Until 10 years ago, Americans thought West Nile was a disease found Africa, Asia and the Middle East, said Chester Moore, an infectious disease researcher at Colorado State University.
"As disease vectors, mosquitoes have the ability to significantly change the health or health threats to a community - much as we've seen with the slow advancement of mosquito-transmitted West Nile virus across the United States," Moore said.
West Nile can infect people and horses. Only 20 per cent of people are likely to suffer symptoms, but in its most severe form it can attack the nervous system and cause dangerous brain inflammation.
About a third of horses infected by the virus develop severe disease and die, or are so affected that euthanasia is eventually required. A vaccine is available for horses.
Moore has kept a database since 1986 of incidents involving the accidental importation of mosquitoes to the US.
His monitoring is part of efforts to ensure that new diseases don't become a threat to the United States.
Mosquitoes infect one billion people and countless animals around the globe each year with diseases and cause millions of deaths.
There are more than 3000 varieties of mosquitoes in the world. Only about 150 of them are native to the United States, yet only a few species carry and transmit certain infectious diseases, and an invasion of non-native mosquitoes can open up a new population to an infectious disease that hasn't been established in that area or country before.
"The introduction of a new variety of mosquito into any population of humans and animals can pose a significant health threat because they may introduce a new disease or strain of a disease," Moore said, citing the 1999 arrival of West Nile.
It's not hard for a mosquito to invade new territory in today's global society.
For example, about 10 years ago the Asian tiger mosquito - technically known as the Aedes albopictus - caught a ride on bamboo imported into the Los Angeles area. They infested shipping ports and local florist shops. It was not the first time the insect had invaded; since 1984, the Asian tiger also has been discovered in 25 states, including Colorado, Texas, Chicago, Minnesota, New Jersey, Ohio, Indiana, Nebraska and Pennsylvania, and the District of Columbia.
In 1994, it was documented in all counties of Georgia, Florida, South Carolina and Tennessee, and it's now well established in the South.
It carries an exceptional number of diseases for a mosquito species, including Chikungunya fever, dengue fever, St Louis encephalitis, yellow fever, Cache Valley virus, West Nile virus, and dog heartworm.
Moore, a professor in the Department of Microbiology, Immunology and Pathology at Colorado State University, began tracking the invasion of mosquitoes while working at the Centers for Disease Control and brought the work with him when he began researching vector diseases at the university in 2003.
Moore is contacted when an official discovers and confirms a foreign mosquito in the US. While most invaders don't survive, some become established only in a small area, but their population can explode if the conditions change in their favour, as evidenced by the Asian tiger and the Aedes aegypti, the yellow fever mosquito.
Several invading species, including the Asian tiger and the yellow fever mosquito, lay eggs that can survive winter conditions to hatch in the spring. These eggs also survive drying, which makes it easier for them to be carried around the world on tyres, bamboo and other plants and other surfaces.
The Aedes aegypti, which spreads yellow fever and dengue, or breakbone fever, was most likely the first stowaway to the Americas. It is native to Africa and scientists believe that it was brought to America through slave trade in the 1500s.
In 1501, the Spanish crown authorised Nicolás de Ovando, at that time the governor of the Indies, to begin importing African slaves. From its initial introduction into the Caribbean, the yellow fever mosquito spread rapidly via the Spanish shipping routes, according to Moore.
It quickly colonised Central and South America, Mexico and the coastal regions of the US. At one point, it extended as far north as Philadelphia and Boston during the summers.
Benjamin Rush, a physician and signer of the Declaration of Independence, described a dengue epidemic in Philadelphia in the late 1700s.
Scientists now know it was transmitted by this mosquito. It remains an important mosquito, in terms of human and animal health, and is a focus of Moore's monitoring work. Today, it is mostly found only in the southern regions of the United States.
Another significant invader is the Ochlerotatus japonicus, which was first introduced into New Jersey and New York among scrap tyres imported from Asia.
While fewer than 10 foreign varieties of mosquitoes have been documented in the US since Moore started maintaining the database, he and others have noticed other invasions, and he now tracks ticks and other disease vectors moving into the country.
Moore also tracks how mosquitoes indigenous to regions within the United States are expanding their territory.
"Several indigenous mosquito species are changing their distribution across the states if you look at patterns from the last 30 years," said Moore. His database has documented several species on the move, becoming established in new areas. The species are moving in a different direction - some toward the south, some northward and some eastward.
The reason for the movement remains a mystery.
"If you look at old distribution maps from the 1950s and 1960s for the Culex coronator, you'll see they once were only found in the lower Rio Grande Valley of Texas. Now, they've moved across the Gulf States into Florida and the Carolinas - they've really moved around.
"I've been looking at the distribution to see if global warming has caused the changes, but we just had an email from someone in the Minnesota who found a Canadian species of mosquito - so that species is moving south. We need to do a lot more work before we can answer questions about why mosquitoes are moving around so much.
"Increased international commerce and travel will likely lead to more introductions," Moore said. "And there are multiple paths of introduction. The fact that a species is not an important vector in its native environment does not mean it will not be a vector in a new environment."
Moore has a significant role in tracking the presence of West Nile virus in the community. He runs the laboratory at the university that tests all mosquitoes gathered in Larimer County for the virus. Thousands of mosquitoes are caught around the city and the two known to carry West Nile virus are sorted out and brought to Moore's laboratory. Each week a newly captured batch is tested for the West Nile virus by lab technician Kamiey Price.
This year, Price and Moore have encountered a low number of West-Nile-positive mosquitoes, although that number will likely grow significantly with the warming weather and decline in rain.
Despite recent rain, Moore anticipates that it will still be a year with a low level of transmission of the virus to people and animals because cooler weather slows down the development of the virus. Next year could be a different story, depending upon the weather conditions.