States, there were nearly 3,000 cases reported in this country (Fenner et al., 1988), all of which occurred in or were the result of transmission from infected foreign travelers. The same pattern holds for dengue (see the earlier discussion), which is currently hyperendemic in parts of Asia and the Caribbean. In 1990, the U.S. Public Health Service (PHS) reported 102 suspected cases of imported dengue, although only 24 cases could be confirmed (Centers for Disease Control, 1991c).
Many other diseases common in other countries are periodically introduced into the United States by travelers. Lassa fever, an acute viral illness, is endemic to West Africa. It first came to the attention of U.S. health officials because of a series of epidemics in Africa between 1969 and 1974 (Carey et al., 1972; Monath et al., 1973; Fraser et al., 1974). Many of the original outbreaks, including almost all of the secondary cases, involved health care personnel who had cared for infected patients. The best known of these instances was an outbreak at an American mission hospital in Jos, Nigeria (Frame et al., 1974), which is the subject of a popular book, Fever! (Fuller, 1974).
Because Lassa fever is endemic to parts of Africa, sporadic introductions into the United States by travelers returning from that country are likely (see Box 2-5). However, since the virus is maintained in a rodent species not normally found in the United States, it is unlikely that the disease will become established in this country (barring the emergence of a suitable rodent host in the United States or changes in the host range of the virus). Nevertheless, each imported case of the disease has the potential to be followed by a significant number of secondary infections among close contacts.
Malaria, considered one of the greatest contemporary killers among infectious diseases, is no longer endemic to this country but is one of the diseases that is most frequently imported. The CDC reported 1,173 imported malaria cases in 1991 (Centers for Disease Control, 1992b). Figure 2-5 shows malaria incidence data for 1930 through 1990. During this period, there were four major peaks of malaria, including one that began in 1980. During this latest peak, virtually all of the cases were imported.1
Blood transfusions are an occasional source. Spread of the disease as a result of needle sharing by heroin users led to an outbreak of 47 cases in California in 1971 (Friedman et al., 1973). In addition, two imported cases in 1990 were iatrogenic, occurring in patients with late-stage Lyme disease who went to Mexico for malariotherapy. Malariotherapy is not recognized in the United States but is available in some foreign countries as an unconventional treatment for spirochetal infections (e.g., syphilis and, in this case, Lyme disease) in which patients are inoculated with blood containing Plasmodium vivax, one of the four species of malaria parasites (Centers for Disease Control, 1990a). The fever resulting from the malaria infection is supposed to cure the individual by killing the spirochetes, but scientific proof of the efficiency of this procedure has not been demonstrated.