Tumors in Rats and Mice

Tumors are unfortunately very common in pet rats and mice, due to poor breeding practices, laboratory, and their sensitivity to the world around them. Most of them are benign, but that doesn't mean they are not harmful. There are things that we can do as owners to reduce the chances for them and prevent them from becoming life-threatening.

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Types of tumors:

• Mammary - hormone-related, most common in females

  • removable if caught early, might recur

  • anywhere on belly side from chin to tail

• Uterine

• Testicular

• Pituitary - mostly female, aggressive, can travel to other parts of body

  • linked to high calorie diets

  • head tilt and behavior change common symptoms

• Zymbal's gland - base of ear canal, poor prognosis

Distinctions:

• Benign- slow, localized, non-invasive, often well-demarcated

• Malignant/cancerous- invasive with no demarcation

It's important to note that tumors can either be caused by genetics or their environment. It's important as pet owners to be mindful of:

• Where you get your rats from

• What you feed them

• Their natural circadian rhythm, ensuring:

- night time is completely dark

- not waking them outside of dawn/dusk

Things we can do to reduce the chances of tumors:

• Get them spayed or neutered while young, before 6 months old

• Adopt from breeders who work to reduce tumors genetically

  • Ethical breeders don't breed rats that present with tumors or have genetic predisposition to health issues like cancers

• Conduct daily health checks

• Take your rats to an exotic veterinarian yearly for professional health checks

• Caloric restriction to prevent overconsumption and obesity

  • measuring daily food to about 10-15g a day per rat

  • don't free feed over 6 months

  • practice fasting, only feeding 1 or 2 times a day

Things we need to be mindful of as they can increase the chances of tumors:

• Don't feed too much sugar

  • limit treats like cheerios and fruit to a couple times a week

• Keep rats a healthy weight

  • Prevent over-feeding

  • Don't feed high-fat high-calorie diets

    • adults should be between 4-6% fat

    • seniors should be 3-4% fat

  • Keep rats active

    • daily free roam for 1+ hour

    • active cage layout with climbing opportunities, properly sized wheel, items to balance, aim for 2.5+ cubic feet per rat

      • YKD 15 Inch Wheel, Amazon

      • Climbing Ropes, Etsy

    • if your adult rats are lazy, introducing 2+ babies safely can motivate the older rats to play

• Don't leave lights or electrons on at night near the cage

Rats are crepuscular, meaning they are most active at dawn and dusk, sleeping through most of the day.

• Ensure 12 hours of dark, 12 hours of light

  • if in dark basement, add artificial light for 12 hours

    • we use these as they double as grow lights for air purifying plants: Grow Lights, Amazon

  • if in bedroom with blackout curtains, open them during the day

• Keep electronic lights off during the night, no night lights

Chronic exposure to bright lights at night disrupt their circadian rhythm, which increases the risks of health problems. Circadian misalignment is shown to be connected to increased risk of obesity, diabetes, and cardiovascular disease.

• Avoid diets high in corn, peanuts, and their products

Dried corn, peanuts, are their products are susceptible to contamination by aflatoxins, a toxic compound produced by a type of mold when it becomes wet. Dried corn is most susceptible, fresh corn doesn't carry the same risks. Aflatoxin B1 is a known carcinogen in animals.

GMO corn is high in pesticides like Glyphosate which can also can cancer. We can reduce the risks by throwing out uneaten food daily, keeping food in air tight containers, keeping humidity below 60%, and buying organic. Only buy enough food to last a month as grains can become rancid or infested with bugs over time.

Corn is high in carbs and lacks complete proteins so really it's a cheap filler, no need to feed it to our rats at all. That's why we make our own organic rat food without corn soy or cheap fillers:

• Our Organic Rat food made without corn or peanuts, Etsy

Another good option that does not have corn:

• Science Select Rat Food, Etsy

What to do when your rat has a tumor:

• Get them removed as soon as possible by a rat-savvy exotic vet

The sooner it is removed, the easier it is and often the faster the recovery. Larger tumors can be difficult or impossible to remove. They can press on nearby organs and make it difficult for them to get around. Other serious complications of not intervening with a tumor include the tumor bursting, depletion of nutrients, sores, infections, death.

• If they are deemed a poor candidate for surgery or have a pituitary tumor, ask about Cabergolin to slow the growth of the tumor. It is a human medication that your exotic veterinarian can prescribe for pets. It come in a pill that can be broken up, crushed, and mixed to be fed to your pet.

Sources

Karen. (2025, November 10). Tumor. Rat Guide. https://ratguide.com/health/neoplasia/tumor.php

Roberts, N. T., MacDonald, C. R., Mohammadpour, H., Antoch, M. P., & Repasky, E. A. (2022). Circadian rhythm disruption increases tumor growth rate and accumulation of myeloid-derived suppressor cells. Advanced Biology, 6(9), Article 2200031. https://doi.org/10.1002/adbi.202200031 

Quesenberry, K. E., & Boschert, K. R. (2026, May). Disorders and diseases of rats. Merck Veterinary Manual. https://www.merckvetmanual.com/all-other-pets/rats/disorders-and-diseases-of-rats#Cancers-and-Tumors-of-Rats_v3230248