Organisms are grouped in categories from general type (such as plant or
animal) through smaller and more inclusive taxa (singular: taxon) that
vary by fewer and fewer characteristics. The taxa, from
largest to smallest, are kingdom, phylum, class, order, family, genus
and species.
The naming system must conform to a rigid set of rules, reducing
confusion that may arise from using local names and providing a uniform language to ensure accurate communication
throughout the world. The genus-species combination is called the
scientific name or species of an organism. These two names are
either underlined or italicized. The generic (genus) name
is always capitalized and the specific (species) name is always written
in lower case. The species name is never used alone: it is
meaningless without the genus.
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Taxonomic Groups
Kingdom:
Many research organisms
are animals--they come from the Kingdom Animalia. Some researchers
use living things from the other kingdoms, Plantae, Fungi, Monera and
Protista.
Phylum
(plural- phyla):
There are several phyla
within each kingdom. Organisms are broken into smaller, more
recognizable groups based on their general body plan. Most
research animals belong to the phylum Chordata, animals that have a
dorsal central nervous system, a notochord during embryonic development
(a stiff cartilage structure under the dorsal surface of the body), and
gills or lungs. Most chordates have a spinal column (and are
called vertebrates). Mammals, birds, reptiles, amphibians, and fish belong
to the phylum Chordata. Other animal phyla include Arthropoda (insects and spiders), Mollusca (snails and clams), Annelida
(earthworms), and Echinodermata (starfish and sea urchins).
Class:
The next level of taxonomy is the class. Several classes are found in each
phylum. The classes of phylum Chordata are:
- Mammalia (mammals)
- Aves (birds)
- Reptilia (snakes, lizards and
turtles)
- Amphibia (frogs and salamanders)
- Osteichthyes (bony fish, such as
trout and goldfish)
- Chrondrichthyes (cartilaginous
fish, such as sharks and rays)
- Agnatha (jawless fish, such as
lampreys)
Mammals are the most
commonly used class of animals in research. They are vertebrates
that nourish their young with milk from the females' modified sweat
(mammary) glands and have skin that is (more or less) covered with
hair.
Order:
Several orders make up
each class. Some common orders of Mammalia used in research
include:
- Primates (monkeys, apes and
humans)
- Lagomorpha (rabbits and hares)
- Rodentia (mice, rats, hamsters
and guinea pigs)
- Carnivora (dogs, cats and
ferrets)
Family:
Several families make up
each order. Family names always end in -idae.
Families in the order Rodentia used for research include Muridae (rats
and mice), Cricetidae (hamsters and gerbils), Cavidae (guinea pigs) and
Chinchillidae (chinchillas).
Genus (plural- genera):
Each family contains from one to many
genera. Animals within the same genus are very closely related.
Genera within the family Muridae include Rattus (the
"Norway" rat) and Mus (the
"house" mouse).
Species:
A species is a group of animals that is reproductively isolated from all
other groups of animals; one species can't, in other words, breed
with another species. This is not an absolute, however, because
some similar species can interbreed (the horse and donkey, for
example, produce a mule when bred together). The species name of the
common laboratory rat is Rattus norvegicus (it can also be
written Rattus norvegicus).
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II The
Laboratory Rodents
 Recommended
readings: Follow the in-text links
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A. Order Rodentia
Characteristics:
The unifying
characteristic of Order Rodentia is their teeth. All rodents have
one pair of upper and lower incisors, a diastema and then at least one
pair of premolars or molars. Rodent incisors have enamel on the
anterior surface, but not on the posterior of the teeth. The
incisors grind against each other as the rodent gnaws, wearing down the
back of the tooth and forming a sharp edge at the front of the
tooth. This tooth structure, along with the incisors' open roots,
ensure that an effective gnawing surface is always available. If
the teeth do not meet properly, it can lead to malocclusion, as the
teeth grow past each other.
Other characteristics
shared by many rodents are a large cecum
and cecotrophy. A short gestation and large litter size are typical of most rodents.
Rodents in Research:
Over 90% of all
animals used in research are rodents, primarily mice and rats.
Guinea pigs and hamsters comprise approximately 2% each and the
remaining 1% are gerbils, chinchillas, degus, woodchucks and other
rodents.
