Risk Wise

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] 51

Responding to the spread of

avian influenza H5N1:

a wildlife conservation perspective

Rebecca Lee, Scientific Task Force on Avian Influenza & Wild Birds

vian influenza is a highly contagious disease caused by

influenza A viruses, affecting many species of birds. Avian

influenza affecting poultry is classified into two recog-

nized forms according to disease severity: low pathogenic avian

influenza (LPAI) and highly pathogenic avian influenza (HPAI).

LPAI viruses are generally of low virulence, while HPAI viruses

are highly virulent in most poultry species, resulting in nearly

100 per cent mortality in infected domestic flocks.

The natural

reservoir of LPAI viruses is in wild waterbirds – most commonly

in ducks, geese, swans, waders/shorebirds and gulls – in which

they cause few apparent clinical consequences.

To date, influenza A viruses representing 16 haemagglutinin (HA)

and nine neuraminidase subtypes have been described in wild birds

and poultry throughout the world.

Viruses belonging to the anti-

genic subtypes H5 and H7, in contrast to viruses possessing other

HA subtypes, may become highly pathogenic after having been trans-

mitted in low pathogenic form from wild birds to poultry and

subsequently circulating in poultry populations.

Since it was first recognized in 1997, HPAI H5N1 has infected

domestic and wild birds in more than 60 countries across Asia, Africa

and Europe. Over 200 million domestic birds have died from disease

or been slaughtered in attempts to control its spread. There have

been serious human health consequences – by March 2009, the

World Health Organization (WHO) had confirmed more than 400

human cases, over 60 per cent of these fatal – and there remains

potential for the virus to adapt allowing for human-to-human trans-

mission and the emergence of a human pandemic. In 2005 before

the spread of the virus out of Asia, the Food and Agriculture

Organization (FAO) of the United Nations estimated impacts on the

economies of the worst affected countries in Southeast Asia of over

US$10 billion.

Prior to the emergence of the current Asian strain of HPAI H5N1,

outbreaks of HPAI in wild birds were extremely rare. The spread of

HPAI H5N1 from poultry into wild bird populations and the broad

geographical scale and extent of the disease in wild birds is both

extraordinary and unprecedented, and the conservation impacts of

HPAI H5N1 have been significant. It is estimated that between five

and ten per cent of the world population of Bar-headed Goose

Anser

indicus

died at Lake Qinghai, China in April-May 2005. At least two

globally threatened species have been affected: the Black-necked

Crane

Grus nigricollis

in China and the Red-breasted Goose

Branta

ruficollis

in Greece. However, the total number of wild birds affected

has been small in contrast to the huge number of domes-

tic birds affected, and many more wild birds die of more

common avian diseases each year. Perhaps a greater

threat than direct mortality are the indirect threats,

including the development of public fears about water-

birds and misguided attempts to control the disease by

disturbing or destroying wild birds and their habitats.

Such responses are often encouraged by inflammatory

and misleading messages in the media and the political

need to show action is being taken.

This paper presents an overview of HPAI H5N1 and its

impacts on wild birds, along with the international

consensus on what should be done to reduce the spread

of the virus, specifically where wild birds could be

involved either as vectors or victims.

The emergence and spread of HPAI H5N1 via

multiple vectors

The HPAI H5N1 virus almost certainly originated from

the mutation of an LPAI virus on poultry farms in East

Asia (a precursor to the virus was identified in samples

from domestic geese that died in Guangdong, China in

1996). The virus then spread rapidly within and between

farms, taking advantage of local practices in the feeding,

housing, slaughtering and trade of domestic ducks,

chickens and geese including practices at mixed live bird

markets. Lack of hygiene, overstocking and mixing of

different domestic animals greatly increases the risk of

viral spread.

The role of Asian domestic ducks in the epidemiol-

ogy of HPAI H5N1 has been closely researched and

found to be central not only to the genesis of the virus,

but also to its spread and the maintenance of infection

in several Asian countries.

Typically this has involved

flocks of domestic ducks used for cleaning rice paddies

of waste grain and various pests, during which they are

exposed to wild ducks using the same wetlands.

Research in Thailand demonstrated strong associations

between HPAI H5N1 and abundance of free-grazing

ducks.

Gilbert et al (2006), concluded that in Thailand:

“wetlands used for double-crop rice production, where

free-grazing duck feed year round in rice paddies, appear

to be a critical factor in HPAI persistence and spread.”