Though most rabies fatalities in the United States stem from bat bites, far more people are treated for raccoon rabies. In 1990, raccoons topped the list of most often reported rabid mammal. Controlling the spread of rabies depends on predicting the spatial dynamics of the disease - where new outbreaks might occur and how the virus might spread. In a new study reported in the freely-available online journal PLoS Biology, Real and colleagues apply a mathematical model to predict the likely spread of rabies across Ohio - a potential gateway for spread throughout the Midwest - and find that raccoon rabies could spread throughout the state in just three years, far faster than previously thought.
One strategy for limiting rabies spread is to establish vaccine corridors by distributing vaccine baits - vaccine doses hidden in fishmeal - to wild raccoons. This cordon sanitaire strategy limited rabies in Ohio to sporadic cases from 1997 until 2004, when a single rabid animal was detected - 11 kilometers beyond the buffer zone - in northeastern Ohio. By modelling the spread of past outbreaks, the authors had already shown that local transmission was significantly reduced when townships were separated by geographical barriers. The authors incorporated the likely effect of Ohio's five major rivers on transmission from local points along the Pennsylvania or West Virginia border but also adjusted their model to estimate the potential impact of long distant translocations, such as the occasional garbage truck ride. They estimated that rabies would take just 33 months to spread across central Ohio - compared to 48 months to cross the much smaller state of Connecticut - and cover the state within 41 months. This transmission rate - 100 kilometers/year - significantly surpasses previous estimates, which range from 30 to 60 kilometers/year. The potential for such rapid spread, if unchecked, "is quite alarming," the authors warn.
Given the unpredictable nature of rabies transmission - challenging efforts to identify potential leaks in vaccine corridors and sites of dispersal - the authors' simulations provide a valuable resource for anticipating alternate outbreak scenarios and preparing multiple game plans to prevent or contain them. They also indicate the best sites for establishing a new vaccine barrier. And given how fast raccoon rabies could spread, Real and colleagues make a strong case that halting its western march depends on a strategy based on early detection and high-powered intervention programs - a sensible approach for any infectious disease.
Citation: Russell CA, Smith DL, Childs JE, Real LA (2005) Predictive spatial dynamics and strategic planning for raccoon rabies emergence in Ohio. PLoS Biol 3(3): e88.
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