Now that spring seems to be arriving (more slowly in some places than in others) in the Northern Hemisphere, more people are venturing out into nature. But when we wander around in wooded areas or wild spaces, we are exposing ourselves to the possibility we may become infected with Lyme disease.
Lyme disease is caused by the bacteria, Borrelia burgdorferi. This corkscrew-shaped bacteria is spread through the bite of an infected black-legged tick, or deer tick, lxodes scapularis, in the northeastern and mid-Atlantic states (from northeastern Virginia to Maine), and in the north-central United States (mostly in Wisconsin and Minnesota). In the Pacific Coast states (particularly northern California), this bacteria is spread through the bite of the western black-legged tick, l. pacificus. These ticks are active from spring through autumn.
Although most cases of Lyme disease can be cured with a 2-4 week course of oral antibiotics, even those who have recovered from this potentially debilitating disease can suffer shooting pains, fatigue or brain fog for longer than 6 months after the infection has (presumably) resolved. If not treated, the spirochete bacteria can survive in localized recesses of the body for several years, causing arthritis or severe neurologic consequences. The Centers for Disease Control and Prevention (CDC) estimates that each year, around 476,000 people are diagnosed with and treated for Lyme disease, making it the most common vector-borne disease in the United States.
The chances of getting Lyme disease are highest during times of the year when ticks are most active — which are the same months when people and their pets are most active outdoors. Traditionally, tick season was from March to mid-May and from mid-August to November, but climate change is expanding tick season so that it lasts almost throughout the entire year. This means that the best way to deal with Lyme disease is to be constantly alert to ticks and to not get bitten by them in the first place.
Humans are not the only animal that can become infected with Lyme disease. Dogs, horses and occasionally cattle can get it, too. A variety of wildlife can also become infected with Lyme disease, particularly mice, chipmunks, squirrels, opossums and raccoons.
Curiously, although deer, such as white-tailed deer, can become infected with B. burgdorferi, they are considered to be ‘noncompetent reservoirs’ for this pathogen because they are cannot efficiently harbor and transmit B. burgdorferi. Of course, this raises an important question: why don’t white-tailed deer get Lyme disease too?
To answer this question, a team of researchers based at the University of Massachusetts Amherst (UMass) investigated, hoping that by solving this mystery, they may discover a new way to successfully treat this infection in humans.
“Deer are vitally important to the survival of deer ticks, but they are not involved with transmitting the Lyme bacteria, Borrelia burgdorferi”, senior author of the study, Stephen Rich, a professor of microbiology at UMass and executive director at the New England Center of Excellence in Vector-Borne Diseases (NEWVEC), said in a statement. Professor Rich is an expert in zoonotic diseases, which are diseases that originate in or are maintained in natural animal populations, but which can spill over into human populations. His main focus is on zoonotic diseases that are spread by blood-sucking arthropods such as mosquitoes and ticks.
“We are the accidental host”, Professor Rich explained. “The ticks that bite us are actually looking for a deer because that’s where they breed. Without the deer, you don’t have ticks. But if you had only deer, you wouldn’t have any Lyme.”
Curiously, we’ve known that ticks removed from deer are not infected with Lyme bacteria.
“We’ve known for some time that ticks taken from white-tailed deer are not infected, and we speculated that something about the deer prevented those ticks from becoming infected”, Professor Rich explained. “But until publication of our paper, no one had done the experiment to show that deer blood — specifically the serum component of white-tailed deer blood — kills Lyme.”
To do these experiments, Professor Rich and his collaborators obtained blood samples from a herd of semi-captive white-tailed deer living at the Deer Lab at Auburn University. This herd of deer were selected to participate in this experiment because it is believed that they have had no previous exposure either to ticks or to the bacteria that causes Lyme disease.
The initial experiment was quite simple: after growing the bacteria that causes Lyme disease in liquid cultures, the researchers added deer serum to those cultures.
“And lo and behold, it killed the bacteria”, Professor Rich said. “Whatever it is in the deer that’s killing the germ is part of the innate immune system, a part of the immune system that precedes antibodies.”
The innate immune system is the defense system that is inherited, that an individual is born with. It forms the first line of defense in the immune response. This differs from the innate immune system in people.
“The Lyme bacterium has proteins on its surface that protect it from the human innate immune system”, lead author of the study, Patrick Pearson, a Ph.D. student at the NEWVEC, said in a statement. “Deer blood is somehow different such that Lyme bacteria are apparently unable to protect themselves from the innate immune system of white-tailed deer.”
But why? The next research step is to identify the precise mechanisms in place in deer blood that kill these bacteria.
“In these experiments, we determined that white-tailed deer serum kills the Lyme bacterium”, Mr Pearson said. “The next important question will be to understand exactly how deer blood kills Lyme bacteria.”
Mr Pearson noted that the results of these studies may one day lead to new strategies and approaches for Lyme disease prevention and treatment in people.
“We’d like to determine if it’s something we can induce in humans”, Professor Rich said. “Or maybe we could use this somehow to our advantage to reduce the incidence of Lyme disease in the wild.”
Patrick Pearson, Connor Rich, Martin J.R. Feehan, Stephen S. Ditchkoff, and Stephen M. Rich (2023). White-Tailed Deer Serum Kills the Lyme Disease Spirochete, Borrelia burgdorferi, Vector-Borne and Zoonotic Diseases| doi:10.1089/vbz.2022.0095