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Avian Influenza

Teun Fabri is a poultry veterinarian and expert in the field of Avian Influenza. He keeps you up to date on the latest developments concerning bird flu. Any questions?

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Avian Influenza in poultry can be divided in to two disease form: High pathogenic Avian Influenza and Low pathogenic Influenza. High pathogenic avian Influenza (AI), also known as bird flu or avian flu, is a viral disease that generally progresses extremely acutely, giving general clinical signs. AI causes high mortality rates in fowl and many other species of birds. (Wild) water birds are particularly considered to be a virus reservoir. AI is a compulsory notifiable disease according to the Animal Health Regulation, and is actively controlled.


Animal disease information Avian Influenza


AI is caused by the Influenza virus type A, and is one of the Orthomyxoviridae.

The virus

Each influenza virus has a specific name based on the constitution of the two protein spikes which are present on the surface of the virus,. The two spikes are Hemagglutine spike and the Neuramidase spike, of which there are various types. We can distinguish between a total of 18 different H types (H1 to H18 and 11 different N types (N1 to N11). The H types 1 to 16 occur in a variety of mammals but also in birds, including poultry, in combination with the N types 1 to 9 such as H5N1 and H5N8. The H types 17 and 18 occur in bats, in combination with N types 10 and 11.

Clinical signs

The incubation period (time between infection and development of symptoms in a individual bird) of the Avian Influenza Virus (AIV) is generally rather short, and varies from a few hours to a number of days in a single chicken and three to fourteen days within a flock. The clinical signs caused and the species of birds affected by the virus, vary per type of virus. There can also be significant differences within each virus type. The low-pathogenic (LPAI) virus generally causes very few clinical signs and may be limited to a drop in egg production in egg producing birds.

The clinical signs of high-pathogenic strains (HPAI) are more severe than those of LPAI. The symptoms are not all specific for AI: lethargy, reduced feed consumption, decline in laying, respiratory noises and nasal excretions. The classical signs are theswelling of the mucus membranes and oedema can occur in the head and neck, as well as blue discolouration of the comb and lobes, haemorrhages and diarrhoea. At a later stage, neurological symptoms may be observed. The mortality rate can be as high as 100%. In some cases, there is extremely acute mortality, without prior clinical signs. In the last decades we see however non classical HPAI were the classical lesion are absent. Until now, the high-pathogenic viruses have always been of the H5 or H7 types

Low-pathogenic H5 and H7 viruses reproduce in respiratory and intestinal cells. Hi-pathogenic viruses penetrate not only respiratory and intestinal cells, but also other cells in the body, such as those of the blood vessels. This is what makes a high-pathogenic variant so dangerous: it causes haemorrhaging and very sudden death of the animal.

Low-pathogenic viruses are generally much less problematic. They can cause mild disease and affect the animals’ feed and water intake. They can also result in a drop in egg production and increased sensitivity to concurrent bacterial infections. A low-pathogenic virus in itself is not particularly dangerous. However, it is important that these H5 and H7 variants are still detected, because as the virus spreads, there is a risk of a high-pathogenic virus developing ‘accidentally’.

Just like most viral infections, influenza is an infection that can naturally pass, and the virus will disappear after a certain period of time. However, the virus spreads extremely rapidly during that period, and the farm becomes a risk for poultry farms in the area and possibly even for human infection. It is therefore essential that we quickly discover the source of the virus, and take measures to prevent it from spreading further. This applies not only to the compulsory treatable types (HPAI H5 and H7) but also to other types.

Disease progression and development of immunity

Following infection with the virus, it will spread throughout the body and may result in clinical signs becoming apparent. Antibodies become detectable following the infection. Infected animals transmit the virus via the airways, mucus membranes and faeces. The virus remains infectious in faeces for a prolonged period (up to 30 days or more).

Compulsory reportable

The Dutch Animal Act and the European animal health legislation contain rules on detection and combating of this compulsory reportable, infectious disease. Any suspicion of AI must be reported to the Netherlands Food and Consumer Product Safety Authority (NVWA). Upon suspicion of avian influenza, the NVWA will visit the farm and will have material from the suspect farm tested by Wageningen Bioveterinary Research (WBVR) in Lelystad as quickly as possible.

