The last two winters have seen the UK’s largest ever avian influenza outbreak, with mutations in the H5N1 virus leading to prolonged environmental survival and greater infection levels. It is highly likely that vets seeing birds in practice will have to consider avian influenza as a differential diagnosis in captive birds and when managing wildlife casualties.
It is important to remember avian influenza is notifiable in the UK, and any suspicion of disease in poultry or other captive birds must be reported to the competent authority.
What is avian influenza?
Avian influenza is a highly infectious virus, most frequently associated with disease in poultry. The recent outbreaks in 2021/2022 and 2022/2023 have caused significant mortality and large-scale economic loss in the commercial poultry sector.
As well as farm cases, several backyard flocks, zoo collections and other captive birds have suffered outbreaks of infection. This article will focus largely on managing avian patients outside of the commercial poultry sector.
Classifying the disease
Avian influenza infections are split into forms: highly pathogenic (HPAI) and low pathogenic (LPAI). Low pathogenic disease can quickly mutate into highly pathogenic disease during an outbreak. LPAI can be challenging to differentiate from other common avian respiratory diseases. The avian influenza virus is classified into HPAI and LPAI based on the particular strain’s effect on chickens and does not necessarily predict the disease severity in other avian species.
The avian influenza virus is classified into HPAI and LPAI based on the particular strain’s effect on chickens and does not necessarily predict the disease severity in other avian species
The disease is classified based on the properties of its surface proteins haemagglutinin (H1 to H16) and neuraminidase (N1 to N9). To date, only H5 and H7 infections have caused highly pathogenic disease.
What is the current situation?
There have been 172 confirmed cases of highly pathogenic avian influenza (HPAI) H5N1 in the UK since 1 October 2022:
- 145 cases in England
- 21 cases in Scotland
- 5 cases in Wales
- 1 case in Northern Ireland
Current control zones can be seen on the interactive map published by the Animal and Plant Health Agency (APHA) (Figure 1).
How is avian influenza transmitted?
Traditionally, migrating waterfowl have been responsible for much of the global spread of avian influenza. In certain instances, waterfowl can carry both LPAI and HPAI without suffering severe clinical disease, although in recent outbreaks, significant waterfowl mortality has been seen in wild birds.
Other wild bird species, including gulls and corvids, can act as bridges of transmission to captive birds. Many species previously rarely affected by avian influenza have suffered significant population declines – with colony sea bird species severely hit in recent outbreaks.
The virus is transmitted through direct contact with infected birds or infected urates, faeces or respiratory secretions. Aerosol transmission, other than over very short distances, is not considered an important form of transmission. Contaminated footwear, vehicle tyres and bedding are common fomites for transmission of disease.
The recent H5N1 virus is particularly environmentally stable – remaining infectious in the environment for up to 55 days in favourable conditions. Environmental persistence, even at summer temperatures, allowed infection to remain circulating in wild birds throughout 2022, with infections in captive birds persisting through the summer and peaking again in autumn/winter.
What are the clinical signs of avian influenza?
HPAI should be included as part of the differential diagnosis for any acute and unexplained mortality
Knowing the common clinical signs of avian influenza will help identify potential clinical cases in practice (Table 1). However, there is a significant overlap with other common avian diseases. If avian influenza cannot be reasonably excluded as a differential, a suspicion of disease must be notified. Bear in mind that HPAI should be included as part of the differential diagnosis for any acute and unexplained mortality.
| Highly pathogenic avian influenza (HPAI) | Low pathogenic avian influenza (LPAI) |
|---|---|
| Sudden mortality up to 100 percent | Potentially asymptomatic |
| Severe drop/stop in egg production | Mild to moderate respiratory symptoms |
| Respiratory signs and swelling of sinuses | Drop in egg production |
| Oedema and subcutaneous bleeding | Increase in secondary infections |
| Diarrhoea and neurological signs | Can mutate into HPAI during flock infection |
| High number of infected individuals/high disease number – highly contagious form |
How do we diagnose avian influenza?
Most cases will be confirmed following PCR testing on sick birds or carcasses. Post-mortem findings are highly variable and not pathognomonic, but petechiae and haemorrhage are frequently seen – particularly affecting the pancreas.
How can we prevent infection?
Vets will likely be asked how avian influenza can be prevented in their birds. It is worthwhile for vets to understand and advise clients on the biosecurity measures available for backyard and other birds. Individual biosecurity programmes can also be written but are best formulated after a site visit.
General biosecurity guidance, as required by the Avian Influenza Prevention Zone currently in place in England, Wales and Scotland, is available on the APHA website. A housing order is also in place in England and Wales. Additional biosecurity measures, movement restrictions and other legislation may apply in the control zones placed around areas with outbreaks. (Specific rules for each disease zone can be found on the APHA website.) The most significant measures are the potential movement restrictions applying to captive poultry and other birds (some pet birds are excluded). Movement licences or home/site visits may be required to provide veterinary care for birds in control zones.
With the exception of zoo birds, vaccination is not currently a prevention option for most captive birds in the UK. An application to vaccinate zoo birds can be made, although the efficacy of available vaccines against the current strain is unknown for most species.
Considerations for veterinary practices
Considerations for patients
There are several significant concerns for vets treating birds in their practice. The risk of transmission between patients if an infected bird is treated or admitted to the practice premises is of primary importance. Furthermore, clients and staff can bring the virus into the practice building on their clothes and shoes.
The risk of transmission between patients if an infected bird is treated or admitted to the practice premises is of primary importance
On top of footbaths, the same biosecurity measures required at avian premises should be implemented at the veterinary practice. Minimising or restricting client access to the building (implemented widely during the COVID-19 outbreak) could also be considered in practices frequently treating birds.
Practice processes and operating procedures need to be in place so birds can still be treated and their welfare protected without risking other patients. This may involve conducting home visits or triaging patients outside the practice for signs of avian influenza before they are admitted for further investigation/treatment. Having a covered, secure and lit outdoor triage area would be the best practice for practices treating a significant number of birds. The practice will also have to decide how emergency care and/or first aid can be provided to wild birds while preventing the risk of transmitting the disease to other avian patients.
Practices regularly treating birds should have hospital accommodation that keeps birds in functionally separate air spaces using extraction and positive pressure ventilation. Isolation protocols should be in place between hospitalised patients to reduce the risk of transmission between inpatients.
Practices regularly treating birds should have hospital accommodation that keeps birds in functionally separate air spaces using extraction and positive pressure ventilation
In the worst-case scenario of a suspect or confirmed case entering the veterinary premises, it is feasible that formal restrictions will be served to prevent further birds (and potentially other species) being treated in the contaminated building. These restrictions could last several days until cleaning and disinfection have been completed.
Considerations for staff
There is also a health and safety consideration for practice staff. While the current strain of H5N1 is considered a low zoonotic risk, practice personnel need to be provided with adequate PPE when examining suspect cases. PPE should include FFP3 masks, nitrile gloves, disposable coveralls, eye protection and rubber boots/shoe covers.
While the current strain of H5N1 is considered a low zoonotic risk, practice personnel need to be provided with adequate PPE when examining suspect cases
Conclusion
Avian influenza continues to be a constant threat to captive birds in the UK. Sensible planning, biosecurity measures and management practices can help to reduce the risk of infection.
