Mathematical Model May Explain Sudden Long Distance Outbreaks
An epidemiologist from the University of Maryland School of Medicine has written a mathematical analysis of a rabies epidemic that may help predict where and how fast infectious disease will spread. The analysis also describes how raccoon rabies tends to jump from a local area of infection to a distant, previously uninfected region. The model quantifies, for the first time, the rate of disease movement at natural barriers, such as rivers or mountains, and attempts to estimate how frequently rabies will spread into uninfected areas. The model is described in the current issue of the Proceedings of the National Academy of Science.
"Our model may make it easier for health officials to control infectious diseases and could help reduce the impact of animal disease outbreaks on humans," says lead author David L. Smith, Ph.D., assistant professor of epidemiology and preventive medicine at the University of Maryland School of Medicine. "The methods would be useful for other diseases as well, such as foot and mouth disease," says Dr. Smith.
The study concludes that major rivers can slow the spread of rabies by almost a year, resulting in a seven-fold reduction in the transmission rate. It also suggests that unwitting human intervention is the likely cause of infection that erupts well beyond where the disease has already been established.
"We were struck by how often infected raccoons were relocated to distant places by humans," says Dr. Smith. "The animals may be picked up accidentally by garbage trucks. And in some cases, trappers have helped the disease spread by transporting raccoons to restock hunting grounds."
The researchers thought the rabies epidemic might be helped along by the size of the human population and the tons of garbage people generate. "Human garbage is a great food resource. It helps the raccoon population density grow to record levels," says Dr. Smith. But the study found human population was no more than a minor factor in the spread of rabies, a finding, they say, that warrants further study.
On the other hand, they say a river's impact in slowing the spread of rabies is quite strong, similar to the effect of vaccine-laden bait distributed for raccoons along a strip of land.
But the researchers warn that the long distance human transport of infected animals poses a serious threat to measures to contain rabies.
The mathematical model links a large database with geographic information and randomized simulations. The database tracks reported cases of animal rabies during an epidemic in Connecticut in the early 1990s. The researchers compared that data, recorded at the township level, with details of the Connecticut landscape -- rivers, vegetation, population density -- and determined how long it took for the rabies wave to spread throughout each of Connecticut's 169 townships.
The researchers then examined the effects of five scenarios that might explain why the disease slowed down in some areas, sped up in other areas and apparently leaped long distances in still others.
Rabies is a preventable viral disease most often transmitted through the bite of rabid wild animals, especially raccoons, skunks, bats, and foxes. According to the Centers for Disease Control and Prevention, domestic animals account for less than ten percent of reported rabies cases.
Rabies virus infects the central nervous system and, without prompt treatment, can ultimately cause death within days of the onset of symptoms. Early symptoms in humans are nonspecific -- fever, headache and general malaise. In its advanced stage, rabies symptoms may include insomnia, anxiety, confusion, slight or partial paralysis, hallucinations, agitation, hypersalivation, difficulty swallowing and fear of water.
The study team included Brendan Lucey, Lance A. Waller and Leslie A. Real from Emory University and James E. Childs at the Centers for Disease Control and Prevention. The National Institutes of Health and the National Center for Ecological Analysis and Synthesis supported the research.
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