El Niño Special Report:
Could El Nino Cause an Outbreak of Hantavirus Disease in the Southwestern United States? Could
El Niño, the weather phenomenon that has been reported in the news headlines, cause an
outbreak of hantavirus pulmonary syndrome (HPS) in the southwestern United States during
1998 and 1999? The short answer is, we're unsure. Nevertheless, as one potential
consequence of El Niño, there may be more deer mice than usual carrying the Sin Nombre
hantavirus, one of the strains of this virus type that has been associated with cases of
HPS in the Southwest. Therefore, people who live in areas of the southwestern United
States where hantavirus infection has been reported and confirmed should pay special
attention to hantavirus prevention recommendations.
El Niño and Its Connection to Hantavirus
"El Niño," also called the El Niño-Southern Oscillation (ENSO), is a
cyclically recurring disruption of the oceanic/atmospheric system in the tropical Pacific.
The El Niño phenomenon has important consequences for weather around the globe, including
increased rainfall across the southern regions of the United States as well as parts of
Central/South America, and drought in parts of Southeast Asia and Australia.
In the past, these changes in weather patterns have been associated with a myriad of
environmental impacts on humans, from destructive floods in Peru to crop failures and
starvation in parts of Asia and Africa. It is possible that the phenomenon may also
influence the number of cases of HPS in the Southwest in the coming years. How would this
occur? It all comes down to the rodents deer mice that carry the Sin Nombre hantavirus. If
increased rainfall results in more carrier rodents than usual, there may be more contact
between humans and infected rodents, and thus more exposure to the virus and more cases of
disease.
The 1991-1992 El Niño and Hantavirus
Let's look at this chain of events in more detail, using the 1991-92 El Niño event as
an example. At that time, a similar weather situation contributed to a deadly outbreak of
HPS, a previously unknown disease in a region known as the "Four Corners" in the
southwestern United States (an area shared by New Mexico, Arizona, Colorado, and Utah).
Available evidence suggests that dramatic increases occurred in
rodent populations in the Southwest during 1991-1993. How did this occur? Scientists have
hypothesized that increased rainfall brought by El Niño resulted in improved rodent
habitat quality in a region that is normally quite dry, and therefore rodents could find
greater cover, more possibilities for shelter, and a much greater food supply. Pinyon
nuts, and many other types of fruits, seeds, succulent vegetation, and arthropods (insect
populations also respond to increases in rainfall) were available in abundance. When the
El Niño phenomenon first began to occur during the summer of 1991, the population
densities of deer mice in New Mexico increased from less than one deer mouse per hectare
to over five per hectare 8 months later (A hectare is equivalent to about 2.5 acres).
Continuing El Niño conditions led to a second mild winter and abundant moisture, so that
by the spring of 1993 there were 20 to 30 rodents per hectare. It was during this period
that the well-known outbreak of HPS occurred.
What do increases in the sizes of rodent populations mean in terms of hantaviruses?
Field studies suggest that hantaviruses may be transmitted among rodents through
encounters between adult rodents (perhaps when an infected rodent bites an uninfected
rodent). As population density increases, rodents are more likely to encounter each other,
resulting in virus-transmission events. This pattern may have happened prior to the HPS
outbreak in the Four-Corners area. The prevalence of hantavirus infection among deer mouse
populations in the outbreak area in 1993 was very high 30%. When rodent density is higher,
the chance of encounters between rodents and humans is also greater, so the chance that
rodents are going to encounter and enter rural homes is greater.
Prospects for 1998 and Beyond
CDC-sponsored long-term studies of rodents and their environment in the Southwest
indicate that rainfall has been unusually heavy for the past 6 months, and the vegetation
has been much more abundant than usual. The density of deer mice populations at these
locations is six mice per hectare. Nevertheless, it is still too early to say that these
changes in climate and vegetation will result in a still larger increase in rodent
populations. The unusual rainfall patterns and resulting habitat improvements would have
to continue for many more months for this to occur.
In addition, the current prevalence of hantavirus infection among deer mice in the
southwestern United States is 10%-15%, still much lower than during the 1993 outbreak.
However, if rodent populations continue to rise, rates of contact among rodents could
result in higher prevalence of the Sin Nombre hantavirus within reservoir populations.
Does all this mean that there is going to be another outbreak of HPS as a result of
these conditions? Making such projections is an inexact science, and there is no
guarantee that weather conditions and resulting rodent population densities will follow
the 1991-1992 El Niño pattern. Nevertheless, based on what we know about the previous El
Niño occurrence, if the unusually warm and wet conditions continue, rodent population
densities could continue to increase, so that the period of highest potential rodent
population density, and the period of highest potential human risk, could occur in spring
1999. CDC will continue to monitor rodent population densities and the prevalence of the
Sin Nombre hantavirus among these rodents to refine our projections.
If You Live In An Area Where Deer Mice Live, Here's What You Can Do
Since the risk for hantavirus infection may be somewhat higher than usual next spring
in the southwestern United States, people living in this area should pay extra attention
to recommendations for preventing hantavirus infection. Your chances of coming into
contact with infected deer miceand your chances of exposure to hantaviruscan
be lowered if you use common-sense precautions. This means keeping a clean home and /or
workplace to discourage rodents from entering these areas, rodent-proofing these areas as
much as possible, and trapping and disposing of rodents properly if they do enter.
CDC has put together a set of guidelines and tips that you can use to help lower your
chances of coming into contact with deer mice. This information is found in the Prevention
section of this "All About Hantaviruses" web page. It is presented in several
parts:
Please click on each subject in which you have an interest.
Other El Niño Resources
The web has good information describing El Niño, and we have identified here several
sites you may find useful.
NOAA: El Niño Forecasts,
Observations and Research From the National Oceanic and Atmospheric Administration.
Links to all aspects of the phenomenon.
El Niño
Loss Reduction Center. From the Federal Emergency Management Agency
(FEMA). Information on preparedness tips, how to mitigate
El Niño-related losses, weather forecasts, and so on.
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