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The Laboratory Mouse and Rat
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Taxonomy: |
Kingdom
Animalia
Phylum Chordata
Class Mammalia
Order Rodentia
Family Muridae
Genus Mus
Species Mus musculus |
Kingdom
Animalia
Phylum Chordata
Class Mammalia
Order Rodentia
Family Muridae
Genus Rattus
Species Rattus norvegicus |
Mice, Rats and Regulations:
Mice and rats
are not regulated by the USDA. New York State, the
Public Health Service, Food and Drug Administration and AAALAC, however,
do regulate laboratory rats and mice and this results in government (or AAALAC)
oversight of these animals.
The History of Mice in Research:
The first recorded use of mice as research animals occurred 1664 when
Robert Hooke studied the reactions of mice in experiments on air. Dr Charles
Little developed the first inbred strain, DBA (dilute, brown, non-agouti)
in 1909. The first albino strain was
developed by Halsey Bagg in 1915 for behavioral experiments; he named
his strain balb/c (Bagg albino). Nude mice were discovered in
Glasgow, UK, in 1962; these resulted from a natural mutation and were
immune defiicient. Recombinant DNA research
began in the 1970s, with the first successful transgenic mice produced in 1976. Knockout mice were first
created in 1987. In the mid-1990s, mapping of the mouse genome
began, with sequencing to be completed by soon.
Common stocks of outbred mice are
the ICR (which stands for the Institute of Cancer Research, where these
mice were developed), the NIH Swiss and the Swiss Webster.
Frequently used strains include the balb/c and C57/Bl6 (which is a black
mouse). Nude and scid mice are strains developed from natural
mutations. Hundreds (or even thousands) of transgenic and knockout
strains of mice have been created, and more are created daily
.The Role of Mice in
Research:
Mice are the workhorse of
research--the model biomedical mammal. Millions of mice are used
every year. Humans and mice are genetically very
similar, with
only a few dozen genes difference between us.
Mice are used as models
for many different diseases and for many different purposes. Some
mice are used for basic research--how the body works and the effects of
noxious stimuli on body function. Most mice are used for applied
research, however, such as:
The History and Role
of Rats in Research:
Like mice, rats are
excellent biomedical models and like mice, large numbers of rats are used
in research every year. Although we often hear about the "lab
rat", they are outnumbered in the research laboratory by
mice. The rat genome has
not been examined as closely as that of the mouse and fewer transgenic
rats are available. The most commonly used stocks of rats are the
albino Sprague Dawley and Wistar rats and the hooded Long Evans
rat. Inbred rats include the Fischer, Buffalo and Spontaneous
Hypertensive rat. Rats are used for many similar biomedical
purposes as mice. In addition, they are often used in basic
psychology research and training, for dental caries, obesity,
diabetes mellitus and hypertension, and the relationship between aging
and nutrition.
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Rat and Mouse
Statistics:
| Criterion |
Mouse |
Rat |
| Life
span |
1.5 - 3
years |
2 - 3.5 years |
| Adult
size |
15 (F)
- 40 (M) grams |
250 (F)
- 800 (M) g |
| Temperature |
98.0 -
101.5 °
F |
99.0 -
100.5 °
F |
| Pulse |
325 -
780 BPM |
250 -
450 BPM |
| Respirations |
60 -
220 RPM |
70 -
115 RPM |
| Estrous
Cycle |
4 - 5
days |
| Gestation |
21 days |
| Weaning
Age |
21 days |
Anatomy and Physiology of Rats and Mice:
Male rats exhibit a
prolonged period of growth and ossification of the long bones is not
complete until into their second year; this makes the male rat
significantly larger than the female. Mice do not show this wide a
sexual variation. The tail of both species is hairless and the rat's
tail is characterized by ringlike striations.
The fur of albino rats yellows as
they age. Male rats male have tan flakes throughout their fur;
this is sometimes mistaken for lice, but it doesn't move!
Mice tend to be primarily herbivorous; rats are natural
omnivores. They have a monogastric digestive system and are
hindgut fermenters. Most rodents, including rats and mice, are incapable of
vomiting. This inability to vomit is very useful: people,
dogs and cats must be fasted prior to anesthesia. If an
animal vomits when anesthetized, they may aspirate their vomitus,
resulting in pneumonia and even death. Rodents, however, do not
need to be fasted.
