February 16, 2022
I remain amused by the naïve beliefs people often have about wildlife and nature. The idea that wild animals acquire diseases and die, or even that they commonly make mistakes and have fatal accidents, just does not seem to sink in! Actually, bad things happen quite frequently to wildlife. The list includes various diseases.
As whitetail populations soared to around 40 million by the turn of the 21st Century, we became concerned about potential impacts of diseases that logically would accompany population growth. In 1994, Dr. Harry Jacobson and I traveled to Edinburgh, Scotland, to deliver a plenary address to the 3rd International Congress on the biology of deer; entitled, “The White-tailed Deer: the Most Managed and Mismanaged Species.”
During our presentation, we warned about future problems in deer management, including emergence of new diseases and future impacts. Although our warnings went unheeded, in the last two decades, it did not stop disease from happening!
There are two kinds of diseases: density-dependent and frequency-dependent. A density-dependent disease is one in which spread and impact is directly related to how crowded is the population. Frequency-dependent diseases impact the population, regardless of density. Dealing with these two types of diseases calls for completely different management strategies.
With this short introduction to disease, I turn to the ones I feel have an impact on deer populations. Deer diseases can be lumped into the following categories: bacteria (including rickettsia), viruses and proteins.
I recently asked three colleagues who study wildlife diseases to rank them, in proportion to impact on deer. It is hard to get two scientists to agree on anything, but their response was universal: viruses, bacteria and proteins.
Hands down, viruses have caused and will continue to cause more deer deaths than any single factor! In the last 20 years, deer harvest has declined about 20 percent, and viruses certainly are causal factors. Comparing deaths from viruses to Chronic Wasting Disease (CWD), I am reminded what it says in the Bible about David: “Saul has slain his thousands, but David has slain tens of thousands!”
To date, untold thousands of deer have died from just one group of diseases, collectively called Hemorrhagic Disease. Until recently the general public remained unaware of these deaths! More recently, die-offs in many areas have brought the public to full attention.
What are Hemorrhagic Diseases? There are two main groups, Epizootic Hemorrhagic Disease (EHD) and Bluetongue Disease (BT). Hemorrhagic means producing a hemorrhage, in this case throughout the body.
Bluetongue is named for the common symptom of a swollen tongue that is bluish in color. They are in the Orbivirus group, which are transmitted by biting insects (midges or gnats), generally of the genus Culicoides. The gnats live as maggots in mud next to ponds and streams, emerging in the evening, when deer come to water.
An infected whitetail will have very high fever and disintegrating organs, hence the name “hemorrhagic.” They go to water to cool off, where they are bitten by gnats, that in turn bite healthy deer. Between 36 hours and a few days later, that deer also becomes infected. The problem with these viruses, and others, is they come in many variations (strains or serotypes), some more virulent than others!
Although there are multiple strains of EHD, three remain problematic. EHD-1 was first reported in New Jersey and Michigan in 1955, while EHD-2 first appeared in Alberta (2013). EHD-6 reassortant virus was reported from Indiana in 2007, probably representing a combination of EHD-2 and EHD-6, from Australia, by the process called reassortment.
There is no known cure for the diseases, and you cannot inoculate deer against them. We have found the best approach is management, using sound herd management, improved nutrition and strategically placed artificial water sources. This spreads deer out and reduces the production of gnat maggots.
Whitetails are beset with a host of bacterial infections, which on rare occasions can be quite devastating; however in a localized context. Anthrax (Bacillus anthracis), an ancient disease going back as far as one of the Egyptian plagues in the Bible, periodically kills thousands of deer in southern Texas.
Anthrax spores remain viable in the soil for 50-100 years, emerging during extremely wet years, followed by prolonged drought. I have witnessed losses of up to 90 percent in local deer populations.
Another somewhat geographically limited bacterial disease is bovine tuberculosis (Mycobacterium bovis), which as the name implies came from cattle. I have been involved in BTb management in the Northern Lower Peninsula of Michigan since 2003, and found this disease to be manageable, and having little impact on deer populations per se.
Other bacteria such as E. coli, Clostridium, Pseudomonas, Fusobacterium, and Pasturella (Mannheimia) cause significant deaths. Combined with predation, bacteria are a significant mortality agent of fawns.
Older deer contract several diseases, including hoof rot, caused by Fusobacteria, which can lead to death. For some reason, bucks seem to have this disease more often than does; generally, it begins with an injury between the toes.
Bacteria cause abscesses, puss-filled bags under the skin or inside the body. Deer recover from most, but brain abscesses in bucks are almost universally lethal. They are caused by skull damage from fighting. Late stage symptoms include staggering, circling and apparent deafness and blindness. Collectively, though, the greatest impact of bacteria is on fawn survival, which can be quite dramatic.
What hunter has not heard of Chronic Wasting Disease (CWD)? Since the late 1960s, millions of dollars and a great deal of manpower have been spent attempting to eradicate or control CWD. To date, there is not a single demonstrable strategy that has been shown to work!
There are isolated cases where an infected deer was destroyed, resulting in no further cases; but, these are few and far between. As the Deer Trustee of Wisconsin, I concluded efforts to eradicate CWD in southern Wisconsin failed, after spending millions of dollars and killing at least 200,000 deer!
CWD is caused by aberrant proteins, called prions (pree-ons). Normal prions are found in the nervous system and are “thought” to function in everything from nerve formation to regulating stress. Like the “transformers” of Hollywood, they can become distorted in shape, rendering them insoluble in the nervous system.
They reproduce and ultimately lodge in critical portions of the brain, where they erode critical tissues. Prions are long-lasting in the environment and are difficult to destroy. Deer can become infected through contact with other deer and some environmental agents, but the transmission route remains poorly understood.
We do know CWD is not a density-dependent disease; rather, it is a frequency-dependent disease, the spread of which is through multiple, prolonged contact with prions. Yet, what constitutes an “infective dose” evades scientists. Yet, the media reports would lead you to believe the woods are full of single prions, waiting to pounce on a deer!
We have yet to prove CWD has caused significant population decline. The name “chronic” is instructive, since unlike many diseases, deer do not die in a short time. This chronic disease takes as much as four years to become clinical. In that time, a doe will produce up to 6 fawns; mitigating population impacts. The true impact of CWD will emerge over the next decades. For now, we feel it should not be included in the top five deer mortality agents. We may be wrong, but time will tell!
Whitetails are ultimate survivors; they have been on this earth for over 2 million years! Countless species have become extinct, yet the whitetail remains with us, the challenging adversary we all love.