Lyme disease is one of a dozen or so infectious diseases that have "emerged" in the past 25 years (others include West Nile virus in North America, HIV/AIDS, Ebola, and Hantavirus). Some of these diseases, including Lyme, are caused by pathogens that have existed quietly for millenia. What makes them suddenly cause outbreaks? Why does disease incidence vary so dramatically in time and space? How can disease risk be reduced? Because they involve interactions among a number of species (pathogen, vector, reservoir, human) in nature, infectious diseases are viewed by researchers in Dr. Richard Ostfeld's program as ecological systems. Ostfeld's primary approach is to unravel the roles played by the various species involved and then use that knowledge to inform managers and citizens about how to reduce contact between humans and the disease agents, and how to apply ecological knowledge to controlling the spread of disease.

Lyme disease, caused by a bacterium (the "pathogen", Borrelia burgdorferi) transmitted during tick bites, is by far the most common vector-borne disease in the United States and Europe. Ticks (the "vector") hatch from eggs free of Lyme bacteria and remain benign until they feed on an infected mammal or bird host (the "reservoirs"), which can transmit the pathogen to the tick. Ticks that feed on the white-footed mouse, a ubiquitous denizen of U.S. forests and fields, are much more likely to acquire Lyme bacteria than are those that feed on other hosts. Ostfeld and colleagues Drs. Felicia Keesing, Kathleen LoGiudice, Brian Allan, and Ken Schmidt, have found that fewer infected ticks are produced in habitats having a high diversity of alternative, non-mouse hosts for ticks, such as opossums, shrews, and ground-foraging songbirds, which tend to dilute the impact of mice. A major lesson for managers and citizens is that biodiversity has a tangible, protective function concerning human health. In agricultural and suburban landscapes, the best recipe for increasing biodiversity is to increase the size of remnant forest patches, because some species of birds and mammals don't persist in small patches of forest habitat.

Studies of the ecology of ticks by the Ostfeld group also provide clues for ways to reduce tick numbers. One such approach, based on observations of tick questing behavior, was tested recently by collaborators Michael Benjamin and Elyes Zhioua. They found in carefully controlled field trials that a native fungus, Metarhizium anisopliae, can significantly reduce the abundance of ticks when sprayed on forest understory vegetation. This fungus may provide a relatively safe and effective alternative to the use of chemical pesticides, which are known to kill non-target insects and which may have toxic effects on vertebrates.

Lyme is not the only disease that is changing rapidly in our region, and about which ecologists are making key discoveries. Following a 1999 outbreak of a protozoan disease, Leishmaniasis, in some foxhounds in Millbrook, Ostfeld and interns Lauren Canter and Wendy Haumaier spent the summer of 2001 determining whether the sandfly vector (genus Lutzomyia) of this disease occurs locally. Although occasional records of sandflies in Connecticut and Quebec have been published, Lutzomyia is generally considered a tropical and subtropical group. Sandflies were not detected at the site of the hound kennel; however, abundant and widespread populations were noted on the grounds of IES. So far, the role of this fly in transmitting disease to either humans or non-humans is unknown. Determining what role they might play will require continued ecological sleuthing.