Rats and mice
have typical rodent dentition: one pair of incisors, no canines
and at least one pair of premolars or molars. Due to
open roots, malocclusion of the incisors may occur, but it is minimized by
continuous gnawing on their hard food pellets. When malocclusion
occurs, the teeth can be trimmed.
Most rodents
have exophthalmic (prominent bulging) eyes, and rats and mice are no
exception. The Harderian gland is located behind the eye and
produces a thin reddish lubricating fluid. When rats and mice
are stressed, the Harderian gland produces increased amounts of this
fluid, which overflows and stains the face and nose. This is
called chromodacryorrhea,
and is sometimes mistaken for blood.
The Jacobson's
organ located on the floor of the nasal cavity detects pheromones, an
extremely important method of communication between rodents, especially
mice.
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Reproduction:
Mice and rats
are very prolific: they produce many young very quickly.
Female rats and
mice have nipples associated with their mammary glands...males do
not. The female is continuously polyestrous with minimal seasonal
variations, particularly if she is exposed to a natural light cycle of
12 hours light and 12 hours of darkness. The estrous cycle of both mouse and rat is 4 - 5 days
long, with estrus lasting about twelve hours.
A vaginal (copulatory)
plug is formed in the female mouse or rat's vagina. This is a
straw-colored mucoid plug formed from secretions in the semen of the
male rodent. It confirms mating--not pregnancy--and is usually found on the floor
of the cage.
After a
gestation of approximately 21 days, the female gives birth to a litter
of 7 - 11 pups. These altricial pups are sometimes called
"pinkies"--they look like little pink rubber erasers.
They have no fur and their eyes are ears are closed. The maternal
instinct of rats and mice is strong, although rats will frequently share
nursing duties with other females in their cage. Cannibalism is
rare, but may occur if the mother and newborns are disturbed in the
first few days of life. In a week, fur begins to grow
on the pups, at two weeks their eyes and ears open and they are weaned at three weeks.
The inguinal
canal of male rodents, through
which the spermatic cord passes from the abdominal cavity to the
scrotum, stays open for life and a threatened rodent is capable of
retracting the testicles to the relative safety of their body.
Testosterone results in a strong odor in the urine, especially of male
mice.
A monogamous or
polygamous mating system can be used, depending upon the need to
document the exact parentage of the offspring. The colony mating
system, in which the males and several females are housed together
continuously, is commonly used.
The Whitten,
Bruce and Lee-Boot effects can all occur, especially in mice (see
the Animal
Reproduction notes for a review).
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Temperament:
In the wild state, rats and mice are prey animals. They are
timid, nocturnal and like to burrow. In captivity, rats are
very social and affectionate.
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Aggression is frequently seen among male mice. Intermale
aggression is a serious problem, particularly in balb/c mice that were not
raised together. It may be necessary to separate males to protect
the less dominant males; it may be difficult to return a separate rodent
to its original cage. A dominant mouse will bite or chew
the fur off of subordinate mouse, usually in the facial area; this
is called "barbering." Another sign of bullying in a
cage of mice is bite wounds on the tail and hind quarters of the
submissive mice. Severe infections and even death can occur.
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Technical
Procedures:
Rats and mice
are easier to handle if they have been socialized. A few minutes
spent gently handling a rodent will make the subsequent procedures less
stressful for both of you!
The Animal
Welfare Act does not require individual identification of small rodents,
but it is useful to be able to identify the individual animals being
used in an experiment. Ear notching is easily done and usually
reliable. Ear tags can be used, although they are sometimes
lost. Nontoxic dyes and markers can be used for temporary
identification, but grooming will remove all traces within a few
days. Subcutaneous transponders (microchips) are available, permanent and may
be used to determine biologic parameters, such as body temperature.
There are
numerous sites from which to collect rat and mouse blood. The
lateral tail vein can be used either to collect a small quantity of
blood or to give an intravenous injection. The saphenous vein
will also yield a few drops of blood, and the tail tip can be amputed
and blood collected. The jugular vein and retro-orbital venous plexus
are used for collection of large volumes of blood. The maximum volume of blood is
available via cardiac
puncture. This should only be performed on an anesthetized
rodent; after collection, the rat or mouse should be humanely
euthanized.
