Outbreak Investigation Report on H5N2 Avian Influenza in Ontario, 2015
5. Summary of Findings and Working Hypotheses on Source and Transmission of NAI
5.1 Source of the Virus
Waterfowl are well established primary reservoirs for a wide variety of strains of avian influenza. A recent report from the CFIA entitled "Qualitative Risk Assessment of a Highly Pathogenic Avian Influenza Virus in Canada" examined multiple potential sources of the H5N2 virus. Based on the epidemiological investigation conducted in this response, no other source of infection beyond introduction of the virus into the environment by migratory waterfowl provides a viable explanation. The existence of reports of waterfowl in the vicinity of the outbreak, particularly near IP1, is supportive of this interpretation.
Given the lack of identified connections between IP1 and the IP2/IP3 cluster, there is no evidence to support any spread from IP1 and IP2, which is supportive of IP2 being treated as a second incursion of virus mediated by migratory waterfowl.
5.2 Means of Spread
Waterfowl contaminate the environment by shedding the virus via their feces. The likelihood of exposure to indoor-reared poultry by migrating waterfowl is subject to several factors: the density of the poultry population in the affected area, the strength of biosecurity measures, the level and frequency of contact between farm workers and birds, and the proximity of the poultry to the migration routes of wild birds. While poultry production systems in Canada are designed to reduce or prevent contact between wild birds and commercial poultry, the potential does exist for the introduction of the virus from the environment if a breach in biosecurity occurs.
The majority of previous outbreaks of HPAI in Canada have occurred in the Fraser Valley of British Columbia, which is populated densely with poultry operations. This has resulted in significant local spread in previous HPAI outbreaks. In contrast, poultry production is much less dense in Oxford County, with few large operations in close proximity. Tracing investigations were conducted on all trace-in and trace-out activity around the IPs in our critical period, and no evidence of fomite or other activity-mediated transmission was found.
5.3 Field Epidemiology – Summary of Findings
Given the different production types and lack of identified connections between any of the three properties the outbreak is being viewed as most likely the result of two separate incursions of virus, mediated by migratory waterfowl (into IP1 and IP2) with limited local/environmental spread (IP3).
All possible linkages were examined to discover any other possible connection between the IPs, including veterinarians, feed suppliers, deadstock operators, common personnel and bedding suppliers. To further examine linkages, all IPs were re-interviewed by CFIA personnel. No linkages were identified in either the initial or follow-up investigation. In the absence of any new data, it is recommended that this outbreak continue to be viewed as two separate incursions of virus with limited local spread.
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