Chemotherapy is essential in intensive poultry production all
over the world. It has played a significant role in helping to
meet the ever growing demands for animal protein.
have been used for therapy, prophylaxis and for growth promotion
in poultry. However, choosing between the available drugs and
using them to the best advantage is not always straightforward
and the rational use of drugs is particularly complex for
A vet should come up with an effective flock health programme
which combines management, hygiene, quarantine and chemotherapy
in the most rational and profitable way.
A chemotherapeutic programme should be efficacious, safe, practical and legal and
ethical. Most importantly, the vet must not panic or allow
himself to be rushed into poorly planned, ineffective,
unwarranted or unethical medication programmes.
In our poultry industry, in spite of heavy reliance on
medication, poultry health is deteriorating; there are new
diseases, high drug dependency and susceptibility to epidemics.
Medicate or not – a critical decision ?
Medication is justified in bacterial outbreaks. However, it is
essential that a diagnosis, even if only tentative, is made
before embarking on medication. Important questions such as
whether a treatment is required and with what drug, hinge on the
Without a reasonably accurate diagnosis, chemotherapy
becomes a trial and error or hit and miss affair.
Medication is not justified when it will be ineffective such as
uncomplicated viral infections, uncomplicated management induced
problems, disease caused by environmental factors, nutritional
diseases and toxic diseases. Ineffective medication will reduce
the credibility of the veterinarian or the drug.
Furthermore, medication my create a false sense of security, or
detract from the real cause of the problem and how it should be
Medication is unwarranted if the only reason for
using it is to provide non-specific cover over a stress period,
be seen to be doing something, bring piece of mind (in the
absence of any real disease risk), satisfy the argument that “it
won’t do any harm but it just might do some good”, use up drug
stocks. Sometimes a drug is used as the scapegoat for poor flock
Medication can sometimes be counterproductive. Examples are:
1. It may delay desirable exposure and immunity to
2. Medication of mycoplasmosis during rearing may simply
delay the inevitable exposure until a less desirable time, such
as point of lay.
3. Prolonged use of antibacterial can upset the normal
flora of the digestive tract and lead to secondary fungal
4. Medication may hinder or delay the detection of
infection with Myco. gallisepticum or salmonella.
5. Medication may encourage the development of microbial
resistance to the drug, especially if sub therapeutic levels are
administered repeatedly or for a prolonged period.
6. Many antibiotics including chloramphenicol,
chlortetracycline, gentamicin, oxytetracycline and tylosin are
immunosuppressive and their use will prolong uncomplicated viral
Medication is not justified if it is illegal or unethical.
SELECTION OF A DRUG
The selection of the most effective drug should be based on
a combination of theoretical knowledge, laboratory sensitivity
testing, historical records, and practical experience. There is
no point in using a drug it won’t work, no matter what its other
advantages may be. Even in case of a broad spectrum of activity,
the principal activity of most drugs is directed against a
fairly narrow range of infectious organisms. Some diseases have
a drug of first choice and a guide must be considered.
In theory, bactericidal is preferable to bacteriostatic
activity, particularly in severely ill and immunosuppressed
patients. In practice, it makes little difference, as a
functional immune system is essential to the resolution of all
infections irregardless of the mode of action of the drug used
to treat it.
Multiple drug therapy is a very poor substitute for accurate
diagnosis and selection of the most efficacious treatment.
Complicated infections may require the use of two drugs at the
same time. Some drug combinations are synergistic, increasing
both the efficacy and spectrum of activity.
Given the large number of birds in intensive systems, when a
small proportion of the birds become ill, mass medication with
therapeutic antibiotics is practiced because treating only the
infected birds is considered impractical.
DOSE RATE AND FREQUENCY OF
Ideally, the dose rate of each drug should be based on the
minimum inhibitory concentration (MIC) which is required,
attained and maintained in infected tissues. Unfortunately
little work is done on drug absorption, tissue distribution,
metabolism and excretion of drugs in the chicken. Also, the dose
rates should be adjusted to allow for spillage or wastage.
Dose rates should be calculated based on body weight rather than
on concentration in water or feed as this gives accurate and
consistent dosage. Flock dosage based on water and feed
consumption will fluctuate from day to day and from flock to
flock, potentially resulting in under- or over-dosage and a
variable and unpredictable clinical response. For greatest
efficacy, treatment of disease outbreaks should always commence
at maximum dose rates. Remember that doses used in excess of
those recommended will not be any more effective and may be
toxic. Chickens have been accidentally paralyzed by overdosing
ionophores. For long term prevention, use the minimum effective
dose rate. Remember that doses below the recommended will be
ineffective. When controlled challenge and immunity are desired,
such as with coccidiosis, a suboptimal dose rate may be
In theory, the MIC of a drug should be maintained or exceeded in
tissues throughout the course of treatment. In practice, it is
not critical. Continuous medication is more desirable with
bacteriostatic drugs. On the other hand, bactericidal drugs are
probably as or more effective when given in two divided doses
ONSET AND DURATION OF
Normally treatment of a disease outbreak should commence
within 24 hours. The sooner medication is commenced the more
effective it will be. Sometimes, treatment is best delayed in
order to allow the whole flock to become exposed and immune
e.g., early treatment of mild coccidiosis or coryza in
replacement breeders or pullets may result in uneven flock
immunity and subsequent relapses. Prophylactic medication should
commence just before the anticipated disease challenge.
