HPS (Hantavirus Pulmonary Syndrome)

In 1993, a previously unknown infectious disease agent was discovered by a task force of scientists in New Mexico.

This disease agent occurs naturally throughout most of North and South America; it is airborne, and in the absence of prompt medical attention, its infections are usually fatal.

This disease is called Hantavirus Pulmonary Syndrome (HPS). It can affect anyone, but given some fundamental knowledge, it can also be very easy to prevent.
# What is Hantavirus Pulmonary Syndrome?

Hantavirus Pulmonary Syndrome (HPS) is an infectious respiratory disease endemic to North and South America. It is caused by a virus generally known as the hantavirus. While the disease is frequently fatal, is can be very easily prevented.

The hantavirus has a reputation for being a "rare" virus, which is a clear misunderstanding. In the United States, the virus is ubiquitous, being found in over half of the lower 48 states. In fact, cases of the disease have occured in at least thirty states.

The disease itself is considered rare, as the virus is not very infectious except under certain circumstances. As such, when someone contracts HPS, the incident is frequently considered to be a random "freak accident".

In this regard, contracting the hantavirus is very similar to being struck by lighting: (a) it doesn't happen very often, (b) it is worth avoiding, and (c) it can be prevented very easily if you understand how it works.

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# What is the hantavirus?

A "hantavirus" belongs to a group of RNA virii related to the family Bunyaviridae and, depending on its nature, may be the etiological agent for one of two acute illnesses: hantavirus pulmonary syndrome (HPS) and hemorrhagic fever with renal syndrome (HFRS). The HFRS-causing hantavirii are endemic to east Asia, while HPS-causing hantavarii are endemic to the New World. But like all virii, their distributions are only dictated by the range of their natural hosts.

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# What is the host of the hantavirus?

The natural host of the hantavirus appears to be rodents, which are thus considered vectors for both HPS and HFRS. In the United States, the hantavirus is typically carried by the deer mouse (Peromyscus maniculatus). It can also be found in other rodent hosts, such as the cotton rat (Sigmodon hispidus), the marsh rice rat (Oryzomys palustris) and the white-footed mouse (Peromyscus leucopus), so other carriers may exist.

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# What different types of hantavirus exist?

There are dozens of types of hantavirii, being carried by rodents all around the world. However, the majority of these strains of hantavirus do not cause disease in humans.

In North America, the most prevalent type of hantavirus that causes HPS is the Sin Nombre virus (SNV), also called the "Four Corners virus". This is the virus carried by the deer mouse. Others found in the United States are the New York-1 virus, Black Creek Canal virus, and the Bayou virus.

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# What is the history of the hantavirus and HPS? (Abbreviated version)

The hantavirus was originally discovered in Asia, during the Korean War. Technically, it was discovered vicariously, through the discovery of the disease it caused: hemorrhagic fever with renal syndrome (HFRS). The actual virus wasn't isolated until 20 years later, in 1976; it was discovered in a striped field mouse that was trapped near the Hantaan River in Korea. This prototype virus was thusly christened the Hantaan Virus. This virus was eventually classified under its own genus, "hantavirus", when others forms were discovered in rodents throughout Asia, even extending into Eastern Europe and Scandinavia.

Americans had no reason to fear the hantavirus until mid-May of 1993, when several healthy young members of the Navajo Nation in New Mexico died within a short period of time. Their cause of death was a mystery, enigmatically described by health officials as "unexplained adult respiratory distress syndrome" (ARDS). This cluster of peculiar, unexplained deaths caught the attention of the world, prompting a research endeavor of remarkable haste. The effort involved numerous health agencies, including the Centers for Disease Control and Prevention (CDC), the Indian Health Service (IHS), the University of New Mexico, the Navajo Nation Public Health Center, the New Mexico State Department of Health, and the Office of the Medical Investigator (OMI).

On June 3, as the death toll of the Four Corners epidemic reached twelve, researchers made their critical discovery: this infectious form of ARDS created antibodies that were also produced by the hantavirus, even though no known forms of hantavirii produced respiratory distress, or were believed to exist in North America. While many researchers were skeptical of this claim, the identification turned out to be correct; this alone enabled health professionals to accurately diagnose cases of the disease before conditions became extreme, and it helped epidemiologists determine the virus' natural hosts with relative ease.

