Factors associated with endemic raccoon (Procyon lotor) rabies in terrestrial mammals in New York State, USA

Abstract

This study evaluated characteristics associated with raccoon (Procyon lotor) rabies in New York State (NYS), USA, where this disease has been endemic for the last 15 years. The study included 4448 cases of raccoon rabies in terrestrial mammals reported across 1639 census tracts of NYS during 1997–2003. A Poisson-regression model with census tract-year as the unit of analysis revealed a higher number of raccoon-variant rabies cases per square kilometer in census tracts with each percent increase in the proportion of low-intensity residential areas (those with a lower concentration of housing units) (RR = 7.68) and a lack of rivers/lakes (RR = 1.20) and major roads (RR = 1.10), while the number of cases decreased with each 1-m increase in land elevation (RR = 0.998), and each percent increase in the proportion of wetlands (RR = 0.01). The model was adjusted for county, ecoregion, and latitude to help control for unknown spatially dependent covariates. The model may be used in prioritizing areas for rabies control based on differential risk, including use of costly intervention methods such as oral rabies vaccine.

Introduction

Raccoon rabies has seriously affected the East Coast of the United States, reaching New York State (NYS), USA in 1990. Rabies was laboratory-confirmed in 14,892 terrestrial animals in NYS from 1990 to 2004; 10,980 (74%) of these cases were raccoons (Wadsworth Center, 2005). The New York State Department of Health (NYSDOH) received reports of 20,031 people receiving rabies treatment due to exposures to suspected rabid terrestrial animals or by wildlife that could not be tested for rabies, from 1993 to 2004 (Zoonoses Program, 2005). Costs for the health-care system to respond to rabies exposures are high and increasing (Chang et al., 2002, Recuenco et al., 2007).

The use of oral rabies vaccines (ORV) is one strategy recommended by the Centers for Disease Control and Prevention (US Department of Health and Human Services) to reduce raccoon-variant rabies, exposures to humans, and spread to new areas. Although costly, ORV has had some success in the last decade (Willsey et al., 2001, Nunan et al., 2002, Centers for Disease Control and Prevention, 2003). The vaccine is contained in baits that can be dropped by air or distributed by hand (USDA-APHIS, 2003). ORV containment barriers at the epidemic ‘wavefront’ are in place in many states in the U.S. (Robbins et al., 1998, Foroutan et al., 2002, USDA-APHIS, 2003, Slate et al., 2005).

Persistence of the raccoon-rabies epidemic in NYS for >15 years indicates that there are probably many factors associated with maintaining the infection even with reduced populations of the reservoir species due to the disease. Identification of these and other risk factors might assist efficient application of ORV and other control measures for raccoon rabies (Anthony et al., 1990, Jones et al., 2003). Preferences of raccoons to live close to forested landscapes and to adapt to urban settings where food is more available need to be considered when studying raccoon rabies (Prange et al., 2004, Zeveloff, 2002, Chamberlain et al., 2007).

New York State (a large area endemic for raccoon rabies) includes diverse land-use types, elevations, and human population densities. Previous studies have identified factors of interest at the regional, state, and county levels (Wyatt et al., 1999, Childs et al., 2001, Raczkowski and Trimarchi, 2001, Chang et al., 2002, Guerra et al., 2003, Recuenco et al., 2007, Russell et al., 2004). Census tracts are small geographical subdivisions of a county used by the U.S. Census Bureau in the national population census each decade in the USA. The area of the census tract is defined with the intent to be relatively homogeneous regarding population characteristics, economic status, and living conditions (U.S. Census Bureau, 1994). Data to study rabies at this geographical level have not been analyzed previously in NYS. We modeled raccoon rabies at the census tract level to provide more detailed estimates of risk.