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Due to continual antigenic drift and shift specificity and sensitivity of diagnostics is a big problem.
Due to continual antigenic drift and shift specificity and sensitivity of diagnostics is a big problem.
==Treatment==
There is no treatment for HPAI in birds, and all affected birds should be culled as well as any birds in contact with diseased stock.
== Control ==
== Control ==
Within developed countries a combination of vaccination, culling, movement restrictions and disinfection are used. Once the disease is endemic it is harder to control. During the H5N1 outbreak in Egypt these methods were unsuccessful so mass vaccination (with inactivated H5 vaccines), surveillance and pre emptive culling of birds was carried out alongside the above methods. A problem was encountered with the disposal of carcasses post culling (both the speed of disposal and the methods used) which may have led to increased spread. Dead infected birds are often eaten by stray dogs and cats which can increase circulation of the virus. Since the 1st of July 2010 there has been a ban on selling live poultry in Egypt and only licensed slaughter houses are allowed to handle poultry.
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The presence of live bird markets and live bird trade in many countries has made control of AI harder. Live bird markets can act as viral reservoirs to maintain and increase the spread of the disease, both through fomites and movement of infected birds. This increase in circulation can aid the development of more, potentially zoonotic, strains. Fragmenting the live bird market network through disinfecting both the market (daily) and the vehicles coming in and out would greatly reduce the spread. The concept of ‘rest days’ to limit spread has also been introduced in some areas. Previously a ban on live bird markets has only served to result in illegal bird markets, which have resulted in further spread of AI both due to the lack of knowledge of where these were taking place and the lack of control over biosecurity. Education of the local population plays a key role in the reduction of the spread of AI. Bioexclusion (excluding the virus) in farms and villages should be the aim, whereas biocontainment (containing the virus) should be the aim with markets.
==Vaccination==
Vaccination against AI is usually only used in order to help control an outbreak. A majority of the vaccines used are inactivated vaccines, though some are starting to use vectored vaccines. A reverse genetic engineered reassortant (rg) vaccine is also available. This vaccine has been updated twice due to changing field viruses.
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The duration of immunity from vaccination is largely unknown due to the logistical problems of studying this. Current studies vary from 12 weeks to 138 weeks with different vaccines. The effect of repeat dosing is also unclear, though dosing twice has shown an increase in immunity from 1 to 10 months.
Within the commercial sector breeding birds and layers are usually the only birds vaccinated due to the high cost. Also, the addition of adjuvants (e.g. oil emulsion) requires a withdrawal time that is usually longer than the lifespan of a meat chicken thus meaning that, in most cases, it is economically unviable to vaccinate. Egypt has however vaccinated meat birds as part of a routine vaccination program. Hong Kong is the only place that has achieved national flock immunity. In developing countries the vaccination rate within village poultry is too low to achieve population immunity, and proper vaccine storage is also a problem.
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Suboptimal vaccination, and thus incomplete immunity, can lead to an increase in antigenic drift by increasing immune pressure exerted in hosts and can also increase the rate at which the virus is able to cross species. It could also lead to clinical symptoms being masked and thus increasing disease spread. Suboptimal immunity can occur through a number of ways: host immunosupression, vaccine spoilage, inadequate dosing, poor antigenic match between vaccine and virus, low vaccination uptake in the population or exposure to the virus before full immunity has developed.
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AI vaccines are constantly being updated to cover for new strains of AI due to antigenic drift. Vaccines are currently being developed that target the M2 protein, which is conserved among different subtypes of virus. Subunit vaccines are being developed but the cost would inhibit use in poultry and there is a lack of data on whether it would provide protection in chickens.
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rNDV (recombinant Newcastle Disease virus) vaccines may offer a solution to the labour intensive administration of traditional vaccines in that they can be administered through drinking water. NDV vaccination is routine worldwide, and NDV replicates in the same tissues as AI therefore can confer better immunity at smaller doses than would otherwise be required. NDV will also replicate in chickens and turkeys. However, maternal antibodies to the vector will interfere with immunity. Fowl poxvirus vectored vaccines are licensed for use in the US and have been used in China and Mexico.
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HI tests can be used post vaccination to monitor the immune response, though HI tests used after vaccination in Egypt showed the antibodies produced by the vaccine to have no cross reactivity with field antigens. Serum from vaccinated birds from one strain found in Egypt showed no reaction with other H5 based antigens of vaccine strains found in Egypt, thus showing the importance of using the correct field antigens present when developing vaccines. Monoclonal antibody ELISA tests based on the H5N1 Asian strain did not detect the Egyptian variant either.
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Inactivated vaccinations have played an important role in H9 control in China, though it still persists in chicken populations, even those that have been vaccinated, due to genetic variation. Vaccination of poultry can decrease disease and virus shedding if birds become infected, and can also lead to an increase in the amount of virus needed to infect vaccinated birds.
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Within the countries that have endemic H5N1 Bangladesh and India are the only countries not to have vaccination programs, though Bangladesh is in the process of developing a program.
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A majority of the vaccine research being performed is related to human vaccines due to the public health risks associated with AI. DNA vaccines are currently being developed, though due to cost and the need for multiple doses to be individually administered it is unlikely they will be used in poultry.
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The development of DIVA system (differentiating infected from vaccinated animals) is currently an important focus of research due to trade and political issues.
== References ==
== References ==