According to the American
Veterinary Medical Association (AVMA), euthanasia is "the
act of inducing humane death in an animal." This
statement and standards for humane killing of animals are contained in
the 2000 Report of the AVMA Panel on Euthanasia.
Essential requirements for euthanasia are rapid loss of consciousness, cardiac or
respiratory arrest and cessation of brain function.
There are several reasons why an animal must be euthanized. In
some cases, the only way to interpret experimental results is by post
mortem tissue examination. When an experiment has been completed,
animals may be euthanized because they are no longer needed.
Depending upon the experimental procedures, it may not be humane
to allow an animal to live out its natural life.
Regardless of the reason
for euthanizing an animal, it is essential that the animal be treated
with the maximum amount of respect and care.
Some facilities adopt out
animals from completed protocols or donate them to zoos, wildlife
rehabilitators or snake owners for use as food (all animals used
in the Medaille College Veterinary Technology program are available for
adoption).
Any method of euthanasia
must be submitted to the Institutional Animal Care and Use Committee
(IACUC) for consideration and approval. If the euthanasia protocol
is listed as "conditionally" acceptable by the AVMA, the
investigator must justify selection of this method and explain why an
"acceptable" method was not chosen. Under no
circumstances will a method listed as "unacceptable" by the
AVMA be allowed.
Pain and Distress:
Pain is an noxious
sensation detected by nerves that is transported to and recognized by
the brain. For pain to be felt, the animal must be conscious; an
anesthetized animal does not perceive pain.
Distress is anxiety and
apprehension that is experienced by an animal. Gentle handling and
talking to animals often help minimize distress. Animals waiting
to be euthanized should not be exposed to other animals that are being
euthanized.
Criteria for
Evaluating Methods of Euthanasia:
The AVMA lists numerous
criteria that they used to evaluated methods of inducing euthanasia,
including:
- Rapid loss of consciousness and death
without causing pain and distress
- Reliable (i.e. same effect on animals
every time)
- Safety for personnel
- Irreversible
- Compatible with subsequent evaluation,
examination or use of tissues
- Emotional effect on personnel (and
perceived public response)
- Drug availability and human abuse
potential
- Compatibility with species, age, sex
and health status of the animal(s)
- Ability to maintain equipment in
proper working order
Euthanizing agents cause death by inducing hypoxia (oxygen deprivation),
depression of neurons needed for life or the physical disruption of brain
activity. The selection of the most appropriate method depends
upon the experimental requirements, species, size and number of animals
and skill of personnel.
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Chemical Methods of
Euthanasia:
Inhalant gases can be
used to euthanize animals, especially halothane (anesthetic gas
overdose) and carbon dioxide. Gases must be administered in a
closed chamber that protects the operator from the possible toxic
effects. Sufficient oxygen must be delivered with the euthanizing
gas to prevent hypoxia and, therefore, distress to the animal. It
is important to expose the animal to a high concentration of the agent to
produce a rapid onset of unconsciousness and to be aware that neonates and
animals with compromised breathing are refractory to the effects of the
gas. If animals are euthanized as a group, they must be protected
from the activities of the other animals and animals should not be
placed in a dirty euthanasia chamber that may contain pheromones and
odors from previously euthanized animals.
Halothane is the inhalant
anesthetic of choice for euthanasia, as it results in rapid induction of
anesthesia. It occasionally causes seizures in deeply anesthetized
animals. Other anesthetic gases that may be used are ether
(irritating and explosive), isoflurane (slow onset of anesthesia),
methoxyflurane (slower onset) and nitrous oxide (causes hypoxemia and
has a high human abuse potential).
The primary value of
anesthetic overdose for euthanasia is its use in small animals (<7
kg) or other animals in which venipuncture may be difficult. It is
generally not used in farm animals because of its cost. All anesthetic vapors are irritating and may cause anxiety and
struggling. Nitrous oxide should only be used with other
anesthetics and it increases the rate at which unconsciousness occurs.
Carbon dioxide is the
preferred nonanesthetic gas for use in small animals, including rodents, rabbits and
birds that weigh less than approximately 500 grams. It is supplied
by compressed carbon dioxide cylinders pumped into a closed chamber, as
used with anesthetic gases. Carbon dioxide causes rapid
depression, analgesia and anesthesia in animals. It is
inexpensive, nonflammable, nonexplosive and the hazard to the operator
is generally low. In addition, there is no accumulation of carbon
dioxide in the tissues and the euthanized animals can be used for
testing and food without any secondary effects to the hawk or snake that
eats it. There are disadvantages to its use, however, that must be
addressed. Carbon dioxide is heavier than air and if the chamber
is not filled properly, an animal may be able to avoid
exposure. Some animals, such as fish, reptiles and amphibians, are
able to tolerate high concentrations of carbon dioxide without loss of
consciousness. Concentrated carbon dioxide may be irritating for
some animals, because it combines with water on moist mucous
membranes to form carbonic acid. Gas flow must be continued
beyond apparent death and death must be verified before the animal is
removed from the chamber.
Other gases have been
used to euthanize animals, including nitrogen, argon and carbon
monoxide. These gases lack the reliability and safety of carbon
dioxide, however, and human deaths have resulted from faulty carbon
monoxide chambers.