The Four Corners outbreak occurred because of a combination of unusual environmental conditions: El Niño in 1991-1992 led to a warm winter and a rainy spring in 1993. This contributed to the explosive growth of vegetation, providing food and cover for a burgeoning rodent population. The region experienced a tenfold increase in the numbers of deer mice from the year before. This population explosion, in turn, exacerbated the spread the disease.

Later that year, the virus itself was given a name: Muerto Canyon Virus, which was eventually changed to Sin Nombre virus. The disease was thusly called Hantavirus Pulmonary Syndrome. At the start of 1994, over 55 HPS cases had been documented, 32 of which were fatal.

For a version of this story with more detail, read about the history of HPS at the CDC's web site.

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# Is HPS a "new" disease?

No, definately not. The Sin Nombre virus and other New World strains of hantavirii have probably been living happily in their rodent hosts for thousands of years, long before the first immigrants even settled here. There are even references to HPS in Native American folklore -- where if you let mice live in your dwelling, they'll "take away the breath" of your children.

HPS is a "stealth" disease. Its incident rate is low enough -- and its symptoms are nondescript enough -- that it went unidentified as a specific malady for centuries. It was only identified during the 1993 outbreak because several cases occurred in the same region within a short period of time. Now that the presence of the virus can be determined with antibody tests, health officials have identified HPS patients -- both survivors and victims -- from as far back as 1959.

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# Is HPS a "notifiable" disease?

Yes. By being a "notifiable disease", HPS is one of over 50 diseases that is under national surveillance; whenever a case occurs, it is reported to the CDC's National Notifiable Diseases Surveillance System (NNDSS), to facilitate prevention and control of the disease.

This reporting is voluntary, unless it is mandated on the state level. (More than half of the continental United States mandate the reporting of HPS cases to the NNDSS.) The information compiled by the NNDSS is compiled in the Morbidity and Mortality Weekly Report (MMWR), a weekly serial publication by the CDC.

Whether or not a disease is considered "notifiable" is determined by the Council of State and Territorial Epidemiologists (CSTE) and this list is revised periodically. Currently, other notifiable diseases include: anthrax, cholera, encephalitis, gonorrhea, hepatitis, HIV/AIDS, Lyme Disease, malaria, plague, rabies, rubella, syphillis, tuberculosis, and yellow fever.

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# Where have cases of HPS occurred?

Within the United States, cases of HPS have occured in at least thirty of the southern 48 states.

Outside of the continental United States, cases of HPS have occurred in the following countries: Argentina, Brazil, Canada, Chile, Paraguay, and Uraguay.

However, HPS carrier rodents can also be found in Bolivia, Costa Rica and Mexico, but have not yet been linked to any incidents of the disease.

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# How do people contract HPS?

Humans don't usually contract HPS directly from rodents. Rodents shed hantavirus particles in their saliva, urine and droppings. Humans usually contract HPS by inhaling particles that are infected with the hantavirus.

HPS is an airborne infectious disease. The virus becomes airborne when the particles dry out and get stirred into the air (especially from sweeping a floor or shaking a rug). Humans then inhale these particles, which leads to the infection.

Other possible methods of contracting HPS include: (a) being bitten by a rodent that is carrying the hantavirus, (b) eating food or drinking water that has been exposed to a hantavirus carrier, or (c) bringing hantavirus-infected particles or droplets into contact with your nose, eyes, or mouth (e.g. licking your hands).

Ticks, fleas, and other biting insects have not been found to trasmit HPS from rodents to humans. In fact, no other animals (apart from the carrier rodents) are believed to be directly involved in HPS transmission to humans. However, it is possible for domestic dogs and cats to bring infected rodents into contact with humans.

It is generally believed that humans cannot spread HPS to other human beings, but cases from an HPS outbreak in Argentina (in late 1996) suggest that this may be a possibility. At any rate, human-to-human transmission is considered the least-likely method of contracting the disease, especially in the United States.

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# What conditions usually lead to contracting HPS?

Since HPS is not considered a highly infectious disease, people usually contract HPS from long-term exposure. If rodents can be found in your home or workplace, you may be at risk for contracting HPS.

Since transmission usually occurs through inhalation, it is easiest for a human being to contract HPS within a contained environment, where the virus-infected particles are not thoroughly dispersed. Being in a small house, a crawl space, or a barn where rodents can be found poses elevated risks for contracting HPS.

