A 76-year-old man in Montreal found a rat in his toilet bowl. He reached in to remove it, and it bit him on two fingers. He went to an emergency department, got basic wound care and a tetanus booster, and was sent home.
Eighteen days later he was back, carrying three days of fever, headache and abdominal pain. His blood pressure was low, his heart rate high, his kidneys already injured and his platelet count falling. He was admitted to intensive care with sepsis of unclear origin.
The infection that nearly killed him was not the one most clinicians would name first. It was leptospirosis, a disease rats spread almost entirely through their urine. His doctors published the case in the Canadian Medical Association Journal in January 2024, and its central point is a transmission puzzle that still has no clean answer.
The Diagnosis That Was Not Rat-Bite Fever
Given a bite from a wild urban rat, the Montreal team weighed two candidates: leptospirosis and rat-bite fever, caused in the Americas by Streptobacillus moniliformis. Rat-bite fever is the more intuitive answer after a bite.
They ranked it lower. The man had no maculopapular rash and no joint pain, the two features that usually accompany rat-bite fever. Ordinary wound infection from the rat's mouth flora also looked unlikely, since nearly three weeks on there were no local signs of infection at the bite site beyond a little redness.
Laboratory confirmation took time. A polymerase chain reaction test targeting the LipL32 gene came back positive for Leptospira on a urine specimen and was later identified as Leptospira interrogans. Blood cultures grew nothing, but genetic sequencing on those same bottles found Leptospira as well. An antibody test drawn on day two of the hospital stay was negative, which is expected. Antibodies are often undetectable during the acute phase, and a single negative result does not rule the infection out.
Rats Do Not Shed These Bacteria in Saliva
Here is the part that makes the case unusual. Leptospira colonizes the kidney tubules of infected rats and is excreted in urine. The CDC notes that the bacteria can then survive in contaminated water or soil for weeks to months. People are normally infected when that urine, water or soil meets broken skin or a mucous membrane. Rats do not shed leptospires in saliva.
So how does a bite transmit it? The leading explanation cited in the report is mundane and slightly grim: the rat's mouth had been temporarily contaminated with its own urine. Transmission through animal bites is documented but uncommon, and the authors suggest one reason it stays uncommon is that people bitten by animals frequently receive antibiotics anyway.
Steroids, Immunoglobulin and a Kidney That Kept Getting Worse
The man was started on intravenous piperacillin-tazobactam with aggressive fluids. His blood pressure responded well, and he never needed vasopressors.
His kidneys did not cooperate. Creatinine climbed from 162 to 518 micromoles per liter, though he never met criteria for dialysis. His platelets fell to a low of 17 billion per liter, a severe drop. His liver enzymes stayed normal throughout, which is itself a clue: severe leptospirosis classically produces disproportionate kidney failure alongside relatively mild liver enzyme elevation.
Because the kidney injury and the platelet crash both looked out of proportion to the rest of his course, the team added corticosteroids for possible leptospirosis-associated vasculitis, and a hematologist gave intravenous immunoglobulin in case immune-mediated platelet destruction was involved. Both decisions came with caveats the authors state plainly. Evidence for steroids in severe leptospirosis is low quality. The role of immunoglobulin is unknown, routine use is not advised, and it interferes with antibody testing afterward. His improvement may simply have been the antibiotics.
He left intensive care after three days and finished 14 days of treatment, ending on oral amoxicillin.
Why This Case Still Matters in American Cities
Leptospirosis is not a tropical curiosity. It was reinstated as a nationally notifiable condition in the United States in 2014, and a CDC-authored surveillance summary in PLOS Neglected Tropical Diseases counted 1,053 case reports from 34 jurisdictions between 2014 and 2020. Among cases with outcome data, 85% were hospitalized and 10% died. Reports rose across the period by an average of 13 a year.
New York City has been the clearest urban signal. The health department's most recent advisory on the infection, issued in April 2024, recorded 24 cases in 2023, more than in any prior year, against an average of three locally acquired cases a year from 2001 through 2020. The species involved is the same one found in Montreal, and the reservoir is the Norway rat. The CDC puts the usual gap between exposure and illness at 2 to 30 days and notes the disease can arrive in two phases, with a deceptive stretch of feeling better in between.
The unresolved question the Montreal authors raise is preventive. Rat bites make up roughly 1% of an estimated 2 million annual animal bites in the United States, and children are often the ones bitten. Historically, rat bites have been reported to cause rat-bite fever about 10% of the time. Some clinicians recommend preventive antibiotics on that basis, though the benefit has not been proven, and studies of wound infection rates after rat bites conflict, ranging from about 2% to about 10%. As the authors put it, "rat bites could warrant antibiotic prophylaxis because they regularly result in rat-bite fever." They call for better studies before anyone writes a firm rule. Anyone bitten should still be seen, and the clinical overview most doctors will reach for treats early antibiotics as the priority.
Key Questions Answered
What is leptospirosis?
A bacterial infection caused by Leptospira species. Infected animals, mostly rats in American cities, shed the bacteria in urine. People are usually infected when contaminated urine, water, or soil contacts broken skin or mucous membranes.
How did a bite transmit a urine-borne infection?
The explanation cited in the report is that the rat's mouth was temporarily contaminated with its own urine. Rats do not shed these bacteria in saliva, and bite transmission is documented but uncommon.
How long after the bite did he get sick?
He arrived at the hospital 18 days after the bite with three days of symptoms behind him, so illness began about two weeks after the exposure. The CDC gives an overall range of 2 to 30 days.
What symptoms should prompt medical attention after a rat bite?
Fever, headache, chills, muscle aches, vomiting, or diarrhea in the weeks after an exposure warrant prompt evaluation. Severe cases can involve kidney damage, liver failure, meningitis, or breathing problems.
Was this rat-bite fever?
No. Rat-bite fever was considered and ranked as less likely because the man had no rash and no joint pain. Testing confirmed Leptospira interrogans.
How common is this in the United States?
Uncommon but rising. CDC surveillance recorded 1,053 reports across 34 jurisdictions from 2014 to 2020, with an average annual increase of 13 cases.
