Avian influenza (AI) is an infectious viral disease of birds, commonly known as bird flu. Wild water fowl such as ducks and geese are mostly affected by AI, but often show no apparent signs of illness. Poultry birds are also susceptible to AI infection that can cause large outbreaks and epidemics in poultry.
Humans are not usually affected by AI virus, however some subtypes of AI such as A(H5N1) and A(H7N9), have caused serious infections in people. Other avian influenza subtypes, including H7N3, H7N7, and H9N2, have also infected people.
Avian influenza A(H5N1) subtype virus is highly pathogenic virus. It was first recognized in humans in 1997 during the poultry outbreak in Hong Kong, Special Administrative Region of China. Since its re-emergence in 2003, outbreaks of A (H5N1) have reported from poultry in Asia to Europe, and Africa. According to World Health Organization (WHO), 16 countries have reported 846 laboratory-confirmed human cases of avian influenza A (H5N1) virus, including 449 deaths since 2003 through 20 January 2016.Out of 16 countries, 4 were in South-East Asia Region; Bangladesh, Myanmar, Indonesia and Thailand.
10 laboratory-confirmed human cases with avian influenza A (H5N6) virus including 6 deaths have been detected in China since 2013.
In March 2013, a subtype of influenza virus, A(H7N9) was detected first time in humans in China. A total of 722 laboratory-confirmed cases of human infection with avian influenza A(H7N9) virus, including at least 286 deaths , have been reported to WHO.
A total of 28 laboratory-confirmed cases of human infection with avian influenza A(H9N2) viruses, with mild symptoms have been detected globally.
Infection of avian populations with certain subtypes of avian influenza A virus poses continuing global public health risks because of two reasons, first- occurrence of sporadic human infections and second- emergence of a pandemic influenza strain (disease occurring over a wide geographic area and affecting an exceptionally high proportion of the population).
All human infections caused by a new influenza sub type are reportable under the International Health Regulations (IHR, 2005).
Signs And Symptoms Of Avian Influenza A Virus Infections In Humans-
In Infection caused by low pathogenic avian influenza (LPAI) A virus, sign and symptoms ranged from conjunctivitis to influenza-like illness like fever, cough, sore throat, muscle aches to lower respiratory disease (pneumonia) requiring hospitalization.
Highly pathogenic avian influenza (HPAI) A virus infections in people have been associated with high grade fever (higher than 38oC) with influenza like symptoms as cough with sputum (sometimes bloody) and sore throat. Some patients may show symptoms of lower respiratory tract involvement early in the illness.
Some patients may suffer from diarrhea, vomiting, abdominal pain, chest pain, and bleeding from the nose and gums and sometime neurological changes (altered mental status, seizures).
Respiratory distress, a hoarse voice, and a crackling sound during inhalation are common findings.
Incubation period for A (H5N1) is ranging from two to eight days and may be as long as 17 days and for A(H7N9) it is between two to eight days, with an average of five days. WHO recommends that an incubation period of seven days can be used for field investigations and for the monitoring of patient contacts.
The case fatality rate for A (H5N1) and A(H7N9) virus infections in people is much higher as compared to seasonal influenza infections.
Disease is caused by avian influenza virus belongs to the Influenza A genus of the orthomyxoviridae family.
AI viruses are divided in to two groups according to severity of the disease: high pathogenic viruses and low pathogenic viruses.
High pathogenic viruses are responsible for high death rates (up to 100% mortality within 48 hours) in some poultry species. Low pathogenic viruses, which are not, associated with severe disease but cause outbreak in poultry species.
Highly pathogenic avian influenza (HPAI) viruses are mainly restricted to H5, and H7 subtypes which are in circulation in poultry. These viruses have potential to cause serious disease in people and have the potential to change into a form that is more transmissible among humans.
Transmission- Infected birds shed avian influenza virus in their saliva, mucous and faeces. Following direct close or prolonged contact with sick or dead infected poultry, viruses enter into a person’s eyes, nose or mouth, or are inhaled.
Risk factors for human infection-
Direct or indirect exposure to infected live or dead poultry or contaminated environments, such as live bird markets are the primary risk factors for getting the infection.
Consumption of dishes made of raw, contaminated poultry blood can cause infection.
