Infectious Diseases 2021

Infectious Diseases 2021

HPAI H5N8 and H5N1 outbreaks in GB: Nov. 2020 to Apr. 2021

DEFRA 18 August, 2021

The outbreak of high pathogenicity avian influenza virus in Great Britain from November 2020 to March 2021 consisted of two types of virus, H5N8 (twenty infected premises) and H5N1 (two infected premises). Infection was disclosed across England, Scotland and Wales in commercial layer and broiler flocks, small-holder flocks, game flocks, captive bird collections and birds of prey – including a conservation centre and two animal rescue centres. There was no apparent spatial nor temporal clustering of infected premises.
Extensive epidemiological, tracings and genome sequencing investigations revealed no evidence of spread between any of the infected holdings, neither within the United Kingdom, nor to trading partners. The only spread between infected premises occurred on one large holding, where the three separate premises were located in close proximity and were operated as one business that shared staff and equipment across the sites. The epidemiological, tracings and genome sequencing evidence strongly supports that all of the infected premises became infected as a result of independent, direct or indirect, introductions from wild birds, apart from the three premises that were immediately adjacent to each other, shared staff and equipment and were functionally part of the same business.
A number of common risk factors for the introduction of infection were identified: bedding management, building maintenance (especially roofs), flooding events, staff and visitor biosecurity discipline, and proximity to large water bodies. The assertions above were supported by full genome sequence data demonstrating greater than 98.7% sequence identity (including with H5N8 wild bird cases from the same period) with other H5N8 UK report case isolates detected and characterised during the autumn/winter 2020/2021 period. Genetically these viruses cluster together and cannot be distinguished. The haemagglutinin gene of the H5N8 isolates detected across GB during autumn/winter 2020/2021 is closely related to the H5N8 viruses detected across Europe in the months preceding the GB outbreak events. Despite the high level of genetic identity, the geographical split of cases across the UK suggests that there is no direct relationship between infected premises and supports multiple independent introductions from wild birds with minimal viral divergence. This was also supported by the results of the tracing investigations, i.e. there were no epidemiological links established between the IPs.
The most likely ancestral virus has been determined to be a common ancestral virus to that responsible for spread across Middle East/Central Asia. Furthermore, the current isolates are distinct from those H5N8 viruses detected in the UK in 2016-2017. Again, this assessment supports the conclusion that this virus has been recently introduced into the UK through migratory wild birds entering the UK as partof their winter migration.
The unusual detection of H5N8 of avian origin in four seals and a fox that had been brought as casualties from the wild and which were being held in close proximity to infected wild swans, while all were undergoing treatment in a rehabilitation centre, indicated that cross-species transmission can occur should conditions allow. However, thorough analysis of genetic data generated from samples taken in the investigation of this isolated event, indicated that no significant adaptive genetic changes had occurred to increase the affinity for mammalian tissues, and that the risk of human infection from this virus remained low. 

 

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