Mouse nutrition is a complex and multifaceted topic, especially in preclinical research. Diets have an enormous impact on the overall health and welfare of mice, their development, and changes to their microbiomes, and this has a significant effect on research reproducibility and outcomes. What may appear as a simple statement, “mice need to eat a well-balanced diet,” can quickly become confusing with the number of available options. There are multiple commercial vendors, different diet types, formulations, and sterilization techniques available.
Wild mice are opportunistic omnivores and will eat both plant and animal-based food. Wild mice eat a wide variety of seeds, grains and other plant material, as well as invertebrates, small vertebrates and carrion. Fortunately for the owners of pet mice, the nutritional requirements of mice are well established from work done in mice kept in laboratories (the same species as those we keep as companions). A good-quality, balanced diet will help prevent nutrition-related problems in your mice.
Diet Formulations and Types
There are three main diet types and two different ways to formulate these diets.
- Natural ingredient diets are the most commonly used mouse diet type as these diets contain agricultural products and by-products.
- Purified diets contain refined ingredients (starch, cellulose, etc.).
- Chemically defined diets are chemically pure compounds (amino acids, fatty acids, etc.).
These various diet types may be needed depending on research goals and questions.
Purified diets can help evaluate nutrient requirements, nutrient deficiency, nutrient-toxin relationships, or other interests that evaluate specific nutrients. However, as the nutrients become more accurate, the costs increase, and palatability decreases.
Read also: The Hoxsey Diet
Chemically defined diets are a type of purified diet, where individual amino acids are used in place of a protein source, and specific fatty acids replace oils.
All of these diet types can be provided with an open formula, meaning that the nutrients do not vary, and the exact proportion of ingredients is published. Natural ingredient diets are often available as a closed formula diet, where the manufacturer lists the ingredients but not the specific formulation. The formulation will change depending on the ingredient market price and availability. Therefore, protein, fat, and fiber will stay as consistent concentrations between batches, but the specific ingredient types will change.
At JAX, several standard mouse diets are used. For most production colonies, a closed formula, custom diet upon the NIH-31, 6% fat (w/w) or 10% fat (w/w) diet (LabDiet® formulations 5K52, 6%; 5K20, 11%) is used. The diets for the production mice are available on the strain datasheets under “Technical Support/Dietary Information.”
Essential Nutrients for Pet Mice
The key to a healthy life for a pet mouse is species-appropriate nutrition. Even though the small rodents eat almost everything (and are certainly not vegetarians), caution is advised. An incorrect or imbalanced diet damages the health of the animals and further reduces their naturally short life expectancy.
A nutritious and well-balanced mouse diet consists of high-quality rodent lab blocks or pelleted food that’s formulated especially for mice. Good quality pellets or cubes formulated for mice should be the foundation of their diet (ensure they have a protein content of approximately 14-16% and a fat content of approximately 5%; note that animals who are breeding will need higher protein levels).
Read also: Walnut Keto Guide
Seed mixes are not a good choice of diet for mice as the mice tend to selectively feed on the bits they like the most (usually high-calorie foods, such as sunflower seed) and ignore formulated pellets, resulting in dietary imbalance.
Water
Water must always be available and replenished to keep it fresh. Foods rich in water (melons, cucumber) are not enough on their own. Provide clean fresh water at all times. It is preferable to provide water in good-quality water bottles rather than bowl (as bedding, droppings etc will end up in water bowls and soil them). It is important that water bottles are checked and tested to ensure that the bottles are functioning normally (i.e., not leaking or blocked). Have more than one water bottle in the enclosure in case one malfunctions.
Mice need constant access to clean drinking water, as they can die if they don't have water for even short periods of time. Give your mice their water in bottles with metal sipper tubes and check them everyday for leaks and blockages. Change the water regularly and clean the bottle and nozzle properly each time.
Treats and Supplements
You can feed treats, but these should be given infrequently in small amounts (no more than 10% of the total calories the mice need); for example, fruits like apples, berries, and kiwi.
Here are some safe options for treats:
Read also: Weight Loss with Low-FODMAP
- Herbs, flowers, and leaves: basil, dill, echinacea, daisy, chamomile, dandelion, mallow, red clover, sorrel, ribwort, dead-nettle, white clover, rough hawksbeard, chickweed, cornflower blossoms, yarrow.
- Cereals: amaranth, buckwheat, barley, oats (in grains or flakes), various types of millet such as Chinese/German/Hungarian/Italian millet, rye, wheat (in grains or flakes).
- Fruit: apples (without seeds), bananas, pears, rose hips, raisins, blackberries, strawberries, gooseberries, raspberries, watermelon, kiwis, grapes (without seeds), persimmons cherries, mirabelles.