With acute disease outbreaks, medication should continue until
mortality stops and clinical signs are no longer present in the
flock. Usually this takes at least 3 days. Usually, an acute
disease outbreak is under control within 5-7 days. However, if a
response is not apparent within 3-5 days, then the diagnosis and
treatment regime should be carefully reassessed.
After an initial period of treatment at maximum dose rates, and
once the outbreak has subsided, it may be necessary with some
diseases to continue medication at lower levels for a few weeks
to allow controlled exposure and immunity to develop throughout
the flock. However, remember prolonged exposure to low levels of
drug is more likely to encourage acquired resistance.
Chronic flock infections such as Mycoplasma or Haemophilus,
often respond poorly to medication, so an initial period of
treatment may be followed up with intermittent medication at
treatment dose rates, or more prolonged medication at control
ROUTE OF ADMINISTRATION
In general, the most effective route depends on the site of
infection. Parenteral medication is the most effective route of
treating systemic diseases. IM injection is recommended for
long-acting drugs. Oral medication is the most effective route
for alimentary infections. Remember some drugs are not absorbed
from the digestive tract and will be ineffective against
systemic infections if given by this route. DW medication is
usually more effective in disease treatment than in feed
medication. Remember sick birds continue drinking longer than
eating. Enteric absorption of many drugs is delayed in the
presence of food. Topical medication is the most effective way
of treating external parasites.
DRUG SENSITIVITY AND RESITANCE
All infectious organisms have an inherent pattern of
sensitivity and resistance to specific drugs. Resistance to
certain drugs may also be acquired. This can occur through
mutation and selection and cross resistance. Multiple drug
resistance may be to a number of unrelated antibiotics.
R-plasmids (transferable drug resistance) may be transferred
between individual bacteria within a strain, between strains or
species, and between genera, e.g., E.coli and salmonella.
Antibiotics are selecting an ever widening range of resistant
bacteria and this is being potentiated by gene transfer. We must
expect the incidence and types of resistant bacteria to continue
to increase if the overall rate of antibiotic usage continues.
Antibiotic resistant E. coli are particularly prevalent in the
gut flora of poultry. Antibiotic resistant E. coli persist long
after the antibiotic has been withdrawn since many of these
strains are good colonisers of the gut. There is no evidence
that E. coli of animal origin cause disease episodes in man.
Their importance lies in the fact that they constitute a highly
versatile pool of R plasmids which may be transferred ultimately
to important pathogens.
Safety is an important prerequisite for any flock health
programme. There is no point in using a drug if it will cause
more problems than it solves. Safety referes to both poultry and
Drug toxicity to poultry may occur due to inherent features of
the drug, overdosage, prolonged usage and interaction with other
drugs or chemicals. Lack of safety in poultry may manifest as:
1. Clinical signs or mortality caused directly by the
2. Reduced growth rate, feed conversion efficiency, egg
production, fertility or hatchability.
3. Reduced feed or water consumption.
4. Secondary or super-infections.
Drug toxicity to people may occur due to consumption of drug
residues in poultry meat and eggs. This may result in an
immediate clinical reaction (allergy to penicillins and
sulophonamides), gradual development of a fatal aplastic anaemia
(in people sensitive to chloramphenicol), acquisition of an
allergy or sensitivity to the drug or acquired resistance by
human bacteria. Furazolidone, and chloramphenicol are
Antibiotic-resistant bacteria in food pose a substantially
greater risk to human health than antibiotic residues.
Consumption of drug resistant bacteria on poultry meat can cause
food poisoning in humans. Poultry enteric bacteria may transfer
their resistance plasmids to human pathogens. Multiple-drug
resistance is increasing at an alarming rate.
The possible adverse effects on people can be avoided by:
1. Using antibiotics correctly and only when necessary.
2. Avoiding prolonged use of subtherapeutic doses which
encourage bacterial resistance.
3. Adhering to withdrawal/product withholding periods
before poultry are processed or eggs are sold.
4. Not using chloramphenicol.
5. Avoid contact with drugs when they are handled.
6. Minimising salmonella infection in poultry flocks and
salmonella contamination of poultry carcasses and products.
7. Cooking poultry thoroughly and handling it
Cost has a large bearing on the practicality of
chemotherapy. Medication cost has 3 main components: drug cost,
administration cost and cost of adverse effects. Sometimes it
may be cheaper to live with a minor or refractory disease than
to attempt to control it through medication. Cost may be
expressed as an amount per kg, per egg or as a proportion of the
total flock value.
There is a high expenditure on veterinary drugs in modern
poultry production in Pakistan.
1. The establishment by government of a surveillance
system for antimicrobial resistance.
2. The use of antibiotics for growth promotion should be
3. Fluroquinolones, and 3rd generation cephalosporins,
should no longer be permitted for mass medication. This is to
prevent the further development of resistant food poisoning
4. Poor prescribing by vets should lead to retraining and
excessive prescribing should result in legal action.
5. The Government should help establish a School of
Preventative Veterinary Medicine. It should research, collate
and disseminate reliable information to farmers, vets and
6. Veterinary and agricultural colleges should place
greater emphasis on the teaching of drug-free preventative
7. Practical and technical help should be given free of
charge to producers to encourage them to alter production
methods in order to reduce dependency on antibiotics.
8. The Concept of Positive Health: we need to shift away
from pathogen-targeted fire-fighting towards proactive,
health-targeted policies and practices. Organic farming is a
holistic system for achieving positive animal health with proven
a. Short-term biosecurity or treatment measures.
b. The long-term goal of positive animal health.