An infestation of Aedes albopictus (Asian tiger mosquito), a mosquito known to transmit epidemic dengue in its native Asia, was discovered in 1985 in Harris County, Texas. The mosquito was probably introduced in standing water in used tire casings imported from Asia. The mosquito is a competent vector for the yellow fever, dengue, and West Nile viruses. Figure 1 shows the current range of A. albopictus in the United States.

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Tick-borne diseases.
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Arboviral Encephalitis
Encephalitis caused by mosquito-borne viruses (called arboviruses) is common in the United States. (Arbovirus is short for arthropod-borne virus.) Figure 2 shows the mosquito life cycle. Figure 3 shows the incidence over a sequence of years. Sentinel animals, such as caged rabbis or chickens, are tested periodically for antibodies to arboviruses. This gives health officials information on the incidence and types of viruses in the area.

A number of clinical types of arboviral encephalitis have been identified (Table 1); all can cause symptoms ranging from subclinical to severs, and even rapid death. Active cases of these diseases are characterized by chills, headaches, and fever. As the disease progresses, mental confusion and coma occur. Survivors may suffer from permanent neurological problems. Horses as well as humans are frequently affects by these viruses; thus, there are strains causing Eastern equine encephalitis (EEE) and Western equine encephalitis (WEE).

Viral Hemorrhagic Fevers
Most hemorrhagic fevers are zoonotic diseases, they appear in humans only from infectious contact with their normal animal hosts (Table 2). Yellow fever has been medically familiar for so long that it is considered a "classic" hemorrhagic fever. The yellow fever virus is injected into the skin by the Aedes aegyptii mosquito. Monkeys are the natural reservoir for the virus, but human-to-human transmission can maintain the disease. Local control of mosquitoes and immunization of the exposed population are effective controls in urban areas.

In the early stages of sever cases, the person experiences fever, chills, and headaches, followed by nausea and vomiting. This stage is followed by jaundice, a yellowing of the skin that gave the disease its name. This coloration reflects liver damage, which results in the deposit of bile pigments in the skin and mucous membranes. The mortality rate for yellow fever is high, about 20%.

Yellow fever is endemic in many tropical areas (Figure 4). At one time, the disease was endemic in the United States and occurred as far north as Philadelphia. The last U.C. case of yellow fever occurred in Louisiana in 1905 during an outbreak, resulting in 1000 deaths. Mosquito eradication campaigns initiated by the U.S. Army surgeon Walter Reed were effective in eliminating yellow fever in the United States.

Dengue is a similar but milder viral disease also transmitted by A. aegyptii. The disease is endemic in the Caribbean and other tropical environments, where an estimated 100 million cases occur each year (Figure 5). It is characterized by fever, severe muscle and joint pain, and rash. Except for the painful symptoms, which have led to the name breakbone fever, classic dengue fever is a relatively mild disease and is rarely fatal.

The countries surrounding the Caribbean are reporting an increasing number of cases of dengue. The disease does not appear to have an animal reservoir. The mosquito vector for dengue is common in the Gulf states, and there is some worry that the virus will sooner or later be introduced into this region and become endemic. (Figure 6 shows dengue cases in the United States.) Health officials are concerned about the American introduction of the Asian mosquito, A. albopictus. It transmits the virus by transovarian passage and from person to person.

Dengue also occurs in a second form, dengue hemororrhagic fever (DHF). DHF can induce shock in the victim and kill in a few hours; it is a leading cause of death among southeast Asian children.

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Dengue
Yellow fever
West Nile encephalitis