- Vegetables: broccoli, leaf spinach, chicory, iceberg lettuce, endive, lamb's lettuce, lettuce, fennel tubers, cucumbers, carrots, Jerusalem artichokes, beetroot, parsnips, pumpkin, corn, chard, radishes, courgettes.
- Branches: maple, apple tree, birch, ash, lime, shrubs (hazelnut, currant, blueberry).
- Animal protein: mealworms, crickets, freshwater shrimps, grasshoppers, fish, zophobas, eggs, dairy products (low-fat curd cheese or yoghurt).
Mice are foragers - they love to dig and search for food and tasty treats! Try scattering their daily allowance around their cage and hiding treats in toys or interesting items. You can also make your own toys to hide some of their food in. For example, you could put the food in an empty cardboard box stuffed with bedding or wrap it in paper and stuff it in an egg box.
Healthy treats are fine - in moderation! Avoid colourful chew sticks, mineral and salt blocks as these can in some cases be harmful. Instead, stick to healthy, natural food options, adding a variety of treats to their weekly diet: Cooked or raw pasta such as macaroni and orzo. Or, break up larger pasta into smaller pieces Small pieces of fresh chopped vegetables such as broccoli, curly kale, cabbage, peas, cauliflower, carrot, cucumber. As a guide, a portion the same size as the mouse’s ear is about right. Small pieces of fresh chopped fruit such as grapes, strawberries, blackberries, blueberries, raspberries, pear, melon Small amount of boiled egg Pulses Wholegrain low sugar cereal (less than 5% sugars) such as oat Cheerios Fresh mealworms - one or two at a time, unless your dry food already has these included Apple wood - or other fruit tree wood to gnaw on Millet seed spray for birds, or other dried seed sprays marketed for hamsters Dried forage such as dandelion or strawberry leaves (you can usually find them being sold for rabbits and guinea pigs).
Foods to Avoid
Some components such as corn are healthy but can cause constipation and should not be given too often. Regular pumpkin is fine, but never give decorative pumpkin. Shepherd's purse must not be given to pregnant pet mice, because it can induce labour. Apple seeds contain prussic acid and must be removed.
Some foods can be harmful to your mouse’s health and should not be fed to them. These include:
- Fruit pips and stones
- All citrus fruits
- Avocado pits and skin
- Human foods high in fat and salt
- Chocolate, caffeine, or alcohol, because these are toxic and can cause death or serious illness.
Feeding Recommendations
As nocturnal species, food and water consumption occurs mostly during the night. It is essential that the diet of your mice provides them with ample opportunities to chew/gnaw.
We recommend feeding your mice around 5g of dried food each, per day (around 1 teaspoon each).
Homemade Mouse Food Mix
Some mouse owners choose to make their own mouse food to make sure their mice get everything they need. You can find a great recipe we use at Woodgreen below. If you’d like to learn more about making your own mixes, we recommend the book, ‘The Scuttling Gourmet: A Guide to Wholesome Nutrition for Rats’ by Alison Campbell. Keep in mind that mice are much smaller than rats, so they’ll need a little bit more protein. They’ll also need larger pieces of food broken into smaller pieces.
Looking to make a healthy food mix for your mice? Try out the recipe below, based on the Shunamite diet from The Scuttling Gourmet. This mix is made up of the food groups your mice need for a balanced diet.
This recipe suits most mice of all ages. But make sure to follow your vet’s advice if they say your mouse has other needs. Once you’ve made your food mix, give it a good stir and store in a container with an airtight lid. Note down the date you made it and use within three months. After this point, the ingredients can go stale and lose their nutritional value, so they should be thrown away.
What it includes The food mix is made up of the following:
- Nine scoops of base mix
- Five scoops of processed grains
- Three scoops of protein source
- Two scoops of dried herbs and vegetables
- One scoop of healthy seeds
We’ve given the recipe in scoops. This makes up the amount your mouse needs from each of the food groups. Make sure to use the same scoop (or size) throughout, so the mix stays nutritionally balanced.
Base mix
You can use a mixture of the following or just one type. Why not rotate the base mix you use each time you make up a new batch of food to give your mice variety?
- Mr Johnson’s Special ‘No Added Pellets’ Rabbit Food
- Allen & Page Green Pea Mix for rabbits
- Extra Select Premium Large Pea Rabbit
- Dodson and Horrell Conditioning Mix
- Food4wildbirds Fruity Rabbit Food
- Red Mills Conditioning Mix
Processed grains
A mixture of at least three of the following:
- Low sugar (less than 5% sugar) breakfast cereal
- Dried egg noodles
- Dried pasta (small pieces like macaroni or spaghetti broken up)
- Wholegrain rice
- Plain rice crackers
- Plain popcorn (no added sugar, salt, oils etc)
Protein source
You can use a mixture of the following or you can use a different protein source each time you make up a new batch of food:
- Mealworms (only in small amounts due to fat content)
- Fish-based dog kibble (no meat/animal derivatives and at least 20-25% protein)
- Dried insects/shrimp/fish
- Roasted soya beans
- Lentils
Dried herbs and vegetables
Feel free to use a mixture of the following ingredients below, or rotate which source you use each time you make up a batch of food. It doesn’t matter which dried forage or dried vegetables you choose as long as they’re safe for mice. Here are a few examples to inspire you.
