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Human Health Issues related to Avian Influenza in Canada

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2 Background

Avian influenza is an infectious disease of birds caused by type A strains of influenza virus. Since the clinical signs of avian influenza can resemble other diseases, such as Newcastle Disease in poultry, the diagnosis of avian influenza must be made on the basis of laboratory confirmation.

In birds, all of which are thought to be susceptible, influenza viruses may cause asymptomatic infection or a wide spectrum of symptoms, ranging from mild illness to a highly contagious and rapidly fatal disease. The occurrence of predominantly mild illness or asymptomatic infection in birds due to influenza is referred to as "low pathogenic avian influenza" or LPAI. When influenza has resulted in severe epizootics, characterized by sudden onset, severe illness and rapid death of affected birds/flocks, with a mortality rate that can approach 100%, the strain is referred to as "highly pathogenic avian influenza" or HPAI based on the observed morbidity and mortality in affected bird population(s). Laboratory testing (e.g., genetic analysis and in vivo biologic studies in chickens) of the avian influenza virus can also result in a particular virus being labeled as "highly pathogenic". The degree of correlation between laboratory-determined pathogenic potential, and pathogenic behaviour as evidenced by the negative outcomes in the affected bird population(s), is unclear.

For the purposes of this document either a clinical or laboratory finding should trigger the recommended control measures with the duration and aggressiveness of the control measures being determined by the epidemiological data available at the time of the outbreak.

The World Organization for Animal Health (OIE) defines an outbreak of disease or infection as:

" the occurrence of one or more cases Opens in a new window [an individual animal (mammal, bird, bee) infected by a pathogenic agent, with or without clinical signs] of a disease or an infection in an epidemiologic unit or group of animals with a defined epidemiological relationship that share approximately the same likelihood of exposure to a pathogen. This may be because they share a common environment (e.g. animals in a pen), or because of common management practices. Usually, this is a herd or a flock. However, an epidemiological unit Opens in a new window may also refer to groups such as animals belonging to residents of a village, or animals sharing a communal animal handling facility. The epidemiological relationship may differ from disease to disease , or even strain to strain of the pathogen."⁽⁶⁾

This includes the first occurrence of an OIE-listed disease or infection in a country or zone/compartment; the first occurrence of a new strain of a pathogen of a listed disease in a country or zone/compartment; an emerging disease with significant morbidity/mortality or zoonotic potential; or evidence of change in the epidemiology of a listed disease (e.g. host range, pathogenicity, strain of causative pathogen), in particular if there is a zoonotic impact. The scope of this document has been expanded from the previous version which provided recommendations during poultry outbreaks, to include the human health issues that may arise from confirmed cases of AI in wild birds or single cases in domestic birds.

2.1 Avian Influenza in the Wild Bird Population

Wild birds can serve as a silent reservoir for avian influenza viruses. If infected wild birds come into contact with, or contaminate an area populated by, commercial/domestic birds the virus may spread to the domestic flock providing an opportunity for the virus to proliferate and possibly mutate. Viruses introduced in this manner may start out as low pathogenic strains and mutate into highly pathogenic strains.

The first Canadian wild bird survey, conducted in 2005, identified many different AI viruses including four H5 subtypes - H5N1, H5N2, H5N3 and H5N9. In all cases, these viruses were clearly characterized by scientists as low pathogenic, North American strains. The low pathogenic strain of H5N1 was determined to be similar to strains previously identified in North America^[4]. These laboratory results were supported by the absence of any increased mortality observed in Canadian birds. All North American samples originated from healthy wild birds which can also be attributed to the low pathogenic nature of the North American strain, as distinguished from the outbreaks of highly pathogenic H5N1 Asian strain seen in Eurasia and Africa. A 2006 survey will also be carried out to provide an early warning for the possible entry of highly pathogenic avian influenza into Canada.

