How to raise Backyard Chickens: [CHICKENS-101] chicken vaccines--in australia

Monday, October 29, 2012

[CHICKENS-101] chicken vaccines--in australia

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A ProMED-mail post
<http://www.promedmail.org>
ProMED-mail is a program of the
International Society for Infectious Diseases
<http://www.isid.org>

Date: 12 Jul 2012
Source: Science [edited]
<http://news.sciencemag.org/sciencenow/2012/07/chicken-vaccines-combine-to-prod.html>

Scientists have found that 2 virus strains used to vaccinate chickens
in Australia may have recombined to form a virus that is sickening and
killing the animals. "This shows that recombination of such strains
can happen, and people need to think about it," says Glenn Browning, a
veterinary microbiologist at the University of Melbourne, Parkville in
Australia and one of the co-authors on the paper.

Chickens worldwide are susceptible to a group of herpesviruses called
ILTV, which target their upper respiratory tract. The resulting
disease, called infectious laryngotracheitis, reduces egg production
and can kill up to 1/5th of those infected. "The birds effectively
choke to death on blood and mucus," says Browning. The disease is not
known to infect any animals other than chickens and chicken-like
birds.

To combat ILTV, farmers vaccinate their chickens with attenuated
herpesviruses that can still infect and replicate but do not lead to
disease. Australia has used 2 vaccines, which are produced by Pfizer
and called SA2 and A20. In 2006, however, the country purchased a new
vaccine from the European company Intervet called Serva. 2 years
later, new strains of ILTV, called class 8 and 9, appeared. They are
just as deadly as other strains. "But they seem to be dominating over
the strains that were reported prior to 2007," says Browning.

Because the new strains appeared shortly after the European vaccine
was introduced, scientists thought that the new vaccine strain might
have reverted back to a disease-causing form. But when the researchers
sequenced the genomes of the 2 new strains and the 3 vaccine strains,
they found that the new viruses were actually stitched together from
the European and Australian vaccines. Although it is not clear what
mutations keep the vaccine strains from causing disease in the 1st
place, they were probably lost when the viruses recombined, says
Browning, whose team reports its findings online today [12 Jul 2012]
in Science.

"This is quite possible but a bit surprising, since it would imply
that both vaccines have gone into the same animal, which would be
required for recombination to occur," Paul Farrell, a virologist at
Imperial College London, wrote in a statement released by the Science
Media Centre. Farmers do not deliberately vaccinate with both
vaccines, Browning agrees. But the SA2 strain might have spread into
an unvaccinated population that was later vaccinated with the Serva
strain, he suggests.

The data for the recombination is "convincing," says Walter Fuchs, who
heads the National Reference Laboratory for Infectious
Laryngotracheitis of Poultry on the island of Riems in Germany. The
combination of vaccine strains to form a new virus is "a problem that
needs to be taken seriously," adds Thomas Mettenleiter, head of the
Federal Research Institute for Animal Health also on Riems. Only
well-characterized live vaccines rendered harmless by mutations in the
same or overlapping regions should be used in order to minimize the
risk of recombination to a new virulent strain, he argues.

Live-attenuated vaccines are also used in humans, but a lot less than
in poultry, and their sequence is usually known. "This is not a
panic-button on vaccines," says Browning. And Farrell stresses
vaccines have been one of the great success stories of medicine. "The
type of important technicality raised in this article should not be
allowed to detract from the enormous health benefit generally provided
by vaccines," he wrote.

- --
Communicated by:
Thomas Monath <tpmonath@gmail.com>

What strikes me as most important is that there are modified live
viruses in these vaccines. If vaccines with different strains can
recombine to produce a different strain of the disease in chickens,
then it stands to reason it could happen in any animal, including in
people. Scientists at all levels should be looking at this when a
vaccine with a new strain is introduced for any disease. Veterinarians
and physicians need to be keeping accurate records of which strain was
used for vaccination in a particular animal or human.

Furthermore, reversion of an ITLV vaccine to virulence has happened
before; see ProMED-mail post 20050308.0689

Australia may be located on the interactive ProMED HealthMap at:
<http://healthmap.org/r/1z_*>. - Mod.TG]

[see also:
2005
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Avian influenza, poultry vaccines (02) 20050308.0689
Avian influenza, poultry vaccines: a review 20050307.0680]
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