- Burgess Luscious leaves forage
- Burgess wildflower forage
- Pets at Home Dried Vegetable Mix
- Pets at Home Crunchy Dried Beetroot and Parsnip Mix
- Naturals Rosewood Herbs Plus
Healthy seeds
Choose two or more of the options below. Remember, this should make up one scoop of the whole mix - keep this in mind when creating your feed. It’s not healthy for mice to eat too much fat, so avoid choosing both sunflower seeds and peanuts.
- Conditioning bird food mix
- Wild bird seed mix
- Parrot seed mix
- Sunflower seeds (only small amounts due to fat content)
- Peanuts (only small amounts due to fat content)
- Pumpkin seeds
- Linseed
- Hempseed
Sterilization Techniques for Laboratory Mice Diets
To ensure reproducibility and reduce variability, institutions have moved toward housing mice in barrier facilities and maintaining colonies in a specific pathogen-free (SPF) status. Many murine pathogens are known to result in immune perturbations that can affect research outcomes. It has recently been shown that the pellet manufacturing process itself does not eliminate all rodent pathogens in the feed. This highlights the importance of sterilizing feed, as well as any items coming into contact with personnel and the mice. There are two main sterilization techniques available for mouse feed: autoclave and irradiation.
Autoclave
The autoclave has been in use and evaluated for its effectiveness for decades, using steam and time to disinfect. Sterilization effectiveness depends on load size and the ability to penetrate all surfaces. It is essential to have quality control measures in place when maintaining and operating autoclaves. This ensures that the loads have been properly sterilized, and all machine components continue to work appropriately. At JAX, autoclaved diets are preferred for the mice as there is extensive process validation and routine monitoring of nutrient quality.
Autoclaves have a high upfront cost and require an initial significant time investment to develop and validate procedures for various load types. However, once these have been established, and the machines are maintained appropriately, autoclaves are very effective at sterilizing feed. It is crucial to validate autoclaves for your operational uses. For instance, if you were to autoclave feed and you want to sterilize whole bags at a time, you would need to validate the machine to handle the number of packs you want to disinfect. A validation process for one method is not guaranteed to transfer to all potential uses. For example, if your facility decides to switch to autoclaving whole cage setups with feed in a hopper, the process would need to be re-evaluated. The same would be true if the facility decided to process fewer bags at a time, as compared to the initial validation. Therefore, even basing your facility’s temperature and time on previously published data may not be relevant for your machine. Here at JAX, each facility is strongly encouraged to validate their autoclaves for their unique facility needs.
There are some aspects of diet quality that are affected by autoclaving, such as clumping, which makes it more difficult for the mice to access food within a hopper. Clumping is also known to alter nutrient quality. Autoclaves are known to reduce the availability of heat-labile vitamins, including vitamins A, B12, E, thiamine, pantothenic acid, and pyridoxine. At The Jackson Laboratory, an extruded diet is utilized, which results in less pellet hardness when autoclaved.
Irradiation
Food can be irradiated via Cobalt 60 gamma rays, electronic beam (E-beam), or X-ray delivery. Rodent diets are commonly irradiated with cobalt 60 or E-beam. This has proven to be effective at eliminating pathogens, similar to autoclaving. However, depending on the starting dose of contamination, irradiation does not guarantee full sterility. A recent study demonstrated that the pelleting process for feed does not eliminate viral pathogens and that autoclaving feed is effective at eliminating murine parvovirus and murine norovirus, specifically. However, the irradiation process did not prevent infection of mice with murine parvovirus if the feed had a high viral load. Moreover, the manufacturers who irradiate diets do not guarantee sterility of the product regardless of the irradiation method.
Irradiation dose delivered to diets is measured in the unit Gray (Gy), where a typical diet for barrier-housed mice is often exposed to 25kGy, and diets for germ-free rodents are exposed to 50kGy. The irradiation process, utilizing any method, does not leave residual radioactivity on the product, ensuring it is safe for mice to consume.
Irradiation does generate effects in the diet that may confound research results, such as an increase in oxidized lipid metabolites and glucosinolates, which may affect studies in mammary cancer or diabetes. Here at JAX, irradiation is only used for diets that cannot be sterilized via autoclave, such as high-fat diets or diets with drug additives. However, all irradiated diets are tested via culture before entering any mouse facility.