Abstract
BackgroundIn 2008, 800 rural Thai adults living within Kamphaeng Phet Province were enrolled in a prospective cohort study of zoonotic influenza transmission. Serological analyses of enrollment sera suggested this cohort had experienced subclinical avian influenza virus (AIV) infections with H9N2 and H5N1 viruses.MethodsAfter enrollment, participants were contacted weekly for 24mos for acute influenza-like illnesses (ILI). Cohort members confirmed to have influenza A infections were enrolled with their household contacts in a family transmission study involving paired sera and respiratory swab collections. Cohort members also provided sera at 12 and 24 months after enrollment. Serologic and real-time RT-PCR assays were performed against avian, swine, and human influenza viruses.ResultsOver the 2 yrs of follow-up, 81 ILI investigations in the cohort were conducted; 31 (38%) were identified as influenza A infections by qRT-PCR. Eighty-three household contacts were enrolled; 12 (14%) reported ILIs, and 11 (92%) of those were identified as influenza infections. A number of subjects were found to have slightly elevated antibodies against avian-like A/Hong Kong/1073/1999(H9N2) virus: 21 subjects (2.7%) at 12-months and 40 subjects (5.1%) at 24-months. Among these, two largely asymptomatic acute infections with H9N2 virus were detected by >4-fold increases in annual serologic titers (final titers 1∶80). While controlling for age and influenza vaccine receipt, moderate poultry exposure was significantly associated with elevated H9N2 titers (adjusted OR = 2.3; 95% CI, 1.04–5.2) at the 24-month encounter. One subject had an elevated titer (1∶20) against H5N1 during follow-up.ConclusionsFrom 2008–10, evidence for AIV infections was sparse among this rural population. Subclinical H9N2 AIV infections likely occurred, but serological results were confounded by antibody cross-reactions. There is a critical need for improved serological diagnostics to more accurately detect subclinical AIV infections in humans.
Highlights
After detecting the first highly pathogenic avian influenza (HPAI) poultry outbreaks in 2003 and the first human cases in 2004 in Thailand [1], detections continued until 2006 when intensive bird and human surveillance efforts, poultry culling, poultry vaccination programs, and several other interventions prevented further HPAI transmission [1,2,3,4,5,6,7]
As influenza surveillance in Thailand is often conducted in urban areas at the best medical facilities [10,11], people living in rural settings, or people with mild influenza infections who do not seek medical care, may be missed
Because serologic responses to avian influenza virus (AIV) infection can rapidly wane [19], as we have reported previously [12,20], we chose a low threshold of antibody titer ($1:10) as evidence of previous infection with an AIV strain
Summary
After detecting the first highly pathogenic avian influenza (HPAI) poultry outbreaks in 2003 and the first human cases in 2004 in Thailand [1], detections continued until 2006 when intensive bird and human surveillance efforts, poultry culling, poultry vaccination programs, and several other interventions prevented further HPAI transmission [1,2,3,4,5,6,7]. To better examine the incidence and prevalence of avian influenza transmission in Thailand, adults with poultry exposure living in rural north-central Thailand, as well as their family members, were prospectively followed for 2 yrs for evidence of avian influenza virus (AIV) infections. A previously published report detailed the study methods of enrolling the cohort and presented findings from the serological investigation of enrollment sera [12]. Enrollment data suggested that people in rural central Thailand were experiencing subclinical H9N2 and H5N1 AIV infections as a result of yet unidentified environmental exposures. In 2008, 800 rural Thai adults living within Kamphaeng Phet Province were enrolled in a prospective cohort study of zoonotic influenza transmission. Serological analyses of enrollment sera suggested this cohort had experienced subclinical avian influenza virus (AIV) infections with H9N2 and H5N1 viruses
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