CDC answers your questions about
HANTAVIRUS
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Part One:
Tracking
a Mystery Disease: A brief history of hantavirus pulmonary syndrome
How Is the Virus That
Causes HPS Transmitted? The rodent connection
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Tracking a Mystery Disease: a brief history of hantavirus pulmonary syndrome
An outbreak of unexplained illness occurred in May 1993 in the "Four
Corners," an area of the Southwest shared by New Mexico, Arizona, Colorado, and Utah.
A number of previously healthy young adults suddenly developed acute respiratory symptoms;
about half soon died. The New Mexico Department of Health, the Arizona Department of
Health Services, the Colorado Department of Health, the Utah Department of Health, the
Indian Health Service and CDC, with the assistance of the Navajo Nation Division of
Health, rapidly mounted an intensive investigation. Researchers soon suspected that they
were dealing with a form of hantavirus, which is transmitted by rodents. Researchers then
investigated the possible rodent connection, trapping rodents in the
affected area, doing tissue studies both of rodents and hantavirus victims, until the
virus and its principal carrier—the deer mouse—were positively identified. Why
the Four Corners area? Simply because there was a "bumper crop" of rodents
there, due to heavy rains during the spring of 1993, which produced an extra-plentiful
supply of the foods that rodents eat. Early on, it was also established that
person-to-person spread was unlikely. It was also determined that this "new"
hantavirus had actually been present, but unrecognized, at least as early as 1959. Since
the 1993 outbreak, hantavirus pulmonary syndrome (HPS) has been identified in over half of
the states of the U.S.
Tracking a Mystery Disease: The detailed story of hantavirus pulmonary syndrome
The "First" Outbreak
In May 1993, an outbreak of an unexplained pulmonary illness occurred in the southwestern United States, in an area shared by Arizona, New Mexico, Colorado and Utah known as "The Four Corners." A young, physically fit Navajo man suffering from shortness of breath was rushed to a hospital in New Mexico and died very rapidly.
While reviewing the results of the case, hospital personnel discovered that the young man's fiancee had died a few days before after showing similar symptoms, a piece of information that proved key to discovering the disease. As Dr. James Cheek of the Indian Health Service noted, "I think if it hadn't been for that initial pair of people that became sick within a week of each other, we never would have discovered the illness at all."
An investigation combing the entire Four Corners region was launched by the New Mexico Office of Medical Investigations (OMI) to find any other people who had a similar case history. Within a few hours, Dr. Bruce Tempest of OMI had located five young, healthy people who had all died after acute respiratory failure.
A series of laboratory tests had failed to identify any of the deaths as caused by a known disease, such as bubonic plague. At this point, the CDC Special Pathogens Branch was notified. CDC, the state health departments of New Mexico, Colorado and Utah, the Indian Health Service, the Navajo Nation, and the University of New Mexico all joined together to confront the outbreak.
During the next few weeks, as additional cases of the disease were reported in the Four Corners area, physicians and experts worked intensively to narrow down the list of possible causes. The particular mixture of symptoms and clinical findings pointed researchers away from possible causes, such as exposure to a herbicide or a new type of influenza, and toward some type of virus. Samples of tissue from patients who had gotten the disease were sent to CDC for exhaustive analysis. Virologists at CDC used several tests, including new methods to pinpoint virus genes at the molecular level, and were able to link the pulmonary syndrome with a virus, in particular a previously unknown type of hantavirus.
Hantaviruses
Hantaviruses are part of the viral family Bunyaviridae. They represent one of the
groups of zoonotic viruses, which can be transmitted from animals to humans. Hantaviruses
are spherical, about 100 nm in diameter. Interestingly, unlike humans, their genetic
material is not composed of double-stranded DNA. Instead, it is composed of a single
strand of RNA in three segments. These viruses are surrounded by a lipid (fatty) envelope,
so they are somewhat fragile. The lipid envelope can be destroyed and the virus killed by
fat solvents like alcohol, ordinary disinfectants and household bleach. That is why one of
the most important ways to prevent transmitting HPS is to carefully wet down dead rodents
and areas where rodents have been with disinfectant and/or bleach. When you do this, you
are reducing the chance that any virus will get into the air, as well as killing virus
particles themselves.
