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. Last Updated: 07/27/2016

Unusually Warm Winter Increases Health Risks

MTA police officer avoiding puddles of melting snow on a square in Voronezh, one of the cities hit by hantaviruses.
WASHINGTON -- Experts have long feared that Earth's warming climate would cause tropical diseases such as malaria to spread into more temperate zones, but a dramatic example of an apparently climate-related disease outbreak cropped up this winter in a cold place -- Russia.

More than 3,000 cases of infections caused by hantaviruses have been reported so far in Russian cities and towns, including many that are within a few hundred kilometers of Moscow, such as Voronezh and Lipetsk. The viruses can cause a serious, and sometimes deadly, disease known as hemorrhagic fever with renal syndrome, or HFRS.

During Russia's more typically frigid winters, scientists believe, HFRS-causing viruses die off in the consistently below-freezing temperatures. But this winter has been anything but cold.

HFRS was last on a rampage in Russia in 1997, coinciding with another very warm winter. By mid-spring that year, the number of cases exceeded 20,000.

The viruses are transmitted to humans when infected mice set up house in homes, barns, sheds and other buildings. If droppings left by the mice are disturbed, the viruses waft up and out of the excretions like a miasma, infecting people who breathe the air.

Biologists estimate that the current population of rodents in Russia is 10 times as high as in previous years, and that one in three mice is infected with an HFRS-causing virus. Most researchers attribute the spike to the unusually warm weather, although some think a natural cycle in mouse populations may play a role.

"Global warming has tipped a balance," said Irina Gavrilovskaya, a scientist and physician at the State University of New York who has conducted research on HFRS at the Russian Academy of Medical Sciences in Moscow. "Because of the lack of snow cover on Russian fields, the country has had an explosion in numbers of virus-carrying mice."

With the coming of spring, Lyudmila Kirillova, the regional epidemiologist in Lipetsk, is predicting a new outbreak as the little snow that fell this winter melts and hibernating mice awaken.

Over the past decade, unusually warm winters and large populations of mice have also been responsible for outbreaks in New Mexico and nearby states of a related illness, hantavirus pulmonary syndrome.

More than 20 other hantaviruses threaten people in China, Korea, Northern and Western Europe, Argentina, Chile, Brazil, Panama, and Canada. Some of those outbreaks have also been linked to climate change -- higher temperatures or altered patterns of rainfall -- and its effect on rodents.

"Climate change is about more than a warming Earth," said David Blockstein, senior scientist at the National Council for Science and the Environment, a scientific advocacy group in Washington. "Climate change is turning environmental issues into public health issues."

HFRS is caused by one of four hantaviruses -- Hantaan virus, Puumala, Dobrava and Seoul. In Russia, Hantaan and Puumala are the culprits. Hantaan, from which the term hantavirus is derived, is named for the Hantaan River near the border between North and South Korea. The first hantavirus was identified there in 1978.

Symptoms of infection usually develop one to two weeks after exposure, but in rare cases may take up to eight weeks. Headaches, fever, chills and nausea, or a rash or redness around the eyes are early signs. "If left untreated, HFRS quickly leads to hemorrhaging of small blood vessels throughout the body and to kidney failure," Gavrilovskaya said.

"The risk this year is high," said nephrologist Martin Zeier of the University of Heidelberg in Germany, who specializes in HFRS. "Russia hasn't had a winter this warm since the mid-1990s. Infected mice are having a field day, running around the streets when they should be hibernating under a blanket of snow. Many more people are being exposed to the virus."

Zeier is seeing the same trend in other European countries. "HFRS is a growing problem in historically 'cold places,'" such as Scandinavia, he said.

The trend concerns Alla Bernshtein of the Russian Academy of Medical Sciences. "There is a definite change in plant life in areas where HFRS outbreaks have occurred," Bernshtein said. "The climate is warming. With it, tree and flower types of warmer regions are moving north. Unfortunately, those plants provide additional shelter and food resources for the mice that are carriers of the virus."

If climate change continues at its current pace, "we might easily see HFRS cases again reach the tens of thousands," Zeier said.

Zeier has treated HFRS in farmers and loggers living in small huts where infected mice had camped out. When the rodents' human roommates moved in, they inhaled dust from mouse droppings and contracted the virus. "All but one of the patients lived," said Zeier, "because they were treated in time." Ribavirin, the sole treatment for HFRS, is effective only if given early.

Most victims survive, said Pierre Rollin, a scientist at the U.S. Centers for Disease Control and Prevention, "although they might be very sick for a while. Complete recovery can take weeks or months. Some types of HFRS infection have higher mortality rates, however."

The death rate can be as high as 15 percent, said Karl Johnson, a retired virologist from the University of New Mexico. Johnson was one of three scientists who identified the Hantaan virus. "Ever since we found out exactly what it was," he said, "we've been hoping for a way to prevent the disease."

Almost 40 years later, researchers are getting close. "We're working on a vaccine that would protect people against all four of the viruses that cause HFRS," said Connie Schmaljohn, chief scientist at the U.S. Army Medical Research Institute of Infectious Diseases. "We soon should have [U.S. Food and Drug Administration] approval to test the vaccine in humans."

For now, Zeier said, there isn't much that people in affected areas can do but try to keep rodents out of their homes by setting traps, and using latex gloves and bleach when removing dead mice or mouse droppings. "Above all," he said, "forget the dusting and sweeping."

In Russia, said Zeier, "it all has come down to having the best mousetrap."