Why Has There Been Such a Dramatic Resurgence of Vector Borne Diseases

The dramatic global reemergence of epidemic vector-borne diseases in the past 25 years is4 closely tied to global demographic, economic, and societal trends that have been evolving over the past 50 years. Complacency and deemphasis of infectious diseases as public health problems in the 1970s and 1980s resulted in a redirection of resources and ultimately to a decay of the public health infrastructure required to control these diseases. Coincident with this trend, unprecedented population growth, primarily in the cities of the developing world, facilitated transmission and geographic spread. This uncontrolled urbanization and crowding resulted in a deterioration in housing accompanied by a lack of basic services (e.g., water, sewer, and waste management). Population growth has been a major driver of environmental change in rural areas as well (e.g., deforestation, agriculture land use, and animal husbandry practice changes). All of these changes contributed to increased incidence of vector-borne infectious diseases.

Many urban agglomerations (population >5 million) have emerged in the past 50 years, and most have an international airport through which millions of passengers pass every year (Wilcox et al. 2007). In addition, globalization has insured an equally dramatic increase in the movement of animals and commodities between population centers. The jet airplane provides the ideal mechanism by which pathogens of all kinds move around the world in infected humans, vertebrate host animals, and vectors. A classic example of how urbanization combined with globalization has influenced the geographic expansion of disease is illustrated by the DENVs (Fig. 11). In 1970, only Southeast Asian countries were hyperendemic with

Fig. 11 The global distribution of dengue virus serotypes, (a) 1970 and (b) 2007. Source: Adapted from Mackenzie et al. (2004)

multiple virus serotypes co-circulating, as a result of World War II. The rest of the tropical world was hypoendemic with only a single DENV serotype circulating, or nonendemic (Fig. 11a). In 2007, the whole of the tropical world is hyperendemic as a direct result of urbanization, lack of mosquito control, and increased movement of viruses in people via modern transportation (Fig. 11b). The result has been increased frequency of larger epidemics, and the emergence of the severe and fatal form of disease, DHF, in most tropical areas of the world. Globalizaton and modern transportation were also responsible for the recent spread of WNV to and throughout the western hemisphere (Fig. 6). Increased transmission is a major driver of genetic change in all of these viruses, which can result in virus strains with greater virulence or epidemic potential being spread around the globe. The concern is that YF or RVF will be the next vector-borne diseases to spread because of globalization and modern transportation.

There are many other vector-borne diseases that have the potential for geographic spread. As an illustration of movement of infectious disease pathogens, Table 2 lists some of the exotic diseases introduced into the United States in recent years. It should be noted that the majority of these pathogens are vector-borne, zoonotic, and viruses. In addition, five species of exotic mosquitoes have been introduced and have become established in the country in the past 25 years. Some of the more important epidemic vector-borne diseases affecting humans at the beginning of the new millennium and which have the potential to spread via modern transportation are shown in Table 3. Again, it should be noted that most are zoonotic viral diseases. There is reason to believe that, sooner or later, one or more known or unknown pathogens will cause devastating epidemic disease.

Table 2 Exotic infectious diseases that have recently been introduced to the United States

Diseases Autocthonous transmission

Table 2 Exotic infectious diseases that have recently been introduced to the United States

Diseases Autocthonous transmission

West Nile fever

Yes

Yellow fever

No

Mayaro fever

No

Dengue fever

Yes

Chikungunya

No

SARS

No

Monkeypox

Yes

CJD/BSE

No

HIV/AIDS

Yes

Lassa fever

No

Malaria

Yes

Leishmaniasis

Yes

Chagas disease

Yes

Cyclospora

Yes

Cholera

No

Table 3 Principal epidemic vector-borne diseases affecting humans at the beginning of the twenty first century

• Leishmaniasis

• African trypanosomiasis

• Relapsing fever

• Dengue fever and dengue hemorrhagic fever

• West Nile encephalitis

• Japanese endephalitis

• Rift valley fever

• Venezuelan equine encephalitis

• Chikungunya

• Epidemic polyarthritis

• Other arboviruses

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