As far as human disease is concerned, hantaviruses are divided into two major groups. The Hantaan, Puumala, and Seoul viruses are found mostly in the Asia and Europe. They cause a group of illnesses called hemorrhagic fever with renal syndrome (HFRS). In people with HRFS, the kidneys may not function properly, so the body is not as able to produce urine. People with severe cases of HFRS may have circulatory problems, including shock, bleeding, and fluid buildup in the lungs, and abnormal liver function is sometimes seen. In general, these hantaviruses cause death in less than 10% of the people who become infected with these viruses.
So far, the second group of hantaviruses has been found only in the Americas. In the United States, this group includes the Sin Nombre virus (SNV), carried by the deer mouse, which caused the well-publicized "Four Corners" outbreak. Three other hantaviruses have been recently discovered that have caused several cases with the same pulmonary symptoms. The Black Creek Canal virus, carried by the cotton rat, was found in Florida, while the Bayou Virus, carried by the rice rat, was found in Louisiana. In New York, the New York-1 virus was linked to the white-footed mouse. Several other hantaviruses, Prospect Hill, Rio Segundo, Rio Mamore, and El Moro Canyon, have also been discovered, but they have not yet been associated with any disease in humans.
HPS cases have also been confirmed elsewhere in the Americas, in Argentina, Brazil, Canada, Chile, Paraguay and Uraguay. In addition, rodents carrying viruses similar to Sin Nombre have also been found in Bolivia, Costa Rica and Mexico, but these hantaviruses have not been associated with disease in humans.
Researchers Launch Investigations to Pin Down the Carrier of the New Virus
Researchers knew that all other known hantaviruses were transmitted to people by rodents, such as mice and rats. Therefore, an important part of their mission was to trap as many different species of rodents living in the Four Corners region as possible to find the particular type of rodent that carried the virus. From June through mid-August of 1993, all types of rodents were trapped inside and outside homes where people who had hantavirus pulmonary syndrome had lived, as well as in piņon groves and summer sheep camps where they had worked. Additional rodents were trapped for comparison in and around nearby households as well. Taking a calculated risk, researchers decided not to wear protective clothing or masks during the trapping process. "We didn't want to go in wearing respirators, scaring...everybody," John Sarisky, an Indian Health Service environmental disease specialist said. However, when the almost 1,700 rodents trapped were dissected to prepare samples for analysis at CDC, protective clothing and respirators were worn.
Among rodents trapped, the deer mouse (Peromyscus maniculatus) was found to be the main host to a previously unknown type of hantavirus. Since the deer mouse often lives near people in rural and semi-rural areas—in barns and outbuildings, woodpiles, and inside people's homes—researchers suspected that the deer mouse might be transmitting the virus to humans. About 30% of the deer mice tested showed evidence of infection with hantavirus. Tests also showed that several other types of rodents were infected, although in lesser numbers.
The next step was to pin down the connection between the infected deer mice and households where people who had gotten the disease lived. Therefore, investigators launched a case-control investigation. They compared "case" households, where people who had gotten the disease lived, with nearby "control" households. Control households were similar to those where the case-patients lived, except for one factor: no one in the control households had gotten the disease.
The results? First, investigators trapped more rodents in case households than in control households, so more rodents may have been living in close contact with people in case households. Second, people in case households were more likely than those in control households to do cleaning around the house or to plant in or hand-plow soil outdoors in fields or gardens. However, it was unclear if the risk for contracting HPS was due to performing these tasks, or with entering closed-up rooms or closets to get tools needed for these tasks.
In November 1993, the specific hantavirus that caused the Four Corners outbreak was isolated. The Special Pathogens Branch at CDC used tissue from a deer mouse that had been trapped near the New Mexico home of a person who had gotten the disease and grew the virus from it in the laboratory. Shortly afterwards and independently, the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID) also grew the virus, from a person in New Mexico who had gotten the disease as well as from a mouse trapped in California.