The monitoring system for AI was established to detect suspect cases and outbreaks as quickly as possible. This system consists of the following:

  • Regular serological monitoring.
  • Obligation to report any clinical signs.
  • Possibility of submitting EWS swabs to exclude AI (so-called exclusion swabs) by the veterinarian.

Diagnosis of Avian Influenza

Differential diagnosis

It is difficult to determine a differential diagnosis due to the large variety of clinical signs. Diseases causing clinical signs similar to AI (differential diagnosis) include:


  • Infectious Laryngotracheitis (ILT)
  • Infectious Bronchitis (IB)
  • Newcastle Disease (NCD)
  • Turkey Rhino-Tracheitis (TRT)


  • Haemophilus paragallinarum
  • Ornithobacterium rhinotracheale
  • Escherichia coli
  • Pasteurella multocida
  • Riemerella anatipestifer
  • Mycoplasma gallisepticum

Risk factors for Avian Influenza


Prevention of an AI infection should mainly focus on preventing contact with wild birds. In addition, it is important to prevent introduction to the farm via new animals, people, pests and fomites, by applying a strict access policy.

Preventing introduction of the virus

The Avian Influenza virus can spread via dust particles in the air. However, any spread over a larger distance is always the result of contact with people, means of transport or borrowed equipment. Poultry becomes infected with the virus through inhalation or contact with infected materials. The virus can enter farms in the following manner:

  • From your own farmyard. Regard your own farmpremises as a ‘dirty site’! Your farmpremises may house infectious material (originating from infected birds, for example);
  • Via contact with (migratory) birds. Infected birds spread the virus via their faeces, for example;
  • Via infected materials such as crates, means of transport and people whose shoes or clothing have been in contact with the virus;
  • Via dust particles from a neighbouring infected barn;
  • Via infected rats and mice. These animals can transport the virus via their coat and feet, and can also multiply and excrete the virus.
  • Via infected poultry that was already infected upon arrival.

To exclude every possible risk of infection, we advise poultry farmers to take the following measures:

  • Ensure that your farm premises and poultry housespoultry houses are not accessible  to unwanted and not registered visitors;
  • Ensure that no people enter the poultry houses other than those necessary for animal care or veterinary care.
  • Apply a hygiene protocol for visitors to the farm;
  • Establish a clear demarcation between the farm premises and the area around the private house;
  • Ensure that your farm premises are kept clean and are disinfected regularly, not forgetting the square at the rearside of the poultry houses;
  • Remember that you yourself, as the poultry farmer, can also transport infected particles. Make sure you also comply with the rules;
  • In order to avoid any possible risk, it is essential to not use any equipment from other (poultry) farms;
  • Apply strict quarantine and  health monitoring of newly introduced poultry a number of times per day. In case of doubt, immediately consult your veterinarian.

We will now take a closer look at measures applicable to a number of important aspects of farms: the farmyard and the barn, location of the farm, visitors and means of transport.

The farm premises and the barn

  • Close off the access route to your farm premises and to the poultry poultry houses and use signs to clearly communicate that access is prohibited.
  • Ensure that the farm premises can only be entered after changing in to farm related clothing and footwear. This footwear must only be worn on paved pathways and certainly not on grasslands.
  • Install a hygiene sluice (minimum: mobile threshold) behind the entrance door to the poultry house and place your footwear in front of this threshold, put on overalls only intended for use in the barn, and step over the threshold into footwear for use in the poultry house. Once again change footwear before moving from the front access area to the animal housing area. Preferably shower before entering the barn, minimum: washing of hands.
  • Do not allow any household pets in the poultry house.
  • Remove any dead rats and mice as quickly as possible, washing and disinfecting your hands directly after contact.
  • Remove any bird faeces from the farm premises in such a manner that there can be no contact with the poultry.
  • At the end of the day, ensure that all feeding systems are emptied, to avoid any contact with pests.
  • Disinfect and clean the access paths to the poultry houses, on a daily basis.

Location of the farm (extra attention when situated in a wetland area)

  • Screen off your farm premises from water birds and all other types of wild birds where possible (electric fencing or netting along access routes) and avoid direct contact, but also avoid contact with bird faeces.
  • Use bird scaring equipment and/or objects to keep these animals at a distance from your poultry houses.
  • Make the premises unattractive for water birds and birds of prey (short grass, no other sources of food such as pellets or wheat, minimum greenery, no water).
  • Do not wear house related boots to walk in grasslands or along ditches, and avoid grasslands when driving farm vehicles.
  • Do not remove any wild bird corpses yourself, but rather report them to the (local) authorities.
  • Inspect the access routes to your farm premises at the start of the day and treat any bird faeces with disinfectant before removing them.
  • Only use registered disinfectants, which are effective against AI virus. These have a pH <2 or pH >11.

