1. Department of Immunology and Infectious Diseases, School of Hygiene and Public Health, Johns Hopkins University, 615 North Wolfe Street, Baltimore, Maryland 21205, USA
2. Pest Management Pesticide Monitoring Division, U.S. Army Environmental Hygiene Agency, Aberdeen Proving Grounds, Aberdeen, Maryland 21010, USA
3. Department of Epidemiology, U.S. Army Medical Research Institute of Infectious Diseases, Fort Detrick, Frederick, Maryland 21701, USA