Transmission of Tick-Borne Diseases and How to Diagnose Them


By Barry C. Fox, M.D., University of Wisconsin

Several years ago, a young woman who had been at a summer camp in Southern Indiana was bitten by a tick and developed symptoms of a tick-borne disease. Her symptoms included a fever, headache, muscle aches, dizziness, and nausea. What happened next?

Young woman looking at a tick sitting on her left arm while in the forest.
Tick-borne diseases are a serious public health problem in the United States. (Image: Jens Rother/Shutterstock)

The Testing of Tick-Borne Diseases

After 24 hours of worsening symptoms, she ended up in the emergency department, about to endure a perplexing five-day ordeal in the hospital. They discovered a small bite wound on her chest, suggestive of a possible tick bite, although she didn’t remember getting bitten. She’d been hiking in a heavily wooded area, with many forest animals, mosquitos, and potentially hidden ticks.

In the early 2000s, diagnostic tests for tick illnesses were much less reliable than they are today, and false-negative tests were common. Often a diagnosis had to be made on clinical manifestations and clues from the epidemiologic history—rather than relying on a positive blood test. This can happen with any blood test performed during the first week of an illness, since there may not be sufficient time for the person’s immune system to mount an antibody response.

But the general consensus of the infectious disease specialists’ was that the woman had what was known as the ‘summertime flu’. This is caused by intracellular parasites known as either Ehrlichia or Anaplasma—both of which are transmissible by ticks.

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Getting the Disease Is Not that Hard

This woman’s scenario could happen to anyone who goes camping or hiking, or even just lives or works in wooded areas or prairie with long grass. Tick-borne diseases have now been deemed to be a serious public health problem in the United States. 

The incidence of infection is rising and accounts for tens of thousands of cases each year. Ticks are also expanding their ranges into new areas across the country. Ticks are known to be the vectors for at least 14 different illnesses—with a wide range of illnesses. New tick-borne illnesses are even still being discovered.

Here are some of the different types of ticks that cause diseases in the United States: the western blacklegged tick, the Rocky Mountain wood tick, the lone star tick, the brown dog tick, the blacklegged deer tick, and the American dog tick.

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How Are Ticks Attracted to People?

Remarkably, ticks can sense human breath and smells, as well as body heat and vibrations. They have special sensory structures known as palps, and these enable ticks to detect an approaching host. Then they place themselves in a convenient place to hitch a ride when a host comes by.

Most people don’t know that ticks insert a small amount of saliva through the skin that has an anesthetic effect so that the tick bite isn’t felt. Ticks also have a pair of knife-like structures known as chelicerae that cut open the skin and a barbed structure called a hypostome that they insert through this opening.

Tick in the fur of a dog.
Sometimes ticks come from animals. (Image: chanon khunkitti/Shutterstock)

In the Indiana wilderness, both white-footed mice and white-tailed deer are common asymptomatic reservoirs carrying tick-borne diseases.

A tick latched onto the white-tailed deer, bit the deer, and acquired the germ. While the deer was sleeping, the tick sought shelter in the grass. Next, sensing a new host, the tick hopped on board and proceeded to bite the woman in the example, passing the Ehrlichia on to her.

This is an example of the zoonotic cycle of transmission from a reservoir, i.e., the deer, through the vector, i.e., the tick, and into the host, i.e., the human.

There are many diseases that can be transmitted from tick vectors. The type of tick that bit the woman was probably the lone star tick, which is easily identifiable due to a white spot on its back.

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They Come in All Shapes and Sizes

Ehrlichia and Anaplasma have similar tick hosts and similar clinical characteristics. But the geographic domains of these two species are somewhat unique in the United States.

Ehrlichia is found most commonly in the central-eastern and central-midwestern states and Anaplasma in the northeast and north-central states. Both Ehrlichia and Anaplasma, which are neither bacteria nor viruses, are intracellular organisms in white blood cells, with Ehrlichia living in macrophages and Anaplasma in neutrophils.

A tick on human skin.
The symptoms of tick-borne diseases are similar to flu symptoms at first. (Image: KPixMining/Shutterstock)

Both cause similar clinical symptoms including a fever, headache, and other flu-like symptoms, and neurological symptoms can be more severe and can include confusion, mimicking encephalitis, which is an infection of the brain, or even meningitis.

The lone star tick is the primary vector for Ehrlichia, while Anaplasma has both the western blacklegged tick and the blacklegged deer tick as vectors. The two diseases were not differentiated from one another until 1990.

How Do These Ticks Work?

After inoculation, the germs enter the circulation where they multiply inside the target white blood cells. They reprogram the host cell mechanisms similar to viruses. The germs occasionally cluster together in the white cells in the shape of mulberries, forming micro-colonies, known as a morula. This structure, although rarely visible, is diagnostic of these infections.

The diagnosis often relies on clinical judgment since the rapid diagnostic tests are often not available, and antibody formation may take weeks. One such rapid test is called polymerase chain reaction or PCR technology, which amplifies DNA.

Antibody tests to confirm the diagnosis can be performed at serial time points, four weeks later, for example, to look for an increase in the antibody immune response. Fortunately, the treatment with the antibiotic doxycycline, which affects the ribosomal protein synthesis of the organisms, is effective.

Since these organisms lack a cell wall, antibiotics that attack the cell wall such as penicillins are not effective and, as the prognosis worsens if the treatment is delayed, it’s important that the doxycycline be started immediately if the clinical illness is supported by the findings.

Common Questions about the Transmission of Tick-Borne Diseases and How to Diagnose Them

Q: How do ticks choose a host?

Ticks detect body heat and vibrations, helping them sense when a host is approaching. They can also sense smells and when the host is near enough they get the opportunity to transmit tick-borne diseases.

Q: What are some of the similarities between Ehrlichia and Anaplasma?

They are both intracellular organisms that live in white blood cells, causing similar tick-borne diseases with symptoms like headaches and fevers.

Q: Why aren’t antibiotics like penicillin used against ticks?

The organisms that cause tick-borne diseases don’t have a cell wall, meaning using an antibiotic that targets cell walls, like penicillin, would be ineffective.

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