A Model for Experimental Exposure of Humans to Larval Ixodes scapularis Ticks

Podsumowanie

This article presents the methodology for exposing humans to larval Ixodes scapularis for clinical research. The technique is relatively simple, tolerable by the research volunteers, and can be modified according to experimental needs. Such research involving human subjects must be conducted under clinical study protocols approved by the appropriate regulatory authorities.

Streszczenie

Tickborne diseases are a significant public health problem in the United States and worldwide. Ticks are obligate blood-feeding arthropods; an ixodid tick must remain attached to the skin of the host and complete its multi-day feeding process to acquire its blood meal. Exposing animals to ticks is a common practice for studying host responses to tick bites and tickborne diseases. We developed the procedure, conducted the first human research study, and published the findings on exposing human volunteers to uninfected larval Ixodes scapularis ticks. This article describes the methodology used to construct the containment dressing, how to apply and secure the ticks to the host, how to maintain the dressing, and how to remove the ticks from the host. Exposing volunteers to tick bites is an experimental procedure and must be performed under a clinical research protocol approved by the appropriate regulatory authorities. This method allows for translational research to better understand the human response to tick bites and foster the development of diagnostics, prevention, and therapies for tickborne diseases.

Wprowadzenie

Hard ticks (Ixodidae: Acari) are obligate blood-feeding ectoparasites that occur worldwide and are capable of transmitting a broad range of pathogens, including bacteria, viruses, and parasites, of major medical and veterinary importance. Ixodid ticks must remain attached to the host for days to complete a blood meal, and they have the capacity to stay attached to the skin while avoiding recognition, preventing local blood coagulation, and facilitating long-term feeding^1,2,3. Animal studies have demonstrated that non-permissive hosts acquire resistance to tick bites with repeated tick exposures, which can lead to a decreased ability to transmit a pathogen, while ticks can repeatedly parasitize permissive hosts. Acquired tick resistance is dependent on the nature of the host immune response^4,5,6,7.

Tickborne diseases are an increasing threat in the United States (US), with the number of reported cases more than doubling between 2004 and 2016^8,9. Due to climate change, the geographic ranges of different ticks continue to expand^10,11. The leading tickborne diseases in the US include Lyme disease, anaplasmosis, ehrlichiosis, spotted fever rickettsiosis, babesiosis, tularemia, and Powassan virus disease⁸. Lyme disease, caused by infection with Borrelia burgdorferi sensu lato, is the most common tickborne disease in the US and Europe¹². With approximately 476,000 individuals diagnosed with Lyme disease annually in the US, there is both a public health and economic burden to individuals and to society^13,14,15.

Ixodes scapularis (the black-legged or deer tick) is the primary vector for Lyme disease, as well as anaplasmosis, babesiosis, Borrelia miyamotoi disease, and Powassan virus disease in the US. Other medically important tick species in the US include Amblyomma americanum (Lone star tick), Dermacentor variabilis (American dog tick), Ixodes pacificus (Western black-legged tick), Dermacentor andersoni (Rocky Mountain wood tick), Ixodes cookei (Groundhog tick), Dermacentor occidentalis (Pacific Coast tick), Rhipicephalus sanguineus (brown dog tick), and Amblyomma maculatum (Gulf Coast tick)¹⁶.

Developing a method to expose research volunteers to tick bites supports studies using the natural vector to search for evidence of infection, a procedure known as xenodiagnosis^{17,18,19,20,21}, and to learn more about immunity induced by exposure to ticks, which can contribute to the discovery of an anti-tick vaccine^5,6,7. The procedure described here was developed and used in the first human research study using laboratory-reared Ixodes scapularis larva for xenodiagnosis of B. burgdorferi infection after antibiotic therapy (NCT01143558), published in 2014¹⁹. The system has been successfully used in a phase 2 study investigating whether a positive xenodiagnosis correlated with the persistence of symptoms after antibiotic treatment of Lyme disease (NCT02446626) and in a study exploring the host response to tick bites (NCT05036707).

