A report published in the Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report said in 2007 the New York City Department of Health and Mental Hygiene began its active surveillance program of about 36,000 privately owned properties with historically high rates of rat infestation.
Inspectors walked every block using handheld computers with maps to record fresh tracks, fresh droppings, active burrows, active runways and rub marks, fresh gnawing marks and live rats.
Inspectors recorded a severity score for each sign and also noted and recorded conditions conducive to rats including accessible garbage, poor containerization of food waste, or clutter and dense vegetation promoting the nesting of rats, the report said.
Owners with high scores were sent letters detailing the inspection and a rodent control educational guide with advice tailored to the problem. Property owners were given five to 10 business days to comply with the order and a compliance inspection was conducted.