Visitors in a situation of the (suspected) presence of AI

  • Visitors must park their vehicles on the public road or on the addressed location.
  • Visitors must always report to the home address, Hand washing is required at the home address.
  • At the private house or building that forms the demarcation between the farm and private premises, visitors step into farm clothing and farm related footwear in combination with disposable socks (small overshoes). Showering is preferable, and hand washing is the minimum action required.
  • In the barn, poultry house overalls are worn over the disposable overalls, and visitors step into poultry house footwear. In the poultry house, safety goggles, a face mask, hairnet and disposable gloves must be worn.
  • When leaving the barn, the poultry houseclothing is removed and poultry housefootwear switched for farm footwear, along with hand washing.
  • In the home or intermediate building, the farm footwear is removed, the hands washed again and disinfected using alcohol.
  • The disposable overalls and other materials are left behind in a plastic bag close to the car. Hands are then once again disinfected using alcohol.

Means of transport

  • Vehicles must only be allowed access to the premises following disinfecting of, minimally, the wheels and wheel arches. This is most effectively done by the poultry farmer, as the driver should remain in their vehicle.
  • when leaving the vehicle cabin, the driver must put on overshoes before touching the ground. He/she must then put on disposable overalls.
  • The feed truck should not be placed directly under the poultry house inlet.
  • On departure, the hoses and any other truck equipment used must be disinfected, as well as the wheels and wheel arches.
  • The driver must leave the used working clothing and suchlike behind at the farm premises, and must change his seat cover before departure.

Approach to Avian Influenza

By law, the farmer, the laboratory and the veterinarian must report any suspicion of the presence of the AI virus to the Netherlands Food and Consumer Product Safety Authority (NVWA). The NVWA has provided a telephone number for this purpose, which can be reached 24 hours per day: 045-5463188. Once the NVWA receives a report of a possible outbreak of Avian Influenza, the farm is declared suspect and thereby quarantined, and a specialist team will be formed to initiate the process of combating the disease according to EU legislation.

Specialist team

Upon reporting a possible outbreak of avian influenza, a specialist team is formed, consisting of the veterinarian of the suspect farm, a GD poultry veterinarian and an animal disease prevention officer of the NVWA. This team will initiate the control process according to EU Regulation 2020/687. The specialist team will visit the farm, assess the situation and take samples if necessary. Based on the clinical inspection, the following results may emerge:

  • Strong suspicion of AI
  • AI cannot be excluded
  • AI highly unlikely

Any samples will be submitted to Wageningen Bioveterinary Research (WBVR) in a transport organised by the NVWA. According to agreements between the NVWA and WBVR, in the event of a suspicion of HPAI, the samples will be tested immediately; an M-PCR test will be conducted, followed by a further H5 and H7 specific PCR test if the results of the M-PCR are positive. In the event of a suspicion based on positive serology, the decision may be taken to conduct the M-PCR and the H5/H7 specific PCR during working hours. Should laboratory testing show the presence of an H5 or H7 type virus, the farm will be declared infected.

Besides the information regarding AI in poultry, the specialist team will also enquire after the health of the people directly involved in working with the poultry, and will refer them to the Area Health Authority (GGD) in the event of possible infection.

Approach to suspect and infected farms

Commercial poultry: the decision to cull the poultry/birds at the HPAI-infected farm will be taken as quickly as possible, and the following measures can be taken for all or part of the Netherlands: a general or limited stand-still, extra restriction arrangements for visitors, compulsory protective screening and/or indoor segregation, a prohibition for bird exhibitions and bird fares  and a hunting prohibition. A restricted perimeter will be established around an infected farm: a protective zone of 3 kilometres around the infected farm in which a regular monitoring will take place and a monitoring zone of 10 kilometres around the infected farm. It may also be necessary to designate culling areas and possibly free areas (regionalisation).

In the case of an outbreak of HPAI in non-commercial poultry at locations housing more than 50 animals, a protective zone will always be established. Other measures will depend on the individual situation. When HPAI is detected among wild birds, a government risk analysis will determine the appropriate measures to be taken.


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