El Niño could bring rain to the Western States - and disease

Rain over the Grand Canyon El Niño
CC BY 2.0 Corey Leopold

The possibility that El Niño will develop over the summer months could bring good news for those suffering from drought in the Southwest, but it may also increase the incidence of rodent- and mosquito-borne diseases, some researchers say.

Earlier this month, the International Research Institute for Climate and Society and the National Oceanic and Atmospheric Administration (NOAA) released a statement that there is a 70 percent likelihood that El Niño would emerge by the middle of this year. The prediction was based on observations that water just under the surface layer of the Pacific Ocean is warmer than usual. Warm water will likely come to the surface and travel eastward, sparking an El Niño, as it has in the past.

“El Niño’s only happen once every four to seven years. On most occasions, the chances for El Niño are less than five percent, since the background state in the Pacific has to be extremely favorable and that is often a state that we do not observe,” said Michael Ventrice, an operations scientist at Weather Service International, a division of The Weather Company. “There have only been a hand full of times when the government has raised the odds to this level.”

El Niño years are characterized by rainfall in areas that are normally dry, and drier conditions in areas that are normally wet. The last two El Niño events, in 2009 to 2010 and 1997 to 1998, brought floods to California. Texas, Arizona and Mexico also experienced increased rainfall, but not flooding. Precipitation from El Niño could provide temporary relief after what the University Corporation for Atmospheric Research reported as the driest year in the Southwest. The region is in an extreme drought, with rainfall far below average for the past three years.

Some public health experts are concerned that rainfall will cause population growth among virus carrying pests such as rodents, which can transmit Hantavirus Pulmonary Syndrome and mosquitoes, which can transmit dengue. But other researchers say that the link between El Niño and health risks remains unclear.

Tim Watts - Field mice are carriers of Hantavirus Pulmonary Syndrome/CC BY 2.0

Though these diseases are extremely rare in the United States, some researchers at Colorado State University and Texas A&M say that people should be prepared for the risk.

Paul Roundy, associate professor of atmospheric and environmental sciences at the University of Albany, warned that drought relief would only be temporary, however. He added that increased vegetation growth from heavier rainfall would likely dry up in the following year, increasing the risk of fire. Even so, he said, “many of El Niño's impacts might be welcome this year.”

The plant growth triggered by rain can lead to more seeds and insects such as grasshoppers and beetles, which provide more food for rodents.

Hantavirus Pulmonary Syndrome, spread through rodent feces and urine, can be deadly. According to the Centers for Disease Control and Prevention, the disease has a 36 percent mortality rate. In the last 20 years, about 230 people have died from the disease in the U.S.

Mosquitoes lay their eggs in pools of stagnant water after the rains. Stagnant pools don’t have to be large: mosquito larva can survive in water bowls for pets or in the water that collects in the dish under a potted plant.

On April 10, Texas announced its first case of Hantavirus Pulmonary Syndrome of the year. If breeding conditions for rodents improve because of El Niño, more cases could occur, say some public health experts.

Colin and Sarah Northway - Mosquito larvae can survive in difficult conditions/CC BY 2.0

Several studies have shown that countries with higher rates of dengue are more at risk of having disease prevalence affected by El Niño and this has some implications for south and western parts of the U.S. Dengue has gained a foothold in Southern Florida over the past few years. But despite the presence of the Aedes egyptii mosquito, the carrier of the disease, dengue has not spread to the region. Outbreaks of about 20 infections have been recorded in Texas and Hawaii in recent years. Dengue has a mortality rate of 2 to 40 percent, depending on the severity of the infection.

Not all scientists are concerned about El Niño’s effects on disease prevalence. There is some debate in the scientific community about the extent of the correlation between disease prevalence and El Niño conditions in the United States and other parts of the world.

The CDC said in an email that El Niño will not change how they monitor diseases such as Hantavirus Pulmonary Syndrome.

Part of the complication is that multiple factors contribute to disease prevalence. Studies have shown that increased time indoors and better housing have led to fewer cases of dengue in areas where dengue has been previously reported.

Furthermore, recent studies show that climate change has a larger effect on disease prevalence than El Niño does: as global temperatures increase, dengue-carrying mosquitos move into areas they could not survive in previously. “In the big picture, [El Niño] events will likely take a second seat to global climate change as important determinants of future outbreaks,” said Brian Hjelle, professor of pathology at the University of New Mexico.

The effects of El Niño are always unpredictable, but some scientists and state health departments say they will be monitoring disease incidence as El Niño comes in.

“The fact that El Niño advisories have already been issued should give folks ample time to prepare,” said Mike Halpert, acting director at the Climate Prediction Center.

Tags: California | Florida | Insects | Malaria | New Mexico

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