wo types of linkages between the terminal sialic acid residue and the penultimat
ID: 915604 • Letter: W
Question
wo types of linkages between the terminal sialic acid residue and the penultimate galactose residue are found in nature: alpha(2,3) linkages and alpha(2,6) linkages (Figure 11.3). lnfluenza viruses isolated from humans preferentially bind alpha(2,6) linkages. lnternational surveillance programs monitor influenza outbreaks and analyze the viruses for their antigenic properties in an effort to prevent the occurrence of a pandemic. To obtain sufficient material for analysis, virus isolated from patients is often grown in chicken eggs, cells that predominantly express the alpha(2,3) type linkage. Do you think the virus purified from chicken eggs will accurately reflect the virus isolated from the infected patients?
Explanation / Answer
H9N2 viruses are endemic in terrestrial poultry (e.g. chicken, quail) and are geographically widespread throughout Asia (Cameron et al., 2000; Guan et al., 2000; Liu et al., 2003a; Liu et al., 2003b). These viruses have repeatedly transmitted to humans although only causing mild flu-like respiratory illness (Butt et al., 2005; Guo et al., 1999; Lin et al., 2000; Peiris et al., 1999). Although human disease was less dramatic than that resulting from H5N1 infection (de Jong et al., 2005; Tran et al., 2004; Yuen et al., 1998), these H9N2 viruses have a haemagglutinin with greater affinity for the alpha 2’6 linked sialic acid receptors found in human cells (Ha et al., 2001) and may pose a potential pandemic threat. Studies of influenza surveillance in live-poultry markets have demonstrated that the isolation rate of H9N2 viruses was highest in quail (Guan et al., 2000). It was therefore important to understand the ecology of H9N2 in farmed quail.
Results Study 1: In this cross-sectional study, conducted on the 6 local quail farms, 305 tracheal, 152 cloacal and 16 faecal swabs were collected during the Aug-Sep 2000 period. Forty of these 473 samples tested positive for avian influenza virus, of which 10 were H6 and 33 were H9 subtype viruses. All the 10 H6 virus isolates were from quail tracheal swabs in the 20 day age group from one breeder farm. Thirty two out of the 33 H9 isolates were from quail under 10 days old and only one H9 isolate was from quail at age of 20 days. No H6 or H9 was isolated from quail older than 20 days. In the farms (Farm No 4, 5, 6) where both tracheal and cloacal swabs were collected, the prevalence of H9 viruses from trachea and cloaca was 19.0% and 2.0% respectively. Similarly the prevalence of H6 viruses in trachea and cloaca was 6.5% and 0% respectively. Study 2: In the second cross-sectional study, there were in total 309 tracheal, 309 73 cloacal, 48 faecal samples and 134 blood samples collected during May-Aug 2001 from Farm No 4 in Study 1. Among all the samples collected, 25 H6 and 9 H9 subtype virus were isolated from tracheal swabs, 1 H6 isolated from cloacal swabs (same bird also secreted H6 from trachea) and 1 H6 isolated from faecal samples in this study. Twenty-two out of 25 of the H6 subtype virus isolates were from quail between 15 to 20 days old, and 7 out of 8 H9 virus isolates were from quail were in the 10 day age group Eight birds (age group between 17 to 40 days) with H6 subtype viruses isolated from tracheal swabs also showed the antibody titre against H9 subtype virus. Twelve quail had antibodies against both H6 and H9 subtype viruses (age between 17 to 100 days old). One quail at the age of 5 days had low antibody titre against H9 subtype virus.
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