Hantavirus Pulmonary Syndrome (HPS) is a severe respiratory illness caused by New World strains of hantaviruses, most commonly the Sin Nombre virus in the United States. HPS first drew national attention following an outbreak in the American Southwest in 1993. Though rare, HPS carries a high case fatality rate, often ranging between 30 and 40 percent. Understanding its incidence and transmission pathways is crucial for public health. This article explores the statistical reality of HPS, the biological factors that drive its transmission, and the specific environments where the risk is highest.
The Rarity of Hantavirus Cases
Hantavirus Pulmonary Syndrome is rare, particularly when compared to other infectious diseases. Since its initial identification in 1993, the Centers for Disease Control and Prevention (CDC) has tracked all confirmed cases in the United States. The total number of HPS cases reported through 2016 stood at 688, demonstrating a very low cumulative incidence over more than two decades of surveillance.
The annual number of confirmed HPS cases across the country is typically low, ranging from approximately 20 to 40 cases per year. The case count averaged around 30 cases annually between 1993 and 2009. This low incidence rate underscores that the chance of any single person contracting HPS is minimal, even in areas where the virus is known to be present in the rodent population.
The annual case count for HPS is statistically insignificant compared to common diseases like seasonal influenza, which affects millions each year. The low number of cases highlights that the virus requires a very specific set of circumstances to successfully jump from its animal reservoir to a human host. The disease’s impact is localized and heavily dependent on specific environmental and behavioral factors that facilitate exposure.
Primary Reservoirs and Transmission Pathways
The persistence of hantavirus depends entirely on its natural hosts, which are specific species of wild rodents. In North America, the primary reservoir for the Sin Nombre virus, which causes the majority of HPS cases, is the deer mouse (Peromyscus maniculatus). These rodents carry the virus without showing signs of illness, shedding it continuously through their urine, droppings, and saliva.
Transmission to humans occurs primarily through the inhalation of tiny airborne particles, known as aerosols, contaminated with these infectious materials. When rodent nests, droppings, or urine are disturbed, the dried matter becomes aerosolized and subsequently breathed in by humans.
While the deer mouse is the most significant carrier, other US rodent species also host hantaviruses that can cause HPS. These include the cotton rat and the rice rat, reservoirs for viruses like the Black Creek Canal virus and the Bayou virus, mainly found in the southeastern states. Infection can also occur, though rarely, if the virus enters broken skin, such as through a rodent bite, or by touching contaminated surfaces and then touching the mouth or nose. Crucially, hantaviruses in the United States cannot be transmitted from person to person.
Geographic Concentration of Risk
The risk of hantavirus exposure is highly concentrated in specific geographical areas and environments. The vast majority of HPS cases in the United States have occurred in the western states, particularly west of the Mississippi River. The Four Corners region (Arizona, New Mexico, Colorado, and Utah) is the most well-known hotspot.
This concentration is directly linked to the distribution and population density of the primary host, the deer mouse. Typical environments associated with transmission are rural settings, including barns, sheds, cabins, crawl spaces, and other vacant buildings. The accumulation of rodent droppings and nesting materials in these undisturbed, enclosed spaces creates a high-risk environment for aerosolized viral particles.
Certain occupational groups face an elevated risk due to their frequent entry into these environments. These include construction workers, utility personnel, pest control technicians, and individuals involved in farming or maintenance of outbuildings. Exposure at home remains the most common setting for infection, especially in the western US, where peridomestic exposure accounts for the largest proportion of cases.
Practical Steps for Risk Reduction
Given that the primary risk comes from disturbing contaminated rodent materials, prevention focuses on safe cleanup and exclusion practices. Before entering a high-risk area, such as a vacant shed or cabin, it is important to first ventilate the space. Doors and windows should be opened for at least 30 minutes to allow fresh air to circulate and help inactivate the virus particles, which are sensitive to sunlight and fresh air.
When commencing cleanup, dust-generating activities must be strictly avoided; sweeping or vacuuming dried rodent droppings is unsafe. Protective gear, including rubber, latex, or nitrile gloves, should be worn before applying a disinfectant solution. A common and effective solution is a freshly prepared mixture of one part household bleach to nine parts water.
All contaminated materials, including droppings, nests, and dead rodents, must be thoroughly soaked with this bleach solution for five to ten minutes before removal. This “wet cleaning” method prevents the virus from becoming airborne. The materials should then be wiped up with disposable paper towels or rags, double-bagged, and placed in a sealed garbage receptacle. Finally, sealing potential entry points around homes and outbuildings—any gap larger than a quarter inch—is an effective long-term strategy for rodent exclusion.