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. Transmission through food is not possible.
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. Transmission through food is not possible.
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.
In 2023, there have already been numerous outbreaks of avian influenza in wild birds in all provinces of Austria. However, individual poultry farms and hobby farms (captive birds) were also affected by the highly pathogenic A(H5N1) virus. Information on the international situation and spread of AI is assessed and compiled in the monthly Animal Disease Radar Austria.
Poultry farmers should pay special attention to the observance of biosecurity measures: direct and indirect contacts between poultry and wild birds should be prevented at all costs. In case of health problems of the animals in poultry farms, a veterinary examination should be carried out without fail and avian influenza should be ruled out. For early detection and prevention of further spread, all wild waterfowl and birds of prey found dead must be reported to the locally competent district administrative authority (official veterinarian). Such animals should not be touched and left at the site where they were found. The recovery and examinations will be arranged by the authority.
We would also like to point out that every poultry holding must be reported to the competent district administrative authority.
Avian influenza surveillance in Austria
The Europe-wide AI surveillance program consists of an active part (farm poultry) and a passive part (wild birds).
In the course of the active surveillance program in 2022, a total of 4,105 poultry blood samples were tested for avian influenza virus in Austria. Slaughter blood from 1,420 laying hens from 134 farms (of which 60 were free-range), from 400 parent birds, broilers, and pullets from 40 farms, from 550 fattening turkeys from 55 farms, from 1,682 geese and ducks from 83 farms, and from 53 ostriches from 2 farms were serologically tested. No antibodies to influenza A virus of subtypes H5 and H7 were detected. 1365 samples from poultry and captive birds were tested for the AI virus genome during clarification testing. Highly pathogenic AI virus (HPAIV) of subtype H5N1 was detected in 3 hobby farms in the districts of Rohrbach and Graz-Umgebung.
In the context of passive surveillance in 2022, 338 samples from wild birds found dead were tested for the avian influenza A virus genome by PCR. The highly pathogenic A(H5N1) virus was detected in 35 dead wild birds from 5 provinces.
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).
Last updated: 19.09.2023