On a warm spring day, Daniel Salkeld is hiking the hills of coastal scrub and chaparral of Marin County, north of San Francisco. It is his favorite spot to collect ticks.

As he walks, he trails a white flannel blanket attached to a pole, and every 20 meters, he stops, scrutinizes the flannel and picks up any ticks that have latched on. They wait for a mouse, deer or person to brush past the blade of grass they are clinging to to be a passive predator of blood. The scientists who track them are easily fooled by wool fabric.

As he walks, Salkeld places the ticks in a container for further examination in his laboratory at Colorado State University. He wants to know if there are high or low risk areas for tick-borne diseases in California. He says that when his tick count is zero, it's a useful insight.


tick biologists like him are uncovering an unnerving trend: Many tick species are expanding their ranges and picking up new pathogens that can deliver disease to people should a tick bite.

The reason to worry is that ticks bring more types of pathogens to people from animals than any other creature. They are on the march. The number of tick-borne diseases in the United States has doubled since 2004, with most of the increase occurring in the form of the tick-borne illness, Lyme disease.

Rebecca Eisen, a research biologist with the Centers for Disease Control and Prevention, says that it's important to know when ticks have spread to new places.

Predicting where the blood-feasting arachnids will move next is important. It's more difficult than grabbing ticks onto flannel. Land development, climate change, and the availability of blood are some of the drivers that scientists are trying to untangle. All will have consequences for the number of ticks in a given area, as well as for the likelihood that a tick's saliva will carry at least one of the 18 tick-borne pathogens identified in the US.

Lucy Gilbert is an ecologist at the University of Glasgow.


Ticks are expert spreaders of pathogens

There are two main types of ticks: hard ticks, which have visible mouthparts and a hard plate on their back, and soft ticks, which lack the hard plate and have mouth parts hidden on their undersides. Hard ticks are the more serious of the two types of arachnids. The origins of hundreds of species in this family are ancient, and they are scattered all over the world.

There are a number of diseases caused by hard ticks, both familiar and obscure. The most serious spreaders are a few tick species. The black-legged tick can harbor six other diseases. The black-legged tick, the lone star tick, and the American dog tick are responsible for most cases of tick-borne illness in the United States.

Because they live a long time, ticks are able to transmit disease. Most of the ticks that carry disease live two to three years and feed on the blood of multiple hosts across their four-stage life cycle. They have a lot of time to pick up a pathogen that can be transmitted through saliva when they bite someone.

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The ecology of tick diseases is difficult to understand because they can tap the blood of many animals. She points to the black-legged tick and the Ixodes ricinus, both of which spread the disease.

Life cycle of a tick.
Most ticks that spread pathogens to people have a two-to-three-year life cycle with four stages: egg, larva, nymph and adult. The timing of these is approximated in this graphic; depending on the particular tick species and where the tick is located, the length of stages and the overall length of a tick’s life can vary. Credit: CC BY-ND

Not every host species is capable of producing disease-causing organisms in their blood. This helps to explain some geographic patterns. Black-legged ticks prefer lizards for their blood meals in the southeastern US. The blood of one species even kills the pathogen, so the reptiles rarely pass it on to ticks. The southern US has a low incidence of the disease.


Other patterns are hard to understand. The lone star tick, which is most common in the southern US, has made it into Canada and scientists are trying to understand why. The CDC says that the species can transmit at least six diseases, including one that can cause death from an allergic reaction to red meat.

They want to know why black-legged tick populations have spread to new areas. The number of US counties where the tick has been found has more than doubled in the past two decades.

Since the late 1990s, the US has seen a threefold increase in the number of cases reported to the CDC. The five-year average for the number of cases in Ontario was three times higher in 2017).

Why are tick-borne diseases increasing? There are many theories.

Confirmed cases of Lyme disease in the United States.
Lyme disease diagnoses have risen over the past couple of decades. Experts also believe that the illness is vastly underdiagnosed. The Centers for Disease Control and Prevention estimates that actual cases could be more than 10 times higher than diagnosed cases. Credit: CC BY-ND

Climate change, land development and ticks

Climate change may be contributing to the expansion of tick habitat for several species of arachnids. It has the potential to affect tick range because ticks are exotherms and their bodies don't maintain consistent internal temperatures. Since they spend most of their lives away from animals that they bite to get blood, they are sensitive to environmental temperature shifts.


Gilbert says that increasing temperatures allow their life cycles to speed up, shortening a three-year life cycle to a two-year one. ticks can survive in habitats that would have killed them in the past.

Ixodes ricinus, the castor bean tick, is 1,300 feet higher in elevation than it was in the 1950s. Under some scenarios of global warming, the range of the lone star tick will move northward and eastward. A review of climate modeling studies found that black-legged ticks are expected to expand northward with warming temperatures across the Dakotas, northern Minnesota, Canada and Alaska.

The area of tick habitat is projected to grow as temperatures rise in higher altitudes and more northerly countries, but some areas may become too dry for the ticks.

We know that climate change has an effect on ticks, but we don't know how important it is relative to other factors.

The most studied case is that of black-legged ticks and the research suggests that land use changes have been critical.


The spread of ticks seems to have been caused by a pattern of development in the Northeast. The settlers felled a lot of trees in the Northeast to make way for farms. They hunted white-tailed deer, a major source of blood food for ticks. The deer fell in numbers.

