Anticoccidials
Introduction
Coccidiosis is an infection of the intestinal tract
caused by a single cell parasite. All livestock species, as well as wild
animals, can be infected and it is especially prevalent when animals or
birds are grouped together in significant numbers. However it can occur in
less intensively managed situations, including outdoor flocks and herds.
The disease is characterised by an invasion of the
intestinal wall by the parasite. The parasite then undergoes several stages
of growth and multiplication, during which there is damage to the mucosal
and submucosal tissues. Severe haemorrhage may result and mortality in an
unprotected poultry flock may be extensive.
Although cattle, sheep and pigs can become infected
causing significant depression of performance, it is in poultry species that
the parasite causes the most devastating losses. For this reason, it is
essential in most poultry rearing situations to use an anticoccidial agent
during the rearing period to prevent illness and control infections.
The production of affordable, quality poultry meat owes
much to the development of effective antiparasitics used in the prevention
and control of coccidiosis.
The Development of Anticoccidial
Drugs
The development of the broiler (chicken meat)
industry in the 1950s required the urgent availability of anticoccidial
compounds. Intensive screening activities by several companies soon
produced a range of medicines that were effective in the control of
coccidia.
However, these chemical products were so efficient at
destroying coccidia that they prevented treated livestock from building
up any natural immunity to this parasite, but they were not so potent
that all exposed coccidia were killed. The result was that the surviving
coccidia quickly became resistant to the products and since there was
little natural immunity in the livestock, severe outbreaks of disease
occurred.
For this reason the development of new chemical
anticoccidials proceeded throughout the 1960s and by selective use of
anticoccidial compounds it was possible to rear broilers reasonably
satisfactorily, with only occasional breakdowns.
Since 1970 there has been a European system for
approval of anticoccidials. Initially based on approval of the molecule
(the active substance) it has now been modified so that now each
specific product must be approved ("Brand specific approval"). This
system involves scrutiny by European experts in the Commission and the
European Food Safety Authority.
In the past few years all the anticoccidial products
have a had a complete re-review by a specialist scientific body under
the European Food Safety Authority. Major new studies were conducted by
the sponsor companies to comply with current guidelines; the review was
extremely thorough, focusing mainly on human safety from both residue
and microbiological aspects. In 2006 MRLs
(maximum residue levels) were published on the molecules that
completed the review. Also, in 2008, the Commission published a report
on the future of the anticoccidial products both from a technical (need)
point of view and well as regulatory (should they be under veterinary
control?) In agreement with most stakeholders, including producers and
practising veterinarians, this recognised that the products are
essential for animal health and welfare, and proposed that the current
mode of regulation should be maintained.
The authorised products are now listed in a Register,
published in the European Official Journal at the end of 2005. This can
be located at:
http://europa.eu.int/comm/food/index_en.htm
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Anticoccidials since 1970
Although several chemical structures have remained to
the present day and continue to provide a key role in the prevention and
control of coccidiosis, the need for constant checking that they are
still active adds to the management burden. During the 1970s a new class
of antibiotics was discovered, the ionophores, which is now an important
component of coccidiosis control.
The ionophores are unique in that they permit a small
“leakage” of coccidia to enable the bird to develop a certain level of
immunity. This allows a greater degree of protection against the
parasite and is a much more efficient method of control. Resistance to
ionophores develops very slowly and is more of a tendency for increasing
levels of tolerance. This means that the broiler producer is in a
position to make changes before there is an acute outbreak of disease.
Control programmes
In the first few weeks or days of a chick's life the immune system
is not fully developed. It is not unusual for protection in this early stage
to be provided by a chemical agent followed by a switch to an ionophore.
Such “shuttle programmes”, as they are known, provide an adequate balance
between control of infection and the development of immunity in the older
bird. In some countries, shuttle programmes may comprise two types of
ionophore, but this is less common. Other authorities are known to propose a
“reverse” shuttle, where the starting anticoccidial is an ionophore followed
by a chemical based product. Some anticoccidials are based on a combination
of an ionophore with a chemical based product.
Vaccines to prevent coccidiosis are available and are
used in replacement/breeding stock. A vaccine for use on broilers is also
available.
Vaccines are an important tool, but they are not always
the answer. It is important that there is a full range of solutions, so the
best option can be chosen to prevent this disease on a particular farm.
In other livestock species, the disease is more sporadic
and difficult to predict. However, effective anticoccidials are available
and the disease can be kept under control to permit natural immunity to
develop. These anti-coccidials may be incorporated into feed, or some are
administered as an oral drench.
Treatments
In the event of a clinical outbreak of coccidiosis, it is
vital that effective products are available for treatment. These are usually
water soluble treatments for poultry, which are convenient and can be easily
administered. In other livestock, treatments may be given as an oral drench
or added to feed.
Conclusion
It would not have been possible to develop the modern
broiler and turkey industries without the discovery and use of anticoccidial
agents - those who were farming poultry in the 1950s may well remember the
death, illness and devastation the disease can cause if it takes hold in a
flock of birds. Other species for which anticoccidials are important include
replacement layers, game birds and rabbits. When used in a structured and
monitored programme, the anticoccidial agents are very effective and permit
the optimum performance of the animal without compromising health status. No
new in-feed anticoccidial agent has come onto the market since the 1980s, which is
perhaps a reflection of the level of success that the ionophores have
brought to the control of the disease.
Updated October 2008
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