Changes

== Introduction  ==

== Introduction  ==

Highly Pathogenic Avian Influenza (HPAI) is caused by H5 and H7 isolates. The disease is notifiable in the UK and is found worldwide. H1-15 and N1-9 have all been isolated, but these are of varying pathogenicity.  

Avian influenza (AI) is a notifiable disease. It is part of the [[Orthomyxoviridae]] family, possessing a single negative sense RNA strand. Within the influenza [[viruses|virus]] family there are 3 subtypes; A, B and C, with only A causing disease in birds.  Type A can then be further subdivided based on the haemagglutinin (HA) and neuraminidase (N) envelope glycoproteins present, with subtype antigens H1-17 and N1-9. Each virus possesses one HA and one N antigen. Each isolate can then be further subdivided into viral lineages called clades.

 

The disease has caused so much concern over the last few years due to the fact it can mutate in the pig and gain attachment proteins for humans. This means that the deadly H5N1 strain of HPAI can be transmitted to humans, making it a feared zoonotic disease. A 2003 Dutch outbreak of a pathogenic H7 virus caused widespread conjunctivitis and flu-like symptoms with recovery among poultry workers and more recently, rural chinese children became infected with H5N1 via aerosol transmission, which was limited to upper respiratory symptoms and did not show horizontal spread.  

Influenza A viruses affecting birds are divided into two groups based of the severity of clinical disease; highly pathogenic avian influenza (HPAI) and low pathogenic avian influenza (LPAI). HPAI viruses are found within (though not all of) H5 and H7 subtypes. HPAI viruses are thought to be a result of mutations within an LPAI strain. HPAI is defined by the ability to infect and kill chickens using a standardized dose given intravenously (World Organization for Animal Health, 2006). The mutation occurs after the virus has moved from the wild bird host into the poultry population and may take days to months to occur. The longer the virus persists the more likely it is to adapt and mutate into a highly pathogenic strain, and once in poultry in can then spread to other species.

 

In the USA, many farmed turkeys die each year from avian influenza and this is due to migrating birds contaminating their housing. With the current and severe H5N1 strain, Chinese poultry are the main reservoir, not wild birds. HPAI is spread by faeces and infected viscera (kidney/spleen are 100X more infectious than faeces). H7 and other viruses are carried by 6% of the wild bird population and outbreaks mirror migrating patterns, this poses significant risk to free-range flocks.  

Initially AI cases were found to decrease as the ambient temperature increased but in 2009 cases were documented all year round with increased cases during the warmer months of the year. AI viruses can persist for long periods of time at low temperatures in water, and therefore can reinfect migratory water fowl in the spring and lead to further spread. Geographical separation of the virus can also increase independent evolution of the virus and potentially increase virulence.

Wild birds (in particular aquatic birds) are the main reservoir for the virus, and hence are responsible for its continuous spread and maintenance, which is easy and quick. The migration of many wild birds across distances has led to long distance spread and the introduction of AI into other countries.  Different species of wild birds have different genetic pools of virus and different susceptibilities. Most infection within Europe has been detected in dead wild birds. A majority of cases, especially in developing countries, have been the result of secondary spread within poultry, most of which is human mediated. Most AI viruses are well adapted to their host species, so when they infect a new species they replicate and transmit poorly. Domestic poultry (especially ducks) are reservoirs for HPAI, and, in some cases, birds can produce the virus for weeks with no clinical signs.

AI is spread by movement of birds, direct contact with respiratory secretions or faeces as well as via fomites (inanimate objects). AI can also be transmitted by eating uncooked infected bird products, including uncooked eggs. AI virus has been found on the surface of eggs but so far there has been no evidence of transmission to people by the consumption of egg products. The practice of manure spreading also increases the possibility of disease transmission.

H5N1 is an HPAI strain that can persist in wild bird populations. In April 05 this strain caused high mortality in ducks and gulls in Qinghai Lake in China. Wild bird migration led to the virus being carried to Europe and Africa and over winter 2006 a large number of wild birds were found to be infected. Migratory birds in Egypt tested positive for H5N1 three months before the outbreak in poultry. The virus is thought to still be present in wild bird populations but at a lower level. This subtype proved harder to control due to the maintenance of infection within wild birds and the increased susceptibility of poultry to the strain (as shown by a low MID50, meaning that only a small amount of virus is needed to produce infection). H5N1 has spread to over 60 countries and is currently endemic in China, Egypt, Vietnam, India, Bangladesh and Indonesia.

The most recent cases reported in China on the 29th March 2013 involve the H7N9 subtype, which has been shown to be more virulent in people than poultry. Currently no animal outbreaks have been identified in the area surrounding the confirmed cases and only a small proportion of birds have tested positive for the virus; however 77% of those people infected have been exposed to poultry/swine (including live bird markets). Three family clusters of 2-3 cases each have been identified where limited human to human transmission may have occurred.

At the moment it is thought that H7N9 was transmitted from healthy poultry or swine to people either directly or through contaminated environments. As few H7N9 positive birds have been detected, this may indicate that the virus is widespread in poultry and is asymptomatic which could lead to silent spread of the virus.

The complete virus is a recombination of three viruses found in Asia, H7 of the virus has been found to be closest to that found in domestic ducks in Zhejiang and the N9 closest to the wild bird strain in South Korea. Genetic changes have also been found that lead to increased virus binding and replication in mammalian respiratory cells and thus increased severity of infection. The virus has been shown to be weakly pathogenic in poultry and testing of different populations for H7N9 specific antibodies may be helpful in finding the source, though viruses with H7 HA may not trigger a strong antibody response. No cases have been reported outside of China.

== Clinical Signs  ==

== Clinical Signs  ==