TRAVEL AND IMMIGRATION
Historically, migration of populations has often led to epidemics of disease in countries where people have no immunity to the disease. Because of trade, immigration, and wars, yellow fever, malaria, hookworm, leprosy, smallpox, measles, mumps, syphilis, and many other infectious diseases have been brought to the Western Hemisphere. More recently, the HIV epidemic was trans-mitted worldwide by means of travel and immigration.
Few diseases carried by travelers spread efficiently in the United States environment because of enforced vaccination, clean water, and insect and rodent control. However, there is growing concern that vector-borne diseases, such as dengue, may be transmitted by mosquitoes if a reservoir of infected humans is established. The CDC maintains an active surveillance system to prospectively monitor and halt the incidence of many diseases.
The fact that AIDS reached pandemic proportions in less than a decade after its recognition attests to the efficiency of world travel in spreading disease. The significance of such rapid transmission rates is especially dramatic in that HIV essentially requires inti-mate contact between two people through sexual activity or shar-ing blood through needles.
The reservoir of HIV-1 in the United States is estimated to be approximately 800,000 to 900,000 people, with approximately 40,000 new cases each year. It was probably first introduced in the 1970s when asymptomatically infected travelers returned to the United States after having acquired the virus in other coun-tries. HIV-2, which is similar to HIV-1 in causing immunodefi-ciency but less contagious in the early stages, is most prevalent in West Africa. The public health challenge is to set up surveillance and control mechanisms for this disease so that it is not regularly introduced into the United States. Because HIV-2 is rare, patients with signs of immune dysfunction do not need to be routinely tested for it. However, patients with immunodeficiency should be tested for HIV-2 if they have negative HIV-1 confirmatory test results or have traveled from countries where HIV-2 is prevalent. All donated blood must be screened for HIV-2. Routine sero-prevalence studies are conducted to validate low prevalence in the United States (Grant & DeCock, 2001).
Although there are substantive plans to eliminate TB in the United States, it remains a growing epidemic in developing nations. Im-migration has always been an important influence in the dynamic epidemiology of TB in the United States. In 2001, the incidence of TB was eight times greater in the foreign-born than in the native-born population of the United States (CDC, 2001h).
The association between immigration and transmission risk is greatest in urban areas because these locations are frequently heavily populated and visited by foreign-born people. These locales are also often the epicenter of the HIV epidemic, a population with suppressed immunity to TB. The combination of social, financial, and immunologic risks makes the goal of TB elimina-tion in the United States very challenging.
A positive purified protein derivative (PPD) skin test estab-lishes that TB infection has occurred at some time in a personâ€™s life. Because it does not provide information about current in-fectivity, it cannot be used to determine transmission potential. The complexity of PPD interpretation is increased because of the common use of the bacillus Calmette-GuĂ©rin (BCG) vaccine in many foreign countries. After receiving BCG, individuals are often PPD positive for a prolonged time, decreasing the ability of the PPD to serve as a TB screen.
Malaria, yellow fever, and dengue are diseases that cause signifi-cant morbidity and mortality throughout the developing world. These diseases are spread by infected mosquitoes. Many other vector-borne parasitic diseases in developing countries rely on mosquitoes and other organisms to complete their life cycles and transmit disease.
Dengue fever is an example of the risk of imported vector-borne disease. The disease is caused by a virus that is spread through human populations by the Aedes aegypti mosquito. The mosqui-toes thrive in tropical zones and breed in stagnant water sources. Travelers, immigrants, and returning military personnel can serve as reservoirs of infection. A recent increase of dengue virus in the Caribbean has caused concern that outbreaks may occur in the United States in areas where there are vector mosquitoes.
Infection from dengue produces flulike symptoms of fever, chills, eye pain, joint pain, and sometimes, a hyperpigmented rash. Symptoms often wax and wane and are generally self-limited. A small proportion of patients may develop hemorrhagic disease, which can be life-threatening in extreme forms. There is no spe-cific treatment for this infection. Control efforts rely on local effective mosquito control.