Raising laboratory mice and rats

Historic, archived document

Do not assume content reflects current
scientific knowledge, policies, or practices.

PEArEET NO} 253
U. S. DEPARTMENT OF AGRICULTURE

Raising Laboratory Mice and Rats

By ORSON EATON, animal husbandman, and CuHaRLES A. CABELL, assistant
chemist, Bureau of Animal Industry, Agricultural Research Administration

INTRODUCTION

The mouse is probably one of the most widely distributed animals in the
world. It has world-wide distribution, having followed man to all of his
environments. The mouse probably originated in central Asia, spread to
other portions of Asia and to northern Africa, then to central and northern
Europe. The mouse came to Mexico and South America on the ships of the
southern European explorers, and to the United States and Canada from
northern and central Europe with the early colonists.

The mouse was known to the Aryan tribes of Asia 4000 B. C. Pottery de-
picting mice has been found in Egypt dating back to 2000 B. C. A cult in
Asia Minor, northern Africa, Greece, and some of the neighboring islands
worshiped the mouse for nearly 3,000 years. Mice were raised in the temples
to be used in connection with worship. The mouse was depicted on many
ancient coins. The mouse is not native to Japan, but stories of it exist in
Japanese folklore. Chinese priests made use of the mouse for many centuries.
During the nineteenth century European zoologists bred mice for use in the
study of inheritance of color varieties. ‘Their results remained uninterpreted
until after the rediscovery of Mendel’s laws in 1900.

Production of the albino rat has become a subject of wide interest, prin-
cipally on account of its many virtues as a laboratory animal. Experimental
techniques involving the use of these animals have been employed for many
years, but extensive use of the rat began with modern scientific methods of
investigation. Experiments with vitamins, hormones, and foods and drugs
concerned with animal and human well-being and growth require large num-
bers of these animals.

The wild gray rat can be tamed and handled, and is sometimes used for
special laboratory work. However, various strains of albino and piebald mu-
tants which are more docile and less easily frightened by strange sounds and
movements are being produced more extensively for experimental and teach-
ing purpose:.

DESCRIPTION

Two species of mice of the family Muridae have been domesticated: Mus
musculus and M. bactrianus. The former is the larger of the two species, has
a longer tail, and differs in skull proportions from the latter. The belly is
gray. WM. bactrianus has fewer young per litter and has a white belly. The
two species cross readily and the offspring are fertile.

The white rat (Mus norvegicus albinus) is a member of the subfamily Mur-
inae of rodents. The genus Mus, including rats and mice, contains about
130 species. They are all exclusively of Old World origin. ‘Two species of
rats have migrated to the United States in large numbers from Europe. First
came the black house rat (M. rattus), which has been supplanted largel by
the brown or Norway rat (M. norvegicus). This latter species now is found

Issued May 1949
834082°—49

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9 LEAFLET 253, U. S. DEPARTMENT OF AGRICULTURE

widely distributed throughout the globe, no doubt largely because it is readily
transported by man. Most laboratory rats are probably mutants and domes-
ticated strains of this species. M. rattus and M. norvegicus are mutually in-
fertile, but available albino strains breed freely with the latter species. Com-
parisons of hemoglobin crystals and skull shape confirm their relationship.

The albino rat has undergone modifications and the various strains which
have been more or less inbred in different laboratories over a period of years
show conspicuous differences in size and other characteristics important in
animals for laboratory use. The albinos now composing the colonies in vari-
ous laboratories are not strictly homozygous, even from the standpoint of
color, since crosses with pigmented forms give offspring with color markings
which could have been derived only from the albino parent.

USE OF MICE AND RATS

Because of its fertility, prolificacy, convenient size, short gestation period,
variations, inexpensive maintenance, resistance or susceptibility to certain
diseases, and ease of production, the mouse has become a favorite research
animal. Its variability has made it a valuable animal for genetic research, and
it has been more intensively studied in this connection than any other mam-
mal. It is widely used in medical research, especially in work on cancer. It
serves as a medium in which the pathologist cultures disease germs and the
physiologist studies yarious life processes, including the interaction of the
hormones involved in reproduction. The embryologist and anatomist have
learned much about the early developmental processes of mammals in general
from the mouse. Thus the mouse has contributed in wide measure to the
biological sciences.

