Officials have issued a health warning over an incurable rat disease after four people died from the infection this year.
Hantavirus, which is fatal in 36 percent of cases, typically causes about 30 cases a year in the U.S., mainly in the western and southwestern areas.
But in some states, there is now an increase in the disease.
In the outbreak hotspot Arizona, authorities have issued a warning after seven cases and three deaths were recorded this year, compared with 11 cases in the six years from 2016 to 2022.
Two cases and one death have also been reported in California, a region where the virus has not been detected for two decades.
The map above shows the number of hantavirus cases by state from 1993 to 2021. Colorado and New Mexico are the major hotspots
Recent extreme temperatures combined with heavy rainfall are thought to be causing a rise in infections in some areas as this causes rodents that transmit the virus to seek shelter indoors from the heat and rain, where they come into contact with humans.
The heavy rainfall can also stimulate vegetation growth and food supply for rodents, which can lead to increased rodent activity and, later, possibly an increase in their population.
Arizona health officials said: ‘Hantavirus is a serious and sometimes fatal respiratory disease.
“It is not limited to one geographic location. It can be present in many areas of the southwestern United States where there is rodent activity, even if mice are never seen.”
Rats and mice can carry the hantavirus, but show no symptoms.
But they can also shed the virus through their feces, saliva and urine, which can then be inhaled by people who touch contaminated surfaces.
Nine to 33 days after infection, patients suddenly develop fever, muscle aches, nausea and abdominal pain.
Hantavirus is carried by rodents such as the deer mouse (pictured). It does not cause symptoms in them, but can be spread through their urine or saliva.
This then leads to breathing difficulties, a condition called Hantavirus Pulmonary Virus.
This is where the virus infects cells that line the small blood vessels in the lungs, causing them to leak and the lungs to fill with fluid. Death occurs from respiratory failure or shock.
There are no specific treatments for the condition. Instead, doctors use infusions and over-the-counter medications to relieve symptoms.
Most hantavirus infections occur in Colorado and New Mexico. Since 1993, 119 cases have been recorded in both countries.
Arizona has the third highest number of cases, with 85 cases, followed by California with 78 cases and Washington with 59 cases.
In April, a person in Sierra County, California, died from hantavirus, the first case in the region in more than two decades.
Officials said at the time: “Although rare, hantavirus can be very serious and fatal.
‘About three cases of hantavirus disease are reported each year in California, and about one in three people with hantavirus disease dies.’
There are concerns that the disease will be detected in new areas as rising temperatures drive rodent movements.
