative histopathological characteristics of highly pathogenic avian influenza (HPAI) in chickens and domestic ducks in 2008 Korea

We compared characteristic lesions occurring in chickens and domestic ducks naturally infected with H5N1 HPAI virus in April and May 2008. Infected chickens generally exhibited pale-green, watery diarrhoea, depression, neurological signs and cyanosis of wattles and combs, and infected ducks generally exhibited neurological signs and watery diarrhoea. Gross petechial or ecchymotic haemorrhage affected the heart, proventriculus, liver, muscle, fat, and pancreas in chickens, and muscle in ducks. Necrotic foci were primarily present in the pancreas of both species and in the heart of domestic ducks. Histopathologically, chickens exhibited multifocal encephalomalacia, multifocal lymphohistiocytic myocarditis, multifocal necrotic pancreatitis and haemorrhage of several organs and tissues; ducks exhibited lymphohistiocytic meningoencephalitis with multifocal haemorrhages, multifocal necrotic pancreatitis, and severe necrotic myocarditis with mineralisation. The characteristic histopathologic findings of 2008 HPAI were multifocal encephalomalacia and necrotic pancreatitis accompanied by lymphohistiocytic myocarditis, and haemorrhage in various organs and tissues in chickens, whereas in ducks, they were severe necrotic myocarditis with mineralisation and necrotic pancreatitis, accompanied with lymphohistiocytic meningoencephalitis. The high mortality of domestic ducks may be intimately associated with heart failure resulting from increased H5N1 HPAI viral cardiotropism.     

Pathogenicity and vaccine efficacy of different clades of Asian H5N1 avian influenza A viruses in domestic ducks

Waterfowl represent the natural reservoir of all subtypes of influenza A viruses, including H5N1. Ducks are especially considered major contributors to the spread of H5N1 influenza A viruses because they exhibit diversity in morbidity and mortality. Therefore, as a preventive strategy against endemic as well as pandemic influenza, it is important to reduce the spread of H5N1 influenza A viruses in duck populations. Here, we describe the pathogenicity of dominant clades (clades 1 and 2) of H5N1 influenza A viruses circulating in birds in Asia. Four representatives of dominant clades of the viruses cause symptomatic infection but lead to different profiles of lethality in domestic ducks. We also demonstrate the efficacy, cross-protectiveness, and immunogenicity of three different inactivated oil emulsion whole-virus H5 influenza vaccines (derived by implementing reverse genetics) to the viruses in domestic ducks. A single dose of the vaccines containing 1 μg of hemagglutinin protein provides complete protection against a lethal A/Duck/Laos/25/06 (H5N1) influenza virus challenge, with no evidence of morbidity, mortality, or shedding of the challenge virus. Moreover, two of the three vaccines achieved complete cross-clade or cross-subclade protection against the heterologous avian influenza virus challenge. Interestingly, the vaccines induce low or undetectable titers of hemagglutination inhibition (HI), cross-HI, and/or virus neutralization antibodies. The mechanism of complete protection in the absence of detectable antibody responses remains an open question.     

The PA protein directly contributes to the virulence of H5N1 avian influenza viruses in domestic ducks

During their circulation in nature, H5N1 avian influenza viruses (AIVs) have acquired the ability to kill their natural hosts, wild birds and ducks. The genetic determinants for this increased virulence are largely unknown. In this study, we compared two genetically similar H5N1 AIVs, A/duck/Hubei/49/05 (DK/49) and A/goose/Hubei/65/05 (GS/65), that are lethal for chickens but differ in their virulence levels in ducks. To explore the genetic basis for this difference in virulence, we generated a series of reassortants and mutants of these two viruses. The virulence of the reassortant bearing the PA gene from DK/49 in the GS/65 background increased 10(5)-fold relative to that of the GS/65 virus. Substitution of two amino acids, S224P and N383D, in PA contributed to the highly virulent phenotype. The amino acid 224P in PA increased the replication of the virus in duck embryo fibroblasts, and the amino acid 383D in PA increased the polymerase activity in duck embryo fibroblasts and delayed the accumulation of the PA and PB1 polymerase subunits in the nucleus of virus-infected cells. Our results provide strong evidence that the polymerase PA subunit is a virulence factor for H5N1 AIVs in ducks.     

