Poultry Vaccination - A Key Tool In The Control Of Avian Flu

With the first consignment of Intervet's Nobilis Influenza vaccine due to arrive in the UK mid March, the company is keen to clarify how the use of the vaccine could form an integral part of a controlled disease prevention programme.


Controlled experimental studies and epidemiological reports have demonstrated that vaccination as part of a control strategy including strict biosecurity measures and monitoring can extinguish avian influenza (AI) outbreaks. It reduces the flock's susceptibility to AI infection, whilst at the same time dramatically reducing the quantity of virus shed post challenge, resulting in transmission being blocked within the vaccinated flock.



"Vaccination of free range flocks and other outside birds can play a vital role in controlling avian influenza when it is combined with strict biosecurity, surveillance and depopulation programmes," says Jim Hungerford, general manager from Intervet UK. "We are not advocating vaccination as a panacea, but by the same token, its protective value must not be underestimated. Vaccination can act as an invaluable cornerstone in a prevention strategy, enabling producers and authorities to limit the damage from this devastating disease should it reach UK shores.


"Preventative vaccination programmes must be implemented within a framework that assures effective application as well as allowing rapid detection of any disease exposure," he emphasises. "The Dutch Government has developed a programme for its free range poultry flocks, incorporating the elements of planned vaccination, response assessment and sentinel birds. We believe this type of integrated programme is critical to the successful use of the vaccine."


Use of AI vaccine is under strict guidelines as set out by the European Community legislation on the control of avian influenza, and in the UK use of the vaccine is strictly under the control of DEFRA.


The evidence for the value of vaccination in avian influenza control is compelling. When the vaccine's efficacy was challenged in the field during the Hong Kong outbreak, studies showed that the vaccine had completely stopped virus circulation on the infected farms within 18 days of its administration(1). In addition to quarantine, depopulation of birds in the affected sheds and increased biosecurity, vaccination of the affected sheds and on surrounding farms was shown to completely control the spread of the disease.


"One of the key concerns regarding use of the vaccine at that time was whether it would control highly pathogenic avian influenza in poultry farms and whether it would prevent asymptomatic circulation of the virus and thus stop the spread of it to other surrounding farms," he continues. "The monitoring and surveillance programmes conducted in the Hong Kong chicken farms showed that use of the killed H5N2 vaccine in the face of high virus challenge was able to protect the chickens from disease and interrupt virus transmission."















Similarly, the densely populated turkey area of the Sanpete Valley of Utah was under threat in 1995 when it experienced repeated low pathogenic challenges in spite of adequate biosecurity measures. The outbreak affected 1.9 million turkeys and cost the industry $1.5-2.0 million. Vaccination with an inactivated H7 vaccine was quickly approved by the authorities and played a key role in eliminating the infection in this closed population of multi-age turkeys(2).


"Non vaccinated sentinel birds were placed strategically within vaccinated flocks, and these were periodically bled to check for seroconversion to AI," he adds. "Monitoring continued until the last vaccinated flock was processed. New outbreaks were rapidly diminished once the vaccination programme was underway and within 19 weeks after the first flock was detected with AI, all clinical outbreaks ceased."


Other challenge studies, soon to be published, have also shown that specific-pathogen-free (SPF) chickens vaccinated with Intervet's H5N2 vaccines and challenged with H5N1 Asian field isolates were protected against mortality and morbidity. All non-vaccinated birds excreted large amounts of virus, however the vast majority of vaccinated birds did not excrete any virus, with the minority excreting 10,000 to 100,000 times less virus than the controls(3).


Overall, the evidence confirms that inactivated vaccines successfully increase a flocks' resistance to AI infection, whilst at the same time reducing the quantity of virus shed post challenge, thereby reducing transmission and markedly lessening disease losses(4).


"It's important to remember that in an outbreak of avian influenza, reduction of virus circulation is a very important factor in preventing its spread to other birds and to people," Jim Hungerford concludes. "Where birds are vaccinated according to the data sheet, the level of shedding from vaccinated birds is so low that no virus is detected by conventional virology techniques. As a result, there is insufficient virus to infect other vaccinated birds and the infection is extinguished within the flock. Reduction of the virus excretion also significantly reduces the chance that an infected farm will infect another farm."


Intervet has been working with many governments to advise on outbreak control and management, including vaccination strategies aimed at helping bring this epidemic to a halt.


Further information and downloadable copies of the relevant research papers are available from Intervet's dedicated website avian-influenza.
Intervet's vaccine (Nobilis Influenza) is suitable for protecting poultry against avian influenza type A, subtype H5 and also subtypes H7 and H9. Its use will only be permitted under strict guidelines set out by the European Community legislation on the control of avian influenza. Use of the vaccine is strictly under the control of Defra.


References


1. Ellis T, Leung C, Chow M, Bissett L, Wong W, Guan Yi, Malik Peiris JS. Vaccination of chickens against H5N1 avian influenza in the face of an outbreak interrupts virus transmission. Avian Pathology, 33(4), August 2004, pp 405-412.


2. Frame D (Extension Poultry Specialist, Utah State University Cooperative Extension, USA). H7N3 outbreak halted by vaccine. World Poultry - Elsevier Special, November 2000.


3. Swayne D (Southeast Poultry Research Laboratory, USDA/ARS, Athens, Georgia, USA). To be published, a summary is available on avian-influenza


Halvorson, D The Control of H5 or H7 Mildly Pathogenic Avian Influenza: A Role For Inactivated Vaccine. Avian Pathology (2002) 31, 5-12


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