Chickens and Ticks and Lyme Disease and…Super Models?

A recent conversation with a relative set me to researching chickens and ticks and what a long, strange trip it’s been.

The conversation was about the efficacy of chickens in controlling tick populations. It struck a chord with me because I’ve been thinking of the advantages of backyard chickens lately and I wondered if I could add “tick control” to the list. (Which is not to say I’m thinking of getting backyard chickens — I face a basically insurmountable obstacle to that, given that I have no backyard).

Of course, by “tick control” I was actually thinking, “Lyme disease control,” and what interested me was the idea of chickens as a first-line defense against that very nasty disease. As May was just declared “Lyme Disease Awareness Month” in the Cape Breton Regional Municipality and the provincial Department of Health and Wellness just called upon Nova Scotians to be “tick-aware,” I decided the time was ripe to follow up my conversation with a relative by talking to an actual scientist.  I contacted Richard Ostfeld, senior scientist at the ​Cary Institute of Ecosystem Studies, who has spent over 25 years studying the ecology of infectious diseases, “especially those carried by ticks in the northeastern US.”

I told him I’d heard that chickens were effective in controlling tick populations. He told me he’d heard the same thing:

[P]eople talk about chickens as tick control mechanisms, agents, whatever, all the time, I get this at parties too, and I occasionally conduct a search with all the possible search terms I can think of and the only paper I’ve ever found looking at chickens and ticks was in East Africa, published…I think it was in the ’90s, on a different species of tick, showing that chickens that are living in and around these kind of temporary livestock corrals that they erect will occasionally eat this species of African tick…But I have not found a single paper looking at chickens and ticks anywhere in North America or Europe.

Okay, that’s not looking good for chickens. But wait, Ostfeld had more:

There was a paper on guinea fowl that was published by a few entomologists and Christie Brinkley, the super model.

Guinea fowl are not chickens and Christie Brinkley is not an entomologist but how could you not be interested in this study? Said Ostfeld:

It was not considered a very well designed study…If you cage a guinea fowl, you stick it in a cage on a lawn, and you put ticks in the cage with it, it will eat the ticks, some of them at least, and that’s basically the extent of what that study said. So, it did not adequately address whether having guinea fowl, you know, a flock of them, on your property, would do anything to the tick population.

Still, the finding was intriguing enough that in the early oughts, a graduate student of Bard Professor Felicia Keesing (Ostfeld’s research partner) named Amber Price set up a more rigorous study involving 20 paired lawns in Dutchess County, New York (the heart of the Lyme disease epidemic). Half the lawns had guinea fowl, half didn’t. Price’s research was incorporated into a 2006 BioScience article co-authored by Price, Ostfeld, Keesing, Victoria L. Hornbostel and Michael A. Benjamin.

Said Ostfeld:

There were some tantalizing signs that the guinea fowl ate some ticks but we did not find a statistically significant difference in tick abundance between properties with and without guinea fowl. So, it is possible that had we had a larger sample size, that same magnitude of a difference might have reached statistical significance, but we can confidently say that they are not a panacea, they do not have a strong effect. They may have a weak effect but we did not detect a significant effect at all.

In fact, said Ostfeld, the notion that chickens control ticks has become something of an urban legend:

[P]eople make assumptions that chickens go round scratching and they eat a bunch of different things and some of those are small arthropods [invertebrates including lice, fleas and mosquitoes] and…why shouldn’t ticks be part of that? And they fail to understand that even if you were able to show that animal A eats Animal B, in this case, chicken eating tick, that does not demonstrate any capacity to control the abundance of Animal B, and this is why we ecologists exist, is to actually do the science, to ask questions like that…In the case of chickens, we don’t, actually, so no one has. It’s a great project, I might try to find a collaborator who would be able to undertake such a study.

I can’t say the jury is still out on chickens as a biological control for ticks because the jury has yet to be impaneled. Perhaps one day the research will be done and it will actually back the urban legend. Until then (and on the bright side), there is a pretty extensive “laundry list,” as Ostfeld termed it, of known biological controls for ticks and he and Keesing are currently running a five-year program testing one of them in Dutchess County.

