Ticks, like many parasites, are not random in their selection of a host; they use sensory tools to find a meal. The perception that ticks prefer some people over others is accurate and stems from variations in the specific signals each person emits. Differences in human biology, particularly in chemical and thermal output, create unique “signatures” that certain ticks find more attractive. This explains why some individuals are more frequently targeted than their companions, even when sharing the same environment.
Chemical Signals Ticks Use to Hunt
The primary method ticks use for long-distance host detection is chemoreception, or the sense of smell. Their most important sensory organ, the Haller’s organ, is located on the first pair of forelegs and is constantly waved in the air to detect chemical signals. This organ is highly sensitive to carbon dioxide (CO2), which acts as the main long-range attractant for ticks, signaling the presence of a breathing animal. Ticks can detect minute changes in CO2 concentration, allowing them to locate a potential host from several meters away.
As a tick gets closer, it detects a plume of volatile organic compounds (VOCs) emitted from the host’s skin and breath. These compounds include specific chemicals such as lactic acid, ammonia, and certain phenols, which are byproducts of metabolism and microbial activity on the skin. Lactic acid is a known attractant that guides the tick toward the skin surface. The unique blend of these VOCs provides a specific chemical fingerprint that ticks can use to distinguish between potential hosts.
Thermal and Physical Cues
Once chemical signals guide a tick into close proximity, secondary thermal and physical cues serve as confirmation signals for host presence. Ticks possess thermoreceptors, particularly within the Haller’s organ, that detect radiant heat emitted from a host’s body. This allows them to home in on a warm target, such as a human at body temperature, from a distance of up to four meters.
Ticks also use hydroreception to detect moisture and humidity, which is a strong indicator of a breathing, sweating host. Physical cues, such as vibrations in the substrate or the shadow of a passing animal, will trigger the characteristic “questing” behavior. Questing involves the tick climbing onto vegetation and extending its front legs to latch onto a host.
Why Human Chemistry Varies
The reason ticks are attracted to some people more than others lies in the biological variations that affect the quantity and type of chemical and thermal signals. Differences in metabolic rate directly influence the amount of carbon dioxide (CO2) and heat a person emits. Individuals with a higher resting metabolic rate, or those who are exercising, exhale more CO2 and radiate more heat, making them a larger target signal for seeking ticks.
The skin microbiome is a major factor contributing to differential attraction. The thousands of bacteria living on the skin break down compounds in sweat, such as amino acids and sebum, into volatile organic compounds. A person’s specific mix of skin bacteria determines the chemical cocktail of VOCs released, and certain microbial communities produce attractant chemicals in greater quantities. This unique microbial fingerprint creates a distinct scent profile that ticks can prefer or ignore.
Genetic factors play a role in determining the composition of sweat and the baseline skin microbiome, contributing to a consistently appealing or unappealing profile. Additionally, diet can temporarily alter the volatile compounds released through breath and sweat. The presence of certain volatile breakdown products from food can modify the overall chemical signature.
Some common theories, such as a preference for a specific blood type, have been explored in limited studies, though the evidence is not conclusive for most tick species. One pilot study suggested a preference for Type A blood and a lower attraction to Type B blood in a laboratory setting. However, this finding has not been replicated in real-world conditions, meaning blood type is not a reliable predictor of tick attraction.