The new virus was called Muerto Canyon virus—later changed to Sin Nombre virus (SNV)—and the new disease caused by the virus was named hantavirus pulmonary syndrome, or HPS.
The isolation of the virus in a matter of months was remarkable. This success was based on close cooperation of all the agencies and individuals involved in investigating the outbreak, years of basic research on other hantaviruses that had been conducted at CDC and USAMRIID, and on the continuing development of modern molecular virologic tests. To put the rapid isolation of the Sin Nombre virus in perspective, it took several decades for the first hantavirus discovered, the Hantaan virus, to be isolated.
HPS not really a new disease
As part of the effort to locate the source of the virus, researchers located and examined stored samples of lung tissue from people who had died of unexplained lung disease. Some of these samples showed evidence of previous infection with Sin Nombre virus—indicating that the disease had existed before the "first" known outbreak—it simply had not been recognized!
Other early cases of HPS have been discovered by examining samples of tissue belonging to people who had died of unexplained adult respiratory distress syndrome By this method, the earliest known case of HPS that has been confirmed has been the case of a 38-year-old Utah man in 1959.
Interestingly, while HPS was not known to the epidemiologic and medical communities, there is evidence that it was recognized elsewhere. The Navajo Indians, a number of whom contracted HPS during the 1993 outbreak, recognize a similar disease in their medical traditions, and actually associate its occurrence with mice. As strikingly, Navajo medical beliefs concur with public health recommendations for preventing the disease.
Navajo Medical Traditions and HPS
In Navajo medical traditions, mice are considered to be the bearers of an ancient illness that even predates the bubonic plague in the Navajo region. Healers say that when mice enter the home, they put people at risk of infection, as people come into contact with mice droppings and urine. The illness enters through the mouth, the nose or the eyes, and it usually attacks the strongest and healthiest of the Navajo people. Therefore, traditional medicine prescribes avoiding mice, keeping them out of the hogans, and isolating food supplies.
Some of the Navajo elders had predicted the 1993 HPS outbreak. In addition, their oral tradition says that in 1918 and 1933-34, there were similar outbreaks, after increases in rainfall produced increases in the piņon crop and the number of mice.
Why did the outbreak occur in the Four Corners area?
But why this sudden cluster of cases? The key answer to this question is that, during the this period, there were suddenly many more mice than usual. The Four Corners area had been in a drought for several years. Then, in early 1993, heavy snows and rainfall helped drought-stricken plants and animals to revive and grow in larger-than-usual numbers. The area's deer mice had plenty to eat, and as a result they reproduced so rapidly that there were ten times more mice in May 1993 than there had been in May of 1992. With so many mice, it was more likely that mice and humans would come into contact with one another, and thus more likely that the hantavirus carried by the mice would be transmitted to humans.
Person-to-person spread of HPS decided unlikely
"Although person-to-person spread [of HPS] has not been documented with any of the other known hantaviruses, we were concerned [during this outbreak] because we were dealing with a new agent," said Charles Vitek, a CDC medical investigator.
Researchers and clinicians investigating the ongoing outbreak were not the only groups concerned about the disease. Shortly after the first few HPS patients died and it became clear that a new disease was affecting people in the area, and that no one knew how it was transmitted, the news media began extensive reporting on the outbreak. Widespread concern among the public ensued.
Unfortunately, the first victims of the outbreak were Navajo. News reports focused on this fact, and the misperception grew that the unknown disease was somehow linked to Navajos. As a consequence, Navajos found themselves at the center of intense media attention and the objects of the some people's fears.
By later in the summer of 1993, the media frenzy had quieted somewhat, and the source of the disease was pinpointed. Researchers determined that, like other hantaviruses, the virus that causes HPS is not transmitted from person to person the way other infections, such as the common cold, may be.
To date, no cases of HPS have been reported in the United States in which the virus was transmitted from one person to another. In fact, in a study of health care workers who were exposed to either patients or specimens infected with related types of hantaviruses (which cause a different disease in humans), none of the workers showed evidence of infection or illness.