The rat has shared with the mouse its reputation as an animal for re-
search. It has been used more extensively in the field of nutritional investi-
gations than the mouse. It maintains an important place in the study of
hormones and in the testing and standardizing of drugs. It also has been
a favorite animal for use in animal-psychology experimentation because of
its relatively high learning ability as compared with other small laboratory
animals.

THE LABORATORY MOUSE

Reproduction and Growth

Ovulation occurs in the mouse at fairly regular intervals of 3 to 4 days, and
is not dependent on copulation as it is in the rabbit and some other animals.
The female’s first estrus occurs at 30 to 40 days of age. The sexes should be
separated prior to this. Estrus reeccurs 6 to 24 hours after a female gives
birth to a litter and continues for 12 hours or less. Gestation normally lasts
19 to 21 days except in females which are nursing a litter, in which case it
may be lengthened from 6 to 16 days. A common practice in the manage-
ment of a mouse colony is to place three to six females in a cage with a male.
After 15 to 18 days the females should be eXamined for signs of pregnancy,
manifested by greatly distended abdomens. ‘Those females «which are preg-
nant should be removed to separate cages, each female in an individual cage.
Females usually make a nest in which the young are placed and Kept until
they are able to run about. The average number of young is 6 to 8, but
litters of 12 to 15 are frequent. A mouse at birth weighs about 1 to 1.5 grams,
depending on the number in a litter, the young in smaller litters having the
greater individual weight. At 4 months of age a mouse weighs about 25
grams, slightly less than an ounce.

At birth the mouse has no hair except the vibrissae, or hairs about the nos-
trils; no pigment in the skin except in the iris of the eye, visible through closed
eyelids; and the ears are thick, bent forward, and attached to the face and
cheek. After 2 to 3 days hair appears, at 4 to 6 days the ears detach from the
face, and at 8 to 10 days the mouse is fully haired. The incisor teeth erupt
at 11 to 13 days and the eyes open at 14 to 16 days. At this time the young

RAISING LABORATORY MICE AND RATS 5

mouse leaves the nest and begins to eat solid food. Young mice may be re-
moved from their dam at 3 weeks of age and the dam rebred. A female is not
usually productive after 15 months, but may live much longer. Male mice
have been known to live 214 to 3 years. :

Care and Management
HOUSING

The temperature in the building in which mice are housed should be kept
at 70° to 80° F. Dampness and drafts should be avoided. ‘There should be
good ventilation, however. A variety of types of cages are used, depending on
the needs and fancy of the individual. Cages should be of metal, however, to
stand sterilization at least monthly by steam or by dipping in a solution of
disinfectant. For general purposes the “shoe box” type of cage is satisfactory
(fig. 1). The sides and bottom are of sheet metal. The dimensions are
7 by 12 by 5inches. ‘The cover fits like the cover to a shoe box. A rectangular
area in the cover is cut out and the metal replaced with %4- to 34-inch mesh
hardware cloth. Some absorbent material such as shavings, sawdust, or
peanut hulls should be kept in the cages to absorb moisture. Pregnant
females should be provided with shredded paper for nesting material. A
Y%-inch mesh hardware-cloth feed cup is hung over the edge by hooks so
that feed is before the mice at all times. This type of feed cup can be used
only with the compressed dog biscuit type of food. A glass drinking tube of
6- to 8-millimeter bore extends from an inverted water bottle, into the cage,
through a reinforced opening in the top (fig. 1).

Figure 1.—A metal ‘‘shoe box’’ mouse cage for general use
having ¥2-inch-mesh hardware cloth in the cover. From
the inverted water bottle a glass tube extends into the cage.
The feed cup is shown at right.

When used to house breeding animals, this cage will accommodate a male
and three to five females. This type of cage is also suitable for housing preg-
nant females and females with their litters to weaning age. Asa holding pen
for weaned mice, it will accommodate 10 animals.