2.2 Avian Influenza in Poultry Flocks

In the 1960's when turkeys were often raised outdoors, cases of low pathogenic avian influenza were often reported in the autumn. One of the viruses isolated in Canada in 1966 was later found to meet the modern criteria of a highly pathogenic influenza virus. Since 1975, low pathogenic avian influenza - subtypes H5 and H7 - has been detected five times in domestic poultry in Canada, most recently in 2005⁽⁷⁾.

Low pathogenic strains (for example H9N2) have occasionally caused minor self-limiting illness in humans. Although no human deaths have been associated with any LPAI virus⁽⁵⁾, these viruses may still have pandemic potential. Also some LPAI H5 and H7 virus strains have mutated to HPAI following circulation in domestic poultry flocks⁽⁵⁾. During the 2004 outbreak in British Columbia the H7N3 virus was initially LPAI but converted to HPAI within days of the first outbreak on the index farm through a minor recombination event⁽⁸⁾. Therefore precautions are warranted regardless of the pathogenicity of the avian influenza virus in the poultry population.

Highly pathogenic strains of avian influenza virus have also caused disease in humans⁽⁹⁾. To date implicated strains have included: H5N1, H7N2^[5] , H7N3 and H7N7. Human fatalities were observed with a H5N1 strain in Hong Kong in 1997 and a single fatality was associated with the H7N7 outbreak in the Netherlands in early 2003⁽¹⁰⁾. In the fall of 2003 a new sub-type of H5N1 started causing poultry outbreaks in Asia and widespread infection in wild birds⁽¹¹⁾. This virus continues to circulate in 2006 and has resulted in human cases and fatalities among people who had close contact with infected birds. Human cases have occurred in Asia, Europe and Africa. Spread of H5N1 into avian populations in Europe has been observed as a result of migration of infected birds. Current Avian influenza (H5N1) affected areas, including a table of human H5N1 and avian H5N1 affected areas can be found at http://www.phac-aspc.gc.ca/h5n1/index-eng.php

2.3 Human Health Context

Avian influenza infection in humans can potentially occur as a result of contact with infected poultry and under- or uncooked poultry products, infected wild or pet birds, manure and litter containing high concentrations of virus, contaminated surfaces, or contact with contaminated vehicles, equipment, clothing and footwear at involved sites (e.g., infected poultry farms). Direct contamination of the mucous membranes by infectious droplets or inhalation of aerosolized viruses are other possible transmission routes. Close contact with sick or dead domestic poultry (especially backyard flocks where biosecurity measures are non-existent or not stringent), has evolved as the predominant risk factor for H5N1 Asian strain infection⁽¹²⁾. In general, the risk to human health from wild birds infected with an avian influenza virus (both low and high pathogenic strains) is considered to be low. Recommendations for safe practices when handling wild birds, including information for hunters, are available at: http://www.phac-aspc.gc.ca/influenza/fs-hwb-fr-mos-eng.php

The main global human health concern is that outbreaks particularly in domestic poultry flocks present an opportunity for ongoing genetic mutation or viral reassortment. Since simultaneous infection with human influenza and avian influenza viruses in an intermediary host, including a human, may provide an opportunity for an exchange of genes, one possible outcome is the development of a new influenza virus subtype with pandemic potential.

According to the World Health Organization (WHO) pandemic phases, identification of a novel virus (e.g. avian) in a human denotes the beginning of Phase 3, the first phase in a period called Pandemic Alert. This situation raises the level of pandemic preparedness activity for all jurisdictions since this finding is considered to be a potential precursor to a pandemic. During this phase there is no or at most rare instances of human-to-human transmission. WHO Phases 4 and 5 are characterized by limited but increasingly efficient human-to-human transmission of the novel virus resulting in clusters of human cases^[6].

Education of all stakeholders, ideally in advance as part of pandemic preparedness activities, but definitely as part of the first response to an outbreak, should be given high priority. Awareness of the potential consequences of these outbreaks may facilitate compliance with recommended control measures.