Experts warn that a developing “super El Niño” in the Pacific might not only reshape weather patterns but also increase hantavirus risks in parts of the United States this summer by boosting rodent populations carrying the disease. A recent hantavirus outbreak linked to the MV Hondius cruise ship has raised public awareness of the virus, prompting monitoring and quarantine efforts globally, including in the U.S.
Hantavirus Monitoring and Surveillance
The U.S. Centers for Disease Control and Prevention (CDC) began tracking hantavirus in 1993 during an outbreak in the Four Corners region, encompassing Arizona, Colorado, New Mexico, and Utah. This event, linked to the Sin Nombre strain, led to the identification of hantavirus pulmonary syndrome (HPS) and launched national surveillance efforts as health officials suspected undetected cases prior. Despite NOAA data showing 1993 was ENSO-neutral, above-average rainfall was reported in the Southwest, a condition that might return this summer under similar weather patterns.
Understanding Super El Niño
A “super El Niño” refers to an exceptionally strong occurrence of El Niño, which involves unusual warming of Pacific Ocean surface waters. While El Niño events happen every few years, a “super” event means ocean temperatures rise significantly, over 2 degrees Celsius above average for months, disrupting wind patterns and global weather effects.
This month, NOAA’s Climate Prediction Center predicted an 82% chance of El Niño emerging between May and July, with a 96% likelihood of continuing through December to February 2027. In the U.S., El Niño affects weather events like a weaker Atlantic hurricane season and increased rains during the Southwest’s monsoon, possibly starting as early as June.
Implications for Hantavirus Cases
Researchers suggest a strong El Niño may increase hantavirus risk by changing environmental conditions. More rainfall and warmth enhance vegetation growth, providing better food and shelter for deer mice, Sin Nombre virus carriers in the U.S.
This “trophic cascade” concept links more rain to more vegetation, supporting more rodents and increasing hantavirus risk, according to research by Yates et al. Other studies suggest milder winters and extended breeding seasons may also contribute to rodent surges, given El Niño’s association with warmer, wetter conditions.
Expanding rodent populations raise human contact risk, especially in rural or agricultural zones, cabins, or sheds, echoing dynamics from the 1993 outbreak that alerted the nation to hantavirus.
Symptoms of Hantavirus
An increased hantavirus risk in the U.S. would likely involve the Sin Nombre virus, primary in North America, not the Andes strain seen earlier this year. Sin Nombre spreads through contact with infected rodent excretions, inhaled when stirred up in enclosed spaces, with no person-to-person transmission.
Sin Nombre is the main cause of hantavirus pulmonary syndrome, starting with “flu-like” symptoms like:
- Fever
- Fatigue
- Muscle aches
- Headache
- Chills
- Gastrointestinal issues
Four to ten days later, a respiratory phase can develop, featuring:
- Cough
- Shortness of breath
- Chest tightness
- Rapid breathing
- Intensified fatigue
In severe cases, symptoms include:
- Low blood pressure
- Irregular heartbeat
- Respiratory failure
HPS carries a fatality rate as high as 60%.
Characteristics of Deer Mice
Deer mice differ from house mice, as house mice typically don’t carry hantavirus. Deer mice, or field mice, can become indoor pests, especially in colder months. Increased rainfall enhances vegetation, boosting food and shelter for deer mice, potentially growing their populations.
According to Orkin Entomologist Shannon Sked, house mice are usually gray or brown, while deer mice have gray or brown fur with a white belly. Care is essential when handling droppings as hantavirus can persist for days.
“Use a sanitizer before cleanup,” Sked advises. Preventing mouse entry requires sealing openings as small as a dime.
Preventive Measures and Future Outlook
Public health experts emphasize that hantavirus infections are rare and preventable with precautions. Avoiding wild rodents, sealing home gaps, and applying safety measures can reduce risks. The impact of El Niño on deer mouse populations and related hantavirus risk is uncertain, though no year since 1993 has been without HPS cases in the U.S.
“People see ‘rare’ diseases as seldom occurring, but HPS cases have existed annually since its discovery,” says associate professor Stephanie Seifert.