Breaking Down the Hemorrhagic Disease Outbreak
chance of contact.
But, it doesn’t stop there — warmer temperatures cause female midges to lay more eggs, thus producing more midges. For example, females produce about 22 eggs at 50 degrees, but when the temperatures increase to nearly 90 degrees, females produce 100 to 200-plus eggs! Additionally, the virus replicates quicker in warmer temperatures, increasing the chances of virus transfer when a midge takes a blood meal.
However, even with all that information we cannot reliably predict when and where an HD outbreak will occur because these conditions do not cause HD, but rather intensify an outbreak. Even if we could predict an outbreak, there are no proven actions that can protect a deer from contracting the virus.
While there is no way to completely protect deer from hemorrhagic diseases, there are a few management practices that could possibly reduce contact rates between midges and deer. These management practices are not absolute and effectiveness is dependent on the prevalence of HD in the area, size of your property, home range of deer in your area, and management on surrounding properties. However, after seeing the potential impact that hemorrhagic diseases can have on an area, these practices might be worth considering.
The most promising efforts for reducing potential contact between midges and deer are practices that reduce favorable conditions for midges around water and minimize unnecessary visits for deer to water sources. Efforts that promote vegetation surrounding water sources decrease the amount of mud flats that are exposed during droughts, thus reducing midge-breeding areas. Also, fencing o livestock from ponds is an ideal practice because livestock ponds are perfect breeding areas for midges. This is because livestock remove surrounding vegetation, thus exposing more mud, and excrete in the water and surrounding area, further increasing nutrient levels. While landowners have good intentions when placing feed or salt near water sources during the summer, this can cause deer to visit these water sources more often, increasing the probability of encountering midges. Most insects are unable to survive in water with high salt content; however, midges are the exception. Therefore, salt blocks near water can eliminate the majority of other insects, thus reducing competition for midges. The removal of salt and feed near water sources can reduce the risk of a deer encountering midges.
While preventative measures might have some a on the severity of the outbreak in a local area, post-outbreak management efforts, such as assessing local conditions and adjusting doe harvest as appropriate, is the most critical.
The way HD affects a population can vary greatly based on the size and make-up (i.e., sex ratio) of the population and severity of the outbreak, which is often not fully expressed in changing harvest until two to three years following an outbreak. Typically, the year following an HD outbreak hunters harvest the same number of deer as before the outbreak resulting in a greater proportion of deer being removed from the population because the pre-season population has already been reduced due to hemorrhagic mortality, thus leading to population declines. The declines will likely be intensified due to the record-low acorn crop, subsequently making deer more vulnerable to harvest in heavily forested areas. Alternately, in some areas where local harvest rates are typically low, the additional HD mortality will have little affect on long-term population size.
In next month’s Conservationist, we will discuss the potential population effects of the 2012 hemorrhagic disease outbreak, along with other factors that are influencing regional deer population trends.