Drivers, dynamics, and control of emerging vector-borne zoonotic diseases

Abstract

Emerging vector-borne diseases are an important issue in global health. Many vector-borne pathogens have appeared in new regions in the past two decades, while many endemic diseases have increased in incidence. Although introductions and emergence of endemic pathogens are often considered to be distinct processes, many endemic pathogens are actually spreading at a local scale coincident with habitat change. We draw attention to key differences between dynamics and disease burden that result from increased pathogen transmission after habitat change and after introduction into new regions. Local emergence is commonly driven by changes in human factors as much as by enhanced enzootic cycles, whereas pathogen invasion results from anthropogenic trade and travel where and when conditions (eg, hosts, vectors, and climate) are suitable for a pathogen. Once a pathogen is established, ecological factors related to vector characteristics can shape the evolutionary selective pressure and result in increased use of people as transmission hosts. We describe challenges inherent in the control of vector-borne zoonotic diseases and some emerging non-traditional strategies that could be effective in the long term.

Fig. 1

Temporal patterns of reported cases (in thousands) for selected introduced vector-borne pathogens (red) and emerging endemic diseases (blue) on six continents. Introduced pathogens may cause striking epidemics followed by lower incidence (WNV in North America (http://www.cdc.gov/ncidod/dvbid/westnile/index.htm), CHIKV in Réunion), sporadic epidemics from repeated introductions and local transmission (dengue in Australia), or establishment and increased case burden (dengue in S. America, http://new.paho.org). The incidence of some endemic zoonotic vector-borne diseases has increased markedly in the past several decades (Lyme disease in North America, http://www.cdc.gov/lyme/stats/index.html, plague in Africa), but may show different trajectories (plague in Africa vs. plague in Asia), even in neighboring regions (TBE in eastern (ex-communist) and western (historically free market) Europe) because of socioeconomic differences. CCHF is shown as endemic to Turkey because there is evidence of its presence there many years before its recent appearance in humans.

Fig. 2

The global aviation network. Lines show direct links between airports, and the colour ramp indicates passenger capacity in persons per day (red = thousands; yellow = hundreds; blue = tens). Routes linking regions at similar latitudes (in the Northern or Southern hemisphere) represent pathways that pathogens are likely to move along to reach novel regions. Note the lack of high volume air traffic to most places in Africa, regions of South America, and parts of central Asia. If travel increases in these regions additional introductions of VBPs are likely. Adapted from with permission.

Fig. 3

Interactions between economic status and disease risk, particularly applicable where contact with infectious agents is due largely to human activities outdoors, such as tick-borne diseases. Increased poverty increases reliance on subsistence activities that sometimes occur in vector-infested habitats. Increased income may be associated with increased recreation (especially in Europe and North America) that may also expose individuals to vectors of disease but also allow increased access to public health. Human activities take place against a backdrop of variable inherent risk from zoonotic vector-borne pathogens, measured as the density of host-seeking infected vectors such that the overall risk curve may rise or fall.

Fig. 4

Seasonal patterns of tick-borne encephalitis (TBE) cases (columns) relative to abundance of questing nymphal ticks (Ixodes ricinus) (lines). The data for ticks are lagged by one month to account for the interval between a tick bite and diagnosis of TBE. In eastern European countries (Poland, Latvia and Lithuania), more TBE cases occur from late summer onwards, after the peak of tick activity, whereas in western Europe (Switzerland, Germany and Slovenia), TBE cases peak in mid summer. This is likely to reflect different principal purposes for human visits to forested tick habitats, either for collecting forest foods (more pronounced in eastern Europe) or for recreation.