An El Niño year in 1993 unmasked a deadly virus carried by deer mice
In 1993 a cluster of sudden deaths in the Four Corners region of the American Southwest led to the discovery of hantavirus pulmonary syndrome. The culprit, Sin Nombre virus, lives harmlessly in western deer mice. People catch it by breathing in dust laced with rodent droppings, and between 30 and 60 percent of those who fall ill die.
The 1993 outbreak was linked indirectly to El Niño, whose weather boosted rodent numbers. Since then many related viruses have been found across the Americas. In South America the main cause is Andes virus, spread chiefly by the long-tailed pygmy rice rat. North America sees dozens of cases a year and South America more than 100, with isolated cases in Europe and Turkey. Rainfall, temperature and humidity all shape the risk because they drive rodent populations.
Symptoms arrive one to eight weeks after exposure and unfold in three stages. The first few days feel like flu, with fever, aches and a falling platelet count. Within about 10 days comes the cardiopulmonary phase: a racing or irregular heart, shock as the heart fails to pump enough blood, and tiny blood vessels in the lungs leaking fluid, which can cause respiratory failure. Death, when it comes, is usually sudden at this stage. Survivors may need months to recover and up to two years for breathing to normalise, but reinfection has never been seen, suggesting lifelong immunity.
The virus mainly targets the cells lining blood vessels and immune cells called macrophages, disrupting clotting and causing the leakage. Its genome comes in three RNA segments, and because segments from different strains can swap, new variants emerge readily. Outside a host the virus can persist around 10 days at room temperature and longer when chilled. Farmers, forestry workers and hikers face the greatest exposure.
No vaccine or specific antiviral exists. Care is supportive: heart monitoring, ventilation and sometimes ECMO, which oxygenates blood outside the body. Public health advice centres on keeping rodents out of homes and cleaning up after them safely. Andes virus is often said to pass between people, but a systematic review found the evidence insufficient.
Source: Hantavirus pulmonary syndrome
