Aetiology
Avian influenza virus belongs to the family Orthomyxoviridae. Viruses are typed into influenza A, B or C on basis of nucleocapsid or matrix antigens which are common for all viruses of same type. Influenza A viruses are subtyped on the basis of haemagglutination antigen and neuraminidase antigen found on envelop. At present, there are 14 recognized 5 H-types and 9N-types. Each virus possesses one H-type and one N-type.
Host
Many avian species domestic or wild can be infected with influenza viruses that may or may not cause disease. Among domestic avian species, turkeys have been most frequently involved in the disease outbreak of influenza whereas chickens are least frequently involved. Isolation rate for wild duck congregating on lakes prior to migration has been very high usually in range of 25%-60%and this indicates the potential for the spread of influenza virus throughout the world but especially along migratory routes. The viruses responsible for reported epidemic of highly pathogenic avian influenza HPAI up till now have been H5 and H7 subtypes. In some area such as Minnesota, USA and England, disease outbreaks due to low virulence influenza viruses have been reported every year since the 1960s and have represented a serious economic burden to turkey producers. Commercial ducks have also been shown to be frequently infected with avian influenza virus but geese have been rarely associated with the because of the marked resistance these birds show even to strains highly virulent for chicken and turkey.
Transmission
Sources of infection include aerosol from nasal discharge, feed and water contaminated with faeces.
Route of infection is either airborne or most likely through the ingestion of infective material.
Primary introduction of avian viruses into an area is usually due to water fowl activity.
H1N1 viruses may also be readily transmitted between pigs, humans and turkeys.
Secondary spread of avian influenza virus has been considered to be by human where virus is carried from location to location on foot, clothing and equipment.
Clinical signs
These are influenced by several factors such as strain of virus, species and age of host, presence of concurrent infectious agents, nutritional deficiencies. Environmental factors such as excessive ammonia and dust. The disease caused by different viruses varies in severity from high mortality with sudden death proceeded by few unknown clinical signs to a very mild and inapparent form. Often the first sign of avian influenza in chicken and turkey is sudden onset of high mortality which may approach 100% within 2 days. Clinical signs which may be associated with high mortality are cessation of egg production, respiratory signs, rales, excessive lacrimation, sinusitis, edema of head and face, subcutaneous haemorrhage, cyanosis of comb and wattle and diarrhea. The less virulent viruses may cause considerable disease. In uncomplicated infection, these viruses may cause drop or cessation of egg laying, respiratory disease, anorexia, depression, sinusitis and low mortality while other concomitant infection present exacerbate infection on birds under stress due to advance environmental condition. Mortality may rise to 50% of the flock and clinical signs showed a marked increase in severity. Ducks and other water fowls tend to be refractive even to viruses that are highly pathogenic to chicken.
Post-mortem lesions
The most severe cases show congestion or haemorrhage of the skin, liver, spleen, heart, kidney and lung. Infection with viruses of low virulence is usually associated with lesions of respiratory tract including the nasal and tracheal passages and mostly notable sinusitis.
Diagnosis
A tentative diagnosis can be reached through clinical signs due to their variability. The definitive diagnosis of influenza virus A is reached by isolation and recognition of virus in 8-10 day virus embryonated hen’s egg.
Samples collected in live animal include tracheal and cloacal swabs. In dead animals, faeces, intestinal contents and cloacal samples are collected. Further characterization to determine the subtype is also important.
Haemagglutination activity in bacterial
The recommended method for demonstration of the presence of influenza virus is by immunodiffusion using specific antiserum.
Control
Most developed countries have policy for prevention of introduction of highly pathogenic avian influenza via trading with other countries and stamping policy and eradication policies at national level in eradication of the disease.
Vaccination is practiced in a place like Minnesota at significant losses due to low virulent influenza viruses. Vaccines are not aim at highly pathogenic avian influenza viruses.
Inactivated oil emulsion vaccine is used to protect against low virulent viruses that cause little disease in absence of predisposing factors like mutation.
Management practices like situating farms away from migratory routes is necessary.
Birds should be reared in bird proof cages.
Prevent movement of people from farm to farm.
Equipment and vehicles should be disinfected.
When disease outbreak occurs, depopulation should be considered.
Restock only after two weeks following thorough cleaning and disinfection.