For 15 years, a laboratory researcher in his 30s in Japan treated the occasional mouse bite as a minor hazard of the job. The bites came about once every two to three years and caused nothing worse than itching. Then a mouse bit his left index finger during an experiment. Twenty minutes later, his whole body had turned red, and his throat felt uncomfortable.
By the time he reached the emergency room, 80 minutes after the bite, his face, neck, and limbs were flushed. Map-shaped hives covered his abdomen, back, and the sides of his chest; doctors could hear him wheezing, and his blood oxygen level had dipped to 90% to 93%. Dermatologists at Oita University diagnosed grade 2 anaphylaxis and described the case in a report published Jan. 21, 2026, in Cureus.
The reaction was only part of the story. Blood tests later showed he had become sensitized to rats and hamsters, two animals he had never handled in his work.
Twenty Minutes from Bite to Hives
The emergency team gave him an injection of adrenaline, also called epinephrine, along with an intravenous corticosteroid, an antihistamine, and an inhaled bronchodilator. His breathing symptoms cleared completely within an hour, and the rash had nearly disappeared within 15 hours.
He had asthma, controlled with a daily combination inhaler and free of flare-ups for two years, and a soy allergy, but no latex allergy. He had not taken any medications such as nonsteroidal anti-inflammatory drugs, had not been drinking alcohol, and had not been exercising, all factors that can make allergic reactions worse. He had never kept pets.
The authors, Maako Kudo, Mayuko Goto-Umeki, and Yutaka Hatano, concluded that his years of mouse bites likely sensitized him to mice. Earlier bites had caused itching but never the swelling seen this time.
Antibodies to Animals He Never Touched
Blood drawn a week after the bite told a more complicated story. His total IgE, the antibody class behind allergic reactions, measured 731 IU/mL, far above the normal ceiling of 170. Allergen-specific testing showed a strong class 5 result for mouse, class 3 for rat, class 2 for hamster, and a weak class 1 for guinea pig.
The mouse result made sense. The rat and hamster results did not, because he had worked only with mice.
The team looked to the facility for an explanation. The mouse, rat, and hamster rooms had separate air-conditioning systems, but every cage was cleaned in the same room, and the researcher routinely took part in that work. The authors suspect he was sensitized to rats and hamsters during cage cleaning, possibly by breathing airborne allergens, while acknowledging that this is difficult to prove.
Cross-reactivity may also play a role. A key mouse allergen, Mus m 1, is structurally similar to the rat allergen Rat n 1, and the two are considered potentially cross-reactive, but the authors said it is unknown whether that mattered here. No cross-reactivity between mouse and hamster allergens has been described.
The report has a clear limitation. The patient declined skin prick testing, so his sensitization to rats and hamsters rests on blood tests alone, which the authors called the report's most important weakness. To their knowledge, it is only the third reported case of combined mouse and hamster sensitization. Both earlier patients were also lab researchers, including one described in a 2025 case report who showed sensitization to hamster and mouse despite working only with mice.
A Workplace Hazard That Often Goes Uncounted
Allergy to laboratory animals is a long-recognized occupational problem. A 1998 alert from the National Institute for Occupational Safety and Health on preventing asthma in animal handlers estimated that about 33% of animal handlers have allergic symptoms and roughly 10% have symptoms of animal-induced asthma.
Anaphylaxis from a bite is far less often documented. A 2023 review by Duke University's Gregg Stave in Allergologie Select found about 40 published cases of animal bite anaphylaxis, mostly from mice, rats, hamsters, and guinea pigs.
Published cases appear to undercount the problem. In a national survey of U.S. laboratory animal facilities, 15 of 198 responding organizations said workers had experienced anaphylaxis after an animal bite. Detailed reports covered 14 cases, 13 of them from rodent bites, and half of the rodent cases involved workers with no prior history of animal allergy. All recovered without complications, and the survey's authors concluded that such reactions occur more often than the medical literature suggests.
The Oita authors noted a trend outside the lab as well: the risk of rodent allergy in homes has been rising, partly because pets have grown more popular, and how people become sensitized in either setting remains poorly understood.
What Changed After One Bad Bite
After the episode, the researcher handled mice only while they were under anesthesia. He had no further bites and no recurrence of allergic symptoms, and the authors said no similar case had occurred at their facility.
Standard protections include gloves to block bites and skin contact, careful removal of rodent waste to limit exposure to urinary allergens, good ventilation and filtration, and masks. NIOSH also recommends that employers provide health monitoring and medical follow-up for workers who become sensitized. The authors added one more lesson: an allergy to one lab species should prompt doctors to consider sensitization to others, even without a clear history of contact. They acknowledged that exactly how much protection is enough remains unclear.
A single case cannot show how common multi-species sensitization is among lab workers. Anyone who develops hives, wheezing, throat tightness, or dizziness after an animal bite should seek emergency care, and workers with allergy symptoms around animals should see an occupational health clinician or allergist.
Key Questions Answered
What happened to the researcher?
A mouse bite triggered grade 2 anaphylaxis, with widespread hives, wheezing, and low oxygen levels, after years of bites that caused only itching.
Why is the case unusual?
Blood tests showed sensitization to rats and hamsters, animals he had never handled. His doctors say it is only the third reported case of combined mouse and hamster sensitization.
How might he have become sensitized to them?
His doctors suspect airborne exposure in a shared cage-cleaning room, though they say this is hard to prove. He declined skin testing, so the findings rest on blood tests.
How common is anaphylaxis from animal bites?
About 40 cases appear in the medical literature, mostly from rodents, but a U.S. survey of lab animal facilities suggests many cases go unpublished.
How often do animal handlers develop allergies?
A NIOSH alert estimated that about one in three animal handlers has allergic symptoms and about one in 10 has animal-induced asthma.
What should someone do after a reaction to a bite?
Seek emergency care for breathing trouble, hives, throat tightness, or dizziness, then ask an allergist or occupational health clinician about testing.