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Husbandry:
Rats and mice are
typically housed in metal or plastic "shoe boxes" with slotted metal
lids. Filter cage covers are used to reduce airborne transmission
of microbes and to minimize the change of disease.
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Solid flooring with direct
contact bedding is preferred. If a suspended
wire floor is used, care must be taken to ensure that limbs are not
fractured because of too close of a mesh and young are not lost between
the wires or dehydrated due to lack of a nest.
Rodents do not have sweat
glands and are unable to pant. This makes them prone to heat
stress if the cage or room temperature is improperly
regulated. The acceptable temperature range for rats
and mice is 64 - 79 °
F. Humidity should be maintained between 40 - 70%. Room temperature and humidity may not adequately reflect the
temperature and humidity in the animal's cage! If
neonatal rats are housed in humidity less than 20%, they may develop
"ringtail", permanent annular (ring-shaped) constrictions on
their tails. Necrosis and sloughing may occur.
Pressed
paper, corn cobs and hardwood shavings can be used as bedding.
Cedar shavings should not be used, because they contain oils that can
cause reproductive, liver and
respiratory problems. Pine shavings also contain oils and
should be used with caution.
There should be sufficient bedding for the rats or mice to burrow and
form nests. Nesting material, such as shredded paper, or
nest boxes provide enrichment
and entertainment.
Mice
and rats are fed herbivorous diets in the laboratory. The form of
the diet varies with the requirement of the experiment, but rodents are
typically fed a high-quality hard pellet. Both rats and mice are
cecotrophic, eating their feces to gain the vitamins produced during
hindgut fermentation. Rats are neophobic--they are cautious
feeders and may avoid strange food, including a new diet specified in an
experimental protocol. Water bottles or automatic watering systems
can be used.
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Health
and Disease:
The
goal of the research facility is to prevent disease from
occurring: prevention through good husbandry is the best treatment
for any disease!
Health
problems in research rodents are often diagnosed postmortem.
Infections can be subclincal (no visible signs of disease), but still affect the outcome of an
experiment due to immune suppression and disruption of normal
physiology. Because of the potential of widespread disease in a
laboratory population, unhealthy rodents are frequently culled (euthanized).
There
are many important diseases of rats and mice: these notes just
provide a glimpse of what the rodents may acquire.
Several
bacterial diseases are important in rodents.
Tyzzer's disease
affects the liver and gastrointestinal tract of rodents and rabbits. It
is caused by Clostridium piliformis which is an obligate
intracellular bacterium. This disease can be precipitated by stress due to
overcrowding, high temperature and humidity, or experimental procedures.
Transmission is by the fecal-oral route, esp. if food is contaminated by
feces from infected animals. Tyzzer's may produce no clinical
signs...or it may result in hemorrhagic diarrhea, lethargy, weight loss,
rough hair coat and death. Diagnosis can be difficult and
treatment is discouraging. Antibiotics may suppress an outbreak, but culling,
sanitation and repopulation is more effective. Prevention is the
best treatment!
Salmonella
enteriditis is a bacterium that causes salmonellosis in rodents and many other
animals,
including man. Man may be the donor or recipient of an
infection! Like Clostridium, Salmonella is primarily
transmitted by the fecal-oral route. Some rodents may become
carriers, not ill, but shedding the bacterium and spreading the
disease. Clinical signs include lethargy, anorexia and weight
loss, ocular discharge, dull hair coat, hunched posture and
diarrhea. Diagnosis is made by culturing the bacteria. Because of the public health
concern, infected rodents should be culled and the premises
sanitized before the animals are replaced.
Numerous
viruses infect rats and mice. Mousepox (ectromelia) is an
Orthopoxvirus that only affects mice. It is transmitted as a
respiratory aerosol, through skin abrasions, by contact with skin
debris or through ingestion of food or water contaminated with feces
(fecal-oral route). In chronic (slowly developing) infections, a
papular rash with swelling, ulceration and eventual amputation of
appendages occurs. Mice with an acute (rapidly developing) infection have a high morbidity
(sickness) and mortality (death rate). They have a hunched posture, rough hair coat,
conjunctivitis, swelling of their face and extremities and
diarrhea. Diagnosis is made the clinical signs,
laboratory testing and necropsy. There is no treatment.
