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Tick-borne disease testing algorithm

 

Presenter

Bobbi Britt, M.D., is the Director of the Clinical Parasitology Laboratory and Vice Chair of Education of the Department of Laboratory Medicine and Pathology at Mayo Clinic in Rochester, Minnesota. She holds the academic rank of Professor of Laboratory Medicine and Pathology.

 

Questions?

Contact us: MMLHotTopics@mayo.edu.

Transcript and References

Introduction

Hello, and welcome to Part 2 of this “Hot Topic” series on tick-borne diseases. I’m Dr. Bobbi Pritt, and I co-direct the Vector-Borne Diseases Laboratory Services area along with my colleague, Dr. Elli Theel.

Disclosures

Before I begin, I’d like to say that I have no disclosures to make.

Before we discuss our first case, I’d like to introduce the algorithm that I will be subsequently referring to. This algorithm for acute tick-borne disease testing is freely available on the Mayo Medical Laboratories website and it contains links to most of the tests that I will be discussing today.

Case #1

Now, let’s move on to our cases. Case #1 is a seven-year-old boy living in Connecticut who presents with several days of fever, fatigue, and body aches. He had been camping with his family at a local state park two weeks prior to presentation, and his mother reports multiple mosquito bites but doesn’t recall any tick bites. Also, no rash was noted.

If we go to our algorithm, we can determine our next steps. The algorithm starts with the clinical scenario—in this case, a scenario where there is a clinical suspicion of tick-borne disease based on patient characteristics. The boy has an illness during tick season with fever, fatigue, and body aches. He also has a recent environmental exposure to ticks, even though his mother doesn’t recall any tick bites. In this situation, the next step is to consider a number of tick-borne pathogens based on his geographic exposure.

Distribution Map

This map from the Centers for Disease Control and Prevention (CDC) shows the distribution of key tick-borne diseases reported in 2015. I like this map because it nicely demonstrates where the main clusters of disease are, as well as the overlap of many tick-borne diseases. It also shows how there are a smattering of cases outside of the main clusters of disease. Although our focus will be on the main areas of tick-borne disease, it’s important to keep in mind that cases can occur outside of these areas.

Case #1, Continued

The next step in our algorithm is to determine if the patient is at risk for Rocky Mountain spotted fever, as shown on the left in the green box, or the other tick-borne diseases shown on the right. Depending on the patient’s residence and exposure history, he or she may be at risk for both groups of diseases. In this case, our patient’s potential tick exposure is in Connecticut, and he is therefore at risk for Lyme disease, anaplasmosis, babesiosis, and Borrelia miyamotoi disease. We will therefore be following the arm of the algorithm shown in the red box.

The next question in the algorithm is whether a classic erythema migrans rash is present.

If yes, then no testing is indicated for Lyme disease since the presence of the classic erythema migrans rash is consistent with Lyme disease. Thus, presumptive therapy for Lyme disease is indicated.

If the answer is no, as it is with this patient, then one or more of the tests listed should be considered. Also, therapy for anaplasmosis should be considered while waiting for the test results since this disease can be potentially fatal.

In this case, the Lyme disease serology was ordered to further evaluate for Lyme disease, and because the patient had systemic symptoms, a PCR panel for tick-borne pathogens was also ordered.

The Lyme disease serology was performed and was negative. However, the tick panel, which is comprised of PCR tests for Ehrlichia species and Anaplasma phagocytophilum, Borrelia miyamotoi, and Babesia species, was positive for Babesia microti.

Babesiosis

Let’s briefly discuss babesiosis.

It’s an interesting tick-borne disease, since unlike the other diseases in this algorithm, it is a parasitic infection. It is primarily due to Babesia microti in the United States, and it’s highly endemic in the northeast and upper Midwest. It is primarily transmitted through the bite of Ixodes scapularis, the black-legged or “deer” tick. Less commonly, babesiosis is due to Babesia duncani—primarily on the west coast, and the Babesia MO-1 strain. Babesia duncani is transmitted through the bite of Ixodes pacificus, the western black-legged tick whereas Babesia MO-1 is possibly transmitted by Amblyomma americanum, the lone star tick.

