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Respiratory Viruses Slide Set

 

H5N1 Avian Influenza
 

The H5N1 avian influenza outbreak in Hong Kong 1997

In the latter half of 1997, an outbreak occurred in Hong Kong whereby 18 persons were infected by an avian influenza A, serotype H5N1. Of these 6 died, and 3 others were severely ill. The source of the outbreak was infected chickens and the outbreak stopped after all the chickens were slaughtered in the territory. Large-scale serological studies carried out showed that workers in the poultry industry were particularly at risk of infection although none complained of any symptoms. There was evidence of limited human to human transmission. It was postulated that the strain of avian influenza involved was unusually virulent; it had multiple basic amino acids near the cleavage site of the haemagglutinin protein, which as a result may render the haemagglutinin susceptible to a wider range of proteases. Since that outbreak, no more cases have occurred. In 1999, there were reports of human infections by avian influenza A H9N2 in Hong Kong and in Mainland China. However, all these cases were very mild and it is thought that the virus was unlikely to pose a large public health risk.
 

2003 Onwards

The virus resurfaced in Feb 2003 to cause 2 infections (one fatal) in a Hong Kong family who had recently traveled to China. It began to cause outbreaks in the rest of Asia that year that were unnoticed. In 2004, Vietnam and Thailand started reporting human infections, followed by Cambodia, Indonesia and China in 2005. The strains exhibited divergence in these localities. It is now thought that highly pathogenic H5N1 is now firmly endemic Asia and has also spread to Europe and Africa. The WHO has put great priority and resources in the surveillance of H5N1 infections worldwide in the hope of averting or diminishing the impact of a pandemic should a pandemic capable strain emerges. http://www.who.int/csr/disease/avian_influenza/en/

 

Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) Reported to WHO

3 April 2008

 

Country

 

2003

2004

2005

2006

2007

2008

Total

cases

deaths

cases

deaths

cases

deaths

cases

deaths

cases

deaths

cases

deaths

cases

deaths

Azerbaijan

0

0

0

0

0

0

8

5

0

0

0

0

8

5

Cambodia

0

0

0

0

4

4

2

2

1

1

0

0

7

7

China

1

1

0

0

8

5

13

8

5

3

3

3

30

20

Djibouti

0

0

0

0

0

0

1

0

0

0

0

0

1

0

Egypt

0

0

0

0

0

0

18

10

25

9

4

1

47

20

Indonesia 

0

0

0

0

20

13

55

45

42

37

15

12

132

107

Iraq

0

0

0

0

0

0

3

2

0

0

0

0

3

2

Lao People's Democratic Republic

0

0

0

0

0

0

0

0

2

2

0

0

2

2

Myanmar

0

0

0

0

0

0

0

0

1

0

0

0

1

0

Nigeria

0

0

0

0

0

0

0

0

1

1

0

0

1

1

Pakistan

0

0

0

0

0

0

0

0

3

1

0

0

3

1

Thailand

0

0

17

12

5

2

3

3

0

0

0

0

25

17

Turkey

0

0

0

0

0

0

12

4

0

0

0

0

12

4

Viet Nam

3

3

29

20

61

19

0

0

8

5

5

5

106

52

Total

4

4

46

32

98

43

115

79

88

59

27

21

378

238

 

Total number of cases includes number of deaths.
WHO reports only laboratory-confirmed cases.
All dates refer to onset of illness.


Risks of a pandemic

The present H5N1 strains do not have the ability to transmit efficiently between humans. To date, there had been no certain cases of human to human transmission although a few suspicious clustering of cases in families have occurred. It is thought an avian influenza may acquire this capability through either

  1. Reassortment with human influenza viruses (1957 and 1968). Reassortments in 1957 (H1N1-H2N2), and 1968 (H2N2-H3N2) are thought to have occurred through an intermediary host such as the pig.

  2. Gradual mutations, as suspected in the 1918 pandemic H1N1 strain. Direct infection of humans by H5N1 opens the possibility that reassortment can occur without an intermediary host.

Many experts believe that a pandemic was stopped in 1997 in Hong Kong by the culling of chickens. The bottom line is that nobody knows when and if a pandemic will arise out of the current H5N1 outbreaks.

Control Measures

A large number of control measures have been implemented or recommended against avian H5N1 infection, at both the animal and human level. These include increased surveillance of livestock, increased biosecurity to prevent contact between livestock and migratory birds, and also vaccination of livestock. At the human level, there is increased surveillance, stockpiling of tamiflu, and also the development of candidate vaccines.

In August 2005, WHO sent all countries a document outlining recommended strategic actions for responding to the avian influenza pandemic threat. Recommended actions aim to strengthen national preparedness, reduce opportunities for a pandemic virus to emerge, improve the early warning system, delay initial international spread, and accelerate vaccine development. Despite an advance warning that has lasted almost two years, the world is ill-prepared to defend itself during a pandemic. WHO has urged all countries to develop preparedness plans, but only around 40 have done so. WHO has further urged countries with adequate resources to stockpile antiviral drugs nationally for use at the start of a pandemic. Around 30 countries are purchasing large quantities of these drugs, but the manufacturer has no capacity to fill these orders immediately. On present trends, most developing countries will have no access to vaccines and antiviral drugs throughout the duration of a pandemic.
 

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Respiratory Viruses Slide Set