The environments that provide the greatest risk are unoccupied buildings, such as an abandoned house, a cabin, or the toolshed in your back yard. Rodents can thrive in such places, especially in cold weather. The gathering dust will only increase the infectiousness of the disease.

A very common scenario for contracting HPS is cleaning out a dirty shed: if the shed has been a long-standing home to any carrier rodents, then sweeping the floor will aerosolize the virus particles and make their inhalation much more likely.

Travelling to a place where the hantavirus is known to occur is not considered a risk factor. Camping, hiking, and other outdoor activities also pose insubstantial risks, especially if steps are taken to reduce rodent contact.

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# Can HPS be fatal?

Absolutely. Untreated cases of HPS are almost always fatal. However, if you can get yourself treated for HPS before the disease progresses to acute respiratory distress, then your chances of surviving are greatly increased. Thanks to improved methods of diagnosis, care, and a greater HPS awareness within the medical community, the mortality rate of HPS has rapidly decreased over the past few years.

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# What are the symptoms of HPS?

The very first symptoms can occur anywhere between five days and three weeks after infection. They almost always include fever, fatigue, and aching muscles (usually in the back, shoulders, and/or thighs). Other early symptoms may include headaches, dizziness, chills, and abdominal discomfort (such as vomiting, nausea, and/or diarrhea).

These early symptoms are very difficult to distinguish, and as such they are usually overlooked. In fact, these symptoms are frequently described as "flu-like", because they indicate that the body's immune system is kicking in to defend itself against a viral infection, flu or otherwise. Most people experience these symptoms at least once a year, and HPS will almost never be diagnosed at this point.

(Conversely, rashes, sore throats, and earaches are not typical symptoms of HPS. These symptoms are sometimes used diagnostically to determine when a hantavirus infection is unlikely. Also, HFRS will lead to hemmorhages and severe kidney disfunction, which HPS does not.)

HPS starts to distinguish itself in its later symptoms, which usually occur between three to five days later. These pronounced symptoms include coughing and shortness of breath. This is known as the "cardiopulmonary phase" of the disease, where the body reacts as the lungs start to fill up with fluid. From here, the disease progresses very rapidly; the shortness of breath leads to acute repiratory distress, often within 24 hours.

Breathing will become extremely labored and difficult, and in many cases, it will eventually become impossible for the victim to breathe unassisted. The heart rate will also slow down considerably. If the victim is not receiving medical assistance during this phase of the disease, they will likely die.

The primary cause of death will be excessive proteinaceous fluid in the lungs. The fluid, essentially plasma, is leaked from capillaries into the lungs' air sacs. Autopsies of HPS victims have found that their lungs were so severely fluid-filled, that they weighed twice as much as expected. However, death is frequently associated with shock and heart failure instead of "drowning"; the body's response to the trauma is actually more damaging than the trauma itself.

If someone survives the cardiopulmonary phase of the disease, they usually recover very rapidly. Sometimes a recovering HPS patient can have kidney difficulties, such as excessive urination ("polyurea"), but usually convalesce quickly. During the course of the disease, if damage happens to occur to the lungs or lung vasculature, then the patient may experience minor respiratory difficulty after recovery.

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# What should I do if I suspect that I have HPS?

Seek medical attention immediately. Most HPS victims who receive prompt medical attention are likely to survive the infection. (However, the overall mortality rate is still near 50%, so even immediate medical attention will not guarantee recovery.)

Haste is a very important consideration with fighting HPS. The disease can become acute very rapidly; people have died within hours of suspecting that they were even sick.

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# How is HPS treated?

HPS is a viral infection; if a severe viral infection cannot be prevented by a vacciene, then it can only be controlled with "aggressive supportive care", where the patient is provided continued medical assistance and (hopefully) kept alive long enough for their body to develop antibody resistance.

In the case of HPS, the patient will usually receive antibiotics initially, until the diagnosis of HPS is certain. Once HPS is proven, the patient will be transferred to an intensive-care unit, where they are carefully monitored for fluid balance, electrolyte balance, and blood pressure.

During the onset of the cardiopulmonary phase, the patient may need to be hooked up to a ventilator, which will hopefully keep them breathing. In some occasions, antiviral medication (such as Ribavirin and Bradycor) will be administered intravenously, although it hasn't shown much promise when fighting HPS. Interestingly enough, Ribavirin does appear to be an effective weapon against HFRS.

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