HPS since the first outbreak
After the initial outbreak, the medical community nationwide was asked to report any cases of illness with symptoms similar to those of HPS that could not be explained by any other cause. As a result, additional cases have been reported.
Since 1993, researchers have discovered that there is not just one hantavirus that causes HPS, but several. In June 1993, a Louisiana bridge inspector who had not traveled to the Four Corners area developed HPS. An investigation was begun. The patient's tissues were tested for the presence of antibodies to hantavirus. The results led to the discovery of another hantavirus, named Bayou virus, which was linked to a carrier, the rice rat (Oryzomys palustris). In late 1993, a 33-year-old Florida man came down with HPS symptoms; he later recovered. This person also had not traveled to the Four Corners area. A similar investigation revealed yet another hantavirus, named the Black Creek Canal virus, and its carrier, the cotton rat (Sigmodon hispidus). Another case occurred in New York. This time, the Sin Nombre-like virus was named New York-1, and the white-footed mouse, Peromyscus leucopus, was implicated as the carrier.
More recently, cases of HPS stemming from related hantaviruses have been documented in Argentina, Brazil, Canada, Chile, Paraguay, and Uruguay, making HPS a pan-hemispheric disease.
How is hantavirus transmitted? The Rodent Connection
So just how do people get hantavirus pulmonary syndrome (HPS)? It all starts with rodents, like the deer mouse and cotton rat, which carry hantaviruses.
The basic transmission cycle
The short story is that some rodents are infected with a type of hantavirus that causes HPS. In the United States, deer mice (plus cotton rats and rice rats in the southeastern states and the white-footed mouse in the Northeast) are the rodents carrying hantaviruses that cause hantavirus pulmonary syndrome.
These rodents shed the virus in their urine, droppings and saliva. The virus is mainly transmitted to people when they breathe in air contaminated with the virus.
This happens when fresh rodent urine, droppings or nesting materials are stirred up. When tiny droplets containing the virus get into the air, this process is known as " aerosolization."
There are several other ways rodents may spread hantavirus to people:
That is another reason why disinfecting rodent-infested areas is so important in preventing transmission of the virus.
Transmission can happen anyplace that infected rodents have infested. (Remember, by "carrier rodent" we mean deer mice plus cotton rats and rice rats in the Southeastern states, and the white-footed mouse in the Northeast. Common house mice do not carry hantavirus.) This could be barns or sheds or other outbuildings, warehouses or summer cottages closed up for the season. But carrier rodents infest homes as well!
Therefore, the most sensible way to avoid contact with rodents is to prevent rodents from infesting the places where you live and work, and to follow safety precautions if you do stumble into a rodent-infested area. The prevention section (text-only) of this web site details all of this for you!
Transmission details: so how does "aerosolization" really work?
For the hantavirus causing HPS to make someone sick, it must travel from the rodents that carry it to a person. A common way this happens is when a person breathes in the hantavirus from the air.
Let's create an imaginary scenario and go through the process step by step. Say you have a storage room in your home that you hardly ever enter. You keep old furniture there, old newspapers and magazines, and so on. At some point, a group of deer mice find their way into the room, looking for places to build nests. They found their way into the room through a crack—deer mice can squeeze through holes as small as a shirt button! Some mice chew through the fabric of an old armchair and build a nest inside it. Other mice shred bits of magazines and build nests under the shredded pieces.
A few of these mice are infected with the hantavirus. The infected mice don't show any signs of being sick. In fact, the virus does not seem to make them ill at all; it simply lives in their bodies. However, the virus is shed continuously from them: into the droppings and urine they leave around the room, into their saliva, that dries on anything that they have chewed, like nesting material. Out in the environment like this, the virus can live for several days.
Meanwhile, you decide to clean up your storage room. You go inside, spend a few minutes moving boxes and furniture. The mice hear you coming and scurry away, leaving a trail of fresh urine! Because you find mouse droppings and some of the furniture stuffing the mice have used as nesting material, you get a broom and sweep up the mess. As you move around and sweep, tiny particles of fresh urine, droppings and saliva, with the virus in them, get kicked up into the air. This is the aerosolization. It is these tiny particles that you breathe in—and this is the beginning of becoming sick with the hantavirus pulmonary syndrome.