Another popular type of cage has the same dimensions as the one just
described except for the depth, which is 7inches. The sides are made of %4-
to ¥g-inch mesh hardware cloth. The top of the cage slopes so that the front
is about 2 inches lower than the rear. In the top of the cage is a metal door
which is slightly smaller than the top. The door is held shut by the weight of
the water bottle, which is clipped to it. The water bottle is of the same type
as that used with the “shoe-box” type of cage. The glass drinking tube ex-
tends from the bottle into the cage through a small circular hole in the metal
door. This type of cage has no bottom, but is set in a galvanized iron pan
342 inches deep, two cages to a pan. Sawdust or shavings are placed in the
pan to absorb moisture. When the cages are to be cleaned, the mice are

4 LEAFLET 253, U. S. DEPARTMENT OF AGRICULTURE

simply transferred by hand from the one to be cleaned to a clean cage. It is
well to have enough cages or pans so that one set may be sterilized with steam
or by dipping in a solution of disinfectant while the others are occupied.
Both types of cages may be arranged on racks five to six tiers high.

FEEDING

A convenient food for mice is the compressed dog biscuit, which is well bal-
anced as it contains minerals and essential food elements required by mice.
A satisfactory mixture also is one composed of 240 parts of rolled oats, 30 of
powdered skim milk, 8 of cod liver oil, and 1 of salt.

Various other grains such as ground barley or yellow corn meal may be used
in addition to the ones named in the mixture given above. Such foods are
fed in a can. Various arrangements may be made for preventing the mice
from wasting and contaminating the food.

A piece of carrot, potato, or green leaf of cabbage or kale may be fed oc-
casionally. These should be clean and free from decay.

THE ALBINO LABORATORY RAT

Reproduction and Growth

The rat will reproduce at all seasons, but larger numbers and more vigorous
animals are generally obtained in the spring months. Reproduction is lowest
through the late fall and winter. The estrous cycle of the rat is 4 to 5 days,
and the gestation period approximately 21 days.

The young are born with closed eyes and ears and without hair. Within
21 days they may be weaned and will develop normally when fed a suitable
diet such as one of those given in the section on feeding. Although they vary
some in the time required to reach sexual maturity, the average animal is
mature at 70 to 80 days. It is generally considered good practice to delay the
mating of these animals until they are about 4 months of age.

One satisfactory method of mating the animals is to place several females,
up to five or six, in a cage with one male, and remove the females to individual
cages when they show signs of pregnancy, such as a plug, rapid increase in
weight, or sign of erythrocyte, or blood clot. The plug consists of a mass of
seminal fluid which has coagulated in the vagina of the female after copula-
tion. It is expelled after a few hours and may be seen in the litter on the
floor of the cage. The plug is not always a sure sign of pregnancy, as the
fetuses may have died before term, but rapid increase in weight, especially
near the end of the gestation period, is usually a good sign of pregnancy.
However, if the number of fetuses is small, the increase in weight will be
scarcely noticeable. The most certain sign of pregnancy, and that which is
routinely used for determining pregnancy in the rat, is the presence of an
erythrocyte, or blood clot, in the vagina on the thirteenth day of pregnancy.
This may be large enough so that it protrudes and is visible from the outside;
or the edges of the vulva may be spread slightly with forceps in order that it
may be observed. After the end of the lactation period females should be
rested several days, up to about 2 weeks, before remating.

With this method females generally produce four to six litters and then
usually are discarded. They are so near to menopause (about 15 months of
age) that reproduction is uncertain.

Considerable variation occurs in the number of young to the litter. From
1 to about 10 is the usual number, the average being between 6 and 8. Ifthe
large litters are reduced to about 7 at birth the young develop better and the
strain of lactation on the female is less.

Other conditions being satisfactory, this procedure should yield about half
as Many young per week as there are female breeders in the colony.

Sexing of young rats at weaning age and even earlier is often necessary.
The most dependable external character by which sex may be determined at
birth is the distance from anal opening to genital papilla. This distance is

RAISING LABORATORY MICE AND RATS 5

always greater in a male than in a female of the same age. In animals 21
days of age the developing scrotum and buttonlike appearance of the genital
papilla may serve to distinguish the male.