Injectable euthanasia
agents are the most reliable and rapid method of inducing
euthanasia and may, when properly performed, result in little pain or
distress in the animal. Animals that find restraint and handling
stressful, however, should be euthanized by an inhalant gas or be tranquilized prior to injection.
The primary injectable
agent used for euthanasia is sodium pentobarbitol (at lower doses it
is used as an injectable anesthetic). This
chemical depresses the central nervous system, causing deep anesthesia,
followed by apnea and cardiac arrest. It can be administered by
intravenous, intracardiac or intraperitoneal injection.
Intravenous injection is the preferred route for most animals.
Intraperitoneal injection can be used with small animals, such as
rodents, or when an intravenous injection would be difficult or
dangerous. The intracardiac route must only be used in animals
that are already anesthetized or are unconscious, due to the technical
difficulty of the injection.
Sodium pentobarbital is a
controlled drug and is strictly regulated by the Drug Enforcement
Administration. An unconscious animal may have an agonal
breath (terminal gasp) which is disquieting to the people administering
and observing the procedure.
Other chemicals can be
used to induce anesthesia, including chloral hydrate (sedated farm
animals only), tricaine methane sulfonate (MS 222 for amphibians and
fish) and potassium chloride (anesthetized animals only).
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Physical Methods of
Euthanasia:
There are a variety of
acceptable methods of physically killing animals. When performed
by a well-trained person, physical methods may be more rapid, painless
and less stressful than chemical euthanasia. Several methods are
classed as "conditionally" acceptable, based on the situation
and justification provided by the investigator. Physical methods
should selected only after acceptable chemical methods have been excluded.
They may also be more
upsetting to witness, and the procedure should be carefully explained to
people the first time that they see it done. Most physical methods
of euthanasia involve trauma, and there is a risk that injury may occur
to animals or handlers.
There are three
situations in which physical methods may be appropriate:
-
Easily handled small
animals with anatomic features compatible with the method chosen
-
Large farm, wild or zoo
animals that may be difficult to restrain or inject
-
Research protocols in
which chemical euthanasia may invalidate tissue and/or body fluid
interpretation
A penetrating captive bolt
gun fires a metal rod (bolt) through the skull of an animal, traumatizing
the brainstem and causing immediate loss of consciousness, followed by
death. Skill is needed to properly place the gun and to restrain the
animal so that it does not move. It is an effective method of
euthanasia for horses, ruminants and swine, particularly when the animals
are slaughtered for meat or used in a research project when chemical
contamination is not acceptable.
A sharp blow to the head
may be acceptable for neonatal animals with thin skulls, such a
piglets. Gunshot may be used for feral or wild animals. Both
require a high level of skill to ensure that death is rapid and painless.
Cervical dislocation is a
physical method of euthanasia that may be used in small rodents, immature
rabbits and some birds. According to the AVMA, cervical dislocation
"...when formed by well-trained individuals, appears to be
humane." Use of this method must be scientifically justified to
the IACUC and consistent proficiency must be demonstrated by
personnel. If personnel are not technically proficient at the
procedure, animals must be anesthetized prior to euthanasia by this
method. When performed properly, cervical dislocation results in
rapid, painless death and leaves no chemical residue in the tissues.
It is most frequently used in mice and small rats (<200 grams). A
small rod is placed at the base of the animal's skull and
the tail is pulled sharply away from the pen, separating the
cervical vertebrae from the skull.
Decapitation is another
method of physically euthanizing small animals, primarily rodents and
rabbits, and is accomplished with a sharp blade that separates the head
from the body at the cervical vertebrae. Like cervical dislocation,
decapitation must be justified in the protocol submitted to the
IACUC. Guillotines are commercially available and produce consistent
results. Adequate restraint is essential and the procedure may be
stressful for the animals and the operator. Electrical activity in
the brain continues for several seconds after decapitation and there is
controversy over the suitability of its use.
Microwave irradiation
is primarily used to fix brain metabolites while maintaining the anatomic
structure of the brain. Its use is limited to rats and mice used in
neurobiological research. Death occurs in less than one
second. The microwave instrument used is specifically designed for
euthanizing small rodents and directs the microwave energy directly toward
the animal's head (kitchen-model microwave ovens are unacceptable
for this use).
Adjunctive Methods Used
with Euthanasia:
Additional procedures
may be used with, but not as the sole means of, euthanasia. Stunning
(unconsciousness) of an
animal can be accomplished with a blow to the head or nonpenetrating
captive bolt. Small rodents can be stunned with a single sharp blow
to the central skull bones to cause rapid loss of consciousness. A
nonpenetrating captive bolt (does not penetrate the skull) causes the same
response in horses, cattle and swine. Both of these methods require
an additional method to produce death. Exsanguination (bleeding out)
is a method used to complete the euthanasia of a stunned, unconscious or
anesthetized animal. It allows collection of the maximum volume of
blood and ensures death.
Confirmation of Death:
Death must be confirmed
after euthanasia by the lack of a heartbeat. Because the heart may
continue to beat after cessation of breathing, apnea is not an acceptable
criterion of death. No animal should be necropsied or discarded
until death is verified.
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