But before I tell you about The Tick Project, I need to tell you something about ticks. And Lyme disease. Possibly not in that order.

 

Ixodes scapularis

Lyme disease is a vector-borne disease. The term “vector,” in this sense, refers to the “vehicle” by which a pathogen is passed from an infected host to a non-infected host. Ostfeld says the vector is “almost always an arthropod, so a mosquito or a flea or a fly, a tick, a louse” that is infected by “the pathogen of interest.” In the case of Lyme disease, the pathogen is the Borrelia burgdorferi or B. burgdorferi bacterium. In Nova Scotia (as in the northeastern United States, where the highest concentration of the disease is found) the vector for the B. burgdorferi bacterium is the blacklegged tick (Ixodes scapularis) also known as the deer tick. It’s one of 14 different types of ticks found in NS and the only one capable of carrying Lyme disease.

Lyme disease symptoms, according to the US Centers for Disease Control and Prevention (CDC) include “fever, headache, fatigue, and a characteristic skin rash called erythema migrans. If left untreated, infection can spread to joints, the heart, and the nervous system.” On the other hand, if caught early, it can be treated with antibiotics. (There is a lot of controversy over how Lyme disease is diagnosed and treated, but that is beyond the scope of this article, which is focused on how it is transmitted.)

Speaking of which, I found this video showing just how ticks go about spreading disease. It’s a European production focused on the transmission of something called Tick Borne Encephalitis (TBE), but the principles apply to the spread of Lyme disease:

If you don’t have time to watch the video (or if you’re eating your lunch as you read this, in which case, you probably don’t want to watch the video) I’ll cut to the chase: there are four distinct stages in the two-year life cycle of the blacklegged tick — egg, larvae, nymph and adult.

When the egg hatches, a tiny larva emerges. To continue its development, it requires a “blood meal,” and must hope it gets picked up by a host (a small mammal or bird) brushing past it. If that host happens to be infected with B. burgdorferi, the bacterium can pass to the tick. When the tick is finished eating, it falls off the host, and over the next weeks, becomes a nymph. (Not in the “beautiful maiden dancing by the river” sense, but in the “Check me out: I’ve shed my old skin and grown an extra pair of legs” sense.) To become a fully mature adult, it requires another blood meal and so, as the video narrator explains:

The nymph waits on grasses that are up to a meter high for a suitable victim to brush against it in passing.

If you are that “suitable victim” and the tick bites you, you can become infected with Borrelia burgdorferi. 

To be clear, adult ticks can also infect you, but you are much more likely to notice an adult tick on your skin than a nymph-stage tick which, as Andrew Hebda, the zoology curator of the Nova Scotia Museum told me, are sesame-seed sized. You probably won’t feel the bite in either case, because while it’s sucking your blood, the enterprising tick also injects you with a “whole range of substances” through its saliva, including a toxin that “anesthetizes the wound.” (That said, I heard an interview with Professor Keesing in which she said that she is one of the not entirely rare people who can actually feel tick bites — her immune system reacts to the insertion of the tick’s mouth parts into her skin with an “intense itching.”)

Larval and nymph-stage ticks have no discernible sex — it’s not until they’ve become adults that they can be divided into males and females. The female sends out scent signals to the male, who mates with her as she eats (I refuse to make any dumb jokes about this on the grounds that this is a newspaper article, not an episode of Married with Children). In our part of the world, this reproductive action is likely to take place on the hide of a white-tailed deer, an animal which is not particularly good at passing along the B. burgdorferi bacterium, but which does provide a breeding ground for most of our ticks.

The female tick, once she’s done, falls off the host, lays about 2,000 eggs and dies of exhaustion.

 

Peromyscus leucopus

I’ve told you that ticks require blood meals to develop, but I haven’t told you much about the source of those blood meals, and that is actually important because (as I hinted at above, in reference to the white-tailed deer) some animals are better B. burgdorferi hosts than others. The scientific term for this is “reservoir competence.”

Ostfeld said a host with a high reservoir competence is “a host from which the vector very efficiently acquires infection” but which does not itself “tend to suffer dramatic ill effects from that infection.”