Care and Management
CAGES

The rat will generally gnaw soft materials; consequently metal cages are
the most satisfactory for housing these animals. Cages about 20 inches square
and 12 inches high with removable screen bottoms and dropping trays are
satisfactory for maturing and adult animals (fig. 2, right). However, wire
floors are not suitable for young animals from birth up to weaning age.

Pregnant females should be transferred to metal cages which may be some-
what smaller. These cages should contain litter such as wood shavings or

Figure 2.—Two types of cages used for rats. The cages at the left have no bottom, but are
set in a pan 11/4, to 2 inches deep. This pan is bedded with shavings, sawdust, or other
absorbent material. When the pan needs cleaning, the cage is lifted to a pan of fresh
bedding material. The cages to the right have wire bottoms, and the droppings fall
through to the pan underneath, which contains absorbent material. The pan may be
removed for cleaning. The sides and back of this cage are of solid metal. Record cards,
held by clips, slide into rectangular holders on the door.

sawdust to absorb moisture and provide nesting material. The practice in
some laboratories is to use this type of cage altogether, but the screen-
bottom type with tray underneath to catch the droppings is more convenient
to clean, as removable paper can be placed in the tray. Breeding rooms
should be well ventilated but free from drafts. Temperature should be
maintained at 70° to 80° F. and preferably as close to 75° as practicable.

Water bottles and feed cups used for rats are not materially different from
those used for mice. :

In handling rats, it is well to wear gloves heavy enough that the rats can-
not bite through, or to use tongs or forceps. Rat bites may be painful and
a source of serious infection unless these precautions are taken, nr

LEAFLET 253, U. S. DEPARTMENT OF AGRICULTURE

FEEDING

The rat is capable of thriving on natural foods of considerable variety. Its
natural diet is similar to that of human beings, and is derived principally
from human-food sources. ‘Table scraps probably were the first diets used for
raising these animals. This source of food is variable and undependable for
more than a few animals, and is not satisfactory for feeding a rat colony
where uniform animals are desired. Nutrition of the rat has been studied
probably more thoroughly than that of any other animal; however, doubt
exists that it is possible to formulate diets that will produce maximum growth
and reproduction in these animals. Numerous combinations of natural foods,
however, will produce good results. The purpose for which animals are to
be used is generally the chief factor in determining a suitable diet. For
example, a standardized animal for use in vitamin aSsays must be produced
on a diet with restricted vitamin content. The vitamins to be assayed must
be present in the diet in sufficient quantities for growth, reproduction, and
lactation, but must be limited in amount so as not to allow storage of the
vitamins in the bodies of the young animals.

A stock ration of this type is generally composed of a mixture of cereal
grains such as corn, wheat, and oats, usually in about equal parts, or only one
or two of these grains may be used. This grain mixture usually composes
from 60 to 80 percent of the diet. The remainder is made up of various
vegetable and animal products intended to supplement the protein, mineral,
and vitamin content of the diet. Among the ingredients often used are mill
concentrates such as wheat germ, alfalfa meal, linseed meal, fish meal, milk
powder, liver meal, calcium carbonate, and trace minerals. Table 1 gives
examples of diets which, with slight modifications from time to time, have
been used successfully for raising albino rats for many generations.

Diet No. 1 is a type that may be fed to produce animals which will be Satis-
factory for vitamin A and B complex assays. Diet No. 2 probably approaches
optimum nutrition more nearly but results in faster growing animals which
are unsuitable for vitamin assays.

A less complicated procedure recommended, especially when only a few
animals are to be raised, is feeding only the amount the animals will eat
of one of the prepared dog feeds produced by commercial feed manufacturers.
This may be supplemented with such vegetables as lettuce and carrots.

