Quantitative transmission characteristics of different H5 low pathogenic avian influenza viruses in Muscovy ducks.

EU annual serosurveillance programs show that domestic duck flocks have the highest seroprevalence of H5 antibodies, demonstrating the circulation of notifiable avian influenza virus (AIV) according to OIE, likely low pathogenic (LP). Therefore, transmission characteristics of LPAIV within these flocks can help to understand virus circulation and possible risk of propagation. This study aimed at estimating transmission parameters of four H5 LPAIV (three field strains from French poultry and decoy ducks, and one clonal reverse-genetics strain derived from one of the former), using a SIR model to analyze data from experimental infections in SPF Muscovy ducks. The design was set up to accommodate rearing on wood shavings with a low density of 1.6 ducks/m(2): 10 inoculated ducks were housed together with 15 contact-exposed ducks. Infection was monitored by RNA detection on oropharyngeal and cloacal swabs using real-time RT-PCR with a cutoff corresponding to 2-7 EID50. Depending on the strain, the basic reproduction number (R0) varied from 5.5 to 42.7, confirming LPAIV could easily be transmitted to susceptible Muscovy ducks. The lowest R0 estimate was obtained for a H5N3 field strain, due to lower values of transmission rate and duration of infectious period, whereas reverse-genetics derived H5N1 strain had the highest R0. Frequency and intensity of clinical signs were also variable between strains, but apparently not associated with longer infectious periods. Further comparisons of quantitative transmission parameters may help to identify relevant viral genetic markers for early detection of potentially more virulent strains during surveillance of LPAIV.

Extensive antigenic changes in an atypical isolate of very virulent infectious bursal disease virus and experimental clinical control of this virus with an antigenically classical live vaccine.

The 99323 Egyptian isolate of infectious bursal disease (IBD) virus (IBDV) was identified during an international survey of acute IBD cases. Its unique antigenicity was characterized by a markedly reduced binding of neutralizing monoclonal antibodies 3, 4, 5, 6, 8 and 9 in an antigen-capture enzyme-linked immunosorbent assay. Nucleotide sequencing of the genome region encoding the VP2 major immunogenic domain in 99323 revealed amino acid changes occurring at positions critical for antigenicity, but phylogenetic analysis demonstrated that 99323 was related to typical, very virulent IBDV (e.g. isolate 89163). Protection experimentally afforded by an antigenically classical live IBD vaccine was investigated in specific pathogen free chickens challenged with 99323 or 89163. Both viruses were similarly controlled, as evaluated by clinical signs, growth retardation, bursa-to-body weight ratios and histological lesions of the bursa after challenge. These results document that an active antibody response to a classical live antigen may clinically control infection by an antigenically atypical very virulent IBDV.