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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. Precipitation extremes map


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).

Peromyscus maniculatus, the deer mouse 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 mice—and your chances of exposure to hantavirus—can 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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This page last reviewed Friday, June 18, 2004

Special Pathogens Branch
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