TaBLE 1.—Suggested diets for laboratory rats

Diet No.—
Ingredient ;
fle 2
Percent Percent
Grounds yellow, Corns ane eae oe We ae a 36. 00 25. 63
GrounGiwheats. jewel ewes Cea Waren: tee os learns 36. 00 25.68
Afaliarlea tim calle wyatt Saree ee ee eas ip eine eae ae 3: 00 ce ee ie
IMSeSC meal es NE en es MR EN a io ce ee 1000) 23288 eae ee
sine ime a beatles ante a a ae Ca eee, ROR ett i eeaetias weap Ouea tee 3; 00 ese
pies eh oa Ces 9 ere a Pan ae, Oe Da set mS PS ei ND Bie Sec: nif OO hain eer ne OC
Commercial’ Casein =e oe tk ae eae gs ete: meaner ever 4.00 |e Se
ed eee Se Sa 8 en ne ata: EBs iO tee NR Ie eas Ae. ean ea ee BNL oO 1. 00
Calcium carb Oma te eet et Sy ieee he a eee pres pel ee fo) 1. 00
Skim ymil ko wl ers. ae So ye a A ee ere aie 4.00 Se
East i (TE WETS). se ee Sees ae Eo cee 8 2. 00 3. 00
EY) Us TG Levoyz i= ee a ee a I Pn eels trea une eee a eae eo er 25: 63
Irradiated yeast so) Sie Ta See Bae ae ape Sea a |g 2. 00
Soditrma phosphate ss sk ce Se se ie a ae ee hg pee eee 1. 00
Potassium iodide. eS ee ise as peed | alg ec SOx
Manganese sulfates 25752 ck Se eS ah Tra in Sea . 10
Dried* peeks ss hye 0S ee EE EO Ae Pe Eh ee | aera 10. 00
Driedehoe livers... 8 ae ee aes Ces |S ee eee 5. 00

1 With diet 1, fresh whole milk is fed separately 3 times a week.

RAISING LABORATORY MICE AND RATS 3 7

RECORDS

Some system of marking animals for identification and a book in which to
keep the records are often desirable so that the mice and rats in each litter
may be distinguished. The type and elaborateness of records will depend
upon the purpose for which they are used. For some purposes recording only
sire and dam and date of birth of a litter may be sufficient. In other cases
more detailed records may be desired. For identification of individual ani-
mals, systems of ear notching or toe clipping or a combination of both may
be used. The young may be marked when 15 days of age. A punch of the
kind used for marking chicks is used for punching the ears. The toes are
clipped with small scissors. Usually the bleeding is slight. The bloody areas
may be wiped with a piece of cotton saturated with iodine, mercurochrome,
or 50-percent alcohol. A numbering system used by the genetics office of the
Bureau of Animal Industry is shown in figure 3. By this system, numbers
from 1 to 12,999 may be used as a means of identification.

“
Bek 3\5
Rr
7 oa Z
6: /
900 NeHOS:

P00" yoo 500 50 up?
700 (300 ee 0S Me
200 20
600 [60
400 70

4000 4/000
5000 de 2000
6000 3000
0,000 7000

11000 ¢. S &000
9000

12000

Figure 3.—Diagrams of feet and ears
showing a method of numbering by
which mice may be identified by toe
clipping and ear marking. This method
may be used for rats also.

DISEASES

In several instances the mouse and rat are susceptible to the same in-
fections, though in varying degree. In the following discussion of the more
common diseases of these rodents, the species affected and degree of suscepti-
bility are given.

Bacterial Diseases

Paratyphoid.—This disease is caused by two species of bacteria, Salmonella
typhimurium and S. enteritidis, singly, together, or with other organisms.
Wild mice and rats as well as laboratory stocks of these animals are sus-
ceptible. Infection takes place usually through the mouth, by eating food
contaminated by carriers of the organism. Carriers within the laboratory
colony sometimes serve as a source of infection. Three to six days after
infection, mice and rats usually develop conjunctivitis and diarrhea. In
animals dying of the disease, the viscera are congested and the liver, spleen,
and mesenteric lymph nodes are enlarged. The intensity of these conditions
depends largely on the duration of the disease. Mortality may exceed 75

8 LEAFLET 253, U. S. DEPARTMENT OF AGRICULTURE

percent. Control of Salmonella infection is difficult. Generally it is ad-
visable to destroy the entire lot, and clean and disinfect the cages and prem-
ises before obtaining new stock. If the animals are too valuable to sacrifice,
they may be divided into small isolated groups and the diseased ones de-
stroyed. Cages should be sterilized. Sick animals that recover may act as
carriers and infect the rest of the colony. Surviving males will usually be
sterile as the disease causes pathological changes which prevent the descent
of the testicles.

