URI researcher Dr. Howard Ginsberg breaks down the latest reports on the northeast tick threat
As coronavirus concerns began to decline earlier this summer (before shooting back up with the Delta variant), fear of another dangerous disease and its infamous vector was rising: ticks and Lyme Disease.
Reports of the dangers of ticks popped up in the news around the northeast and in major national outlets like the Today Show in early summer. These are always concerns, but the combination of high numbers of white-footed mice, which carry the bacteria Borrelia burgdorferi that cause Lyme Disease, and the amount of nymphs, young ticks that are smaller than a pinhead and transmit the disease, had experts particularly worried about this year’s tick season.
While black-legged ticks live all over the eastern United States, southern states did not have the same warnings. The CDC labels the northeast as high incidence: 10 confirmed cases of Lyme Disease per 10,000 persons for three consecutive years, but despite also having significant tick populations, the southeast and southern states are all low incidence.
Dr. Howard Ginsberg is a professor in residence at the University of Rhode Island and a research ecologist who studies the management and transmission of vector-borne illnesses plus variables that affect them. We sat down with him to ask about the north-south difference in Lyme Disease, the factors that lead to a brutal tick season, and the best ways to prevent contracting Lyme Disease.
There are lots of ticks in the northern and southern United States, such as the dog tick or the lone star tick. Why does the black legged tick successfully transmit Lyme Disease?
It’s just that the black legged tick has elements of its saliva that suppress the host immune system so that you know that the bacteria in a flat tick are in the gut of the tick. When the tick starts feeding, [Borrelia burgdorferi] replicate and eventually seek hosts and end up in the salivary glands, at which point they can be transmitted; that process takes a day or two. So [if you’re] bitten by a tick, and you get it off in a couple hours, you don’t get Lyme disease usually. In general, it takes a day or more of attachment. But as an aspect of the saliva of the deer tick interferes with the host immune system, whereas with the other ticks, the host’s immune system kills the bacteria in the tick.
There is a big misconception with Lyme Disease that ticks become infected with Borrelia burgdorferi from deer. Can you tell me what the role deer play in the tick and Lyme Disease life cycle if it’s not about infectious bacteria?
The role that deer play is that adult ticks like larger animals—like deer, like bears, they like things like that. So if you have a lot of deer, then you have a lot of adults feeding for [replenishment]. Each one lays, you know, 1,500 eggs or so. That’s lots and lots of larvae, and lots of food for ticks. They will feed on deer but they also feed on small mammals, which are the ones that are good hosts for the bacteria.
I noticed that a lot of your research revolves around explaining the prevalence of Lyme disease cases in the northern United States, like the northeast, rather than the southern part of the United States. Do the ticks behave differently in each area?
One of the grad students at Michigan State University, a woman named Isis Arsnoe, who has now gotten her PhD, decided to look at tick hosting behavior. … And what she found was, just [by] observing these ticks, the northern ticks always hooked on and climbed on top of the leaves and on the dowels when they’re seeking hosts. The southern ticks did not. So she thought, well, that’s a difference, but these were done in [the lab in] Wisconsin with controlled environments, so we repeated the experiment in Wisconsin and Rhode Island and Tennessee and Florida, and found results were consistent: northern ticks, wherever you put them, would always climb up on top of leaf litter, and the southern ticks stayed underneath the leaf litter surface.
So how does that connect back to the amount of Lyme Disease cases?
I mean, if you walk through the woods from the north, the ticks are up on top of the leaves, they get on twigs and get on your shoes, crawl up and can bite you. In the south they are underneath the surface, which is moist, and you don’t encounter them, you step on top of their environment. So that was one. [Arsnoe] got ticks from all 15 sites scattered around the northern [and] southern states and did these experiments and found that if you look at the propensity of these ticks that seek host above leaf litter, that was directly correlated with the amount of Lyme disease in the county those ticks were collected from.
Clearly tick behavior plays a critical role in the prevalence of Lyme Disease in an area. What causes the black legged ticks in the north and northeast to crawl above the leaves?
[Ticks] need very humid environments. So we did this experiment at 95% humidity, 85%, and 75%, and we found that at the higher humidities, there’s no difference in survival between ticks in the northern and southern temperatures, but at 75% humidity, they always died faster in southern temperatures. So in the south, there’s a desiccation stress at the surface, even at 75% humidity, which is pretty humid to a human but to a tick it’s dry. They may have evolved to stay below the leaf layer to avoid mortality.
Besides tick behavior, your paper published last January also looks into the effects of host choice on Lyme Disease. In the north, voles, shrews and especially white-footed mice are hosts that give Borrelia burgdorferi to ticks. In the South, lizards, specifically skinks, are the host-of-choice. Does this also affect increased Lyme Disease rates in the northeast?
But it’s not just that [ticks are] diffused among different hosts, there’s a selective attachment in the south; they like the skinks more than the mice. And that’s significant because lizards are not good hosts for the bacteria. [There was] no infection of ticks feeding on the fence lizards, even if you infect the first. [For] skinks, there was one study that found 24% or so of infection. We tried and we found no infection. So that difference in infection rate results predominantly from the host that ticks feed on
The summer of 2021 was predicted to be a brutal tick season. Why do you think that is?
If you produce a lot of acorns, you get a lot of deer moving towards the acorns and good food for the rodents. That deer moving in [means] the adult ticks lay eggs. The eggs hatch and they have larvae. The year after that, you have lots of nymphs supposedly, so that’s two years later. So net weather can be a really important effect; hot, dry conditions have caused a lot of tick mortality. Plus, if it’s dry, they can’t see hosts that long because they dry out so they’re not seeking hosts. So if you have dry conditions that will lower success, and tick populations. So if we had a bone-dry June this year, the tick numbers would be lower. Now, we’ve had fairly moist conditions, so that’s pretty good for tick survival.
How can people control Lyme Disease in their homes?
You’ll find a lot in the woods, some at the wood edge, a little bit fewer in vinyl plantings, like flower plantings and stuff, and very few on the lawn. So if you’re throwing a ball back and forth on the lawn, especially, if it was quite short, very little chance of exposure but you chase that ball to the woods, high probability exposure. And so, you can landscape. You may keep cut short, make clean edges, have a fence at the edge so no one goes near the woods.
Clearly you do field work in forested areas with lots of ticks. What precaution do you take to prevent contracting Lyme Disease?
The problem is that these ticks are so small and hard to see and they’re so common in forest environments in the northeast, that you have to be extremely diligent to prevent attacks by transmission. So I take all the precautions. I wear light colored clothes, tuck my pants into my socks and check myself frequently when I get back in from the field. I do three full checks for ticks: right in front of the mirror, in the mirror in the shower, and then when I’m dressed I check again. I’ve been very effective at lowering tick bites but still occasionally one gets through. The one thing about Lyme disease is, since it takes a day or more for the tick to transmit [pathogens], the best protection against Lyme disease is when you come in from places where you might have been exposed to ticks, to check yourself very thoroughly, and immediately remove any ticks that are attached. The way to remove a tick here is to get very fine tweezers, fine forceps, grab the tick as close to the skin line as you can and just slowly pull it straight out.
This interview was conducted for the Boston Institute for Nonprofit Journalism.