Pestology Blog

Lyme disease is the fastest growing arthropod-vectored disease in the US, with an estimated 476,000 people diagnosed and treated for this illnesses each year. We know the bacterium responsible for the disease is primarily vectored by the blacklegged tick Ixodes scapularis. And we know that several other animals in urban environments that serve as hosts for the blacklegged tick, such as white-footed mice, can serve as important reservoir hosts for the bacterium. Reservoir hosts are animals that can become infected with a pathogen without dying, allowing them to be a continuous source of infection for the ticks. The fact that white-footed mice are important vectors is an important factor, so keep that in mind as I run through this summary.  

With the number of annual Lyme disease cases on the rise, the need to better understand the transmission cycle of Lyme disease continues to increase. One important relationship that researchers have used to model disease transmission for other pathogens is the relationship between disease vectors and other wild animal hosts, known as sylvatic hosts. Sylvatic hosts tend to be wild animals that are not as competent at maintaining the disease transmission cycle as reservoir hosts. Researchers refer to this relationship as the dilution effect. The hypothesis behind the dilution effect is that greater wildlife biodiversity in an area could translate to lower disease risk for humans. The logic behind how this theory could apply to Lyme disease transmission is that if you throw a bunch of tick hosts into a community, then infected blacklegged ticks may end up feeding on animals that don't pass along the Lyme bacterium nearly as well as white-footed mice do.  

To understand what the dilution effect could tell us about Lyme disease transmission risks, researchers at the University of Texas developed a modeling framework that would examine compiled data sets from previous studies. Two ways their model measured tick-borne disease risk in a given area were nymphal infection prevalence (NIP), which is the percentage of nymphs in an area that are carrying the Lyme disease bacterium. And density of infected nymphs (DIN), the actual number of infected nymphs per unit of land. These two numbers are often used in disease ecology studies and they usually move together, but not always. You can have a community with a very high number of nymphs but fewer nymphs that are actually infected. This would translate to the actual number of infected ticks in the community being just as high, or higher when compared to another community with fewer nymphs overall. Most dilution effect research has leaned heavily on NIP, so the authors at the University of Texas set out to build a model that tracks both metrics side by side. 

The researchers combed the literature for biological data on 15 common tick host species found in the northeastern and upper midwestern US (including mice, voles, shrews, chipmunks, squirrels, deer, opossums, raccoons, several songbirds, and even a couple of lizards). For each species they compiled numbers like host density, how many larval ticks it typically attracts, and reservoir competence (essentially, how good that species is at infecting a feeding tick). They then built a mathematical model with a twist most previous models skip: their model accounted for the fact that when a host species disappears from a community, ticks don't always successfully relocate to a different host. In the real world, larvae can just fail to find a replacement blood meal, something previous field studies on lizard removals have documented directly. 

Running various host communities through the model turned up some counterintuitive results. Most birds, reptiles, and larger mammals like raccoons, opossums, and deer had little effect on infection prevalence either way, with one big exception: the five-lined skink, which turned out to be a serious dilution host on both nymphal infection prevalence (NIP) and density of infected nymphs (DIN). Skinks attract plenty of hungry larvae but are terrible at passing along the pathogen, so loading a habitat with skinks pulls ticks away from more competent hosts without significantly infecting them. Small mammals, on the other hand, told a much more complicated story. Chipmunks, Sorex shrews, and red-backed voles acted as dilution hosts when added to a mouse-only environment. But, they flipped to amplification hosts once removed from a more diverse community. In other words, whether a species helps or hurts depends entirely on what else is sharing the habitat with it. 

Something else I thought was interesting was that the model also uncovered "tipping point" densities. These were where abundance thresholds in a community suddenly shifted from producing mostly uninfected nymphs to mostly infected ones. For mice, that tipping point arrived at a much lower density in a small-mammal-only community versus a fully diverse community, which suggests biodiversity could buffer against a mouse population boom in terms of infection prevalence. But here's where it gets interesting. When the researchers simulated habitat fragmentation by progressively removing species groups, lowering biodiversity consistently raised percentage of infected nymphs (NIP) but often lowered the actual number of infected nymphs per unit of land (DIN). Fewer host species meant a higher proportion of infected nymphs, but also fewer total nymphs being produced, since there were simply fewer animals around to feed the tick population in the first place. A mice-only community, in fact, produced the highest infection prevalence of any scenario tested, but the lowest overall density of infected nymphs. 

The takeaway here isn't that the dilution effect is wrong, it's that it's incomplete on its own. And, that this disease transmission cycle is a lot more complicated. A habitat that looks "safer" by percentage of infected nymphs could still be dumping just as many, or more, infected ticks into the landscape once you account for total tick abundance. For pest management professionals doing tick work, especially around wooded areas, edge habitats, and properties bordering wildlife corridors, this is a good reminder that host diversity on a property isn't simply defined as "risky" and "safe." What matters is the whole community of animals using that yard, not just whether mice are present, and both how common infected ticks are and how many of them there actually are on the ground. That means providing recommendations for addressing conducive conditions for wide range of wildlife can be just as important as the treatment itself, since an important factor in disease pressure may be everything other than the ticks themselves living on and around your client’s property.