Epizootiology of avian cholera in wildfowl

Pasteurella multocida, the cause of avian cholera, has naturally infected over 100 species of free-living birds. Among wild birds, avian cholera has its greatest impact on North American wildfowl. Epizootics usually are explosive in onset and may involve thousands of birds. The disease has been reported in every month of the year among wildfowl. Disproportionate mortality, with some species suffering proportionately greater mortality than others, has been a common feature of this disease. Presence of animal organic matter plays a significant role in the survival of P. multocida. There are conflicting reports or a lack of information on the role of host sex, age, body size, other physical features, genetic variation or behavioral differences, as predisposing factors to infection by P. multocida. There also are ambiguities on the relationship between season, precipitation, temperature, nutritional stress, water quality, other microorganisms, and environmental contaminants, and the occurrence of avian cholera in wildfowl. Two competing hypotheses for the year-round reservoir of wildfowl strains of P. multocida are ambient soil or water of enzootic sites, and carrier animals; most current evidence favors the role of carrier animals. Transmission most likely occurs by ingestion of contaminated water, inhalation of bacteria-rich aerosols, or both. While many techniques have been proposed to prevent or control avian cholera, none have been rigorously tested to determine their effectiveness.     

Possible waterborne transmission and maintenance of influenza viruses in domestic ducks.

Two duck farms in Hong Kong were examined monthly for 1 year for the occurrence and persistence of influenza viruses within the duck communities. The predominant virus in one community was H3N2, a virus antigenically related to the pandemic Hong Kong strain. This virus was isolated monthly throughout the year from feces or pond water or both, indicating a cycle of waterborne transmission. Viruses of the same antigenic combination were isolated 1 and 2 years after the last sampling occasion, implying persistence in the community. Infection was asymptomatic. Maintenance of virus appeared to be dependent upon the continual introduction of ducklings susceptible to infection onto virus-contaminated water; the feces of ducks 70 to 80 days old were generally free of detectable virus despite the exposure of the ducks to virus in pond water. In the second community, in which ducklings were not introduced after the initial sampling, the prevailing viruses, H7N1 and H7N2, also present asymptomatically, ceased to be detected once the ducks were 70 to 80 days old. The normal practice of raising ducks of different ages on the same farm, wherein the water supplies are shared, as typified by the first community, appears to be instrumental in maintaining a large reservoir of influenza viruses in the duck population of southern China.     

A mathematical model of avian influenza with half-saturated incidence

The widespread impact of avian influenza viruses not only poses risks to birds, but also to humans. The viruses spread from birds to humans and from human to human In addition, mutation in the primary strain will increase the infectiousness of avian influenza. We developed a mathematical model of avian influenza for both bird and human populations. The effect of half-saturated incidence on transmission dynamics of the disease is investigated. The half-saturation constants determine the levels at which birds and humans contract avian influenza. To prevent the spread of avian influenza, the associated half-saturation constants must be increased, especially the half-saturation constant H _m for humans with mutant strain. The quantity H _m plays an essential role in determining the basic reproduction number of this model. Furthermore, by decreasing the rate β _m at which human-to-human mutant influenza is contracted, an outbreak can be controlled more effectively. To combat the outbreak, we propose both pharmaceutical (vaccination) and non-pharmaceutical (personal protection and isolation) control methods to reduce the transmission of avian influenza. Vaccination and personal protection will decrease β _m, while isolation will increase H _m. Numerical simulations demonstrate that all proposed control strategies will lead to disease eradication; however, if we only employ vaccination, it will require slightly longer to eradicate the disease than only applying non-pharmaceutical or a combination of pharmaceutical and non-pharmaceutical control methods. In conclusion, it is important to adopt a combination of control methods to fight an avian influenza outbreak.     

禽流感与禽流感病毒研究进展

对禽流感的症状、传播、感染、流行规律、疾病发生历史、流行监测、诊断、防治以及禽流感病毒的分类地位、命名、病毒粒子形态结构、病毒基因组结构、病毒复制、病毒变异的研究进展作了综合评述,并对该领域的研究热点和方向作了探讨。     

Blood and cloacal swab sampling for avian influenza monitoring has no effect on survival rates of free‐ranging ducks

Concerns about the spread of avian influenza viruses (AIVs) have led to cloacal swab sampling of hundreds of thousands of birds worldwide as part of AIV surveillance schemes, but the effects of cloacal swabbing have not been adequately evaluated. We tested for differences between swabbed, swabbed and bled, and non‐sampled wild ducks in terms of live re‐encounter and dead recoveries for Common Pochard Aythya ferina and Tufted Duck Aythya fuligula, and also determined re‐encounter and recovery rates for Mallard Anas platyrhynchos and Common Teal Anas crecca. No effects of sampling methods were detected, except in Teal. Re‐encounter rates were lower in sampled Teal than in controls, with annual re‐encounter probabilities being 25% and 35% lower in males and females, respectively. Teal possibly left or avoided sampling sites, or sought sites where they were less detectable after sampling. In general, no deleterious effects were found, suggesting that cloacal swabbing and blood sampling are suitable methods for conducting AIV surveillance in ducks.     

