Highly contagious generalised viral disease affecting both wild and domestic birds. Various benign or sub-acute syndromes originate from "Avian influenza". It can be extremely serious and induce a death rate of up to 100%.
Type A, low-pathogenic or apathogenic flu virus strains, found in wild aquatic birds, are present throughout the world. Highly pathogenic strains may emerge through mutation or genetic reassortment and cause outbreaks of the disease.
Currently the Americas are not affected by the highly pathogenic avian influenza virus of sub-type H5N1, which is raging and diffusing to the rest of the world from sources which intially appeared in South-East Asia in 2004.
All domestic bird species are presumed to be sensitive to the infection: chickens, ducks, geese, turkeys, quails, pheasants, etc. However outbreaks of the disease mainly occur in chickens and turkeys.
Ducks and other wild aquatic birds are also sensitive but the infection is generally subclinical (asymptomatic carriers).
Humans, horses, pigs and cats may also be affected.
A segmented RNA virus from the family Orthomyxoviridae, type A influenzavirus.
The combination of haemaglutinin (H) and neuraminidase (N) determines the viral sub-type. To date all highly pathogenic virus strains are of the H5 or H7 sub-type.
Infections by the highly pathogenic virus are uncommon and must not be confused with low-pathogenic virus infections which may also belong to the H5 and H7 sub-types.
Resistance - Sensitivity
Virus inactivated by heat, acid pH, formol and iode compounds, oxidative agents, etc.
It may however persist for long periods in cold water, tissue and faeces.
Incubation period: from 3 to 7 days.
Symptoms vary according to the species infected, to individual resistance to the virus, but also according to the virulence of the infecting strain and to tissue tropism.
Sub-acute form: septicaemia and death occur in 1 to 2 days and affects up to 100% of the population.
Acute form: Death rate between 50% and 100%.
- Invasive phase:
• severe depression
• egg drop syndrome
• anorexia
• ruffling of feathers
Followed by cephalic oedema with tumefaction and cyanosis of the crest and the caruncle.
- Status phase: Visible signs in one animal or in several different animals:
• Intestinal disorders: greenish, bloody, profuse diarrhoea, dehydration and thirst.
• Nervous disorders: signs of encephalitis up to the end of disease progression with convulsions, clonic paralysis, loss of balance, falls. Paralysis of the neck, wings and legs.
• Respiratory disorders: abundant tracheal mucous, discharge from the eyes and nose, respiratory difficulties(polypnea, rales, sneezing, scabs on the nostrils).
- Terminal phase: most often aggravation and death, otherwise clinical improvement with persisting nervous system damage and laying abnormalities.
Chronic, subacute form: respiratory and laying disorders
Asymptomatic form: frequent, especially in wild acquatic birds, detectable by virological examination.
No treatment exists to date.
Sanitary prophylaxis
Defensive measures:
• Avoid all contact with poultry and wild birds, especially wild acquatic birds
• Do not introduce any animals of unknown sanitary status
• Limit the circulation of people on the farm
• Ensure effective cleaning and disinfection of equipment and premises
• Ensure (if possible) that only one age group per farm is present
Offensive measures:
Where an outbreak occurs,
• Killing of all birds
• Elimination of carcasses and products
• Cleaning and disinfection
• 21 day depopulation before introducing any new animals
Medical prophylaxis
Very difficult to implement due to the variability in viral sub-types and the absence of cross-protection bewteen the different sub-types.
Inactive oil-adjuvant vaccines have proved effective in prevention of the disease and/or in reducing the death rate among chicken and turkey stocks.
However vaccination does not guarantee the absence of infection (asymptomatic) in certain vaccinated animals. They may therefore carry and excrete the virus, but in a lesser quantity than if they were not vaccinated.
No cross-protection exists between the various Influenza A virus sub-types. It is therefore essential to recognise the circulating viral strain in order to adopt the relevant vaccine. For the currently circulating H5N1 strain, a vaccine has recently been developped for animals.
A human vaccine against H5N1 is currently undergoing research.