The first author on the paper, David L. Smith, Assistant Professor of Epidemiology and Preventive Medicine at the University of Maryland School of Medicine, hopes the model will help health officials to control infectious diseases and will reduce the impact of animal disease outbreaks on humans. The methods should also be useful for other diseases, such as foot and mouth disease.
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.
Dr. Smith explained how surprised they were at the frequency with which humans relocate infected raccoons to distant places. The animals may be picked up accidentally by garbage trucks. Trappers have also helped the disease spread by transporting raccoons to restock hunting grounds.
The researchers thought the rabies epidemic might be aided by the size of the human population and the tons of garbage people generate. Human garbage provides a great food resource for the animals and helps the raccoon population density grow to record levels. Surprisingly, however, the study found that human population was only a minor factor in the spread of rabies. They wish to study this further.
The presence of a river, on the other hand, can have a strong impact on the spread of rabies, slowing it by a similar factor as vaccine-laden bait distributed for raccoons along a strip of land.
The researchers do stress, however, 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 randomised 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, i.e. rivers, vegetation, population density, and determined how long it took for the rabies wave to spread throughout each of Connecticut's 169 townships. They then examined the effects of five scenarios that might explain why the disease slowed down in some areas, sped up in some and apparently leaped long distances in 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 non-specific (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.
Smith, D.L., Lucey, B., Waller, L.A., Childs, J.E. and Real, L.A. (2002) From the Cover: Predicting the spatial dynamics of rabies epidemics on heterogeneous landscapes. PNAS 99, 3668-3672.
LabVelocity
22 March 2002
Veterinary Sciences Tomorrow invited Dr. Eva Nagy of the Department of Pathobiology, Ontario Veterinary College, University of Guelph, Canada, to comment on the importance of this story. Read below what she has to say:
"This study provides a potential model for the spread of rabies virus which could be used to not only predict how best to restrict the spread of an outbreak, but also provides some clues about strategies to better contain the disease so outbreaks do not occur, particularly in wild populations.
It highlights the importance man plays in facilitating, rather than preventing, virus spread. Using such a model could lead to the development of some proactive approaches, such as trying to prevent raccoons from access to human garbage, which could be applied to better control the disease in the wild, short of mass vaccinations.
The observations that trappers can inadvertently assist in the spread of rabies by transporting raccoons over wide distances should also give cause for concern to wildlife control officers or city officials. Though well meaning, their relocation of raccoons from densely urban areas into the wild, may actually put the raccoon population at greater risk to rabies, compared to other control measures. It would be interesting to see how well their model holds up in other geographic areas and to determine how predictive the model can be.
Undoubtedly the model could be fine tuned to be valid under many different situations and consequently different control approaches could be first modelled to identify those which could most effectively restrict the ability of rabies virus to be a major problem."