Establishment and lineage replacement of H6 influenza viruses in domestic ducks in southern China

Domestic ducks in southern China act as an important reservoir for influenza viruses and have also facilitated the establishment of multiple H6 influenza virus lineages. To understand the continuing evolution of these established lineages, 297 H6 viruses isolated from domestic ducks during 2006 and 2007 were genetically and antigenically analyzed. Phylogenetic analyses showed that group II duck H6 viruses had replaced the previously predominant group I lineage and extended their geographic distribution from coastal to inland regions. Group II H6 virus showed that the genesis and development of multiple types of deletions in the neuraminidase (NA) stalk region could occur in the influenza viruses from domestic ducks. A gradual replacement of the N2 NA subtype with N6 was observed. Significant antigenic changes occurred within group II H6 viruses so that they became antigenically distinguishable from group I and gene pool viruses. Gene exchange between group II H6 viruses and the established H5N1, H9N2, or H6N1 virus lineages in poultry in the region was very limited. These findings suggest that domestic ducks can facilitate significant genetic and antigenic changes in viruses established in this host and highlight gaps in our knowledge of influenza virus ecology and even the evolutionary behavior of this virus family in its aquatic avian reservoirs.     

Antigenic and phenotypic variants of a virulent avian influenza virus selected during replication in ducks

To evaluate the replication of a highly virulent avian influenza A virus in a potential reservoir host, mallard ducks (Anas platyrhynchos) were inoculated with the virulent strain A/Ty/Ont/7732/66 (H5N9). Viruses recovered from the ducks were analyzed by hemagglutination inhibition (HI) and enzyme-linked immunosorbent assay (ELISA) and found to possess antigenically altered viral hemagglutinins. Plaque formation on the Madin-Darby Canine Kidney (MDCK) cell line and on primary chicken embryo cells was investigated, and isolates recovered from the ducks differed from the wild type by being unable to form plaques on MDCK cells without trypsin. This phenotype did not appear to be due to inefficient cleavage of the hemagglutinin by host cell proteases since hemagglutinin immunoprecipitated from cell lysates was cleaved. Although the plaquing phenotype suggested attenuation of the isolates from the ducks, they were not significantly altered in their virulence for chickens shown by infectivity studies in vivo. These results indicate that replication of influenza A/Ty/Ont/7732/66 virus in ducks can produce antigenic and phenotypic variants which are still highly virulent for domestic poultry.     

Homo- and heterosubtypic low pathogenic avian influenza exposure on H5N1 highly pathogenic avian influenza virus infection in wood ducks (Aix sponsa)

Wild birds in the Orders Anseriformes and Charadriiformes are the natural reservoirs for avian influenza (AI) viruses. Although they are often infected with multiple AI viruses, the significance and extent of acquired immunity in these populations is not understood. Pre-existing immunity to AI virus has been shown to modulate the outcome of a highly pathogenic avian influenza (HPAI) virus infection in multiple domestic avian species, but few studies have addressed this effect in wild birds. In this study, the effect of pre-exposure to homosubtypic (homologous hemagglutinin) and heterosubtypic (heterologous hemagglutinin) low pathogenic avian influenza (LPAI) viruses on the outcome of a H5N1 HPAI virus infection in wood ducks (Aix sponsa) was evaluated. Pre-exposure of wood ducks to different LPAI viruses did not prevent infection with H5N1 HPAI virus, but did increase survival associated with H5N1 HPAI virus infection. The magnitude of this effect on the outcome of the H5N1 HPAI virus infection varied between different LPAI viruses, and was associated both with efficiency of LPAI viral replication in wood ducks and the development of a detectable humoral immune response. These observations suggest that in naturally occurring outbreaks of H5N1 HPAI, birds with pre-existing immunity to homologous hemagglutinin or neuraminidase subtypes of AI virus may either survive H5N1 HPAI virus infection or live longer than naïve birds and, consequently, could pose a greater risk for contributing to viral transmission and dissemination. The mechanisms responsible for this protection and/or the duration of this immunity remain unknown. The results of this study are important for surveillance efforts and help clarify epidemiological data from outbreaks of H5N1 HPAI virus in wild bird populations.     

