Image caption Marty Baum, shown with Robin Pitman, worries about getting the fever again

Mosquitoes that transmit dengue fever are spreading across the US. But outbreaks of the disease are only occurring in some areas, and scientists are trying to find out why.

Marty Baum was watching TV at home with his girlfriend, Robin Pitman, one Saturday night in August when he was struck down by a fever so severe he could barely stand.

"I was sweating my brains out and shaking so bad I couldn't control myself," said the 59-year-old man from Jenson Beach, Florida.

"If I get the flu, I have a clue a day or so ahead of time - maybe a swollen gland or an achy joint. But the thing that scared me that night was that I had no idea what may have caused this. I went from feeling great to running a fever of 102F (39C)," he said.

Within days an acute pain had developed behind his eyes.

"It felt like somebody was pushing on my eyeballs," he said.

"And then the rash started. I had to scratch myself with a hairbrush to relieve the itch. It was all encompassing and just miserable."

I'm very concerned about opening up a Pandora's box that could be as dangerous or more dangerous than dengue" Joel Biddle, Key West, Florida

Blood tests soon confirmed that Mr Baum and Ms Pitman had dengue fever - also known as break-bone fever - a mosquito-borne virus.

Dengue fever is a tropical disease that affects 40% of the world's population. Around 300 million people become infected every year. It is rarely fatal.

However, some victims develop dengue haemorrhagic fever, which causes bleeding and low blood pressure. These health issues contribute to an estimated 25,000 deaths a year.

Scientists are particularly concerned about the emergence of the virus in Jenson Beach, Florida. It means that dengue fever is spreading north from Key West, where one of the earliest outbreaks in the US was recorded in 2009. Previously there had been no outbreaks of the fever in Florida for decades.

The virus is transmitted by the Aedes aegypti mosquito, which usually lives in tropical regions. But scientists believe that climate change has made them more prevalent across the southern states of the US. An unexpected number of the mosquitoes have recently been recorded in California's Central Valley and in the San Francisco Bay Area.

Too hot

Not all of these regions have experienced dengue fever outbreaks, however. Scientists are trying to find out why.

In findings presented at November's annual meeting of The American Society of Tropical Medicine and Hygiene, experts studied two cities 2,000 miles apart - Key West and Tucson, Arizona. The mosquito Aedes aegypti can be found in both cities, but Tucson has not experienced an outbreak of the illness.

Mary Hayden, a scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, said that differences in climate may be affecting the mosquitoes' life span.

The dengue virus needs at least eight days to incubate inside the mosquito, she said.

"It may be that the temperatures in Tucson are so hot," she said, "and the relative humidity is so low that the mosquitoes may not be able to survive in that climate long enough for the virus to cycle within them."

The findings are not conclusive, she said, but they appear to be the most likely explanation why people in some areas with the Aedes aegypti mosquitoes get dengue fever - while people in others do not.

Mosquito map

Image caption Larva such as this one, shown in 2010, are helping scientists understand the fever

If climate plays a role, it would have implications for mosquito-control programmes. But scientists also need to know more precisely where the mosquitoes live.

Mosquitoes that infect people with the West Nile virus lay their eggs in ponds and sewers, for instance. These areas can be treated with insecticides.

But the Aedes aegypti mosquito lives near people. The insects are found in water that collects in gutters, flower pots, even in the folds of tarpaulins. These areas are harder to treat with insecticides.

Scientists are hoping that high-resolution satellite images will help detect such habitats and allow them to create a map of mosquito populations. The information will be combined with weather and climate data to assess the number of mosquitoes that are capable of carrying dengue.

"You can see fairly detailed aspects of areas that are difficult to survey [from the ground]," said Paul Bieringer, a scientist with NCAR's National Security Applications Program. The project is one of many tools being used in the US to control mosquitoes and to stop the spread of illness.

In Key West authorities are planning to introduce genetically modified male mosquitoes containing a gene that stops normal development. Their offspring will inherit the gene. It alters the way their cells function and causes them to die before reaching adulthood.

'Pandora's box'

Image caption Florida officials such as Lawrence Smart are collecting data on mosquito larvae

Some residents are concerned about the safety of releasing genetically-modified insects into the wild, however.

"This is very exciting technology, but when you are dealing with a genetically modified organism, there are a lot of things that should be looked into," said Key West resident Joel Biddle, who contracted dengue fever in 2010.

"I'm concerned about dengue - I wouldn't want anybody to get it. But I'm very concerned about opening up a Pandora's Box that could be as dangerous or more dangerous than dengue."

According to the website of the Florida Keys Mosquito Control District, genetically-modified insects have already been used to control pests in the US. There is "nothing inherently dangerous about the process", according to the website.

Like most people who get the fever, Marty Baum and Robin Pitman have recovered. But they - like others who live in the area - are worried. If they come down with the fever again, the consequences could be far more severe because their immune systems are weakened.

"We have to be really careful," said Mr Baum.