Several reports describe an expansion out of the tropics of parasites that directly harm man. The most troubling diseases, perhaps, are connected to mosquitoes that have reached North America. Dengue fever, caused by a virus transmitted by the mosquito, Aedes aegypti has until recently been considered solely a tropical problem. The mosquito is distributed throughout most of the tropics including Puerto Rico and other Latin American countries, and the Paleotropics. The disease occurs in several forms, including one, dengue hemorrhagic fever, which can be fatal to humans. The mosquito, better known as a vector of yellow fever, occurs in 23 states of the U. S. and has recently reached California where it has eluded control and is spreading. At this time the vector is distributed more widely than the virus but the situation is fluid (see map).
Both yellow fever, now rare in the U.S. but not in the tropics, and dengue fever, are caused by viruses called flaviviruses, in a genus that also includes species responsible for West Nile virus, tick-borne encephalitis and other diseases. The viruses live and reproduce within adult mosquitoes the females of which transmit the virus to mammalian or avian hosts when obtaining blood, a necessary step in preparation for egg laying.
Yellow fever eradication programs of the last decade of the 20th century in the eastern and central U.S. and in South America considerably reduced the mosquito’s range but were later discontinued leading to dramatic range recovery.
Presently, A. aegypti is found throughout tropical and sub-tropical areas of the world although its natural distribution was restricted to Africa. The mosquito probably reached the New World via sailing ships during the period of transoceanic exploration and colonization.
Unlike many species of mosquito, A. aegypti lays eggs on unflooded surfaces which are likely to become flooded with fresh, or brackish water in the near future. Some strains may have adapted to salt water. Individual females stretch out egg laying temporally and spatially to include several locales and times, thus increasing the probability of choosing a successful site. Eggs can tolerate desiccation for months before hatching in water. This species often chooses egg-laying sites around homes and other human-dominated areas where it accepts any container such as pots, tires, cans, etc, as well as ditches as long as they hold water long enough to sustain the egg, larvae and pupa stages. (under optimal conditions, eggs can hatch within a day, producing adults 7 days later; conversely, given cool temperatures, hatching of eggs may be delayed for several months). Adults live 2-4 weeks. A. aegypti is also fully capable of reproducing where natural conditions prevail, taking advantage of tree holes, pond edges and puddles of any kind.
So far the dengue virus is absent from all the states where A. aegypti is found except Florida and Texas, where cases of dengue fever have been reported. For the disease to occur, the most likely sequence calls for a human, infected by the virus and having just travelled from the tropics to the U.S. to be bitten by a mosquito in the U.S., a scenario seen in other diseases. The infected human then serves as a source of more viruses for other A. aegypti mosquitoes. The presence of an already-established mosquito population thus makes disease transmission and spread much more likely. If this scenario unfolds in California involving, for example, immigration across the U.S.-Mexican border, it will be the 3rd state to record a locally-originated case of the disease. Dengue fever is also now reported in France and Italy.
Interestingly, A. aegypti has been present in Tucson, Arizona for about 20 years with, despite its proximity to Mexico, no cases of dengue fever known to have occurred. An hypothesis advanced to explain this pattern holds that in Tucson, temperatures rise above the tolerances of the mosquitoes after they reach adulthood, killing them before the virus can incubate in them (it takes 4 to 10 days for an infected mosquito to be able to transmit the virus).
A second mosquito species, Aedes albopictus, the tiger mosquito, may have been introduced from Mexico and, or, from Japan in a shipment of used tires. The species is native to Southeast Asia but has spread widely through human actions. The tiger mosquito resembles the yellow fever mosquito and is also able to transmit the dengue virus. It is a recent invader of the U.S., and is now found in most eastern and central states. The adults will bite a wide variety of mammals or birds and are strongly attracted to humans. The mosquito can complete its life cycle (egg to adult in 7 to 9 days.
A. albopictus was first discovered in Florida in scrapped tires in 1986 and spread throughout the entire state by 1993. Its adults seek sites similar to A. aegypti to lay eggs, and its larvae outcompete A. aegypti larvae for food, thus growing faster. The tiger mosquito may in some places, actually be reducing the population sizes of A. aegypti. But this fortuitous outcome is hardly cause for celebration for the tiger mosquito serves as a vector for yellow fever, dengue fever , certain encephalitis viruses and West Nile virus, not to mention some diseases involving microscopic worms (nematodes) carried by the adult mosquito.
Chikungunya, another tropical viral disease spread by mosquitoes including both Aedes species, turned up in Florida in 2013 with an sudden appearance of 24 cases in Martin Co, (east-central area of the state). Symptoms, which resemble those of dengue fever, include fever and severe joint pain. A victim could suffer from Infection by both chikungunya and dengue viruses and because of the similar symptoms, distinguishing between the two diseases would only be possible by determining the identity of the virus. This year (2014), three new cases have occurred in Florida outside the Martin County hotspot, all of them in persons recently returned from the Caribbean. The virus has spread widely in the Caribbean since its appearance on the island of St. Martin in December 2013. The first known cases of chikungunya outside of the tropics were reported for northeastern Italy in 2007 where 17 cases were confirmed, all traceable back to Indian Ocean countries.
The principal vector in Italy is A. albopictus which was first recorded in the country (and in Europe) in 1990 and quickly spread throughout Italy, extending into southeastern France in 2004. Warmer temperatures may have facilitated survival of the vector which was found at elevations as high as 600 m above sea level. With the vector well-distributed, returning travelers carrying the virus can potentially be bitten by the mosquito(s), thus reintroducing the disease and serving as a platform for its spread. In fact, this occurred in 2010 in southeastern France where two children who had not traveled outside their village were infected.
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