Detection of low pathogenic avian influenza viruses in wild ducks from Canada: comparison of two sampling methods

Surveillance for avian influenza viruses in wild birds was initiated in Canada in 2005. In 2006, in order to maximize detection of highly pathogenic avian influenza viruses, the sampling protocol used in Canada's Inter-agency Wild Bird Influenza Survey was changed. Instead of collecting a single cloacal swab, as previously done in 2005, cloacal and oropharyngeal swabs were combined in a single vial at collection. In order to compare the two sampling methods, duplicate samples were collected from 798 wild dabbling ducks (tribe Anatini) in Canada between 24 July and 7 September 2006. Low pathogenic avian influenza viruses were detected significantly more often (P<0.0001) in combined oropharyngeal and cloacal samples (261/798, 33%) than in cloacal swabs alone (205/798, 26%). Compared to traditional single cloacal samples, combined samples improved virus detection at minimal additional cost.     

Susceptibility of wood ducks to H5N1 highly pathogenic avian influenza virus

Since 2002, H5N1 highly pathogenic avian influenza (HPAI) viruses have caused mortality in numerous species of wild birds; this is atypical for avian influenza virus (AIV) infections in these avian species, especially for species within the order Anseriformes. Although these infections document the susceptibility of wild birds to H5N1 HPAI viruses and the spillover of these viruses from infected domestic birds to wild birds, it is unknown whether H5N1 HPAI viruses can persist in free-living avian populations. In a previous study, we established that wood ducks (Aix sponsa) are highly susceptible to infection with H5N1 HPAI viruses. To quantify this susceptibility and further evaluate the likelihood of H5N1 HPAI viral maintenance in a wild bird population, we determined the concentration of virus required to produce infection in wood ducks. To accomplish this, 25 wood ducks were inoculated intranasally at 12-16 wk of age with decreasing concentrations of a H5N1 HPAI virus (A/Whooper Swan/Mongolia/244/05 [H5N1]). The median infectious dose and the lethal dose of H5N1 HPAI virus in wood ducks were very low (10(0.95) and 10(1.71) median embryo infectious dose [EID(50)]/ml, respectively) and less than that of chickens (10(2.80) and 10(2.80) EID(50)/ml). These results confirm that wood ducks are highly susceptible to infection with H5N1 HPAI virus. The data from this study, combined with what is known experimentally about H5N1 HPAI virus infection in wood ducks and viral persistence in aquatic environments, suggest that the wood duck would represent a sensitive indicator species for H5N1 HPAI. Results also suggest that the potential for decreased transmission efficiency associated with reduced viral shedding (especially from the cloaca) and a loss of environmental fitness (in water), may be offset by the ability of this virus to be transmitted through a very low infectious dose.     

Possible circulation of H5N1 avian influenza viruses in healthy ducks on farms in northern Vietnam

To estimate the prevalence of influenza A subtype H5N1 viruses among domestic ducks in the period between October and November 2006 when H5N1 outbreaks had been absent, 1106 healthy ducks raised in northern Vietnam were collected. Inoculation of all throat and cloacae samples into embryonated eggs resulted in the isolation of subtype H3N8 in 13 ducks, but not H5N1 viruses. Serological analyses demonstrated that five ducks (0.45%) solely developed H5N1 subtype-specific hemagglutinin-inhibiting and neuraminidase-inhibiting antibodies together with anti-non-structural protein 1 antibodies. The results suggested that the ducks were naturally infected with H5N1 viruses when obvious H5N1 outbreaks were absent.     

Serological monitoring of eastern wild turkeys for antibodies to Mycoplasma spp. and avian influenza viruses

From 1981 through 1986, plasma or serum samples were obtained from 322 wild turkeys (Meleagris gallopavo) from Georgia (n = 111), Kentucky (n = 21), Louisiana (n = 22), North Carolina (n = 118), Tennessee (n = 19), Missouri (n = 24) and Iowa (n = 7). These samples were tested for antibodies to Mycoplasma gallisepticum (MG) and in most instances, M. synoviae (MS), M. meleagridis (MM), and avian influenza (AI) virus. All 322 turkeys were seronegative for MG by the rapid plate agglutination (RPA) test. All of a subsample (n = 147) also were negative (titer less than or equal to 1:40) for MG by the hemagglutination inhibition (HI) test. Five of 253 turkeys (2%) were seropositive (+4 reaction) for MS by the RPA test; however, HI tests for MS on these five turkeys were negative as were attempts to isolate MS from trachea and homogenized lung tissue. Three of 253 turkeys (1%) were seropositive (+1 to +3 reactions) for MM by the RPA test. None of 210 turkeys had antibodies to AI by the agar gel precipitation test. These data suggest that populations of native eastern wild turkeys are not important in the epizootiology of MG, MS, MM, or AI.