Avian influenza (also called bird flu) is a notifiable viral disease of birds. The pathogens are influenza A viruses, which are divided into subtypes based on different surface antigens (hemagglutinin, HA, and neuraminidase, NA). In birds, 16 hemagglutinin and 9 neuraminidase subtypes have been reported to date. As a result of gene changes and exchange of genetic material, new virus variants are constantly emerging.

Highly pathogenic (severely pathogenic, HPAIV) and low pathogenic (weakly pathogenic, LPAIV) avian influenza viruses can be distinguished. The distinction relates to the severity of disease in birds. Highly pathogenic to birds are some variants of subtypes H5 and H7. These viruses are highly contagious to birds; in poultry, clinical disease is also referred to as avian influenza. Low-pathogenic variants of avian influenza virus can spontaneously change to highly pathogenic virus variants through mutations, causing severe disease outbreaks.

Mammals (including pigs, horses, cats, dogs, foxes, badgers, martens, otters, or minks) can also become infected with avian influenza viruses and become ill. The (A)H5N1 subtype, which currently occurs in Europe, is poorly adapted to humans, and disease has not been observed to date. In other parts of the world, sporadic transmissions of avian influenza to humans are reported, some of which can lead to fatal disease. Almost all human infections are due to close direct contact with infected or diseased poultry or indirectly through their feces. Human-to-human transmission has not yet been observed worldwide.

Avian influenza was first observed in Italy in 1878. The pathogens occur worldwide.

Wild waterfowl are the natural reservoir of pathogens.

All poultry species, but also many ornamental, raptor and wild bird species are susceptible to avian influenza viruses. The HPAI viruses are highly contagious, especially for poultry. The virus is shed in feces, saliva, and tears. Infection occurs directly from animal to animal or indirectly via objects that have come into contact with the virus. If there is a lot of dust, indirect infection via air is also possible.

Humans and other mammals (including pigs, cats, dogs, foxes, badgers, martens, otters, or minks) can also become infected with avian influenza viruses through intensive contact with infected poultry.

1-3 days

While low-pathogenic AI viruses cause no or only mild symptoms, infections with highly pathogenic AI variants lead to severe courses of disease and high mortality rates. Chickens, turkeys and numerous wild bird species are particularly affected. Respiratory symptoms up to severe respiratory distress, greenish watery diarrhea, bleeding on organs, encephalitis (CNS symptoms), comb tips and stands, edema (swelling) in the head area, pronounced decrease in laying performance, thin and missing egg shells, significantly reduced water and feed intake, languor and fever are common symptoms of an illness with the highly pathogenic avian influenza virus. Ducks, geese, and some wild bird species usually show no or only mild symptoms, but are important for pathogen spread.

In mammals, infection with avian influenza viruses is often asymptomatic or with mild flu-like symptoms. However, infections with certain HPAIV variants can also lead to severe and fatal disease.

From October 2023 to April 2024, avian influenza was detected in a total of four poultry farms (one zoo and three hobby farms) in Austria. In all federal states, with the exception of Salzburg, the pathogen was also found in wild birds. As the situation has improved across Europe and also in Austria in recent weeks, all areas with a highly increased risk of avian influenza will be lifted from 18 April. There are currently still some areas in Austria with an increased risk of avian influenza in which special biosecurity measures apply.

Information on the international situation and spread of AI is assessed and compiled in the monthly Animal Disease Radar Austria.

Poultry farmers should pay particular attention to compliance with biosecurity measures: direct and indirect contact between poultry and wild birds should be avoided at all costs. In the event of health problems in poultry farms, a veterinary examination should always be carried out to rule out avian influenza. For early detection and prevention of further spread, all wild waterfowl and birds of prey found dead must be reported to the locally responsible district administrative authority (official veterinarian). Such animals should not be touched and should be left where they are found. The authorities will arrange for the animals to be recovered and examined.

Given the occasion, we would also like to point out that every poultry farm must be reported to the competent district administrative authority

Information from the Ministry of Health on avian influenza

Notification of poultry farming in the Consumer Health Information System (VIS)

At the National Reference Laboratory for Avian Influenza (NRL, AGES Mödling), we test various bird samples (organs, swabs, carcasses) for the presence of AIV using Influenza A real-time RT-PCR. Positive detections are further subtyped and confirmed by sequencing, egg culture and hemagglutination test (HA). Indirect detection by determination of antibodies is performed by ELISA and hemagglutination inhibition test (HAH).

Real-time RT-PCR and sequencing: Influenza A viruses are tested molecularly in all birds by real-time RT-PCR. If positive, AIV variants are typed directly using specific methods to determine whether an H5 or H7 subtype is present. The NRL can currently differentiate all H1 to H16 or N1 to N9 AIV subtypes. Sanger DNA sequencing can be used to determine the pathotype of the H5 or H7 virus strain (low pathogenic or high pathogenic, LPAI or HPAI). Whole-genome sequencing is performed on selected samples, where all eight segments of the viral genome are enriched and subsequently sequenced. Enrichment is performed directly from clinical samples using a dedicated RT-PCR, and sequencing is performed using modern high-throughput equipment (next generation sequencing).

Hemagglutination test: Certain viruses, such as influenza A viruses, bind erythrocytes to their surface using hemagglutinin. This causes the blood to agglutinate (clump together). Dilution series can be used to determine the amount of virus.

Hemagglutination inhibition test: Special antibodies can prevent the agglutination (clumping) caused by the virus. In this way, antibody titers and specific antibodies directed against individual AI virus strains can be determined.

ELISA: The Enzyme-linked Immunosorbent Assay is an antibody-based detection method. Antibodies present in serum bind to a specific antigen and are measured qualitatively or semi-quantitatively using an enzymatic color reaction.

Egg culture: a potentially infectious inoculum solution is prepared from the samples, with which guaranteed virus-free chicken eggs are inoculated. These eggs are incubated for at least five days. If highly pathogenic HPAI viruses are present, the embryos in the eggs die and the virus in the allantios can be identified by hemagglutination (HA).