Epizootic
diarrhea of infant mice (EDIM) and infectious diarrhea of infant rats (IDIR)
are caused by a Rotavirus. Transmission is fecal-oral or by
aerosols. It is highly contagious, and filter covers were
developed to prevent the aerosol transmission of EDIM.
Typical signs are diarrhea and stunted growth. Treatment is
supportive, not curative.
Sendai
viral pneumonia occurs in rats, mice and many
other rodents. It is caused by a
Paramyxovirus and is spread by direct contact, contaminated
fomites and aerosols. It is extremely contagious and can cause an
acute respiratory infection, with dyspnea (difficulty breathing) and
congestion. Weight loss, anorexia and a hunched posture is also
seen. Infected animals are
usually culled.
Rodents
in contact with wild rats and mice or that come from facilities with
inadequate husbandry may be infected with Polyplax spinulosa,
the spined rat
louse. This is a highly host specific parasite,
only infesting rodents, never man. It is spread by direct contact
or through fomites. Scratching, alopecia and epidermal scaling are
early signs. Because this louse is a blood-sucker, severe anemia
and death may result.
The
most common neoplasm in rats is the mammary
tumor. These tumors are usually benign and generally occur in rats that are at
least 18 months old. The tumors can be surgically removed, but often
reoccur.
There
are a number of public health concerns associated with rats and
mice. Rats (and to some extent mice) have powerful allergens in
their dander and urine. Laboratory-associated allergy and asthma
is an important concern for animal care personnel, and proper protective
equipment, such as gloves and face masks, should be used.
Zoonoses
that may be acquired from laboratory rodents include:
- Salmonellosis
- Streptococcal infections
- Tapeworm infection (Hymenolepis
nana and H. diminuta)
- Haverhill (rat bite) fever
- Hanta virus (from wild rats and
mice)
Small
rodents are not a source of rabies.
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Study Guide- Taxonomy, Rats and Mice
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Define taxonomy.
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Why is taxonomy important?
-
List the taxonomy groups in order from largest
(broadest) to smallest (narrowest), beginning with KINGDOM.
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What is the correct format for writing a scientific
name?
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The animals used in the Medaille College Vet Tech
program all belong to which:
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Kingdom
-
Phylum
-
Class
-
What is the most commonly used order of animals in
research?
-
What characteristic(s) unify this order?
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What is the taxonomic classification of the laboratory
mouse from Kingdom to species (and be sure to write the species
(scientific) name correctly!)?
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What is the taxonomic classification of the laboratory
rat from Kingdom to species (and be sure to write the species
(scientific) name correctly!)?
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Describe important characteristics of the rodent
gastrointestinal (GI) tract.
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Can rats and mice vomit? Why is this an
advantage?
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If a client brings a rat to your veterinary hospital
with the complaint that it's "crying blood," what is
probably wrong with it (hint: it's not blood!) and why does it
occur?
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Complete this chart:
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Characteristic |
Mouse |
Rat |
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Monestrous or polyestrous? |
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Seasonal or nonseasonal? |
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Spontaneous or induced ovulator? |
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Type of mating system? |
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Length of gestation? |
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Altricial or precocial? |
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Age at weaning? |
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What are the results of the Whitten, Bruce and
Lee-Boot effects on small rodents and which species of rodent is most
affected?
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Intermale aggression is more common in which species
of small rodent? What signs indicate that aggression is
occurring?
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What are the Animal Welfare Act standards in regard to
identification of rats and mice?
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What sites can be used to collect blood from rats or
mice?
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Why are solid floors and direct bedding preferred for
rats and mice?
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What problem occurs in a rat breeding facility if the
humidity is too low?
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What is the relationship of disease in a rodent
facility to experimental outcomes?
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What is the etiology (cause) of Tyzzer's disease, what
are the signs of a clinical infection and what factors contribute to
the development of clinical disease?
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What is the importance of salmonellosis in a research
facility?
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What viruses may cause diarrhea in rats and
mice? may cause pneumonia?
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What is the most common cancer of rats?
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