The Hantavirus’s unusual link to El Niño

Guest Blogger: Michael B. Paccassi (MESM 2017) on assignment for ESM 437


Toxic algae blooms, hammerhead sharks, and poisonous snakes; El Niño is here and it is bringing with it a host of El Niño oddities.  These unusual guest appearances—like the rare yellow bellied sea snake that washed up on a beach in San Diego this past January – are the result of changing oceanic and atmospheric conditions brought on by the abnormally warm waters of the El Niño Southern Oscillation (ENSO).  But the latest addition to this list of impacts may be far from the ocean. The Hantavirus, a potentially fatal disease transmitted through contact with infected rodents, may soon become an issue in the aftermath of our latest El Niño .[i]  Is this rodent-borne disease something we have to worry about? Some researchers say yes.

In the spring of 1993, scientists began noticing a correlation between El Niño and an increased occurrence of the disease following a significant ENSO event in 1992. From 1993 to 1994, 52 cases of Hantavirus were confirmed in the “Four Corners” states (New Mexico, Colorado, Utah, and Arizona).i Over the following two non-El Niño years, just eight cases were reported.i

A similar pattern was also recorded during the following large El Niño in 1997. Researchers discovered that there was a five-fold increase in Hantavirus cases in the Four Corners states in 1998 and 1999.[ii] With a 38% mortality rate and symptoms including fatigue, fever, and muscle aches, this disease commonly infects individuals who come in contact with hantavirus-infected rodents or feces.[iii]

Based on the prior two non-El Niño years, only six cases should have been expected, instead of the 33 reported.i Despite early warnings to the public by the New Mexico Department of Health concerning the possibility of outbreak, the years following this El Niño brought a significant increase in Hantavirus outbreak. While the exact reason why such a significant increase occurred is still unclear, El Niño seems to be highly linked.

So, why should we care about this here in California? It turns out that after the 1997 El Niño, there was an especially prominent cluster of Hantavirus cases in southern California and in the foothills of the Sierra Nevada Mountain range as well.i Given this information, it may be reasonable to assume that we can expect to see another major increase in the years following this latest El Niño.

While this may sound frightening to many, the results of this study are not meant to cause alarm. Rather they are meant to shed light on the complex nature of El Niño.

It may be impossible to state with confidence that El Niño causes the increased occurrence of Hantavirus, but the results of these findings should be considered. As we all get ready for the coming winter, a look into the past can prepare us for some of the strange, and sometimes overlooked, effects of El Niño.


Michael Paccassi is a first year MESM student (2017) specializing in Water Resources Management (WRM).


[i]“Hantavirus.” Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 20 Feb. 2014. Web. 29 Nov. 2015. <>.

[ii] Hjelle, Brian, and Gregory E. Glass. “Outbreak of Hantavirus Infection in the Four Corners Region of the United States in the Wake of the 1997–1998 El Niño–Southern Oscillation.” The Journal of Infectious Diseases J INFECT DIS 181.5 (2000): 1569-573. Web.



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