Abstract
In 2008, 800 adults living within rural Kampong Cham Province, Cambodia were enrolled in a prospective cohort study of zoonotic influenza transmission. After enrollment, participants were contacted weekly for 24 months to identify acute influenza-like illnesses (ILI). Follow-up sera were collected at 12 and 24 months. A transmission substudy was also conducted among the family contacts of cohort members reporting ILI who were influenza A positive. Samples were assessed using serological or molecular techniques looking for evidence of infection with human and avian influenza viruses. Over 24 months, 438 ILI investigations among 284 cohort members were conducted. One cohort member was hospitalized with a H5N1 highly pathogenic avian influenza (HPAI) virus infection and withdrew from the study. Ninety-seven ILI cases (22.1%) were identified as influenza A virus infections by real-time RT-PCR; none yielded evidence for AIV. During the 2 years of follow-up, 21 participants (3.0%) had detectable antibody titers (≥1∶10) against the studied AIVs: 1 against an avian-like A/Migratory duck/Hong Kong/MPS180/2003(H4N6), 3 against an avian-like A/Teal/Hong Kong/w312/97(H6N1), 9 (3 of which had detectible antibody titers at both 12- and 24-month follow-up) against an avian-like A/Hong Kong/1073/1999(H9N2), 6 (1 detected at both 12- and 24-month follow-up) against an avian-like A/Duck/Memphis/546/74(H11N9), and 2 against an avian-like A/Duck/Alberta/60/76(H12N5). With the exception of the one hospitalized cohort member with H5N1 infection, no other symptomatic avian influenza infections were detected among the cohort. Serological evidence for subclinical infections was sparse with only one subject showing a 4-fold rise in microneutralization titer over time against AvH12N5. In summary, despite conducting this closely monitored cohort study in a region enzootic for H5N1 HPAI, we were unable to detect subclinical avian influenza infections, suggesting either that these infections are rare or that our assays are insensitive at detecting them.
Highlights
Since 2004, Cambodia has experienced more than 30 outbreaks of highly pathogenic avian influenza (HPAI) H5N1 virus among poultry and at least 47 human infections with 33 deaths [1,2,3]
While new influenza infections were frequent among the cohort and cohort family members, we did not serologically detect AIV infections among 800 rural Cambodian villagers and 364 close family contacts suggesting that either AIV infections are rare or our methods are insensitive
With the exception of the single H5N1 infection, there were no other AIV infections detected from clinically ill participants monitored weekly for influenza-like illnesses (ILI)
Summary
Since 2004, Cambodia has experienced more than 30 outbreaks of highly pathogenic avian influenza (HPAI) H5N1 virus among poultry and at least 47 human infections with 33 deaths [1,2,3]. Despite the numerous documented outbreaks of H5N1 among poultry and the periodic human H5N1 cases that have been identified, previous seroepidemiology studies have estimated the seroprevalence of H5N1 antibodies to be relatively low (0%–2.6%) [6,7,8]. Each of these studies were conducted in areas where recent outbreaks of H5N1 in poultry had been molecularly confirmed, human cases identified, and the majority of participants reporting intense contact with poultry, all of which would intuitively suggest a greater risk for avian influenza transmission to humans. These studies, were limited as they only focused on H5N1 influenza virus and did not test for other avian influenza strains
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