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Unit 5 Principles of
Control of Infectious Disease (Students: Note
hyperlinks below to Web sites with
additional information on the topics covered in this unit)
Use of Drugs to Control Disease
The variety of substances available to treat disease is constantly
changing, with new drugs being introduced and others being removed from the marketplace.
Consequently, we will not discuss specific drugs or substances in terms of recommendations
for their use in treating specific diseases in this course. Any mention of specific drugs
will be done simply as an example to illustrate general principles which must be kept in
mind when using drugs in food animal species.
In chapter 15 of Principles of Veterinary Science, Hoopes and
Thwaits discuss a variety of drug classes, including anesthetics, analgesics, cardiac
drugs and so on. Very few drugs in these classes have been approved for use in food
producing animals and most can only be obtained under veterinary prescription. The
majority of drugs used legally in food producing animals are in the classification of
antimicrobials and antiparasitics, consequently, we will concentrate on these classes of
drugs in our discussions.
When deciding whether to treat or not to treat an animal, always remember
the adage "above all do no harm" and also remember that "there is no
such thing as an absolutely safe and effective drug; safety and efficacy are
relative"1
Basic Principles
The primary goal in using an antibiotic or other antimicrobial drug is to treat
bacterial infections, not to prevent them (that is the goal of vaccine use). Antibiotics
are ineffective against viruses.
Antibiotics are sometimes used to prevent a secondary bacterial infection from
developing when an animal is suffering from a viral disease that compromises the animals
ability to fight off bacterial infections.
Antibiotics are sometimes used prophylacticly to treat animals at high risk of
developing a bacterial infection (example: treatment of stressed beef cattle upon arrival
at a feedlot to prevent shipping fever).
Antibiotics have been included at low levels in feed rations for growth promotion
benefits unrelated to treating specific diseases. This use of
antibiotics is being hotly debated world-wide, and guidelines have been
proposed by the World Health Organization (WHO). For the latest
information on the guidelines, see an excerpt from the August
2000 Pig International Electronic Newsletter.
It is likely that use of antibiotics for any reason other than the treatment of
specific diseases for specified periods of time will be prohibited in the future.
Bactericidal antibiotics kill bacteria; bacteriostatic antibiotics simply
prevent bacteria from multiplying (the host immune system kills them).
The liver and kidney are the primary organs for metabolizing and excreting antibiotics
from the animal. Animals with compromised liver and/or kidney function may have higher
tissue levels of antibiotic than a normal animal, and may have violative levels of
antibiotic even after the appropriate drug withdrawal time has elapsed. Metabolism and
excretion of all drugs varies with the species (cats, for example cannot metabolize
aspirin and acetaminophen), age and sex of an animal.
Guidelines for Proper Drug Use
1. Diagnose the problem accurately
- establish a veterinary/ client relationship
- seek confirmatory culture and drug sensitivity of agent
2. Use a drug that is effective against the agent
3. Administer the drug in the proper form by the correct route
Parenteral
Intravenous (IV) gives immediate blood levels but is cleared from the body rapidly
Intramuscular (IM) blood levels reached more slowly than IV, should not inject more
than 5-10 cc/site depending upon the size of the animal.
Subcutaneous (SC) released into blood even more slowly, less painful than IM and larger
volumes can usually be injected per site.
Oral -- rate of absorption depends upon the formulation; can be by addition to feed or
water (palatability is an issue), by pill, paste, drench
Topical -- usually the least rapid rate of absorption, however this can be altered by
mixing with other chemicals
4. Administer the drug at the appropriate dosage, time interval and for the proper
length of time
5. Store drugs under proper conditions
6. Observe expiration dates, contraindications, withdrawal times
Always read and follow the label directions
Some drugs, termed "over-the-counter' or OTC drugs,
can be purchased by a lay person directly from a veterinary supply house, pet store or
farm supply store. Drugs that are approved for OTC sales by the FDA must have
"adequate written directions for safe and effective
use by a lay person."