It is precisely because the virus is spread when virus-containing particles are stirred up into the air, that an essential hantavirus prevention tactic in areas showing signs of rodents is to avoid actions that raise dust and to carefully wet the area down with disinfectant. The less chance the virus has to get into the air, the less chance it will be breathed in!
Could you get hantavirus from another person?
Remember, the types of hantavirus that cause HPS in the United States stop at the person who has been infected—they cannot be transmitted from one person to another. For example, you cannot get the virus from touching or kissing a person who has the disease, or from a health care worker who has treated someone with the disease. Finally, you cannot get the virus from a blood transfusion in which the blood came from a person who came down with hantavirus pulmonary syndrome and survived.
Could you get hantavirus from animals other than rodents, or from insects? What about pets?
No—researchers do not believe that the hantaviruses causing HPS in the United States are transmitted by any other types of animals besides certain species of rodents. This would include farm animals like cows, chickens or sheep and insects like mosquitoes. Dogs and cats are not known to carry hantavirus. However, they may bring infected rodents into contact with people if they catch infected rodents and carry them home. Guinea pigs, hamsters, gerbils and other such pets are not known to carry hantavirus.
Summing up: How hantavirus is transmitted
Here are the rodents that carry the types of hantavirus that cause HPS in the United States:
The deer mouse (Peromyscus maniculatus) is a deceptively cute animal, with big eyes and big ears. Its head and body are normally about 2 - 3 inches long, and the tail adds another 2 - 3 inches in length. You may see it in a variety of colors, from gray to reddish brown, depending on its age. The underbelly is always white and the tail has sharply defined white sides. The deer mouse is found almost everywhere in North America. Usually, the deer mouse likes woodlands, but also turns up in desert areas.
The cotton rat (Sigmodon hispidus), which you'll find in the southeastern United States (and way down into Central and South America), has a bigger body than the deer mouse—head and body about 5 - 7 inches, and another 3 - 4 inches for the tail. The hair is longer and coarser, of a grayish brown color, even grayish black. The cotton rat prefers overgrown areas with shrubs and tall grasses.
The rice rat (Oryzomys palustris) is slightly smaller than the cotton rat, having a head and body 5 - 6 inches long, plus a very long, 4- to 7-inch tail. Rice rats sport short, soft, grayish brown fur on top, and gray or tawny underbellies. Their feet are whitish. As you might expect from the name, this rat likes marshy areas and is semiaquatic. It's found in the Southeastern United States and into Central America.
The white-footed mouse (Peromyscus leucopus) is hard to distinguish fom the deer mouse. The head and body together are about four inches long. You should note that its tail is normally shorter than its body (about 2 - 4 inches long). Topside, its fur ranges from pale brown to reddish brown, while its underside and feet are white. The white-footed mouse is found through southern New England, the Mid-Atlantic and Southern states, the Midwest and into the western states and Mexico. It prefers wooded and brushy areas, although sometimes it will live in more open ground.
Sometimes, the "country mouse" becomes a "city
mouse"
Both the deer mouse and the cotton rat are usually in rural areas, but can also be
found in cities when conditions are right, such as easy availability of food, water and
shelter. (Remember that when it comes to "discouraging" rodents, which we'll
talk about under " How Do I Prevent HPS").
Other rodents may also carry hantavirus
It appears that other rodents carrying strains of hantavirus that cause HPS are
yet to be identified. In addition, yet other rodent species play host to other types of
hantaviruses that cause a different type of infection, hemorrhagic fever with renal
syndrome, or HFRS.
It is wise, therefore, to avoid close contact with rodents in general.
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1999, Special Pathogens Branch
Division of Viral and Rickettsial Diseases
National Center for Infectious Diseases
The Centers for Disease Control and Prevention (CDC)
U.S. Department of Health and Human Services