In a first, the CDC recently reported an outbreak of a type of hantavirus called Seoul virus among pet rat owners. The first case was in a “patient who owned and operated an in-home rattery,” or rat-breeding facility, with a family member becoming similarly ill soon thereafter. The investigation found rat Seoul virus infections in 31 facilities in 11 states with human and/or rat infections, and six had exchanges with ratteries in Canada. Testing found 24 people (13.1%) with Seoul virus antibodies; three (12.5%) were hospitalized and no deaths occurred. The outbreak was stopped by quarantining affected facilities and killing infected rats, as well as boosting cleaning.
Seoul virus infections can range from asymptomatic to fatal, with death from hemorrhagic fever with renal syndrome (HFRS), or bleeding with kidney failure, occurring uncommonly, in 1-2%. Infections generally occur from brown or Norway rats (Rattus norvegicus), who may remain asymptomatic though infected with the virus.
Related hantaviruses are far more deadly and common, generally transmitted by deer or white-footed mice.
Four mice have tested positive for the virus this year in San Diego County, though no people are ill yet; they had twelve isolates last year. It can take up to six weeks for symptoms to develop.
The first recognized outbreak of hantavirus was in 1993, when there was a sudden cluster of deaths from an overwhelming lung infection in previously healthy young people in the Four Corners area of the southwest.
Investigators looked for the source of infection, and included rodents trapped in and around homes of those infected. They found hantavirus infection in the deer mouse (Peromyscus maniculatus), and the CDC virology branch who isolated the virus named it Sin Nombre virus (SNV); it is the most common strain in the U.S.
What especially intrigued me at the time of this first outbreak was that the clues to the puzzle came from Navaho elders, who had noted similar cyclical clusters of deaths previously. They noted that each cycle followed a year of increased rainfall (El Niño year), which resulted in additional food for a suddenly growing deer mouse population. People living adjacent to the mice then had more exposure, resulting in the spike in cases. This was one of the memorable early studies on climate change and infections.
From 1993 through 2016, there have been 728 infections reported in the U.S. The mean age was 38 years (range, 5-84 years), and more occurred in males. The disease is serious, with fatalities in 36% of those infected.
The other notable cluster was in Yosemite National Park in 2012, among people staying in tent cabins in one section of the park. Epidemiologic investigation found the source to be deer mice nesting in the walls of the tent cabins. Three of the 10 people infected died, and that part of the park was temporarily closed. Complicating the research and tracking was that two of the patients were from the East Coast.
(That emphasizes the value of Infectious Disease training and taking a careful travel and exposure history. Working in Pennsylvania, I picked up one such case in a young construction worker whose job was laying pipes in the ground near Pittsburgh. I was surprised, as hantavirus is uncommon on the East Coast, but was struggling to figure out what was making him ill.)
In 1993, the first case of hantavirus was reported on the East Coast, in a 61 year old man who had been hiking along the Appalachian Trail. He was quite ill with sepsis and multi-organ failure, but recovered. He had noted mouse droppings in shelters he had stayed at.
Several years ago, we started up an old car that had sat for months and, when turning on the heater fan, got blasted with a noxious odor and bits of debris. It turned out that a mouse had made a nest in the engine, and later died there. Fortunately, we didn’t become ill, though hanta certainly crossed my mind, and I told my husband to remember to tell the doc to look for hantavirus, should I become ill, since no one here in Western Maryland would otherwise think to look. Last year, other cases of hantavirus from mice nesting in cars were reported from Washington State.
How is hantavirus transmitted?
Hantavirus has a variety of mice and rats as its host. Although the animals may be asymptomatic, they still shed the virus in their secretions—urine, droppings, and saliva. People usually become infected by breathing in air contaminated with the virus. They can also become infected by contaminating their hands with these infected secretions, and then touching their nose, mouth, or eyes (like many other infections). It is not transmitted from one person to another.
Who’s at risk?
Anyone coming in contact with urine, droppings, or saliva of infected rodents is at risk, but the biggest risk is from aerosolization of the virus, as occurs with vacuuming or sweeping. Hantaviruses are more common in the southwest U.S., but have occurred elsewhere.
With Seoul virus, the initial symptoms are relatively nondescript, including fever, headache, back pain and nausea developing 1-2 weeks after exposure. Some people develop a rash or eye inflammation. A Tennessee teen also developed bloody urine. More severe cases are associated with shock and hemorrhagic (bleeding) fever and kidney failure.
In contrast, the more common Sin Nombre hantavirus typically causes pulmonary (lung) symptoms and pneumonia. It is far more deadly, killing 38%.
How are hantavirus infections diagnosed?
First, someone needs to think of the disease, which is not likely to happen without a detailed history of exposures. Then diagnosis is made by special blood tests looking for antibodies to infection, available at the CDC.
Faced with a new disease outbreak the The National Center for Emerging and Zoonotic Infectious Diseases (NCEZID) at the CDC actually developed a new test for Seoul virus, enabling rapid testing of breeder’s animals and helping to contain the outbreak.
The CDC and private labs (IDEXX and Charles River Laboratories) are also now able to test blood from rats for Seoul virus using a polymerase chain reaction (PCR) test, which looks for bits of genetic material from the virus. Rats are asymptomatic, so the only way to know if one is infected is to have it tested. The CDC has contacted those ratteries known to have been connected with a case.
There is no standard, effective treatment for hantavirus. Ribavirin may be effective if given early, but that is unlikely to happen unless someone is aware of an obvious exposure or during an outbreak. Treatment is largely supportive with fluids and oxygen.
Cleaning up after rodents
Mice and rats transmit the hantavirus to people through infectious secretions or droppings or aerosolization from bedding that has been contaminated.
The most critical way to avoid infection is to not aerosolize material you are cleaning. This means, don’t sweep or vacuum or shake out contaminated materials. Instead, spray the materials 1thoroughly with a solution of 1 part bleach to 9 part water. After five minutes, use paper towels to wipe up the debris. Clean the area again with the disinfectant.
You can follow this for mice in your engine compartment as well, but disconnect the battery cables first. If the mice have gotten in the air ducts, you will likely need a new filter and hoses, and may need a mechanic to help.
Be sure to wash your hands thoroughly when done.
While hantavirus infections are fortunately rare, it’s worth while being cautious if you come into contact with mouse or rat droppings. Be careful not to aerosolize any rodent contaminated debris we do come into contact with. And please tell your physician if you have pets or recent travel.