Babesia divergens is another important pathogen and is the most common cause of babesiosis in Europe and Asia. Of note, all of the species listed on this slide can be detected by the Mayo Clinic PCR test, which is particularly helpful for patients who have travelled throughout the United States or overseas.

Babesiosis—Clinical Presentation and Treatment

Babesiosis is often asymptomatic or causes only mild disease. However, elderly, immunocompromised, and asplenic patients are at risk for severe disease. Treatment is with a combination of agents. The standard therapy for most patients is atovaquone plus azithromycin, whereas clindamycin plus quinine is usually reserved for severely ill patients.

Babesiosis—Diagnosis

Babesiosis is traditionally diagnosed using microscopic examination of thick and thin blood films. This method also allows for calculation of percent parasitemia, which is important for guiding therapy and determining the patient’s prognosis. However, the Mayo Clinic Babesia PCR is more sensitive than blood films and is therefore the initial recommended test for detection of babesiosis.

In looking back at the patient’s case, you may wonder if it makes sense that no one recalled a tick bite.

Well, in fact, it’s not uncommon that patients with tick-borne illnesses don’t recall getting a tick bite. Therefore, the absence of a recalled tick bite cannot be used to exclude a tick-borne illness.

Why Test for So Many Pathogens at Once?

Another question to be considered is why testing for so many pathogens at once is recommended.

That is because multiple organisms can be transmitted through a single tick bite, and clinical features are not usually sufficient to differentiate them.

Case #2

Now, let’s move on to our second case.

The patent is a 42-year-old man living in Minnesota who found an embedded tick on his back. He had initially mistaken it as a skin tag and only later discovered that it was an engorged tick. He had been hiking in northern Minnesota during the past weekend and estimates that the tick had been attached for at least two days. After removing the tick, the patient’s physician submitted it to the laboratory.

This is the specimen that was received in the laboratory. It is clearly alive and obviously engorged. You can see that it’s having some difficulty walking across this petri dish.

Tick Identification Services at Mayo Clinic

The test that the physician ordered is the parasite identification. This broad-ranging test is used for the identification of worms, bugs, and anything that looks like these objects (the ticks). If the object is an arthropod, it is identified to the medically appropriate level, which, where appropriate, includes the genus and species name.

Tick Identification

For ticks specifically, the information that we provide is the degree of engorgement, the gender and life-cycle stage, and the presence or absence of mouth parts. This information can then be used to guide preventative therapy in certain situations.

IDSA Guidelines for Tick Bite Prophylaxis

For Lyme disease, the CDC and Infectious Diseases Society of America (IDSA) recommend prophylactic treatment when all of the circumstances shown on this slide in the red box are met. Note that the second circumstance is that the attached tick needs to be identified as an adult or nymphal Ixodes scapularis tick. By providing this identification service, a laboratory can help health care professionals decide if Lyme disease prophylaxis is appropriate for their patients. Another important take-away message from this slide is that antibiotic treatment following a tick bite is not recommended as a means to prevent tick-borne illness other than Lyme disease, including anaplasmosis, babesiosis, ehrlichiosis, and Rocky Mountain spotted fever.

Case #2, Continued

Now, going back to our case, the patient’s tick was identified as a Dermacentor variabilis female tick. It was fully engorged, and its mouth parts were intact. Given this identification, the patient was reassured that he was not at risk of acquiring Lyme disease from this tick and that no prophylactic antibiotics were indicated. He was, however, advised to watch for signs of tick-borne illness and see his health care provider if they appeared.

The Role of Testing Ticks by PCR

One request that we occasionally receive is to test ticks by PCR to see what they are carrying. However, this is not a test that we offer through Mayo Medical Laboratories. Why is this? Well, it is because finding a tick on your body does not mean that it bit you or transmitted a pathogen. Instead, the identification, gender, and degree of engorgement of the tick are better indicators for risk of disease, and those are all aspects of the identification that we provide.

Summary

In summary:

• There are multiple tick-borne diseases, many of which are clinically indistinguishable.
• The preferred test varies by the time that the patient presents in the course of illness.
• Therefore, an algorithmic approach is useful for optimizing laboratory testing, ensuring that all of the necessary tests are ordered without performing unneeded tests.

Thank you for joining me today in this second part of our Hot Topic series on tick-borne diseases.

 

Related Hot Topic:

Lyme Neuroborreliosis