Hemorrhagic septicemia.—This disease of mice and rats is caused by Pas-
teurella muracida. Infection takes place mainly by contact with sick animals
or carriers which discharge the organism in nasal and conjunctival secretions.
Organisms also are air-borne or may be carried in food. The symptoms in
acutely affected animals are depression, ruffled coat, loss of appetite, dis-
charge from eyes and nose, and rapid respiration. Death occurs sometimes
in afew hours. Infection may cause death even before signs of sickness are
noted. At post mortem examination, the viscera are found to be congested
and numerous capillary hemorrhages are present on the moist surfaces. In
chronic cases, there may be enlargement of the lymph nodes and small, light-
colored areas of necrosis in the liver. The mouse is highly susceptible; the
rat is more resistant. For control, all affected animals should be removed
from the colony and killed. The cages and premises should then be cleaned
and disinfected. Highly resistant stocks should be selected for breeding.

Pseudotuberculosis——A form of this disease, caused by Pasteurella pseudo-
tuberculosis, is sometimes epizootic in rats. Infection takes place through
the intestinal tract. Another form of pseudotuberculosis which occurs in
mice is caused by Corynebacterium kutscheri (murium). Itis a rather infre-
quent chronic disease manifested by tuberclelike lesions in the lungs; liver;
lymph nodes of the cervical region, the mediastinum, and the mesentary; and
isolated nodules in the spleen and kidneys.

Mouse septicemia.—This is a relatively infrequent disease of mice caused
by Erysipelothrix muriseptica. Conjunctivitis, pneumonia, edema of the ab-
dominal tissues, enlargement of the spleen. and small pale gray areas of
necrosis in the liver are the chief characteristics of this disease. The organ-
ism causing this disease cannot be distinguished from E. rhusiopathiae which
causes erysipelas in swine.

Pyogenic infections.—Mice and rats may develop abscesses in the sub-
cutaneous tissue of the head, neck, and other parts of the body. Streptococci,
staphylococci, and other organisms are causative agents and usually gain
entrance through surface wounds. Affected animals should be destroyed
and the cages disinfected. Streptobacillus moniliformis has been found in
the nasopharynx of apparently normal rats but is not responsible for any
serious disease problem in these animals. It is highly fatal in mice, however,
causing abscesses in the joints and internal organs.

Pneumonia caused by pneumococci and Friedlander’s bacilli has proved
highly fatalin mice. The disease is rare and usually due to unusual circum-
stances. Brucella bronchiseptica sometimes causes pneumonia in mice and
certain pulmonary diseases in rats.

Virus Diseases

Infections of the nervous system.—Encephalomyelitis, a somewhat rare dis-
ease, is manifested by paralysis of the hind legs, but not the tail, of mice. It
occurs chiefly in young animals.

Meningoencephalomyelitis is also manifested by paralysis of the hind legs.

Choriomeningitis in mice usually occurs as a latent infection without symp-
toms, although infections resulting in noticeable illness have been reported.
Man is also susceptible to the virus.

Infection of the respiratory apparatus.—Mice can be experimentally infected
with many types of pneumonia virus, but spontaneous illness caused by these
viruses is rare,

RAISING LABORATORY MICE AND RATS 9

Systemic infections.—Infectious ectromelia is an acute disease of mice, and
often becomes chronic. In the acute stage polyserositis and acute hepatitis
are present. In the chronic stage, edema, vesiculation, ulceration of the skin
of the feet, and even sloughing of the feet may result.

The salivary gland disease of rats and mice affects the ducts, salivary glands,
and sometimes the kidneys. Inclusion bodies are present in the hypertro-
phied cells of these organs.

Fungus Diseases

A white ear fungus sometimes grows on the ears of mice. It is difficult to
eradicate. If infection is not great the affected portion of the ear may be
clipped off. Sodium subsulfate powder will stop bleeding. Tincture of iodine
or a 5-percent alcoholic solution of salicylic acid will hasten recovery. Badly
infected animals should be killed.

