A “rat virus” refers to any of several viruses carried by rats and mice that can spread to humans. The most well-known are hantavirus, lymphocytic choriomeningitis virus (LCMV), Seoul virus, and Lassa virus. These infections spread primarily through contact with rodent urine, droppings, or saliva, and they range from mild flu-like illness to life-threatening conditions with a case fatality rate as high as 36% for the most severe form.
Hantavirus: The Most Dangerous in the U.S.
Hantavirus pulmonary syndrome (HPS) is the most serious rat-borne viral disease in the United States. The primary carrier is the deer mouse, though white-footed mice, cotton rats, and rice rats also harbor related strains. Infected rodents shed the virus in their saliva, urine, and droppings. Humans most commonly catch it by breathing in tiny particles when dried contaminated material gets stirred up, such as when sweeping out a shed, cabin, or garage. Less often, the virus enters through broken skin, the eyes, or a rodent bite.
HPS is rare but frequently fatal, with a case fatality rate of 36%. Symptoms typically appear 9 to 33 days after exposure. Early signs look like many other illnesses: fever, muscle aches, and fatigue. Within a few days, the disease progresses to severe breathing difficulty as the lungs fill with fluid. Because the early symptoms mimic the flu, a blood test is often the only way to confirm the diagnosis. There is no specific antiviral treatment, so early recognition and intensive supportive care are critical.
Seoul Virus: A Risk for Pet Rat Owners
Seoul virus is a hantavirus carried by domestic and wild Norway rats, the common brown rats found worldwide. Unlike hantavirus pulmonary syndrome, Seoul virus causes a different pattern of illness called hemorrhagic fever with renal syndrome (HFRS), which primarily damages the kidneys rather than the lungs. Symptoms range from a mild flu-like illness to acute kidney failure. Some people who test positive never develop symptoms at all.
What makes Seoul virus distinctive is its link to pet rats. A 2017 multistate outbreak investigation found Seoul virus infections in people and pet rats across 31 facilities in 11 U.S. states. Among ratteries with at least 10 rats tested, the percentage of infected rats ranged from 2% to 70%. Infected rats show no signs of illness, so owners have no way of knowing their animals carry the virus just by looking at them. Frequent trading of rats between home-based breeders helped spread the virus from facility to facility.
During that outbreak, 17 people in the U.S. were confirmed to have recent Seoul virus infections. Eight reported fever, and three were hospitalized, though none developed full-blown HFRS and all recovered. Pet rat owners can reduce their risk by washing hands thoroughly after handling rats or cleaning cages, avoiding bites and scratches, and routinely disinfecting cages with a 10% bleach solution.
LCMV: Mild for Most, Devastating in Pregnancy
Lymphocytic choriomeningitis virus is carried primarily by the common house mouse, though pet hamsters and other rodents can also harbor it. For most children and adults, LCMV infection is relatively mild and follows a two-phase pattern. The first phase brings fever, headache, muscle aches, nausea, and poor appetite. After a brief improvement, a second phase can develop with symptoms of viral meningitis: intense headache, sensitivity to light, stiff neck, and vomiting. The entire illness typically lasts one to three weeks, though some people feel unwell for months. Most make a full recovery. Less commonly, LCMV can cause encephalitis or other serious neurological complications.
The real danger of LCMV is during pregnancy. When a pregnant person becomes infected, the virus can cross the placenta and infect the developing brain and eyes of the fetus. Congenital LCMV infection causes vision impairment (specifically a type of retinal scarring called chorioretinitis, which appears in virtually every confirmed case), abnormal brain development, intellectual disability, and in some cases fetal death. Babies may be born with an abnormally small head from failed brain growth, or an abnormally large head from fluid buildup in the brain. These effects are permanent. Despite being a serious congenital pathogen, LCMV is underrecognized, and limited data exist on how common it actually is in the U.S.
Lassa Virus: A Major Threat in West Africa
Lassa fever is caused by a virus carried by the multimammate rat, a common rodent found throughout West Africa. The disease is endemic in Nigeria, Sierra Leone, Liberia, Guinea, Ghana, Mali, and Benin, and likely exists in other West African countries as well. Humans become infected through food or household items contaminated with rat urine or feces, or through direct contact with infected rats. Person-to-person transmission can also occur in healthcare settings.
Lassa fever is not a concern for most people outside West Africa, but travelers returning from endemic areas occasionally test positive. Because the early symptoms overlap with malaria, typhoid, and other tropical infections, Lassa fever should be considered in anyone with a fever who has recently returned from the region.
How These Viruses Spread
All rat-borne viruses share a common transmission pattern. Infected rodents shed virus particles in their urine, droppings, and saliva, often without showing any signs of illness themselves. The most common route of human infection is inhalation: when contaminated material dries out and gets disturbed, microscopic particles become airborne and are breathed in. This is why activities like sweeping out a dusty storage area, cleaning a neglected cabin, or shaking out old bedding are particularly risky. Direct contact with contaminated surfaces, rodent bites, and consuming contaminated food or water are less common but documented routes.
The incubation period varies by virus. For hantavirus pulmonary syndrome, symptoms typically develop 9 to 33 days after exposure, with a median of about 18 days. For hemorrhagic fever with renal syndrome (including Seoul virus), the estimated incubation period is one to six weeks.
How to Diagnose a Rat Virus Infection
Because rat-borne viral infections start with generic symptoms like fever and body aches, they’re easy to mistake for the flu or other common illnesses. Diagnosis depends heavily on a person’s history of rodent exposure combined with blood testing. The CDC uses an antibody test to detect acute hantavirus infections, and the same method works for both the lung and kidney forms of the disease. Molecular testing (which detects the virus’s genetic material directly) can also confirm infection. If you develop a fever or flu-like symptoms after known contact with rodents or their droppings, mentioning that exposure to your doctor is the single most important step toward getting the right diagnosis.
Safe Cleanup Around Rodent Droppings
The most practical thing you can do to prevent a rat virus infection is to clean up rodent-contaminated areas safely. The CDC recommends never sweeping or vacuuming rodent droppings, urine, or nesting material, because this launches viral particles into the air where you can inhale them.
- Ventilate first. Open all doors and windows for 30 minutes before you start, and leave the area while it airs out.
- Wear gloves. Use rubber or plastic gloves throughout the process.
- Soak with disinfectant. Spray droppings and nesting material with a bleach solution (1.5 cups of household bleach per gallon of water, or roughly 1 part bleach to 9 parts water) and let it soak for at least five minutes before wiping up.
- Make fresh solution each time. Bleach solutions lose effectiveness, so mix a new batch before every cleaning session.
For pet rat owners specifically, the CDC recommends washing hands after every interaction with your rats and before eating, drinking, or preparing food. Cages and accessories should be routinely disinfected with a 10% bleach solution or commercial disinfectant. If Seoul virus is suspected, the person cleaning should wear a respirator and gloves and cover any open wounds or scratches. Children and people with weakened immune systems should take extra care around all rodents.