"safe use" refers to:
- Safety to animal
- Safety of food products from the animals
- Safety of persons associated with the animals
- Safety of environment
"effective use" refers to:
- Accurate diagnosis can be made
- Drug can be properly administered
- Course of disease and success or failure of drug can be monitored
Other drugs can only be dispensed under veterinary prescription and are
referred to as Rx drugs. Dispensing of these drugs requires a valid
veterinarian-client-patient relationship.
Valid Veterinarian-Client-Patient Relationship
as defined by the American Veterinary Medical Association is: "An appropriate
veterinarian-client-patient relationship will exist when:
(1) the veterinarian has assumed the responsibility for making medical
judgements regarding the health of the animal(s) and the need for medical treatment, and
the client (owner or caretaker) has agreed to follow the instructions of the
veterinarian; and when
(2) there is sufficient knowledge of the animal(s) by the veterinarian to
initiate at least a general or preliminary diagnosis of the medical condition
of the animal(s). This means that the veterinarian has recently seen and is personally
acquainted with the keeping and care of the animal(s), and/or by medically appropriate and
timely visits to the premises where the animals are kept; and when
(3) the practicing veterinarian is readily available for follow-up in
case of adverse reactions or failure of the regimen of therapy."
Instances Requiring a Valid Veterinarian-Client-Patient Relationship
- Dispensing perscription drugs
- Dispensing "extra-label" prescription drugs via AMDUCA
- Dispensing medicated feeds regulated under a Veterinary Feed Directive
Drug Safety Issues
Drug Interactions and Incompatibilities --
Whenever more than one drug is administered to an animal, the potential exists for drug
interactions. Sometimes we purposely use multidrug treatment because the drugs work
synergistically, but in other instances, there may be antagonism between the action of two
drugs. An example of this is the administration of a bactericidal drug in combination with
a bacteriostatic drug. Bactericidal drugs are only effective against bacteria that are
moving through the cell cycle (their mode of action is usually targeted at one or another
stage of the cell cycle). The presence of a bacteriostatic drug will stop the organism
from moving through the cell cycle and cancel out the benefit of the bactericidal drug.
Drug Residues --
There are strict rules
and regulations in place which restrict the use of drugs in food producing
animals. These regulations are designed to prevent the carry-over of drugs into foods for
human consumption. Failure to observe these regulations
[link 1 || link 2] and
the appropriate withdrawal times when using drugs can result in loss of market and severe
penalties including jail time. Cull cows and bob veal calves have been observed to have a
higher prevalence of violative drug residues than other types of animals. It is very
important for the producer to properly identify animals and keep accurate and complete
records of drug use.
Development of
Antimicrobial Resistance --
There are two major ways in which bacteria may develop resistance to any particular
antibiotic.
- Mutation of chromosomal DNA of the bacterium
- Acquisition of resistance factors from another bacterium via transformation or receipt
of plasmids containing resistance factor(s). The continued presence of a large population
of chloramphenicol resistant bacteria in animals after chloramphenicol had been removed
from the market for more than 10 years is thought to be due to it's resistance factor
being included in a "packet" of tightly linked resistance factors for drugs that
continued to be used in the human and animal population.
The complete
prohibition of extra-label use of fluoroquinolones and glycopeptide classes
antimicrobials in food producing animals is due to the concern over rapid development of a
population of bacteria resistant to classes of drugs reserved as "drugs of last
resort" in the human population. Previously approved use of fluoroquinolones
in poultry production are being revoked, due to development of
antimicrobial resistance among campylobacter species, a major foodborne
pathogen in humans.
Drug Use in Food-Producing Livestock
Use of drugs in food producing animals has been a subject of controversy
for quite some time. In addition to concerns about drug residues that
may remain in food products, there is concern over the potential for
development of drug resistance, as discussed above. A good summary of
recent information on this subject can be found in "Livestock
Drugs: More Questions Than Answers?" Agricultural Outlook,
September 2001.
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