Avian Influenza, Vaccines & Vaccination

The Disease.

Avian Influenza (AI) is a viral disease that affects waterfowl, and domestic birds, such as chickens and turkeys, caused by an orthomixovirus type A. The disease has different clinical manifestations, from mild infections that frequently pass underdiagnosed, to a highly pathogenic form that causes high mortality, severe drops in egg production and economic losses.

Avian Influenza Viruses (AIV) are classified in subtypes according to two different antigenic determinants in the virus surface. At the time, there are 15 different Hemaglutinins (usually named as HA or H) and 9 different Neuroaminidases (usually named as Na or N) recognized, and they can be randomly combined with each other, that means that there may exist at least 15 X 9 different serotype combinations, that equals to 135 subtypes.

According to their ability to cause disease, the disease is classified in Low Pathogenic AI (LPAI) and Highly Pathogenic AI (HPAI).

Worldwide is recognized that all the severe outbreaks of AI in domestic hens, have belonged to the H5 and H7 subtypes.

The AIV have the ability to mutate by different mechanisms. These changes include a mutation in the amino acids present at the cleavage site for the HA chain. If basic amino acids are present at the cleavage site, the virus has all the potential to change it’s pathogenicity and to start causing very high mortality in susceptible birds.

Waterfowl birds like ducks and geese are the most important natural hosts for AIV. Chickens, turkeys and mammals (including humans) are considered aberrant (not natural) hosts, but once the virus is adapted to them, they become hosts and shedders of this kind of viruses.

On one hand, AIV are usually inhabitants of waterfowls. That means that during migration time, waterfowls can easily carry and shed the virus anywhere. This is a possible explanation why the disease can appear frequently and in many countries. On the other hand, once the AIV is adapted to and infecting domestic birds, themselves, their products and by-products become an important epidemiologic risk for virus shedding and transmission. Many countries in different continents have reported isolation of AIV, that includes America, Australia, Asia and Europe.

Vaccines.

AI is usually prevented by Biosecurity measures that include good isolation of the flocks. The disease must be always considered as exotic in all countries, and vaccination is not recommended. When present, the focal outbreak must be eliminated, under extreme Biosecurity measures, regularly conducted by Government Authorities. If the infection has gained territory and the disease is considered endemic, vaccination of susceptible birds is an option within an Integral Biosecurity Program that must involve Government Authorities, poultry producers and technicians.

Subunit killed vaccines, recombinant live vaccines and inactivated oil emulsion vaccines have been developed.

Subunit killed vaccines induce humoral immunity, proved to be highly efficacious to prevent mortality, but have a very high cost.
Recombinant live vaccines, usually with the HA fraction inserted in pox virus, are very effective in birds without antibodies against pox virus, and have a very limited protection in terms of time.
Killed oil emulsion vaccines protect birds very well, including birds with previous vaccinations or maternal antibodies, and protection last for longer periods than recombinant vaccines. They fully protect against mortality, drops in egg production and diminish virus shedding.

None of this technologies induce tissular immunity.

The Avimex* Avian Influenza Killed Virus Vaccine.

The product contains inactivated avian influenza virus, subtype H5N2, strain A/CHICKEN/MÉXICO/232/94/CPA, of low virulence, of chicken embryo origin, and emulsified in mineral oil.

The vaccine induces a slow and long lasting antigen liberation, stimulating the immune system in a sustained way for the production of humoral immunity against subtype H5, AIV.

The vaccine is intended to be used only in poultry:
Broiler chickens, breeders & grand parent stock; commercial laying hens, turkeys and Guinea fowl.

Vaccination.

Vaccinate birds by the subcutaneous route, in the midportion back of the neck, with a dose of 0.5 ml/bird. Heavy breeders can be vaccinated with 1.0 ml prior to the onset of lay.

Vaccination Programs:

1. During a HPAIV outbreak, how should the vaccine be used?

During an emergency, all susceptible birds (of any age) under risk of infection with a HPAIV should be vaccinated.
Vaccination is useful to prevent mortality, drops in egg production, drops in fertility and to reduce virus excretion in challenged birds.

2. What is the best vaccination program during an outbreak of HPAIV?

Broilers and pullets should be vaccinated as early as possible, with 0.5 ml per bird.
- When there is no presence of Maternal Antibodies (Mab’s), under field conditions, a double vaccination program has been very useful for broilers starting at day of age with 0.2 ml (two hours after the Marek’s disease vaccine), and re-vaccinating between 10 to 12 days of age with 0.5 ml.
For replacing birds and turkeys, vaccinate as early as possible, and re-vaccinate at least two more times, with 5 weeks between administrations, with 0.5 ml.
For birds in production, vaccinate as early as possible, and re-vaccinate at least one more time, with 5 to 10 weeks between administrations, with 0.5 ml.
In molted birds, vaccinate as early as possible, and revaccinate at least one more time, with 5 to 10 weeks between administrations.

3. During the presence of LPAIV, how should the vaccine be used?

Vaccination is useful to prevent drops in egg production, drops in fertility and to reduce virus excretion in challenged birds.

4. What is the best vaccination program for the enzootic LPAIV?

Broilers should be vaccinated between 8 to 12 days of age, with 0.5 ml per bird.
Pullets, growing birds and turkeys should be vaccinated at 4 weeks of age, with 0.5 ml per bird. Two to three re-vaccinations are recommend, with 5 to 7 weeks between administrations.
For birds in production, with a history of three applications during the growing period, birds will probably not need another re-vaccination.
For birds in production, with a history of two applications during the growing period, birds should be re-vaccinated at 45 weeks of age.
In molted birds, with a history of three applications during the growing period, birds should be re-vaccinate when the molting process already finished and birds already recovered it´s body weight. If necessary, a second re-vaccination should be applied 20 to 30 weeks after the first one.

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