Formulate hypotheses to explain risk of disease.
Test hypotheses with one or more epidemiologic studies.
Implement preventive interventions.
Communicate results of the investigation through written reports or published papers.
Through the application of these principles, investigators attempt to determine the presence, abundance, and ecology of the vector; to identify reservoirs of infection; to evaluate modes of transmission and the ways in which they are influenced by the environment; and to implement disease control and prevention measures.
The plague outbreak in Ecuador occurred in a remote high mountain community with medieval housing conditions, in some ways reminiscent of Europe at the time of the Black Death. Based on their analyses, the researchers concluded that the first people infected had acquired plague from fleas that had previously bitten infected guinea pigs (which are raised locally for meat), and that the pathogen was subsequently transmitted directly among humans, abetted by primitive living conditions and poor access to health care. Hayes said that local climatic conditions, influenced by El Niño, had apparently influenced rodent population dynamics so as to favor the epizootic of plague that preceded the human outbreak.
A post-outbreak comparison of dengue incidence in the contiguous cities of Nuevo Laredo, Mexico, and Laredo, Texas, further illustrated the profound influence of environment on vector-borne disease (Reiter et al., 2003). There, Hayes and coworkers found that although the dengue vector, the mosquito Aedes aegypti, was abundant in the U.S. city, disease incidence was higher in its poorer Mexican neighbor, where far fewer houses were equipped with intact window screens and air conditioners. An investigation of a pneumonic tularemia outbreak on Martha’s Vineyard, Massachusetts, which affected 10 adults, found that mowing lawns or cutting brush was the predominant risk factor for illness. The researchers findings point to small mammals, which presumably contaminated the foliage with the pathogen; the bacteria was then aerosolized and inhaled by workers during mowing. The single fatal case in this outbreak was a man who had limited access to health care. However, the ecological determinants that might explain why this outbreak—the second ever reported in the United States—occurred at that particular time remain unclear.
Turning from outbreak investigation to disease prevention, the authors of the chapter’s first paper, workshop speaker Thomas Scott and Amy Morrison, of the University of California, Davis, present considerable evidence in favor of the use of locally adaptable tools and strategies for dengue prevention, a detailed set of goals for defining and measuring risk factors for human dengue infection, and four “conceptual shifts” in vector control strategy that, they argue, “will substantially improve dengue prevention.” Central to the authors’ recommendations are the observations that (1) dengue transmission risk is strongly associated with