In our part of the world, there is one rodent that is to hosting the B. burgdorferi bacterium what Beijing is to hosting the Olympics: the white-footed mouse (Peromyscus leucopus).

The rodent is incredibly efficient at passing along the Lyme-disease causing bacterium — Keesing said in an that earlier-cited interview that an infected white-footed mouse will infect 90% of the ticks feeding from it.

The other chief culprits are chipmunks and shrews.

 

The Tick Project

From 1995 to 2012, the number of US counties considered Lyme disease hotspots rose from 69 to 260, according to the Centers for Disease Control and Prevention. Roughly 300,000 Americans are diagnosed with Lyme disease each year.

At the same time, the range of the ticks that transmit the disease has doubled.

That increase in range is due to climate change. (Keesing, in that interview cited earlier, floats the interesting idea that it’s not that winters, in places like Vermont — and Nova Scotia — are getting less severe but that the warm period during which the ticks can get their all-important blood meals is growing longer.) Here in this province, which saw a total of 701 cases of Lyme disease between 2002 and 2015 (an average of 54 per year), we had 254 reported cases in 2015 and 115 reported cases in 2014 and the Department of Health and Wellness publishes a “Lyme Disease Estimated Risk Areas” map.

Understandably, authorities are anxious to find a way to control the spread of Lyme disease (and other tick-borne diseases beyond the scope of this article) and that’s the goal of The Tick Project, a five-year, multi-million dollar research study led by Ostfeld and Keesing in Dutchess County, New York.

Under the auspices of the Cary Institute in partnership with Bard College, the CDC, the New York State Department of Health and the Dutchess County Department of Behavioral and Community Health, the study intends to determine whether “two tick control methods, used separately or together, can reduce the number of cases of Lyme disease at the neighborhood level.”

One method involves treating participants’ lawns with Met52, a fungal spray made from a strain of a naturally occurring, tick-killing fungus called Metarhizium ansipoliaee. As Ostfeld explained:

The Met52 is sprayed on people’s property, their lawns, the adjacent forest, their ornamental plants, gardens, you know, whatever they’ll let us spray. If they say, “Well, don’t spray my vegetable garden,” then we obviously honor that…It’s sprayed in a water solution, like a chemical spray would be, although it is not a chemical spray, and then we have a pure-water spray as a placebo control.

The second method is called a Tick Control System (TCS) bait box, which Ostfeld described as:

…a little plastic box with a metal cover — about the size of a bread box, a little smaller maybe, that has a couple of holes that allow small mammals to enter. Small mammals like to enter small, dark holes ’cause that’s what their burrows and escape routes look like. There’s also a little bit of a semi-attractive bait inside. To get to the bait they have to brush by a little wick that places a minute amount of a chemical called fipronil on them. Fipronil is the product in Frontline, which millions of people use on their dogs and cats, and that kills the ticks on the mice and chipmunks and shrews, and those are the main reservoir hosts for tick-borne pathogens.

…So if you’re killing the ticks that are on mice, chipmunks and shrews, you’re killing the ones that are most likely to become dangerous. And you’re not releasing this chemical, fipronil, out into the open, because you’re only delivering a tiny, bitty drop onto the small mammals and it doesn’t affect their health at all. Delivering 100 times as much on a dog or cat doesn’t affect a dog or cat either, it’s a very safe product… [W]e have a placebo control for the bait boxes too, which is a bait box that has no fipronil in it.

Ostfeld said the first year of the project, 2016, was one of “very extensive, aggressive recruitment” of participants which he characterized as “hard work,” because, as he explained, “We’re asking a lot of them, a ton actually.” They ended up with 24 neighborhoods of about 100 households each. The study is a double-blind, like a pharmaceutical study — neither the participants nor the researchers know who is getting what treatment. (There are four possible combinations: active spray and active bait box; active spray and inactive bait box; inactive spray and active bait box; and inactive spray and inactive bait box.)