Slaughter, defeathering, handling carcasses of infected poultry, and preparing poultry for consumption, especially in household settings, are likely to be risk factors.
Properly prepared poultry or eggs usually do not transmit the disease.
Current epidemiological and virological evidence suggests that AI virus is not transmitted human to human.
Diagnosis of Avian influenza is suspected in all persons presenting with acute febrile respiratory illness in those countries or territories where avian influenza A viruses have been identified in animal populations.
Presenting signs and symptoms of AI illness are non-specific therefore a detailed exposure history should be taken including any close/direct contact with sick or dead poultry, wild birds, other severely ill persons, travel to an area with AI activity, or work in laboratory handling samples possibly containing AI virus.
(“Golden Rule:” Clinical specimens from humans and from animals should never be processed in the same laboratory. However they could be processed in the same institution if separation of working rooms for animal and human specimens is clear and strict. This is to eliminate risk of cross contamination of human and animal samples. [WHO])
Avian influenza A virus infection is usually diagnosed by collecting a swab from the nose or throat of the sick person during the first few days of illness.
Following tests may be used to detect avian influenza in human samples-
- Viral RNA detection by reverse transcriptase polymerase chain reaction (RT-PCR) and real time RT- PCR assay-PCR detects viral RNA present in either clinical specimens or virus cultures. RT-PCR assay takes six to eight hours whereas Real time RT-PCR methods provide results in three to four hours and are more sensitive.
- Virus culture- Because of the bio-safety concern, isolation of highly pathogenic viruses is usually performed only in specially qualified and equipped laboratories.
- Rapid antigen detection- Viral antigen detection may be carried out by immunofluorescence or enzyme immunoassay (EIA) methods.
- Serological identification of antibodies against avian influenza A viruses-Serological tests available for the measurement of influenza A-specific antibody include the haemagglutination inhibition test (HI), enzyme immunoassay (EIA), and virus neutralization tests (VN).
Hospital care is recommended in initial stages of the disease (such as Human infection with an A(H5N1) to monitor clinical status. After discharge from hospital appropriate instructions for household members on personal hygiene and infection control measures should be provided.
Along with supportive therapy some antiviral drugs, especially oseltamivir, can be used. It reduces the duration of viral replication and improves prospects of survival.
Antiviral treatment (oseltamivir) is given as early as possible based on clinical suspicion (ideally, within 48 hours following symptom onset) and before confirmation of causative agent. In suspected case a standard five day course should be given, unless any other diagnosis is confirmed. In a confirmed case depending on the clinical status, dose and duration of the oseltamivir therapy can be increased by treating clinician. The use of corticosteroids is not recommended by WHO.
Complications of A(H5N1) and A(H7N9) infection include:
- multiple organ dysfunction,
- secondary bacterial and fungal infections.
Persons who work with poultry or who respond to avian influenza outbreaks are advised to use appropriate personal protective equipment (PPE) and follow proper hand hygiene.
Surveillance with Inter-sectoral coordination- Surveillance committees may be constituted at state and district level comprising experts from health, animal husbandry and other sectors for regular outbreak surveillance (early warning system) in domestic poultry, domestic birds and piggeries.
WHO has advised that travellers to countries with known outbreaks of avian influenza should avoid poultry farms, contact with animals in live bird markets, entering areas where poultry may be slaughtered, or contact with any surfaces that appear to be contaminated with faeces from poultry or other animals.
Travellers should also wash their hands often with soap and water. Travellers should follow good food safety and good food hygiene practices.
Infection with an avian influenza virus should be considered in persons who develop influenza like illness during their travel or soon after returning from area where avian influenza is in transmission.
Post-exposure antiviral chemoprophylaxis of close contacts of a patient with confirmed AI virus infection and/or high risk poultry/environmental exposures is advised.
Control measures in birds-
When outbreak of AI occurs in birds, the immediate priority should be to contain the disease and eliminate the disease by destroying the infected or exposed birds (culling or stamping out), proper disposal of carcasses (burial), decontamination of the affected premises, movement controls, restocking of the bird after a safe period of destocking.
Vaccination strategies recommended by World Organisation for Animal Health (OIE) can be used for preventing AI in poultrya. The OIE recommends eradication of highly pathogenic avian influenza (HPAI) at its poultry source to decrease the disease in avian species and further human infections.