Cancerous Growths

Hereditary tumors, affecting the mammae particularly, are common in cer-
tain strains of mice. These occur mostly in aged animals. Strains of ani-
mals have been bred for high and for low incidence of various types of tumors.
These strains have been used extensively in research on cancer and tumor.

PARASITES

Mice and rats are often parasitized by one or more species of coccidia, but
usually these animals are not seriously affected by the organism.

Rats and mice are often parasitized by the larvae of the tapeworm of dogs
and cats. These larvae are introduced into the animal colony in bedding or
food that is contaminated by the feces of dogs or cats.

Fleas, mites, and lice sometimes infest the mouse or rat colony, but seldom
prove fatal. Pyrethrum powder or pulverized tobacco dusted on the animals
will usually keep these pests under control. Dipping the animals in a warm,
very dilute solution of livestock dip and drying in a warm room are also
effective. Cages should be dipped in a solution of cresylic disinfectant.

SELLING AND SHIPPING

Several of the large institutions that regularly use rats and mice now main-
tain their own animal colonies. However, many institutions depend upon
dealers for regular supplies of strains of these animals standardized for spe-
cial use. There are several producers of standardized strains throughout
the country. Names of dealers and of reference books may be obtained on
request by writing to the Bureau of Animal Industry, United States Depart-
ment of Agriculture, Washington 25, D. C. Persons who require large num-
bers of rats and mice prefer to purchase from well-known dealers or institu-
tions because they are assured of a constant supply of uniform and standard
stock.

Many small producers of laboratory animals sell direct to the institutions
using them or to dealers who gather up stocks to be sold to the laboratories.
Individuals raising these animals should also communicate with the large
hospitals and laboratories in their immediate area, as frequently such insti-
tutions are desirous of dealing with a dependable producer close at hand.

The Department of Agriculture does not purchase guinea pigs, white mice,
or white rats, and has no animals for sale or for free distribution. Persons
expecting to make large profits from the raising of laboratory animals should
be cautious about expending too much capital, since this, like other enter-
prises, is subject to disappointments and failures. Most of those who have
succeeded have probably done so only after years of patience and experience.

Prices of mice and rats vary, depending upon supply and demand, special
purposes for which they were developed; size, age, and various other factors.
Mice for laboratory use are usually sold from about 3 weeks of age and up-
wards and may bring 6 to 25 cents each. Breeding stock is usually sold in

10 LEAFLET 253, U. S. DEPARTMENT OF AGRICULTURE

pairs, or in trios of one male and two females. The price requested by
breeders also varies. Rats for ordinary use vary in price from 75 cents to
$1.50 apiece. Some strains developed for special use may sell for as much as
$2.50 to $3 each. A sexually mature rat weighing about 200 grams sells at
approximately $1.20. As with mice, breeding stock sells at a higher price.

Mice and rats may be shipped safely for considerable distances if properly
crated and if provision is made for food and water. Shipments may be made
across the continent by air express, by air freight, or by a combination of
either and railway express in about 24 to 30 hours. If the destination can be
‘reached in this time enough feed can be placed in the shipping crates to last
the animals until delivery. If a longer period of transit is required, food and
water receptacles should be provided in a separate compartment of the crate
and arrangements made with the express company for feeding and watering
at a stated time. Raw potatoes, carrots, or lettuce leaves should be provided
in addition to the dry food. This will make water less necessary for the
animals.

A crate 10 by 24 inches and 10 inches deep will accommodate 20 rats. If
the trip is for more than 24 hours fewer animals should be put in the crate.
A crate 12 by 15 inches and 6 to 8 inches deep is sufficient for 25 to 30 mice.
The top of the crate or the top and one side should be of %4- to %-inch-mesh
wire screen to provide ventilation. Crates should be supplied with shredded
paper or wood shavings in which the animals can hide, and in which they
can seek protection in cool weather.

U. S. GOVERNMENT PRINTING OFFICE: 1949

For sale by the Superintendent of Documents, U. S. Government Printing Office
Washington 25, D.C. - Price 10 cents

as

Ph esd Oe Sea

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