Epidemiological study of avian influenza in backyard chickens and open field-reared ducks in northern Thailand.

摘要

Avian influenza (AI) is an infectious disease of birds, other animals, and humans caused by type A subtypes of the influenza virus. Highly pathogenic avian influenza (HPAI), caused by the virus subtype H5N1, currently occurs worldwide with the greatest burden in Southeast Asia where the disease was first reported. Even though the major outbreaks of the disease in this region have declined, the disease remains a major threat to the poultry industry and human health. It is generally hypothesized that the main reason for the disease to persist in this region is the existence of traditional backyard chicken and open field-reared duck raising systems. These traditional poultry raising systems are particularly strong in Thailand, but limited research has been conducted to determine their role in maintaining and spreading the AI virus.;This study was designed to test the hypothesis that backyard chickens and open field-reared ducks harbor the avian influenza virus. To test the stated hypothesis, three objectives were designed to address: 1) to determine the prevalence of the avian influenza by a combination of virus isolation and antibody testing; 2) to identify the risk factors associated with the laboratory-confirmed avian influenza by Logistic Regression Analyses; and 3) to generate the Geographic Information System (GIS) mapping of the laboratory-confirmed AI in Northern Thailand.;One thousand oropharygeal swabs of backyard chickens, one thousand cloacal swabs of open field-reared ducks, and two thousand serum specimens from the same individual birds (as well as data via questionnaire) were collected at the time of visit in 87 dusticts of the 6 provinces in the Nothern region of Thailand in 2009-2010.;Avian influenza virus isolation (egg incoculation; confirmed by hemagglutination test) and Agar Gel Immunodiffusion (AGID) test were conducted at Chiang Mai University (CMU) in Thailand; using the guidelines provided by the World Organization for Animal Health (OIE). No active AI virus infection was detected by egg inoculation, and no influenza A antibody was tested positive to AGID (in fact AGID test is fairly insensitive).;To confirm the primary test results, further serological tesing and virus subtyping were carried out at the USDA National Veterinary Services Laboratories (NVSL) in Ames, Iowa. Among 2,000 serum samples of both poultry species, 1.5% (15/1000) (of sera) from backyard chickens and 2.1% (21/1000) of sera from open field-reared ducks tested positive for antibodies against type A influenza virus; using the IDEXX MultiS-Screen ELISA Test Kit (Sensitivity and Specificity of the test is 95.4% and 99.7%). Out of 13 sera that were positive to ELISA and had adequate quantity for AGID test, only 1 chicken serum was tested positive. Thus, AGID is not a recommended test for screening of AI antibody. By way of logistic regression modeling and GIS mapping, the AI antibody positivity in the backyard chickens appeared to be significantly (p-value 0.05) associated with the large flock size (>100 birds/flock) and farming proximity to the other farms. Because of a confounding effect found in the final model, multivariable analyses-risk factors for AI antibody positivity in the open field-reared ducks could not be identified.;AI virus sub-typing was conducted on the 21 adequate sera that were positive for ELISA. Three chicken sera (obtained from the same flock of birds tested positive to AGID) had been definitely identified positive to antibody against H5; using Hemaglutination-inhibition test. The fact that influenza A virus (H5 subtype) remains entrenched in Northern Thailand, it may put the country at risk of disease re-emerging. Therefore, the sustaining ongoing surveillance for early disease detection and preparedness for rapid disease response are still strongly recommended.