Avian influenza: The animal health perspective (Part 3)

Issue: July 2008

ByArnon Shimshony, DVM

The current sequel of this series is addressing the control measures of avian influenza in animals.

Avian influenza, initially called fowl plague/fowl pest, was included in the World Organization for Animal Health’s (OIE) list A diseases since 1950 due to its transboundary, rapid spread and economic implications. International requirements pertaining to such diseases include:

immediate notification (required within 24 hours of initial identification),
stamping out,
quarantine measures,
movement control inside the country,
ban upon the international trade in the relevant animal species or their products, as long as the disease is present,
surveillance,
follow-up reporting.

OIE’s International Animal Health Code provides the rules by which a member country (currently, 172) zone or “compartment” can be regarded “free” of a disease according to detailed, internationally-agreed criteria, allowing international trade in animals and their products.

Arnon Shimshony

Since the mid-1990s, the knowledge of the avian influenza virus, its epidemiology, pathogenicity, and eventually its zoonotic potential have undergone significant advances.

This led to revisions in the Code, including new classification of the disease/pathogen and new guidelines for surveillance. The term “fowl plague” was substituted, in 1996, by the more appropriate term “highly pathogenic avian influenza” (HPAI). Lists A and B have been amalgamated since 2005. A new term – “compartment” – was added in 2005, allowing trade of animals or products from an infected country provided they are derived from establishments recognized as “compartment,” which comply with specific requirements.

A compartment is defined as “one or more establishments under a common biosecurity management system containing an animal subpopulation with a distinct health status with respect to a specific disease or specific diseases for which required surveillance, control and biosecurity measures have been applied for the purpose of international trade.”

Classification

Avian influenza is classified according to disease severity into two recognized forms: 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% 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.

To date, influenza A viruses representing 16 hemagglutinin (HA) and nine neuraminidase (NA) subtypes have been described in wild birds and poultry throughout the world. Viruses belonging to the antigenic subtypes H5 and H7, in contrast to viruses possessing other HA subtypes, may become highly pathogenic having been transmitted in low pathogenic form from wild birds to poultry and subsequently circulating in poultry populations. Therefore, all H5 and H7 viruses have been classified by the OIE as notifiable avian influenza (NAI) viruses, independently of their pathogenicity index.

Although all virulent strains isolated to date have been of the H5 or H7 subtype, most H5 or H7 isolates have been of low virulence.

Any of the other 14 hemagglutinin subtypes found to have high pathogenicity will be regarded as an NAI virus.

The methods used for the determination of strain virulence for birds have evolved during recent years. The traditional method involved a bioassay in chickens to determine an intravenous pathogenicity index but novel molecular procedures have been recognized as well. The obtained results will enable the determination of H5 or H7 isolates as “highly pathogenic notifiable avian influenza” (HPNAI) or “low pathogenicity notifiable avian influenza” (LPNAI) viruses. Non-H5 or non-H7 avian influenza isolates that are not highly pathogenic for chickens are identified as LPAI and are not notifiable.

The significance of H5 and H7 became further emphasized when both were found capable of infecting humans. Out of the 385 H5N1 human cases recorded to date (in 14 countries), 243 were fatal. Fortunately, most likely there have been, so far, no proven human-to-human infections.

The primary aim of an early and rapid response to any occurrence of HPAI in avians is to contain the disease before it spreads and eliminate it by stamping out affected premises. Only if this fails should other measures be considered, including vaccination.

Where it proves impossible to eradicate the disease in the short- to medium-term, it may be possible to aim for “compartment” freedom (eg, freedom within the commercial sector in which birds are housed and protected from infection), or zone freedom (freedom in geographically defined areas). In the case of “compartmentalization,” the poultry industry must take responsibility for its biosecurity performance with the necessary veterinary regulatory oversight in place. In the zonal freedom situation, commercial compliance must follow national restrictions to ensure infection does not enter the free zone, and regulators prove to be trading with partners of the zone’s clean status at any given time.

Disease control

To effectively control the disease, countries should have a complete plan of action and the financial and human resources to implement it under the particular conditions prevailing in the country. A regional approach is also necessary.

HPAI is readily transmitted via contaminated objects, so strict control of movement of anything that may have become contaminated with virus and immediate imposition of tightly controlled quarantine on all places suspected of being infected are essential to a successful eradication program. Ideally, quarantine should be imposed on all farms/villages on which infection is either known or suspected and should be strictly policed to ensure that no one, including the residents, owners, staff and other visitors, leaves without changing clothes and footwear.

Particular attention is paid to workers on poultry farms who keep backyard poultry at home. Strict on-farm biosecurity and hygiene are prescribed to control spread of the disease from wild birds and by humans. Only authorized personnel in protective clothing are allowed to enter. The movements of residents onto and off the property is supervised, and all pets are confined. It is also strongly recommended to ban cockfighting, pigeon racing and other avian concentrations in the outbreak area.

Surveillance may need to be adapted to parts of the country or existing zones or compartments depending on historical or geographical factors, industry structure, population data, or proximity to recent outbreaks.

There are detailed conditions for a country, zone or compartment for regaining the NAI-free status or the HPNAI-free status after being infected. The minimum required period is three months. In the case of HPNAI, a stamping-out policy is obligatory. This means:

immediately imposed quarantine of the affected area (premises or village);
slaughter of all infected and potentially infected birds and disposal of the carcasses;
decontamination of sheds and other poultry housing areas;
rapid surveillance of surrounding areas to determine the extent of possible spread;
closure and disinfection of markets;
keeping sick and dead birds out of the human food chain, banning their sale for feed to other animals (zoos).

In the case of LPNAI infections, poultry may be kept for slaughter for human consumption, subject to specified conditions; alternatively, a stamping-out policy may be applied. In either case, the “free” status may be gained three months after the disinfection of all affected establishments, providing that surveillance has been carried out during that three-month period.

The zoning principles may vary between countries. The EU has established its own zoning system, including ‘protection,’ ‘surveillance’ and, occasionally also ‘restriction’ zones around the affected premises. As soon as the country, zone or compartment fulfill the requirements for being considered NAI-free, they can resume exporting poultry and/or products. Consignments are accompanied by official veterinary certificates testifying their compliance with the OIE requirements.

Since 2003, 48 countries in Asia, Europe and Africa have undergone infection with HPAI H5N1. Most of them have successfully eradicated the outbreaks by stamping out combined with quarantine and animal movement restrictions; some, such as South Korea and Pakistan, have suffered recurrent outbreaks. In several European countries, periods during which the birds are obligatorily kept indoors to prevent their exposure to migratory birds have been applied. Several countries, such as Vietnam, Indonesia, Egypt and China had to apply mass vaccinations. Such vaccines should be certified only if complying with chapter OIE’s “Manual of Diagnostic Tests and Vaccines for Terrestrial Animals.” Differentiating vaccinated from infected animals may allow regaining the free status.

According to recent data between January of 2006 and May 2008 about 42.5 million poultry were culled in H5N1-affected countries.

As of July 1, 2008, H5N1 outbreaks are regarded to be “continuing” in 16 OIE member-countries and declared “endemic” in one country, namely Indonesia.

The end of the epizootic is not yet in sight.

For more information:

Arnon Shimshony, DVM, is Associate Professor at the Koret School of Veterinary Medicine Hebrew University of Jerusalem, Rehovot, and is the ProMED-mail Animal Diseases and Zoonoses Moderator. Dr. Shimshony was Chief Veterinary Officer, State of Israel, from 1974 to 1999.