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Essential Knowledge | Do You Know the Basics About Mice and Rats?

Introduction

Beginners often ask the editor: How do you group mice together? At what age can they start having pups? And if I get bitten by a lab mouse, do I need to get vaccinated?

Today, we’ll provide a detailed overview of the basic knowledge about mice and rats, helping you better understand these animals.

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Origins and Development

The origins of the laboratory mouse can be traced back over 60 million years, and it was not until 1900 that an American researcher, Lathrop, introduced mice into scientific research.

In 1909, Professor Little at the Jackson Laboratory in the United States first established the DBA inbred mouse strain. In 1950, the American Association for Laboratory Animal Science was founded, and in 1956, the United Nations established the International Council for Laboratory Animal Science (ICLAS).

China’s experimental animal research has, since the 1980s, now spanned four decades.

Laboratory mice have captured the attention of researchers worldwide, selflessly serving humanity for more than a century. Out of respect and gratitude, we should deepen our understanding of their behavioral ecology and husbandry practices, so that we may treat them with even greater care.

 

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Basic characteristics of mice

Currently, laboratory rodents are mainly divided into two species: rats and mice. These two species differ significantly in body size, with a typical lifespan of about 18 months; healthy individuals may live up to 18–24 months.

Inbred and closed colony laboratory mice exhibit relatively stronger reproductive capacity, whereas certain disease-model mice show reduced fertility.

Most of them are docile and timid, rarely biting humans (though some strains, such as ICR mice, are lively and active).

It prefers dark, quiet environments, is crepuscular—active at dawn and dusk—and lives in highly social groups.

Incisors continue to grow throughout the animal’s life, so they must constantly gnaw on hard objects to wear them down (which is why mouse chow is quite hard, to help them keep their teeth filed).

 

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Reproduction and breeding

The gestation period of laboratory mice is 19–21 days, and pups can be weaned by the third week; they typically reach sexual maturity at 6–8 weeks. If co-housing for breeding is desired, it is recommended to introduce males and females starting at 9 weeks of age, as this approach tends to be more stable.

The grouping ratio can be 1 male to 2 females or 1 male to 3 females.

Following successful co-housing, the female mouse will form a vaginal plug. The standard method we use to screen for early pregnancy in females is to check for the presence of this plug; its appearance indicates that the female is approximately 0.5 days pregnant.

Figure 1: Vaginal plug in the female mouse

 

However, this method also has limitations: some female mice may experience pseudopregnancy, and in the early stages of gestation, it is impossible to determine whether pregnancy has occurred based on external appearance alone. Pseudopregnancy can only be identified after 12 days of gestation or later.

Each litter of female mice typically consists of 6 to 15 pups. In some cases, the first litter may include instances of neonatal cannibalism, which is a normal behavior; this usually resolves by the second litter. For females that repeatedly consume their offspring, it is advisable to improve their housing conditions and, alternatively, to provide foster pups.

Sometimes, when there are more than ten newborn mice, the mother will eat the weaker ones to ensure the healthy growth of the remaining pups.

 

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Housing Environment Grading

Some of you may have noticed when purchasing mice that some are very inexpensive, while others are quite expensive, with a significant price gap. Why is this the case?

This is because the mice are raised in a hierarchical system, with higher ranks commanding higher prices.

Depending on the level of microbial control, laboratory animals are classified into several grades internationally:

Standard grade ： Animals are raised in a fully open environment, free from pathogens of highly virulent animal diseases and zoonotic diseases, with an unclear status regarding microbial carriage. This can be understood as an outdoor, wild‑type setting.

Clean grade ： In an open‑housing environment, pathogens that pose significant risks to animals or interfere with scientific research are excluded, while the microbial carriage status remains unclear. This can be regarded as a typical laboratory setting.

SPF grade ： Barrier environment, free of specified pathogenic microorganisms and parasites. Currently the most widely used animal grade.

GA grade (germ-free animals) ： A barrier environment housing microorganisms of known and defined identity. (Currently, experimental animals at this biosafety level are rare on the market.)

GF grade (germ-free animals) ： In a quarantine environment, no microorganisms or parasites can be detected using current methods. (The transportation conditions are extremely stringent.)

 

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How to raise them?

Mice lack sweat glands, so they are heat-intolerant and extremely averse to hot, humid conditions.

It is recommended to maintain the housing temperature between 18 and 22°C and the relative humidity between 50% and 60%. Stable temperature and humidity conditions are conducive to the growth, development, and reproduction of laboratory mice; otherwise, they may lead to disease in the mice and compromise experimental outcomes.

At the same time, ensure that mice are housed in a low-noise environment, avoid placing cages directly against walls, minimize human traffic, and designate specific personnel for routine care. Limit handling and environmental disturbances to the mice.

 

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Disease Prevention

When raising laboratory mice, the biggest concern for researchers is: Should they get a rabies vaccine after being bitten by a mouse?

In fact, mice do not transmit the rabies virus, nor does the virus persist in their bodies; therefore, the risk of contracting rabies after a mouse bite is extremely low.

The mice commonly used by researchers are SPF‑grade, which are free from exposure to the rabies virus; even mice housed in a clean‑grade environment have no opportunity to come into contact with the virus.

Figure 3: Excerpt from the “Guidelines for Rabies Prevention and Control”

 

Therefore, if you are bitten by an SPF‑grade mouse that has not been experimentally infected, rabies vaccination is not required; however, the wound should be promptly treated, and tetanus prophylaxis should be considered based on the severity of the injury.

For laboratory mice that have already undergone experiments and received medication, after being bitten, you should still consult a physician and closely monitor your body for any unusual symptoms.

 

 

 

 

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