Reducing the number of accessible food "hotspots" like stored crops, hay bales and grain bins that attract wild deer to farms or fields could inadvertently make the spread of chronic wasting disease worse, according to a recent study published in Ecology and Evolution.
The finding suggests a more comprehensive strategy is needed to help farmers and wildlife managers protect deer by controlling the spread of the fatal disease, explains Kelsey Gritter, who led the research as part of her master's thesis in ecology. If they choose to remove these artificial attractants by making them inaccessible to the deer, they likely need to eliminate all of them rather than just a portion for the approach to be effective.
Chronic wasting disease, or CWD, affects the central nervous system of cervids such as deer, elk and moose. Alberta created an ongoing surveillance program in 1998 to track prevalence, which continues to rise in many areas, with over 75% infection in mule deer males in some local areas where CWD has been present for many years.
It is spread through direct transmission between animals, as well as environmentally as the deer shed prions (misfolded proteins) through saliva, urine and feces, says Gritter. Artificial attractants become hotspots for both forms of disease transmission.
"They cause the deer to congregate and be present in unnaturally high numbers in one location and can affect CWD transmission," says Gritter.
These artificial attractants could encourage deer to leave their home range (the area they typically move within), which would increase the likelihood of deer from different groups infecting one another.
"There's a lot of ways that artificial attractants can cause problems when it comes to disease spread."
For the study, Gritter and her collaborators wanted to examine the impact of the density of artificial attractants in a particular area as well as their configuration. "Density is important because if we're trying to reduce the impact of these sites, the number of sites is one of the things we're able to target," says Gritter. "And configuration is important because there may be better or worse arrangements of these sites that cause more or less contact, so when creating management strategies, we can aim to achieve configurations that are better."
The researchers gathered behavioural data from tagged mule deer, including group size, the home range they typically roam within, and the habitat and landscape characteristics they're naturally drawn to. The researchers looked at data taken every two hours, which also provided information on how far the deer tended to travel and how much they typically deviated from their path.
"When we have the information about the landscape and their general movement patterns, we can use that to sort of simulate their movement," says Gritter.
With the data in hand, the researchers created a computer model to simulate different scenarios: removing all attractants at a particular location, removing a few at random, or removing them based on proximity to the deer's preferred habitat.
They found that making some but not all attractants inaccessible to deer actually increased contact rates, particularly between different groups of deer, because the same number of deer were then gathering around the fewer remaining sites. Removing attractants near their preferred habitat, such as areas with a lot of trees and shrubs, encouraged even higher rates of contact. The only effective strategy was to remove all attractants from a particular farm or field.
While the computer simulations looked at a scenario with moderate mule deer populations, Gritter notes that further research is needed to understand what effect deer population size may have on results.