Hosts ticks feed off of.
The ability of ticks to survive, reproduce and spread disease depends on what hosts are around for them to bite. Some small mammals, like the white-footed mouse, can harbor pathogens. Other animals like lizards, however, don’t carry pathogens reliably—they are called dilution hosts because they dilute the odds of a tick getting infected. Ticks also need reproductive hosts like deer, which ticks both feed off and mate on. Predators affect the population sizes of the small mammal hosts, as do resources such as food—in turn, affecting the chance that a tick in an area will carry a pathogen and spread disease. Credit: CC BY-ND

When northeastern farms were abandoned in the 1800s for farms in the Midwest corn belt, the forests took root again. In a 1993 Science paper, researchers pointed to the rebound of deer as a factor in the rise of ticks and disease. The authors theorize that people in the region may have been affected by the disease hundreds of years before it was recognized by the medical community.

The real story behind the increase in ticks in the Northeast is more complicated than previously thought, according to many ecologists. Not every animal that a tick feeds off carries Borrelia burgdorferi. White-footed mice and Eastern chipmunks are capable of carrying the pathogen and passing it on to ticks, but other animals, such as rabbits and lizards, don't reliably harbor it. Deer and Borrelia burgdorferi aren't the same.

It's not surprising that the range of tick species in a tick habitat is more important than the number of ticks that pick up the pathogen. Having a variety of potential hosts reduces the chance that a tick will feed on an animal that has been bitten by a tick.

If rejuvenated forests and deer aren't the main driver of the rise of Lyme disease in the Northeast, what is? Building patterns that break up forests into isolated chunks is the true culprit.

Deforestation and reforestation in the United States.
Trees have grown back or have been replanted in many parts of the United States following the loss of vast acreages for farms and timber. In the Northeast, the return of forests has been associated with an increase in ticks and Lyme disease. Today, many ecologists say this story is oversimplified—that the rise of pathogen-bearing ticks is the result of a complex web of interactions between creatures and is influenced by land use and climate change. Note: The first three images are based on data for virgin forests, while the last shows all forest cover. Credit: CC BY-ND

Scientists know that white-footed mice are good at helping tick, because they harbor the bacterium without becoming ill, and they feed ticks well. White-footed mice thrive in patches of forest with fewer rodents.

As mouse numbers rise, the odds that a hungry tick will feed on a disease-harboring rodent go up, increasing the odds that a person gardening in their yard or going for a walk in the forest will get bitten.

The density of young, nymph-stage was found in the smallest forest fragments they studied, which were less than 1.2 hectares. 70 percent of the time, the nymphs were bitten by Borrelia, compared to 48 percent in larger patches.

It's clear that the implications of this for catching the disease are clear.

Other studies show that tick-borne diseases go up when predator numbers go down. In the Northeast and Midwest, the numbers of red foxes which eat mice have declined.

Deer counts don't reliably predict where the disease occurs. In some places where deer populations have remained constant for a long time, there has been an increase in the number of Lyme cases.

Nothing is straightforward with ticks and tick-borne diseases. It is possible that a different mouse that is a poor one will predominate in a fragmented patch of green space.

Gilbert says that when tick numbers are high, they pose a risk to people. If Borrelia is only present in one in 10 ticks, then a walk in the woods exposes you to 20 ticks.

Predicting and preparing for tick movement

It's not easy to tell where scientists will go next after making progress on illuminating the causes behind the spread of ticks and the diseases they carry. California is one of the states where this difficulty is clear.

Californians might be forgiven if they don't check for ticks after a hike. The Golden State is not free of the disease. Researchers found that they were able to collect at least 10 ticks in the San Francisco Bay Area. The western black-legged tick is the cause of lyme cases on the West Coast. Health officials have not yet reported a rise in the number of cases of the disease, but under a future scenario of high greenhouse gas emissions, up to a third of the state could becomehabitat by the end of the century.

According to the first author of the study, warmer, wetter winter conditions would make it easier for ticks to survive. Even if the environment dries out, ticks can persist by finding a moist patch.

It's hard to make projections without knowing where ticks like to hang out. The citizen science research project was launched in 2016 and involved volunteers reporting where they encountered ticks and sending them to a lab for identification. The ideal climate niche for Ixodes pacificus was determined based on almost three years of data. They projected how much of the area would be in a warmer world.

The tick habitat in California, Oregon and Washington is going to shrink by 2050. The accuracy of future climate predictions is dependent on the ecology of the tick life cycle, says the study's lead author and an infectious-disease researcher.

Porter and Salkeld say that more citizen science projects like this could give humanity a leg up on ticks. When the team used three years of data from the project to create maps of tick and pathogen prevalence, they looked very similar to the CDC.

The human side of the equation is being tackled by other researchers. Maria del Pilar Fernandez of Washington State University has found that people don't take the same precautions in their yards as they would in the woods. In a study on Staten Island in New York, she found that 40 percent of ticks were carried by Borrelia burgdorferi.

People across the country can help tackle a lot of other tick questions by submitting data on ticks they encounter, and then scientists like him can go out and verify the findings, dragging their tick-gathering blankets along trails.

The job might not appeal to many people, but Salkeld enjoys it.

The article was originally published in Knowable Magazine. You can sign up for the newsletter.