Avian Influenza (AI):

Synonym: Bird Flu

• Avian influenza is viral disease of poultry characterized by heavy mortality.
• Virus also affects respiratory, digestive and nervous system.

Etiology:

• Influenza A virus of Orthomyxoviridae family
• Virus are enveloped, contains RNA as genetic material in 8 segments that encode 11 protein.
• Among them, hemagglutinin and neuraminidase are two antigenic surface protein that forms basis of classification of virus.
• At present, 16 HA and 9 NA have been identified. Each virus has one HA and one NA subtypes.
• Although virus can occur in numerous subtypes; H5N1 seems to be most pathogenic and cause most of outbreaks. It is then followed by H7N7, H7N3, H5N2.

Epidemiology:

• Disease was first reported from Italy as fowl plague in 1878; around 147 years ago.
• Since then, disease is distributed globally
• Disease has been listed under notifiable disease by Government of Nepal.
• In 16 Jan. 2009, Nepal face first outbreaks of HPAI H5N1 among backyard poultry in Jhapa followed by several outbreaks.
• On March 24, 2019 first human case of influenza A was recorded in Nepal.
• Outbreaks of disease are mainly recorded in spring and winter season.
• H5N1 and H5N8 are mainly responsible for HPAI outbreaks in Nepal.
• 6 provinces among 7 provinces have faced outbreak with highest number in Bagmati Province. Karnali and Sudurpaschim province face minimum outbreaks.
• In 2025, first outbreak of disease for year was recorded from Koshi Rural Municipality, Koshi Province in Feb.17 2025. It was confirmed by CVL by testing sample of dead birds.
• Nepal had previously reported a human death from virus in 2019.
• From 2003- May 27, 2025 total number of human cases of infection by H5N1, Gs/GD-related HPAI viruses was 976; out of which 470 were fatal.
• Fatalities are mostly observed in Asia and Africa.

Transmission:

• Virus is excreted from nares (nose), mouth, conjunctiva, and cloaca of infected birds into environment.
• Virus spread by direct contact between infected and susceptible birds.
• Indirect contact through fine droplets suspended in air.
• Through fomites (inanimate objects)
• Virus are readily transported to other farm/premises by people (contaminated shoes and clothing) and equipment.
• Migratory birds play important role in transmission.
• Vertical transmission is rare/absent.

Pathogenesis:

• Virus gains entry through inhalation or ingestion of MPAI or HPAI
• Nasal cavity is major site of initial growth. Virions invade the submucosa of respiratory or intestinal tract and enter blood vessel.
• Virus replicates within endothelial cells of these vessel and spread through lymphatics/vascular system to infect and grow in internal organs, brain and skin.
• Symptoms and death of birds occur due to multiple organ failure.
• With MPAI viruses, respiration is usually limited to respiratory or intestinal tract. Illness or death is usually from respiratory damage.

Clinical Findings:

• Symptoms are extremely variable and depend on factor such as species of bird, sex, concurrent infection, acquired immunity.
• It is described based on pathotype of virus; mildly pathogenic (MPAI) or highly pathogenic (HPAI).

a. Symptoms in MPAI:

• In wild birds, it donot produce any disease.
• In chickens, most common symptoms include mild to severe respiratory symptoms
• Coughing, sneezing, abnormal respiratory sounds (rales)
• Excessive discharge from eyes (lacrimation)
• In layers and breeders, hens may shows increased broodiness and decreased egg production.
• Huddling, ruffled feathers, depression, decreased activity
• Reduced feed and water consumption
• Sometimes diarrhea

b. Symptoms in HPAI:

• In wild birds and ducks, it donot produce symptoms
• Some birds are found dead before any symptoms are seen.
• Nervous disorder if birds survive for 3-7 days. It includes tremors of head and neck, inability to stand, twisting of neck and unusual position of head and legs.
• Sudden drop in egg production in breeder and layer. It continues on increasing and within 6 days, there is total stoppage of egg production.
• Respiratory symptoms are less common than with MPAI viruses, but includes respiratory sounds (rales), sneezing and coughing.
• Morbidity and mortality are very high (50-90%) and can reach 100% in certain flocks.

Other symptoms:

• Swelling of head, face and upper neck of chicken
• Hemorrhages in skin; usually in feet of chicken
• Cyanosis of comb and wattle of chicken
• Comb and wattles are extremely swollen, hemorrhagic and necrotic.
• Cyanosis on feet and toes.

PM Findings:

• PM findings are extremely variable, depending on pathogenicity of virus and presence of secondary pathogens.

a. MPAI:

• Tracheal mucosa is swollen with congestion and sometimes hemorrhage.
• Tracheal exudates may vary from serous to caseous in nature, blocking airway resulting in asphyxiation.
• Air sacculitis may be present.
• Infraorbital sinuses may be swollen and mucus to mucopurulent discharge occurs.
• Bronchopneumonia can result when secondary pathogen such as E. coli and P. multicoda is present.
• Catarrhal to fibrinous peritonitis
• Enteritis
• Inflammatory exudates may be found in oviduct of laying birds.
• Ovaries undergo regression, beginning with hemorrhage in large follicles. Ovary becomes softened and liquified.
• Oviduct may be swollen and contains exudates before undergoing involution.

b. HPAI:

• Swelling of head, face, upper neck
• Eyes may show excessive discharge and swelling surrounding eye is common.
• Small to large hemorrhages below the skin in the feet.
• Necrotic foci, hemorrhage and cyanosis of wattle, comb and legs.
• Hemorrhages are seen on the surface of heart, in breast and leg muscles and in mucosa of proventriculus and gizzard.
• Trachea may be highly congested and show hemorrhages in severe cases.

Diagnosis:

• Based on history and clinical findings
• Based on PM findings
• Direct detection of AI viral proteins or genes in specimen such as tissues, swabs, cell cultures
• Isolation and identification of virus
• HI test
• Immunodiffusion test
• ELISA

Differential Diagnosis:

1. Ranikhet disease (ND):

• Greenish diarrhea
• Hemorrhage at junction of proventriculus and gizzard.
• Torticollis, wing paralysis

2. Infectious bronchitis (IB):

• Mortality is usually low compared to AI
• Respiratory signs include coughing, sneezing, tracheal rales, watery eyes
• Drop in egg production with watery albumen
• No neurological signs are present
• Caseous exudates are present in trachea rather than hemorrhage as in AI
• Nephritis is marked than AI

3. ILT:

• There is characteristic caseous or diphtheritic plugs in larynx and trachea.
• Intranuclear inclusion bodies in tracheal epithelium

4. Mycoplasmosis:

• Onset is slow and chronic in nature
• Culture/PCR for M. gallisepticum or E. coli

Treatment:

• There is no satisfactory treatment.
• Supportive care and treatment with broad-spectrum antibiotics may reduce the effects of concurrent bacterial infections.

Control Measures:

• The most important source of virus for poultry is other infected birds. So, first step is separation of susceptible birds from infected birds.
• Culling of all susceptible flock from poultry houses is essential to control disease.
• After removing birds from houses, all secretions and excretions should be disposed properly.
• Poultry houses should be cleaned, sterilized and fumigated.
• Slaughtered birds or dead birds should be disposed either by burying or burning.
• All the equipments should be sanitized, sterilized before reuse.
• Biosecurity should be maintained at farm; provision of foot bath, separate clothing.
• Vehicles entering or exiting the farm should be disinfected properly.
• Movement of wild birds into poultry farm should be restricted.
• Export and import of poultry and poultry products should be restricted from disease prevalent areas.