November 09, 2023
We actually know a lot about CWD and its impact on deer. I’ve reported on some of this in previous columns. We know from a study done in Wisconsin that deer with CWD had a 75-percent annual mortality rate, while those that did not have CWD had a 24-percent mortality rate. That does not mean that you’ll find CWD-positive deer lying all over the woods, because CWD impacts other forms of mortality.
If a deer has CWD, it has a higher chance of being shot by hunters because infected deer are less aware. If a deer has CWD where there are mountain lions, it has a higher chance of being caught and killed by the big cats, because the deer with CWD are less wary. If a deer has CWD, it has a higher chance of being hit and killed by a vehicle on a highway because they are less aware. No matter what the cause, deer with CWD die at a much higher rate than deer without CWD. That is because they are sick deer and not as alert as healthy deer.
I do not want to be the bearer of bad news, but the more we learn about CWD and what happens to deer that get this disease, the scarier the future sounds. Consider another study done in Wyoming that showed a 68-percent annual death rate for CWD-positive deer, and only 24 percent for CWD-negative deer. This tells me that CWD has the potential to eliminate whitetail deer.
Hunters love to get older, bigger bucks, but research shows that older bucks are more susceptible to getting CWD. The data in one computer study showed that when a high percentage (let’s say 40 percent and above) of the deer in an area had CWD, lots of deer will die. Even with hunting mortality eliminated in the model, the deer population will drop year after year. By the way, there are lots of areas in the country where more than 40 percent of the deer have CWD.
Now let’s talk about how ticks may be helping to spread CWD in deer. We already know there are several ways that CWD is transmitted from one deer to another. Prions that cause CWD pass from infected deer via mucous, urine, and feces. So, when another deer contacts any of those things, it can get the prions. If an infected deer urinates in a scrape and another deer licks that scrape, it can get CWD. If a deer licks a branch that is over a scrape (and they do that all the time), and another deer licks that branch (and they do that all the time), they can get CWD. If a deer licks the mouth, eyes, or anal area of a deer that has CWD, it can get CWD. Plus, when urine or feces end up on vegetation and a deer eats that vegetation, it can get CWD.
Obviously, we don’t need any more ways to spread CWD in deer, but new research has found one that is linked to black-legged ticks. Here is a little background on black-legged ticks. They have a two-year life cycle and spread a number of diseases including Lyme’s disease to humans. They go from eggs and larvae in one year to nymphs and to adults the second year. Female adult ticks take a blood meal in the fall, then drop off onto the forest litter where they overwinter. In May, they lay about 2,000 eggs and then die.
Warmer temperatures up north have allowed black-legged ticks to expand their range, even into Canada. That’s a problem for humans, because it exposes us to several tick diseases, including Lyme’s disease — a disease that is going through the roof in humans in many areas this year. Expanding its range also puts black-legged ticks near deer everywhere.
Part of this new research was a lab study where they set up an artificial membrane made out of silicon for black-legged ticks to feed on blood. They were then able to control the amounts of prions in the blood the ticks were feeding on. The end result of this study showed that this tick could take up prions from the blood and excrete them.
The second part of the study involved collecting ticks from the ears of deer shot by hunters in an area of Wisconsin where CWD is very common. Some of those ticks had CWD prions in them. Although the final data is not out yet, the researchers speculated that those infected ticks can lead to infection in other deer.
They further speculated that grooming might be a pathway for the transmission of CWD. Here’s how that might happen.
Deer with CWD have prions in various body locations, including their blood. When this tick takes a blood meal from a CWD-positive deer, it ingests infectious prions. Deer groom each other. It is common among does; also does will often groom their fawns. Ticks on the bodies of deer increase grooming behavior. They lick and can ingest ticks by accident. If the deer being groomed has CWD, then prions are going to be in the ticks on its body.
Such research is important, because we need to know as much as possible about potential transmission pathways for CWD. Now we know that at least one blood-obligate parasite plays a role in prion transmission.
There is one other piece of bad news about this situation: Ticks concentrate infectious prions. Thus, one tick might have many more infectious prions than would be found in a deer’s urine or mucous. A tick’s excrement puts many prions on the ground. Not good, and it’s just another reason why eliminating CWD in deer may be impossible.