This procedure protocol describes the process for creating the containment dressing, the tick placement procedure, and the tick removal procedure, as well as the site care needed to maintain the containment dressing. The details regarding the pathogen-free I. scapularis tick colony and tick exposure procedures used for the above-cited studies have been previously described^19,22. This methodology offers a flexible research tool that can be adapted to studying different aspects of the human host response to tick bites, the effectiveness of tick prevention medications, as well as Lyme disease and other tickborne illnesses.

Protokół

Exposing volunteers to tick bites is an experimental method and must be conducted under a clinical research protocol approved by the relevant regulatory authorities. The clinical studies (NCT01143558, NCT02446626, and NCT05036707) were approved by the respective institutional review boards, conducted under investigational device exemptions granted by the US Food and Drug Administration, and carried out in accordance with Good Clinical Practice guidelines. Additionally, these studies were registered with ClinicalTrials.gov, and written informed consent was obtained from all participants.

1. Containment dressing preparation

1. To create a measurement template, photocopy the 3" x 3" hydrocolloid containment dressing along the side of the measurement lines (Figure 1A and Table of Materials).
2. Cut the photocopied template along the 1" line. Then, cut along the 2" line, creating a 2" circle with a 1" center (Figure 1B).
3. Using this template, cut the non-adhesive foam dressing to create a 2" circle with a 1" center (Figure 2A,B, and Table of Materials).
4. Using the same template, cut a 2" circle in the center of the 4" x 4" extra-thin hydrocolloid dressing (Figure 2C,D).
5. Cut the hydrocolloid layer of the 3" x 3" containment dressing at the 1½" line.
6. Pull back the paper liner of the hydrocolloid layer of the containment dressing to expose the adhesive. Place the cut foam dressing within the 2" circle of the adhesive (Figure 3A). This will ensure that the foam dressing extends beyond the opening of the containment dressing. Replace the paper liner onto the hydrocolloid layer of the dressing (Figure 3B).
   ​NOTE: These measurements were used for studies involving up to 10 larval I. scapularis ticks. For studies involving up to 30 larval I. scapularis ticks, the template for the foam center was cut between the 1" and 1½" markings to allow for a larger area for placement. The measurements of the containment dressing can be adapted accordingly to the study design.

2. Tick placement

1. Tick storage
   1. Store the larval I. scapularis ticks in a medical-grade refrigerator with the temperature set to 9-10 °C.
   2. Store the ticks in a vial covered with a mesh cap to allow adequate airflow. Package the vial of ticks in a sealable plastic bag with a lightly moistened sponge to maintain humidity and prevent desiccation.
2. Placing the ticks
   1. Remove the ticks from the refrigerator to allow them to warm to room temperature. The ticks can become active in as little as 15 min outside the refrigerator.
   2. Position a white sheet around the placement area to easily visualize any inadvertently dropped tick.
   3. Dampen a 4"x 4" gauze pad with clean water or saline and use it to clean the skin of the placement site. The area should be patted dry or allowed to air dry.
      NOTE: When selecting the body site for tick placement, consider areas that do not restrict the volunteer's daily activities and where the integrity of the dressing can be easily monitored by the participant. Do not place ticks in areas where they could be crushed due to direct pressure (e.g., the back). Evaluate the participant's body hair, and if necessary, carefully trim excessive body hair using scissors at the site of the adhesive, avoiding damage to the skin. Avoid using a razor for hair removal, as it may increase the risk of skin injury and infection.
   4. Once the skin is dry, remove the adhesive liner from the hydrocolloid side of the prepared dressing and firmly affix it to the skin at the placement site, ensuring that the mesh layer remains open.
   5. Break the wooden swab (see Table of Materials) in half to expose jagged ends.
   6. Carefully open the vial of ticks.
   7. Using the jagged end of the wooden swab, transfer active ticks from the vial and place them onto the skin within the containment dressing (Figure 4A). Have an additional team member monitor the tick placement site to ensure the ticks remain inside the containment dressing. Use adhesive tape to trap ticks that escape from the vial during this process.
      NOTE: Breaking a wooden swab and using the jagged end is effective for tick manipulation and transfer without causing damage to the ticks. A small brush with soft bristles in a light color also works for transferring ticks from the container to the skin site.
   8. Once the specified number of ticks for the study is placed, remove the adhesive liner from the mesh layer of the containment dressing and close it over the opening.
   9. After ensuring that the adhesive is securely attached, apply the prepared 4" x 4" extra-thin hydrocolloid dressing over the containment dressing for additional security (Figure 4B).
      ​NOTE: Additional information regarding the pathogen-free I. scapularis tick colony and the duration of the tick exposure procedures used in the cited studies can be found in the methods sections of the published studies^19,22.