Ostfeld said they began the treatments last season and this season is:

…the first year when we expect the full force of both treatments to be achieved, because there’s a lag with the bait boxes. Last year’s bait boxes we expected to kill the baby ticks, the larval ticks on the small mammals, and so the effect is expected to materialize in fewer nymph-stage ticks this year, 2018. And that nymph stage is the dangerous one.

Ostfeld said because of this expected lag, they haven’t yet looked at any of the data; they may do so this year, although he says he and Keesing and their CDC partners are being very cautious of “unblinding” themselves because “you can never go back.” He says they have to be prepared for all possible scenarios when they look at the data and avoid making “ad-hoc decisions for convenience or for some other not very legitimate reason.”

I told him I’d check back in when they’d started examining the data.

 

Biodiversity

I had two more topics I wanted to discuss (briefly) with Ostfeld before I let him go: one was the benefits of biodiversity in controlling Lyme disease. It’s a subject covered in a paper he and Keesing co-wrote with Kathleen LoGiudice and Kenneth A. Schmidt in 2002. Ostfeld told me there had been additional studies since 2002 (two of them undertaken by Canadian researchers) that gave further credence to their conclusions.

Their study showed that the greater the variety of hosts you have in a given area, the lower the percentage of infected nymph-stage ticks. That’s because of that “reservoir competence” effect we discussed earlier — some hosts are just not that great at “amplifying” the bacterium, as Ostfeld puts it (red and grey squirrels, for instance, are very bad at it). So if you have lots of red and grey squirrels diluting your population of white-footed mice, you end up with fewer infected nymph-stage ticks.

The problem is, when environments are degraded — through forest fragmentation or clear-cutting — it’s creatures like the white-footed mouse that tend to survive. As Ostfeld explained:

[T]here is some evidence that clear-cutting does reduce diversity. It drives out certain species of mammals and birds that require larger tracts of undisturbed habitat and the species that disappear when you do things like clear cut or fragment in some way tend to be the species that control the culprits. You tend to lose predators and you tend to lose larger-bodied animals because both predators and large-bodied animals in general tend to require larger habitat areas to make a living and those are the very animals you least want to lose. And the result is that oftentimes the prey, the smaller-bodied things…are the ones that thrive, like the mice and chipmunks. And that is an unfortunate situation when it comes to tick-borne disease risk because they’re the reservoirs, as we discussed earlier.

 

Opossum

And finally, I had to ask about the opossum (or possum, it’s your choice), which we don’t have in Nova Scotia, although one was trapped in New Brunswick in 2014 and they are found in Ontario where they apparently terrified the late Rob Ford, who described them to Toronto councilors as “vicious.” In response, York University animal behaviorist Suzanne MacDonald told the National Post:

There is no need to be afraid of an opossum. I can’t imagine anyone being afraid of an animal that actually falls unconscious when it is threatened.

In addition to being non-vicious — and the only marsupial in North America — the opossum turns out to be a tick-killing machine. That’s apparently because opossums are obsessive groomers who will eat thousands of ticks off themselves. Said Ostfeld:

Yes, opossums are hoovering up ticks and killing them and if they don’t kill the tick they are at least failing to infect it, because they’re not a competent reservoir at all, so they are, acting on our behalf in a way (unwittingly).

So I asked if we should be encouraging the opossum to come to Nova Scotia and he responded like a responsible ecologist:

I’ve been asked whether we should start breeding opossums and air-dropping them into suburban areas and I just try to discourage that. But to be honest with you, I think there is reasonable evidence that opossums are moving northward due to climate change. They evolved in a more tropical area and they’ve been spreading northwards for a decade and so the answer to whether we should be doing more to encourage the spread of opossums… I would say is, “No,” because that would mean that we should…burn more fossil fuels and dump more greenhouse gases into the [atmosphere] and I think for the benefit of the possums we’d be paying a mighty high price.

But:

They will probably get there, I would guess, within a few years, maybe a decade. And maybe they’ll help you out.

So while we shouldn’t do anything to encourage the possums to come here (and honestly, I know that was a terrible idea on my part), and while I’m not calling for any Opossum Recruitment programs, I do want to go on record right now as saying that if they do arrive, we need to welcome them better than New Brunswick did.

 

 

 

 

 

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