Tick-borne diseases are increasing worldwide, and there are currently few effective ways to prevent them. One promising new strategy is to develop a vaccine that targets the tick itself ("anti-tick vaccine") to block the transmission of multiple pathogens. Some animals naturally develop resistance to ticks after repeated tick bites. There are indications that a similar form of acquired tick resistance (ATR) may also occur in humans who have been heavily exposed to ticks. This study investigates whether humans can naturally develop tick resistance and how this affects tick feeding. The investigators will compare people with a long history of tick bites and self-reported signs of tick resistance to people who are tick-naïve. The investigators will also include a small group of volunteers who previously participated in an experimental human tick challenge to evaluate what their reaction on tickbites is 1-2 years after their initial exposures.

Study Objectives Primary Objective To determine whether humans develop naturally acquired tick resistance by measuring tick feeding success, specifically the weight of ticks after feeding.

Secondary Objectives To measure additional tick feeding parameters (tick mortality, attachment rate, duration of feeding, and molting).

To assess clinical skin reactions such as itch and redness. To study immune responses (antibodies and immune cells) against tick saliva proteins in all groups.

To identify tick salivary antigens that may be useful for future anti-tick vaccine development.

Study Design Single-center, open-label human experimental tick challenge study.

Three groups:

ATR group: 11 adults with extensive prior tick exposure and self-reported signs of tick resistance.

Control group: 11 adults with no significant tick exposure. Follow-up challenge group: Up to 4 adults previously challenged in the "TICK ME" study.

Outcome measures Primary Outcome Measure Post-feeding tick weight

Secondary Outcome Measures Tick mortality, attachment rate, days attached, and molting success Clinical skin responses (itch, redness) Immune responses to tick salivary proteins Skin biopsy analyses

Inclusion Criteria:

Naturally acquired self-reported tick resistant individuals:

• Age: 18 - 70 years;
• History of repeated tick bites
• Local reaction (like redness or itch) at place of previous tick bites and/or signs of impaired tick feeding after bite (anamnestic)

Controls:

• Age: 18 years - 70 years;

Experimentally acquired tick resistant individuals:

• Previous participant in the TICK ME study that has completed all 4 challenges
• Age: 18 years - 70 years;

Exclusion Criteria:

• Current suspicion of any tick-borne disease
• Diagnosed with tick-borne disease within the last year
• Chronic skin condition affecting the skin of the arm;
• Inability to maintain the dressing for any reason;
• Known immunodeficiency or autoimmune disease;
• Use of systemic immunomodulating drugs or chemotherapy, current or in the past;
• Chronic use of antibiotics;
• Unable to give informed consent or do not have a thorough command of the Dutch language;
• Refusal to participate in specimen collection and storage for future study related use;
• Pregnant or breastfeeding women;
• Not willing to use adequate contraception during the study period;
• Red meat allergy (both medically confirmed and self-reported);
• Use of investigational therapy and devices during the time of the study;
• Any other condition that, in the opinion of the investigator, would make the patient unsuitable for enrolment or could interfere with the patient participating in and completing the study.

Additional exclusion criteria for controls:

• Known history of tick bites;
• Positive Borrelia serology (VlsE1/PepC10 ELISA);
• Known history or current suspicion on any tick-borne disease

Additional exclusion criteria for experimentally acquired tick resistant individuals:

• History of tick bites acquired outside of TICK ME study;
• Positive Borrelia serology (VlsE1/PepC10 ELISA);
• Known history or current suspicion on any tick-borne disease

The descision about sharing the IPD will follow