3. Site care

1. Provide a water barrier cover and 2" hypoallergenic tape (see Table of Materials) to the research participant.
2. Provide instructions for caring for the tick containment site.
   1. Periodically check the dressing and monitor the edges for secure adhesion to the skin. Apply additional tape to reinforce the dressing edges as needed.
   2. Do not scratch the dressing site, and refrain from opening or removing the dressing during the study period.
   3. Avoid bathing or soaking in water. Also, avoid hot showers and limit the duration of showering. Before showering, protect the containment dressing with the provided water barrier cover and remove it after completing the shower. Gently pat the dressing dry, and inspect it to ensure it remains secure.
   4. Avoid activities that may lead to heavy perspiration and compromise the dressing's integrity (e.g., aerobic exercise, extended hikes).

4. Tick removal

1. Collection vials preparation
   1. Label the tick collection vials with the desired identifiers (e.g., study code, date).
   2. If preservation of live ticks is necessary, puncture holes in the cap of the vials with a 20 G needle. Use a layer of nylon or polyester mesh along with the cap to seal the vial. Place a small piece of moist sponge in a sealable bag with the vial of ticks to maintain humidity.
2. Removing the ticks
   1. Place white sheets around the bandage site and ensure a collection basin is within reach.
   2. Using alcohol wipes, carefully remove the bandage from the skin.
   3. Once fully removed, place the bandage in the collection basin and examine it for the presence of detached ticks.
   4. Assess the placement site for attached ticks. Ticks that have fed to repletion will detach (Figure 5A). For the remaining attached ticks, use fine-tip forceps for removal (Figure 5B).
   5. Clean the skin with alcohol wipes or soap and water.

Wyniki

The study demonstrated that the procedure is safe and well-tolerated, with the primary adverse event being mild itching at the site of the bites, observed in 58% of the procedures. There were no serious adverse events related to the procedure when using clean laboratory-reared larval I. scapularis ticks¹⁹. In the 43 procedures conducted, the mean percentage of recovery of attached ticks compared to placed ticks was 45% ± 27% (SD), with a median percentage of 40% (

Dyskusje

While animal studies involving exposures to ticks^4,5,6,7,21 have been invaluable in increasing our understanding of host response to tickborne diseases and tick bites, these models have limitations in how well they predict the human host response. This model, which describes the methodology for exposing humans to tick bites in a controlled manner, can be easil...

Materiały

Name	Company	Catalog Number	Comments
20 G needle	Any brand		For puncturing the vial cap.
3" x 3" containment dressing	Monarch Labs Names	LeFlap	https://www.monarchlabs.com/ordering
4" x 4" extra-thin hydrocolloid dressing	ConvaTec	DuoDerm	https://www.convatec.com/products/advanced-wound-care/brand-names/pc-wound-duoderm-granluflex/duoderm-extra-thin-dressing/
4" x 4" gauze	Monarch Labs Names		For cleaning skin
Clean water or saline			For cleaning skin
Moisture barrier (e.g. 7" x 7")	AquaGuard	TIDI	For showering, ttps://www.tidiproducts.com/product-listing/aquaguard-shower-cover-sheets
Non-adhesive foam dressing	Coloplast	Biatain	https://www.coloplast.us/biatain-non-adhesive-en-us.aspx
Roll of 2" hypoallergenic tape	Monarch Labs Names	Durapore	For reinforcing containment dressing.
Roll of adhesive tape			For trapping ticks
Vials for collection (e.g. cryovials)	Ependorf	ECC200