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Avian Influenza

Avian Influenza

Causative Agent

Caused by type A strain of influenza. Two types of avian influenza are  defined based on their virulence: a highly virulent type that causes fowl plague (highly pathogenic avian influenza, HPAI) and an avirulent type that causes only mild or asymptomatic disease. All HPAI are of the H5 and H7 subtype and responsible for large avian epidemics to date.

Incubation Period

2-5 days (range 1-9 days)

Infectious Period

No sustained human-to-human transmission documented to date.


Avian influenza viruses spread among susceptible birds through contact with contaminated excretions of other infected birds. They do not normally infect other species with the exception of pigs and horses. Infection of other mammals like cats has also been reported.

Human infection results from close contact with infected poultry (secretions and excrement) or contaminated surfaces.


The virus circulates among birds worldwide.

The first documented human infection with an avian influenza virus occurred in Hong Kong in 1997, when the H5N1 strain caused severe respiratory disease in 18 humans, of whom 6 died. The infection of humans coincided with an epidemic of HPAI, caused by the same strain, in Hong Kong’s poultry population.

The H7N7 subtype was responsible for another outbreak in the Netherlands in 2003 when 83 cases of conjunctivitis and one death from severe respiratory illness were reported.

In the 2004 outbreaks involving H5N1 subtype in Thailand and Vietnam, human cases were associated with severe respiratory disease and high mortality. During the period 2003-2010, a total of 512 cases, including 304 deaths, were reported to  WHO, with most of the cases from Indonesia (171 cases with 141 deaths), Vietnam (119 cases with 59 deaths) and Egypt (115 cases with 38 deaths).

Human cases: The clinical presentations are variable and determined in part by the virus clade. A feature of H5N1 outbreaks is the predominance of infection in children and young adults.

Avian influenza H7N7 and H7N3: Conjunctivitis is the most common presentation.

Avian influenza H5N1:

  • Respiratory illness presenting with fever, cough and shortness of breath is the most common manifestation. Respiratory failure, ARDS and multi-organ failure are the major complications of hospitalized patients.
  • Gastrointestinal symptoms such as watery diarrhoea, vomiting and abdominal pain may be common early in the course of the disease.
  • CNS involvement has been described.
  • Common laboratory findings include bilateral pulmonary infiltrates on chest radiograph (72%), lymphopenia (73%), and increased serum transaminase levels. Almost all human cases had antecedent avian exposure during a coincident avian influenza outbreak among chickens.

Viral isolation, PCR assays or antigen detection from respiratory specimens.

Testing is indicated in the following persons:

  • Patients who have a travel history to a H5N1-affected country; and
  • History of contact with domestic poultry or a known or suspected human case in a H5N1-affected country within 10 days of symptom onset; and
  • Radiographically confirmed pneumonia, ARDS, or other severe respiratory illness of uncertain aetiology.

A legally notifiable disease in Singapore. Notify Ministry of Health (Form MD 131 or electronically via CD-LENS) not later than 24 hours from the time of diagnosis. Notify MOH immediately on suspicion. Call MOH Communicable Diseases Surveillance team at: 98171463.

  • Oseltamivir remains the primary recommended antiviral therapy.
  • Modified regimens of oseltamivir treatment, including two-fold higher dosages, longer duration and possibly combination therapy with other antivirals may be considered on a case-by-case basis, especially in patients with severe disease, diarrhoea or late presentation.
  • Corticosteroids should not be used routinely but may be considered for septic shock with suspected adrenal insufficiency requiring vasopressors.
  • Oseltamivir resistance can emerge during therapy for avian H5N1 and may be associated with clinical deterioration.


  • Surveillance of affected flocks and culling (killing) of sick and exposed birds are the most effective methods of controlling an epidemic.
  • Patients suspected or confirmed to have avian influenza must be isolated and treated.

Current seasonal influenza vaccine does not protect against H5N1 avian influenza but is recommended for selected populations at-risk to reduce opportunities for dual infections.

Recent H5N1 viruses are susceptible to oseltamivir but there are reports of resistance to the M2 inhibitors (amantadine and rimantadine).


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  2. Uyeki TM. Human infection with highly pathogenic avian influenza A (H5N1) virus: review of clinical issues. Clin Infect Dis. 2009 Jul 15;49(2):279-90
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  5. de Jong MD, Bach VC, Phan TQ et al. Fatal avian influenza A (H5N1) in a child presenting with diarrhea followed by coma. N Engl J Med. 2005;352:686–91.
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  10. Chow VTK, Tambyah PA, Goh KT. To kill a mocking bird flu? Ann Acad Med Singapore 2008;37:451-3.

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