Richard Jackson BVMS (Hons) MRCVS
Clinical Director, St David’s Poultry Team
At the time of writing this article there had been a case of High Path Avian Influenza (HPAI) declared in free range layers in Yorkshire. This was the strain H5N5. Prior to this there was a finding of H5N5 in wild pheasants however in previous years the main strain of HPAI was H5N1 rather than H5N5.
In terms of wild bird findings as reported on the APHA website, the main strain found until March 2024 was H5N1 but since then the main strain found became H5N5 (with the occasional H5N1 finding since March). It is worth noting there were a handful of findings of H5N5 last year (2023) in wild birds. The majority of recent wild bird findings of HPAI (H5N5) have been in Scotland and the north of England, however, there have been a few findings in Carmarthenshire and Worcestershire. Outside of the UK, H5N5 has been reported in the Faroe Islands and Canada in wild birds.
Avian influenza is caused by an influenza virus. The virus has two proteins on its surface, a H protein (hemagglutinin) and an N protein (Neuraminidase). There are 13 different H proteins and 9 different N proteins. Each strain of avian influenza will have one H protein and one N protein, and the combinations vary. However, only strains with a H protein of H5 or H7 can be highly pathogenic Avian Influenzas. If the virus has a different H number e.g. H2 then it can only be low pathogenic Avian Influenza (LPAI).
For both the H5N1 and H5N5, as they have the H number of 5 they can be highly pathogenic (note some LPAI have H numbers of 5 or 7 but can mutate from LPAI to HPAI).
The H protein is the part of the virus that is used to attach to the tissues of infected animals. As such most vaccines are aimed at providing immunity against the H protein. (In the case of current vaccines trials, the vaccines used will have the H5 protein).
The current picture is pointing to the H5 in the H5N1 being similar to the H5 in the H5N5. This is not entirely surprising as influenza viruses can undergo what is known as genetic shift whereby if a cell is infected by two different strains of AI, they can produce viruses that recombine the genes for the H and N proteins of the parent viruses.
Given the similarity between the H5 in both strains, there is likely at least some cross immunity between strains, however, for birds that hatched in spring of 2024, they won’t likely have been exposed to the virus and thus will have no immunity.
There are ongoing vaccine trials in several EU countries (Netherlands, France and Italy). The UK have a vaccination taskforce to examine the possibility of vaccination in the UK and no doubt there will be a clearer picture in time.
In the meantime, the key prevention method for farmers is to maintain excellent biosecurity. Wild birds pose a risk (the DEFRA risk assessment at the time of writing this was low for poultry but high for wild birds) meaning Free Range farmers need to ensure wild birds are discouraged from the range areas through the use of bird scarers and fencing off areas of water in the range. As always farmers should not provide feed to poultry in the range.
Bedding storage is important to ensure bedding is stored in a wild bird and rodent proof shed or if stored outside is double shrink wrapped in plastic. It is also important to ensure that sheds are kept in good condition as water ingress can be a route of infection. Wild bird ingress into non-Free-range sheds should be ensured and inlets should be meshed with a mesh of a diameter of no greater than 25mm.
It is important to ensure that accurate production, mortality and visitor records are kept. Accurate production records help with working out when infection started, and visitor records allow APHA to trace visitors if a site becomes infected.
Always ensure visitors wear dedicated PPE and wash/sanitise their hands when entering sheds. The sheds should have separate boots for each shed with a barrier to separate the outside boot zone from the inside boot zone.
Covered boot dips should be replenished at least twice a week with a DEFRA approved disinfectant used at the disease of poultry rate. Similarly, there should be wheel washes at the entrance to the site (with a DEFRA approved disinfected used at the disease of poultry rate). The site should have a boundary fence with a locked gate at the entrance.
The clinical signs for H5N1 and H5N5 are the similar and there can be a combination of the signs below:
- High mortality
- Egg drop
- Egg shell abnormalities
- Nervous signs
- Bruises on the birds’ legs
- Respiratory signs: snicking etc
- Diarrhoea
- Blue colouration of the bird’s face
Because many of the signs above can be caused by other diseases, it is important to discuss any signs of ill health with your vet immediately and if either you or your vet suspect AI it is a legal requirement to immediately inform APHA.