It is becoming increasingly clear that biosecurity
measures on their own are insufficient for controlling
campylobacter and that a vaccine is necessary before the industry
can make significant strides in cutting its
prevalence.
This has parallels with salmonella in eggs, where commercial
launch of an effective vaccine greatly assisted in eliminating it
from laying flocks. Vaccines have also proved successful in the
control of various epizootic viral disease such as Newcastle
Disease or infectious bursal disease (Gumboro).
The interest in reducing campylobacter is being driven by the
European Commission which last year asked the
European Food Safety Authority (EFSA) to consider approaches
and targets to reduce campylobacter in poultrymeat. The poultry
sector is being targeted as it has been suggested that up to 85% of
human infections are caused by campylobacters from poultry.
Campylobacter jejuni and its close relative C coli commonly
colonise the chicken gut in large numbers while the birds show no
obvious symptoms. And these organisms contaminate carcasses during
processing and can infect humans causing moderate-severe enteritis.
Campylobacteriosis is now the most frequently reported cause of
food poisoning in Europe.
Most intensively-reared flocks become campylobacter positive
after two-three weeks of life (the lag-phase in colonisation).
Studies have shown that transmission to flocks occurs horizontally
from the environment via a number of routes including human traffic
into the houses and possible vectors like water, rodents or
flies.
Such routes could be minimised by strict biosecurity, but to
date such levels of biosecurity have proved difficult to
effectively introduce and maintain. Therefore, there is an
increasing recognition for the need for complementary approaches,
such as vaccination, to supplement biosecurity in protecting flocks
from campylobacter colonisation.
Nevertheless to date, no effective campylobacter vaccines have
been commercialised. To discuss progress in this area, an
international scientific workshop was held back in April 2009 under
the auspices of the European Network of Excellence,
Med-Vet-Net.
Immunity to campylobacter
As early as 1992, it was reported that chickens infected with
campylobacter generated detectable levels of antibodies in their
blood and at the gut surfaces. These antibodies were directed
against those parts of the bacterium essential for colonisation
like the flagella (projections).
Importantly, the level of these antibodies was associated with
the elimination of colonisation in older birds and with partial
protection from subsequent challenges. Moreover, it appears that
hens can transfer some of this protection, via antibodies in eggs,
to their chicks and that this explains the observed lag-phase in
flock colonisation. All this evidence provides considerable
optimism that vaccination could generate protective antibody
responses.
Antibodies are not the only way by which protective immunity is
conferred. Certain cellular immune responses are also important in
resistance to infection, especially during the initial stages of
colonisation.
Recent studies have demonstrated that campylobacter stimulates
these cellular responses and that they are genetically determined.
This means that it may be feasible to breed future chickens that
are more resistant against campylobacter.
Vaccination studies
Since the early 1990s, there have been many experimental studies
on vaccination against campylobacter in chickens. Campylobacter is
rarely, if ever, vertically transmitted so the target has to be the
broiler not the parent flock.
In the broiler, the immune response has to be generated at the
gut level, so the approaches used have either involved live
organisms colonising the chicken's gut and producing campylobacter
antigens locally, or parts of campylobacter (so called sub-unit
vaccines) delivered to the gut surface.
Currently two strategies are generating interesting results. The
first strategy involves the use of a live salmonella vaccine
genetically engineered to incorporate campylobacter antigens. The
salmonella vaccine is also engineered to be quickly eliminated from
the chicken's gut, so that vaccinated birds would be
salmonella-negative by slaughter.
To date these experiments have been undertaken using different
salmonella strains and campylobacter antigens, by research groups
in Poland and the USA. Both groups report successful protection
against experimental challenge with campylobacter.
However, the use of a genetically engineered (GM) live
salmonella vaccine in food production raises a number of issues
regarding consumer acceptability and commercial viability, which
will need to be addressed in Europe. Clearly considerable further
research is needed to establish the effectiveness of this approach
on commercial poultry units.
A second strategy, using sub-unit vaccines of campylobacter has
been reported in the UK. Because this approach does not use GM
vaccines, acceptability is not an issue. The problem with this
approach is that such antigens, delivered orally, fail to generate
a response at the chicken's gut surface.
In mice and humans, there are several materials (adjuvants),
which can effectively present the antigens to generate an immune
response at the gut surface. Unfortunately, research has shown that
most of these adjuvants are ineffective in the bird's gut.
However, one such adjuvant, chitin microspheres, was able to
effectively present a major campylobacter antigen, flagellin, to
stimulate both antibody and cellular immune responses in chickens
and that these responses gave protection against experimental
challenge.
Both strategies provide proof that vaccination is a feasible way
of protecting chickens from campylobacter. However, considerably
more research is required to establish the practicality of such an
approach on commercial units.
In particular there are practical issues regarding vaccine
efficacy, safety and stability, as well the timing and delivery
method to achieve protection in an immunologically immature and
short-lived bird. There is also a need for fundamental research to
select the best campylobacter antigens to use and the most suitable
adjuvants or live vectors for delivery.
To conclude, vaccination is a long-term strategy requiring
considerable future investment. But given the extent of the public
health issue, the difficulty in excluding campylobacter from flocks
and the current lack of an alternative strategy, a commitment to
vaccine development may become worthwhile.
• Diane Newell is an independent consultant who established
Food-borne Zoonoses
Consultancy in September 2008. She previously worked for the
Veterinary Laboratories Agency as a senior scientific
consultant.
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